Sues et al, 2002

Page 1

Journal of Vertebrate Paleontology 22(3):535–547, September 2002 q 2002 by the Society of Vertebrate Paleontology

IRRITATOR CHALLENGERI, A SPINOSAURID (DINOSAURIA: THEROPODA) FROM THE LOWER CRETACEOUS OF BRAZIL HANS-DIETER SUES1, EBERHARD FREY2, DAVID M. MARTILL3, and DIANE M. SCOTT4 Department of Palaeobiology, Royal Ontario Museum, 100 Queen’s Park, Toronto, Ontario M5S 2C6 and Department of Zoology, University of Toronto, Toronto, Ontario, M5S 3G5, Canada, hdsues@rom.on.ca; 2 Abteilung fu¨r Geowissenschaften, Staatliches Museum fu¨r Naturkunde Karlsruhe, Erbprinzenstrasse 13, D-76133 Karlsruhe, Germany; 3 School of Earth and Environmental Sciences, University of Portsmouth, Burnaby Road, Portsmouth PO1 3QL, U.K.; 4 Department of Zoology, University of Toronto at Mississauga, Mississauga, Ontario, L5L 1C6, Canada 1

ABSTRACT—The holotype of Irritator challengeri Martill et al., 1996 from the Romualdo Member of the Santana Formation (Lower Cretaceous) in northeastern Brazil represents the most complete skull of a spinosaurid known to date. The now fully prepared specimen provides much new information on the cranial structure in these enigmatic predatory dinosaurs. The skull is remarkably narrow, especially in the region of the elongated snout. The maxillae are in broad contact along the midline, forming an extensive secondary bony palate. The maxillary teeth have straight or slightly recurved, conical crowns, with thin, fluted enamel and distinct but smooth carinae. As in Baryonyx walkeri, the anterior and ventral processes of the lacrimal meet at a more acute angle than in most non-avian theropod dinosaurs. The braincase is short anteroposteriorly but deep dorsoventrally, extending ventrally far below the occipital condyle. Irritator challengeri most closely resembles Spinosaurus aegyptiacus in the structure of its teeth, but more extensive comparisons between the two taxa are currently impossible due to the limited amount of cranial material known for the latter.

INTRODUCTION The Romualdo Member of the Santana Formation in northeastern Brazil is well-known for the abundance, taxonomic diversity, and often exceptional preservation of its vertebrate fossils, especially fishes (Maisey, 1991; Martill, 1993). In recent years, skeletal remains of dinosaurs have been repeatedly reported from this unit (Frey and Martill, 1995; Kellner and Campos, 1996; Kellner, 1996, 1999; Martill et al., 1996; Martill et al., 2000). Although the material recovered to date is fragmentary, it is typically well preserved. The most remarkable among these fossils is the nearly complete skull of an unusual theropod dinosaur, which is housed in the collections of the Staatliches Museum fu¨r Naturkunde Stuttgart (SMNS 58022). In its unprepared state, it was originally identified as the skull of a large pterosaur. Its dinosaurian affinities were first recognized by Martill et al. (1996), who designated this specimen as the holotype of a new taxon, Irritator challengeri, which they placed in the Maniraptora. Kellner (1996) found no evidence in support of maniraptoran affinities and referred Irritator to the Spinosauridae, a group of rather poorly known but distinctive tetanuran theropods. This assignment has been widely accepted (Charig and Milner, 1997; Sereno et al., 1998; Taquet and Russell, 1998), and is fully supported by the present study. The Santana Formation is a poorly defined stratigraphic unit of the Araripe Group in the intracontinental Araripe Basin of northeastern Brazil. Strata of the Araripe Group crop out at the foot of the Chapada do Araripe, a vast plateau on the borders of the states of Ceara´, Pernambuco, and Piauı´, with possible correlative deposits in basins to the south and west (Martill, 1993). The Santana Formation constitutes a diverse suite of sedimentary rocks. In its lower portion, a series of variegated clays and silty clays with channel sandstones, conglomerates, and thin bituminous shales is overlain by silty, greyish-green clays with abundant carbonate concretions. The matrix enclosing the holotype of Irritator challengeri closely corresponds to that of the concretions from the Romualdo Member of this unit.

The Romualdo Member is a well-known source of vertebrate fossils, and there is little doubt that the specimen described here was derived from it. As partial confirmation of this provenance, preparation of the skull yielded fossils of the ostracod Pattersoncypris as well as isolated scales of the ichthyodectid fish Cladocyclus, both of which are commonly found in concretions from the Romualdo Member. D. M. M. recently showed a photograph of the unprepared skull to several fossil dealers in Santana do Cariri. One dealer who had also worked as a collector recalled the specimen and recollected that it was found near Buxexe´, a site near Santana do Cariri. The nature of the concretion as well as its color and texture are consistent with concretions from this region. Buxexe´ is a small farming community located at about 650 m above sea-level on the flanks of the Chapada do Araripe in the south of Ceara´, about 5 km south of the famous fossil-producing region of Santana do Cariri. It is only accessible by rough track. There are several localities in this region where fossils are collected from the Romualdo Member, and teams of local collectors move from place to place. It is possible that the holotype of Irritator challengeri came from one of several localities in the valley of the Caririacu River, and we are unable to determine the provenance of this specimen more precisely than to confirm its derivation from the Romauldo Member of the Santana Formation. Despite the abundance and taxonomic diversity of its fossils, the precise stratigraphic age of the Romualdo Member remains uncertain (Martill, 1993). Gardner (1846) considered the Santana Formation Cretaceous in age based on lithological similarities to the Cretaceous strata in the south of England. Braun (1966) proposed an Early Cretaceous age for this formation on the basis of similarities to the Wealden strata of northwestern Europe. Lima (1978) regarded the Santana Formation as Aptian in age based on palynological data. More recent work by Pons et al. (1990) suggested a late Aptian to early Albian date for the underlying Crato Formation and a mid- to late Albian age for bituminous shales with fossils of the aspidorhynchid fish Vinctifer from the Romualdo Member. Between the Crato For-

535


536

JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 22, NO. 3, 2002

FIGURE 1. Skull and mandible of Irritator challengeri, SMNS 58022 (holotype), in A, left lateral and B, right lateral views. Scale bar equals 5 cm. Most of the dentigerous portion of the right maxilla was broken off during collection and has been reattached to the snout for the purpose of this drawing. Areas in white represent matrix and/or epoxy filler; uniform grey shading indicates opening. Abbreviations used in Figs. 1–4: an, angular; ao.f, antorbital fenestra; ar, articular; b.p, basipterygoid process; bo, basioccipital; bs, basisphenoid; ch, choana; d?, dentary?; e.m.f, external mandibular fenestra; e.n, external naris; f, frontal; f.c, facet for coronoid; f.m, metotic foramen; f.o, fenestra ovalis; g.f, glenoid fossa; it.f, infratemporal fenestra; l, lacrimal; ls, laterosphenoid; ls.c, laterosphenoid condyle; m, maxilla; m.a, maxillary antrum; n, nasal; n.c, nasal crest; n.p, nasal projection; o, orbit; o.c, occipital condyle; os, orbitosphenoid; p, parietal; p.s, process on basisphenoid/parasphenoid rostrum; pl, palatine; pm, premaxilla; pn, lateral pneumatic recess on basisphenoid; pn.o, ‘‘postnasal fenestra’’ (see text for discussion); po, postorbital; po.p, paroccipital process; pr, prootic; prf, prefrontal; pt, pterygoid; q, quadrate; qj, quadratojugal; r, basisphenoid recess; s, stapes; sa, surangular; so, supraoccipital; sq, squamosal; v, vomer; v.f, foramen for V. cerebralis media; x, hole produced by post-mortem damage. Roman numerals represent foramina for passage of cranial nerves; V1 denotes groove for ramus ophthalmicus of N. trigeminus.

mation and the Romualdo Member of the Santana Formation, there exists a significant (if variable) thickness of strata, which includes several intervals of non-deposition (Silva, 1983). Thus the concretion-bearing horizon of the Romauldo Member is probably younger than early Albian. Berthou (1990) suggested a mid- to late Albian age for the concretions from the Romualdo Member, but palynological data for this horizon are inadequate (Maisey, 2000). Dates based exclusively on the vertebrate assemblage are somewhat unreliable due to uncertainties regarding the stratigraphic ranges for most taxa. Furthermore, many vertebrate taxa described from the Santana Formation may have been endemic to the Araripe Basin. However, Vinctifer, which is frequently found in concretions from the Romualdo Member, has also been reported from the upper Aptian of Colombia (Schultze and Sto¨hr, 1996) and the Aptian and lower Albian of Venezuela (Moody and Maisey, 1994). Several fish taxa from the Romualdo Member also occur in Aptian- to Cenomanianage strata of Morocco, including the giant coelacanthid Mawsonia (Wenz, 1981) and the amiid Calamopleurus (Forey and Grande, 1998). On balance, the age of the Romualdo Member is probably late Early Cretaceous (Albian; Maisey, 2000), but it could be as young as Cenomanian. Ever since Stromer’s (1915) original description of Spinosaurus aegyptiacus, the distinctive skeletal structure of these enigmatic predatory dinosaurs has intrigued paleontologists. The most noteworthy features of S. aegyptiacus are the enormously elongated (up to 1.65 m tall) neural spines on the dorsal

vertebrae (Stromer, 1915, 1934a, 1936) and the conical tooth crowns with distinct but smooth carinae (Stromer, 1934b). Although isolated teeth and bones referable to Spinosauridae are not uncommon in the Albian- to Cenomanian-age continental strata of North Africa (Stromer, 1915, 1936; Bouaziz et al., 1988; Buffetaut, 1989; Russell, 1996; Kellner and Mader, 1997; Sereno et al., 1998; Taquet and Russell, 1998; Benton et al., 2000), little has been known to date about the structure of the skull of these theropods. So far, only the fragmentary and largely disarticulated cranial remains of the holotype of Baryonyx walkeri (BMNH R9951; Charig and Milner, 1986) from the Wealden (Lower Cretaceous: Barremian) of Surrey (England) have been described in detail (Charig and Milner, 1997); Sereno et al. (1998:1301, note 18) reidentified several of these bones. D. M. S. has fully prepared the holotype of Irritator challengeri (SMNS 58022) using only mechanical means. Preparation proved to be slow and difficult due to the hardness of the calcareous matrix, which includes numerous spaces filled by dark brown, sparry calcite. It has invalidated a number of observations originally reported by Martill et al. (1996), and the specimen now provides a wealth of information on the cranial structure of this unusual theropod dinosaur. We present here a detailed description of the holotype of Irritator challengeri, which represents the most complete skull of a spinosaurid known to date and thus is of great interest. Anatomical terminology (including positional terms) employed in this paper largely follows the standard English-lan-


SUES ET AL.—SKULL OF IRRITATOR

FIGURE 1.

guage usage of comparative anatomy rather than the nomenclature of veterinary anatomy. Institutional Abbreviations BMNH, The Natural History Museum (formerly British Museum [Natural History]), London; SMNS, Staatliches Museum fu¨r Naturkunde Stuttgart. SYSTEMATIC PALEONTOLOGY THEROPODA Marsh, 1881 TETANURAE Gauthier, 1986 SPINOSAUROIDEA Stromer, 1915 sensu Sereno et al., 1998 SPINOSAURIDAE Stromer, 1915 sensu Sereno et al., 1998 IRRITATOR CHALLENGERI Martill et al., 1996 Holotype SMNS 58022, nearly complete skull, lacking the anterior portion of the snout and preserved together with bones from both mandibular rami in a large calcareous concretion (Figs. 1, 2). Locality and Horizon Romualdo Member of the Santana Formation, Chapada do Araripe, northeastern Brazil. See preceding discussion concerning stratigraphic age and provenance. Diagnosis Nasals with prominent median bony crest that terminates posteriorly in knob-like, somewhat dorsoventrally flattened projection. Dorsal surface of parietals facing posterodorsally and vertical axis of braincase inclined anteroventrally. Posterior surface of basisphenoid with deep, dorsoventrally oval median recess. Surangular with broad lateral shelf. As the holotype of Irritator challengeri represents the only substantially complete skull currently known for any spinosaurid, it is not clear at what taxonomic level the individual features listed above are diagnostic. Furthermore, the cranial structure of related basal tetanuran theropods remains poorly known. Among Spinosauridae, Irritator challengeri most closely resembles Spinosaurus aegyptiacus Stromer, 1915, with which it shares several apomorphic character-states in the dentition: Maxillary tooth crowns straight or slightly recurved and conical rather than labiolingually compressed; carinae of tooth

537

Continued.

crowns distinct but devoid of serrations; enamel thin, with vertical ridges (‘‘fluting’’) on both the labial and lingual surfaces of the crown (Stromer, 1915, 1934b; Sereno et al., 1998). Indeed, Irritator may well prove to be congeneric with Spinosaurus, but the current lack of cranial material for the latter makes detailed comparisons between the two taxa impossible. Comments Kellner and Campos (1996) described a new taxon of spinosaurid theropod, Angaturama limai, on the basis of the poorly preserved anterior end of a snout with broken teeth from the Romualdo Member of the Santana Formation. Direct comparisons with the holotype of Irritator challengeri are not possible because the two specimens represent different regions of the skull and we were unable to examine or obtain a cast of the holotype of A. limai. However, the photographs published by Kellner and Campos (1996:figs. 2A, 4) and Kellner (1996:figs. 2, 3) show that the premaxillae of Angaturama are remarkably narrow transversely and form a median crest dorsally, matching the narrow snout and the dorsomedian crest on the nasals of Irritator. In the preserved portion of the right maxilla (Kellner and Campos, 1996:figs. 2A, 3), the anterior (second and third) teeth are large as in Irritator. Furthermore, as in Irritator, the tooth crowns are round rather than labiolingually compressed in transverse section and have distinct but smooth anterior (mesial) and posterior (distal) carinae at least near the base of the crown (Kellner and Campos, 1996:fig. 2C). Thus we follow Charig and Milner (1997) in considering Angaturama limai a subjective junior synonym of Irritator challengeri, which has priority by one month. Sereno et al. (1998: 1301) even suggested that the holotype of ‘‘Angaturama limai’’ might represent the rostral end of the snout of SMNS 58022. The family-level taxon Irritatoridae Martill et al., 1996 only comprises (and thus, in a phylogenetic classification, is redundant with) Irritator challengeri, which we refer to Spinosauridae Stromer, 1915 sensu Sereno et al., 1998 (including Baryonychidae Charig and Milner, 1986).


538

JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 22, NO. 3, 2002 DESCRIPTION The retouched photograph of SMNS 58022 published by Martill et al. (1996:fig. 2) presented a left lateral view of the then largely unprepared fossil. The principal openings of the skull (shown in black for contrast) were still filled with matrix but the latter has since been removed to reveal additional detail, including previously unrecorded cranial bones. The alleged frontoparietal crest does not belong to the specimen as no actual contact can be established between this fragment of indeterminate bone and the skull roof. The dorsal surfaces of the right frontal and both parietals as well as the posterodorsal portion of the supraoccipital have been largely destroyed, possibly due to exposure along the edge of the concretion containing the skull. SMNS 58022 was preserved lying on its side in an early diagenetic calcareous concretion. The crude initial preparation undertaken by the local collector inflicted considerable damage to the fossil because the hard limestone of the concretion does not always separate cleanly from the enclosed bone. Thus preservation of the bony surfaces on the left side of the skull (including the braincase) is generally poor, with extensive fracturing as well as loss of bone in many places. However, the bones of the right side of the skull are, for the most part, excellently preserved. The skull has been subjected to some crushing, mainly on the left side of the snout and the right side of the postorbital region. The anterior end of the snout, comprising most of both premaxillae and the anterior ends of both maxillae, is broken off. The broken surface is clean and unweathered, suggesting that the rostral portion of the snout was lost only during or after the recovery of the fossil. Numerous large and small fractures, some due to septarian cracking of the surrounding concretion, traverse the cranial bones. A major break extends almost vertically through the skull at the level of the anterior ends of the antorbital fenestrae and was crudely repaired with epoxy car-body filler by the collector. Some bony surfaces sustained damage during a poorly executed attempt by an unknown party to employ acid preparation. The right postorbital, squamosal, and quadrate, the left quadratojugal, and the right angular as well as both coronoids, dentaries (except possibly the posterior part of the left element) and splenials were probably already lost prior to fossilization. The parietals, premaxillae, and supraoccipital are only incompletely preserved. Most of the tooth-bearing portion of the right maxilla was separated from the skull during collecting and is now preserved as a detached fragment. The left jugal was slightly rotated clockwise anteroventrally. The left postorbital became detached along its sutural contacts and dropped into the left orbit prior to burial. Similarly, the left squamosal and quadrate were disarticulated. The former is now preserved medial to the left jugal, whereas the latter was displaced forward and is now exposed through the right antorbital fenestra. The left pterygoid became separated and somewhat rotated clockwise ventrally; its lateral surface was exposed by preparation. The posterior portion of the left mandibular ramus is no longer in articular contact with the skull, but lies close to it and its lateral surface is visible. The posterior portion of the right mandibular ramus was displaced so that the ventral edge of the surangular (which is exposed in both lateral and medial views) is now adjacent to the alveolar margin of the right maxilla. Skull

FIGURE 2. Skull of Irritator challengeri, SMNS 58022 (holotype), in dorsal view. Scale bar equals 5 cm.

In dorsal view, discounting some crushing on both sides, the skull is remarkably narrow throughout its entire length. The long and low snout is subtriangular in transverse section at its broken anterior end where it is much deeper (75 mm) than wide (48 mm). Although the anterior end of the snout is not preserved, the length of the preorbital region of the skull was more


SUES ET AL.—SKULL OF IRRITATOR than twice that of the postorbital region and significantly exceeded the greatest height of the skull. The sides of the snout are relatively flat and steeply inclined dorsomedially. The long axes of the antorbital fenestra and orbit extend posterodorsally. Similarly, the postorbital region of the skull roof faces posterodorsally, and the vertical axis of the braincase extends anteroventrally. The skull has an estimated length of about 600 mm, assuming rostral proportions similar to those for Baryonyx walkeri (Charig and Milner, 1997) and measured from the tip of the snout to the occipital condyle. (The much higher estimate published by Martill et al. [1996] was based on the then largely unprepared specimen including the alleged frontoparietal crest.) It is about 165 mm high at the level of the preorbital bar. As the sutures between the basioccipital, exoccipital-opisthotic, and supraoccipital are still discernable, SMNS 58022 may represent an individual that was not fully mature at the time of death (see Currie and Zhao, 1994). The external surfaces of the cranial bones are smooth where this condition can be confidently assessed on the specimen. However, Carr (pers. comm.) noted the presence of ‘‘immature’’ bone grain on the surangular. The oval external naris is about three times longer anteroposteriorly than tall dorsoventrally (76 mm vs. approximately 25 mm for the left naris). It is situated well behind the anterior end of the elongate snout. The large antorbital fenestra has a longest (posterodorsal) diameter of about 145 mm (left opening). Its anterior margin is rounded, but its dorsal and ventral margins converge posterodorsally, giving the opening an obliquely ellipsoidal outline. The ‘‘preantorbital fenestra’’ and ‘‘maxillary process’’ identified by Martill et al. (1996:fig. 3a, ‘‘mxp’’ and ‘‘p.an.f’’) are, in fact, the choana and part of the palatine and vomer, respectively; these structures are exposed in lateral view through the antorbital fenestra on both sides of the snout. The maxilla lacks accessory antorbital (maxillary) and promaxillary fenestrae. Only the posterior edge of the medial wall of the antorbital fossa was exposed along the anterodorsal margin of the antorbital fenestra. Posteriorly, the antorbital fossa is weakly incised onto the lateral surface of the lacrimal and does not extend onto the lateral surface of the jugal. The tall, somewhat keyhole-shaped orbit is round and widest in its dorsal portion (which housed the eyeball), but, more ventrally, its anterior and posterior margins converge anteroventrally. The longest (posterodorsal) diameter of the right orbit is 130 mm. The small ‘‘supratemporal fenestra’’ identified by Martill et al. (1996:fig. 3a, ‘‘stf’’) represents an artificial hole in the left sidewall of the braincase just behind the dorsal condyle of the laterosphenoid. The medial margins of the actual supratemporal fenestrae are formed by the more or less vertical lateral surfaces of the parietals and laterosphenoids. The greatest anteroposterior diameter of the (right) infratemporal fenestra is 77 mm. Premaxilla Only the posterodorsal extremity of the premaxilla is preserved. Posteriorly, the premaxilla bifurcates to form the anterior and anteroventral margins of the external naris. The short ventral process of this bifurcation contributes to the shelf-like ventral margin of the narial fenestra, whereas the (now largely destroyed) dorsal process contacted the anterior end of the nasal. Maxilla Anteriorly, the maxilla forms a greatly elongated and dorsoventrally deep subnarial ramus, which contributes most of the concave ventral margin of the external naris and separates the nasal and premaxilla below this opening. The lateral surfaces of the maxillae are only slightly inclined dorsomedially towards each other. Most of the lateral portion of the right maxilla has been broken off, exposing a large, conical maxillary antrum (sensu Witmer, 1997a), which extends in the body of the bone forward to the posteroventral margin of the external naris. The maxilla is deeply embayed posteriorly by the anterior and ventral margins of the antorbital fenestra and

539

forms the medial bony wall to the antorbital fossa and maxillary antrum. The medial wall of the maxillary antrum is perforated by a large oval opening, as in Allosaurus (Witmer, 1997a). Medially, the maxillae broadly contact each other to form a secondary bony palate; a distinct ridge, which is clearly visible in transverse section at the broken anterior end of the snout, extends along their median suture. The slender ascending process of the maxilla curves posterodosally and tapers towards the posterior end, which overlaps the anterior ramus of the lacrimal above the antorbital fenestra. The left maxilla preserves nine tooth crowns or stumps of crowns; the first and third tooth show details of the crown. Most of the dentigerous portion of the right maxilla was separated from the skull by the collector but can be readily fitted back onto it, using a major vein of dark, sparry calcite traversing the bone and a broken anterior tooth as landmarks (Fig. 1B). The right maxilla holds nine mostly damaged teeth, and there is the broken base and a partial impression in the matrix of the tenth crown at the incomplete posterior end of the bone. The broken dorsal surface of the posterior part of this jaw fragment shows oblique cross-sections of the functional teeth and of sometimes two replacement teeth in different stages of eruption. The deeply implanted, vertically oriented teeth are well separated from each other more anteriorly. Posteriorly, the maxillary tooth row terminates below the anterior end of the antorbital fenestra, well forward of the orbit. The broken anterior end of the snout reveals that the long, apparently slender root of the first preserved tooth in the left maxilla extends dorsally close to the level of the ventral margin of the external naris. The roots of the teeth from opposing sides converge medially, almost reaching the midline. The first and second preserved tooth in the left maxilla have the tallest crowns, with heights of 32 and over 40 mm, respectively, and a mesiodistal diameter of about 13 mm; their relative position in the maxilla is uncertain due to the loss of the anterior end of that bone. The crowns of the more posterior maxillary teeth are shorter. The left maxilla preserves nine teeth partially or completely, and the complete tooth row probably comprised at least 11 teeth. The crown of the last tooth in the left maxilla was in the process of eruption at the time of death, and only its tip is exposed. All tooth crowns are conical, tapering rather evenly towards the apex, and straight or at most slightly recurved. They are round in transverse section rather than labiolingually flattened as is typical for theropod teeth. The lumen of the pulp cavity is small. The anterior (mesial) and posterior (distal) carinae extend the full height of the crown to its apex. They are distinct but devoid of serrations; their relative position on the tooth crowns does not vary along the maxillary tooth row. The thin enamel is preserved in extensive patches on several maxillary tooth crowns. At higher magnification, it has a granular texture, as in Baryonyx (Charig and Milner, 1997: fig. 19; Sereno et al., 1998:fig. 2E). The labial surface of the left first and the right eighth tooth crown bears fine, short enamel ‘‘wrinkles’’ that extend obliquely toward the posterior carina, and the carina appears ‘‘beaded’’ at higher magnification on the right eighth tooth (Fig. 5). The enamel is distinctly fluted on all maxillary teeth, with vertical ridges extending the height of the crown on both the labial and lingual surfaces. The labial surfaces of the well-preserved seventh and eighth right tooth crowns each bear seven vertical ridges. In Baryonyx walkeri, only the lingual surface of the tooth crowns is fluted (Charig and Milner, 1997), but fluting is present on both the labial and lingual surfaces of tooth crowns referred to Spinosaurus from the Kem Kem beds of southern Morocco (Kellner and Mader, 1997). Nasal The long, narrow nasals form much of the anterior portion of the skull roof, extending from the external nares back beyond the anterior margin of the orbit. Dorsally, a longitudinal crest is developed along the straight internasal suture (which is


540

JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 22, NO. 3, 2002

still discernable more anteriorly). Its height cannot be determined with certainty due to extensive breakage along much of its dorsal edge. The crest appears to reach its greatest depth in the region above the antorbital fenestra. Posteriorly, it decreases in height and terminates in a median knob-like, somewhat dorsoventrally flattened projection (part of which has been abraded) that projects over the anterior end of the frontals and appears to be flanked on either side by a narrow opening in the skull roof. The latter is situated on either side of the posterior end of the nasals at their sutural contacts with the prefrontal laterally and the frontal posteriorly. The presence of a ‘‘postnasal fenestra’’ in Baryonyx walkeri was inferred by Charig and Milner (1997) on the basis of the shape of the posterior end of the fused nasals. However, it is likely that the openings in SMNS 58022 are the result of dorsal displacement of the nasals, which has slightly separated these bones at their sutural contacts with the frontal, lacrimal, and prefrontal. This interpretation is supported by the fact that the left opening is less distinct than the right. Additional cranial material is needed to confirm the presence of the postnasal fenestra in Baryonyx and Irritator. The nasal contacts the maxilla laterally along a nearly straight, forward-sloping suture. Anteriorly, it is emarginated by the posterior margin of the external naris. The nasal does not enter into the dorsal margin of the antorbital fenestra and fossa. The nasals are not fused medially throughout their entire length, and the internasal suture is clearly visible anteriorly where the dorsal surface of the snout was damaged. Lacrimal The lacrimal forms the slightly thickened posterodorsal margin of the antorbital fenestra and the anteroposteriorly broad bony bar separating the orbit from the antorbital fenestra. The long axes of its anterior and ventral processes enclose an angle of only about 408, similar to the condition in Baryonyx (about 358; Charig and Milner, 1997). The ventral process of the lacrimal is distinctly inclined posterodorsally and, in lateral view, flares ventrally to form a broad flange, which reaches its greatest anteroposterior width at the sutural contacts with the jugal and maxilla. The extensive anterior portion of this flange contacts the maxilla in an interdigitating suture; its posterior portion is thicker and forms the anterior margin of the orbit. The slender anterior process of the lacrimal contacts the nasal dorsally and medially, and is overlapped by the tapering dorsal process of the maxilla anteriorly close to the posterodorsal apex of the antorbital fenestra. Posterodorsally, the lateral surface of the better preserved right lacrimal bears a prominent, anteriorly facing recess at the junction of the anterior and ventral rami. This recess is partially concealed in lateral view by a thick bony ridge along the posterodorsal margin of the lacrimal, but it is more exposed than in Baryonyx walkeri. It contains no obvious openings into the body of the lacrimal, but it is possible that several pits within the recess may represent foramina that are still filled with matrix. Dorsally, the lacrimal is narrowly exposed on the skull roof. Its posterodorsal cornual process is lower and smoother than the tuberosity in B. walkeri (Charig and Milner, 1997:fig. 5A). Prefrontal The rather large prefrontal is triangular in lateral view and robust. Its lateral edge forms the rugose, thick anterior portion of the dorsal rim of the orbit, which continues anterolaterally onto the lacrimal. Its ventral process extends far anteroventrally along the medial surface of the lacrimal. Posteromedially, the prefrontal inserts into a broad anterolateral notch on the frontal. Dorsally, it is well exposed on the skull roof and forms the lateral margin of the postnasal fenestra. Frontal The frontals broadly contact the parietals along a transverse suture posteriorly. Dorsally, they form a distinct ridge along their median sutural contact. Their lateral margins converge only slightly anteriorly. The supratemporal fossa extends forward onto the dorsal surface of the posterolateral portion of the frontal. Most of the dorsal surface of the frontal is

gently concave transversely and smooth. Ventrally, low cristae cranii border a narrow median sulcus for the olfactory tracts of the forebrain; the interfrontal suture is discernable in the roof of this groove. The frontal forms the thin posterior portion of the dorsal rim of the orbit. Parietal In dorsal view, the parietals are much expanded anteriorly and posteriorly so that their lateral edges bordering the supratemporal fossae are distinctly concave. They form a nearly straight transverse suture with the frontals anteriorly. The interparietal suture can still be traced more anteriorly, but most of the posterodorsally facing dorsal surface of the parietals has been destroyed. The low occipital wings of the parietals extend posterolaterally as well as ventrally. Distally, each wing tapers and becomes slightly twisted to assume an almost vertical orientation, resting on the dorsal edge of the paroccipital process. The lateral surface of the occipital wing bears a depressed area for contact with the squamosal. Postorbital The postorbital is represented by the disarticulated left element. It formed the posterodorsal margin of the orbit and the anterior portion of the supratemporal bar. The postorbital is more or less T-shaped in lateral view, with a dorsal portion formed by the anterior and posterior processes and a broad ventral process, the distal end of which is still buried in the matrix. The anterolateral surface of the postorbital does not form a ‘‘horn’’ or rugosity. The anterior process curves anteromedially to contact the frontal and parietal. It is more robust than the short, tapering posterior process. The ventral process formed the more dorsal portion of the postorbital bar. It appears to lack any trace of a suborbital process. Jugal The jugal is dorsoventrally deep and mediolaterally compressed. Its much expanded anterior portion meets the maxilla in a deeply interdigitating suture anteriorly and the broad ventral process of the lacrimal dorsally. The sutural contact between the lacrimal and maxilla excludes the jugal from participating in the posterior margin of the antorbital fenestra (contra Martill et al., 1996). The dorsal edge of the jugal is notched by the narrow ventral margin of the orbit. On the better preserved right element, a low ridge extends longitudinally above the ventral margin. The tall, posterodorsally directed dorsal process of the jugal forms a long, steeply forward-sloping surface for articular contact with the ventral process of the postorbital. The slender posterior process of the jugal bifurcates and receives the anterior process of the quadratojugal posteriorly, forming the infratemporal bar. The dorsal prong of this bifurcation is more slender and shorter than the ventral one. Squamosal The displaced left squamosal is preserved medial to the left jugal. The lateral surface of its anterior process bears a posteriorly tapering groove for the reception of the posterior process of the postorbital. The dorsal surface is convex anteriorly and concave posteriorly, indicating a shallow supratemporal fossa that is bordered laterally by a low ridge. A short posterior process arises just behind a ventral concavity on the squamosal for the reception of the proximal head of the quadrate. Its distal end is dorsoventrally expanded and curves ventrally. The ventral process of the squamosal is broad anteroposteriorly. Quadratojugal The L-shaped quadratojugal is represented by the complete right element. It forms most of the ventral and posterior margin of the large infratemporal fenestra. The quadratojugal has a tall, anteroposteriorly broad, and nearly vertical dorsal process, which is divided by a lateral ridge into a broad posterolaterally facing and a narrower anterolaterally oriented surface, which probably contacted the ventral process of the squamosal. It would have contacted the ventral process of the squamosal dorsally. The more slender, tapering anterior process of the quadratojugal inserts into a slot on the infratemporal process of the jugal. Quadrate A displaced bone partially visible through the


SUES ET AL.—SKULL OF IRRITATOR

541

FIGURE 3. Braincase of Irritator challengeri, SMNS 58022 (holotype), in occipital view. Scale bar equals 5 cm.

right antorbital fenestra right behind the palatine represents the ?left quadrate. It is tall and has a vertical shaft terminating in the proximal articular head as well as an extensive, flange-like portion for contact with the pterygoid. The proximal head has a rounded triangular outline in dorsal view and forms a cap that is offset by a lip from the remainder of the bone. Palatine The tall and thin vomeropterygoid process of the palatine (sensu Witmer [1997a]) borders the choana and is exposed in lateral view through each antorbital fenestra. Anteriorly, it forms a deep and stout flange for contact with the vomer. Pterygoid The pterygoid is a long and thin bone. A distinct concavity marks the articular contact for the basipterygoid process of the basisphenoid. A short but dorsoventrally deep, winglike process for contact with the quadrate rises steeply posterior and lateral to the basipterygoid joint. The anteriorly extending palatine ramus of the pterygoid is long, thin, and almost straight. Braincase As in Baryonyx walkeri (Charig and Milner, 1997), the braincase (Figs. 3, 4) is short anteroposteriorly but deep dorsoventrally. The sutural outlines of its constituent bones are, for the most part, still discernable, especially on the well-preserved right lateral surface. The greatest width of the braincase, as measured posteriorly across the (slightly abraded) distal ends of the paroccipital processes, is about 100 mm. The foramen magnum has a concave dorsal and an almost straight ventral margin. It is 25 mm wide and 20 mm high. The dorsal margin of the opening is formed by the supraoccipital, and its lateral and ventral margins are made up by the exoccipitals except for a narrow median contribution to the ventral margin from the basioccipital. The occipital condyle is almost hemispherical but is flattened dorsally. It is largely formed by the basioccipital except for the dorsolateral corners, which are contributed by the exoccipitals. Its articular surface faces slightly posteroventrally. The condyle is set off from the braincase by a short neck. The olfactory tracts extend in about the same horizontal plane as the forebrain, and the hindbrain parallels the forebrain at a more ventral level.

FIGURE 4. Braincase of Irritator challengeri, SMNS 58022 (holotype), in right lateral view. Scale bar equals 5 cm.

FIGURE 5. Crown of the right eighth maxillary tooth of Irritator challengeri (SMNS 58022, holotype) in labial view. Anterior to the right of the figure. Scale bar equals 1 cm.


542

JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 22, NO. 3, 2002

Supraoccipital The supraoccipital is not fused to the exoccipital-opisthotic (otoccipital). In occipital view, its dorsal portion bears a conspicuous, slightly posterodorsally projecting nuchal crest along the midline, which is subtriangular in coronal section and served as the site of attachment for the ligamentum nuchae. The posterior surface of the supraoccipital is deeply recessed on either side of this crest, presumably for the insertion of M. rectus capitis. Breakage reveals that the supraoccipital is V-shaped in coronal section. A foramen in the notch at the dorsal contact between the forward sloping, wing-like lateral process (which presumably represents the fused epiotic, as in extant birds and crocodylians; Walker, 1990) and the dorsomedian process of the supraoccipital on either side represents the posterior opening for a vascular canal. This canal opens anteriorly in a small foramen situated on or close to the suture between the prootic and laterosphenoid, and probably served for the passage of V. cerebralis media (V. occipitalis externa of authors). Ventrally, a short median process of the supraoccipital enters into the dorsal margin of the foramen magnum. Orbitosphenoid The delicate orbitosphenoid is clearly discernable on the right side of the braincase. Dorsally, a large foramen, presumably for the passage of N. trochlearis (IV), is situated ventral and medial to the dorsal condyle of the laterosphenoid on the suture between the latter bone and the orbitosphenoid. Just ventral to this foramen on the right side of the braincase, there are two smaller openings, at least one of which represents the exit for N. oculomotoricus (III). Ventromedially, the orbitosphenoids meet and enclose between them a large opening, which presumably represents the passage for N. opticus (II). Laterosphenoid The laterosphenoid extends anterolaterally from the front of the braincase. It forms a transversely expanded dorsal condyle for articular contact with the frontal, parietal, and presumably postorbital. This condyle has a smooth, rounded articular surface. The lateral surface of the laterosphenoid is concave anteroposteriorly and convex dorsoventrally. Posteriorly, a single large foramen for the passage of N. trigeminus (V) and of V. cerebralis media is enclosed between the laterosphenoid and prootic. A deep sulcus, which probably carried ramus ophthalmicus of V1, and a less well developed groove extend anterodorsally from the foramen along the almost flat anterolateral surface of the laterosphenoid toward the dorsal condyle. This indicates that the three branches of N. trigeminus still left the braincase together through a single common opening and diverged only after exiting the braincase. Prootic The prootic is an irregularly shaped, robust element. Posteroventral to the trigeminal foramen, a slit-like opening for the passage of N. facialis (VII) is situated on a thick crest, which becomes a laterally projecting flange and extends onto the basisphenoid more ventrally. The latter represents the otosphenoidal crest (sensu Witmer, 1997b; crista prootica of authors). Just posterior to the crest, there is an extensive, deep recess that continues laterally as the broad stapedial groove along the anteroventral surface of the paroccipital process. It contains a pair of large foramina, which are separated from each other by a slender crista interfenestralis. The anterior and slightly more dorsally situated opening represents the fenestra ovalis (fenestra vestibuli of authors) into which fitted the head of the stapes, and the posterior foramen is the metotic foramen (sensu Walker, 1990) for the passage of N. glossopharyngeus, N. vagus, and N. accessorius (IX–XI). Although the sutural contact between the prootic and the exoccipital-opisthotic is not clear, at least the posterior and dorsal margins of the metotic foramen are formed by the exoccipital-opisthotic. There is no trace of a fenestra pseudorotunda (cochleae). Basioccipital and Basisphenoid Together with the basioccipital, the basisphenoid (which appears to be indistinguishably fused with the parasphenoid) forms a transversely narrow,

somewhat apron-like structure that extends ventrally far below the occipital condyle. The basioccipital bears a median depression rather than a ridge between the occipital condyle and basal tubera. The basal tubera are indistinct. Just below each tuber, an area for muscle insertion (‘‘oval scar’’ sensu Bakker et al., 1988) is developed along the posterolateral margin of the basicranium. The transversely concave posterior surface of the basisphenoid is marked by a deep, dorsoventrally oval median recess and, more dorsally, a median opening on the sutural contact between the basioccipital and basisphenoid. Both features are part of what is commonly referred to as the basisphenoid sinus, which is part of the median pharyngeal system (Witmer, 1997b). This differs from the condition in Baryonyx walkeri (Charig and Milner, 1997:fig. 9A) where a deep median ‘‘furrow’’ on the posterior surface of the basisphenoid tapers dorsally to terminate on the suture between the basioccipital and basisphenoid. The basipterygoid processes project anteroventrally and diverge only slightly ventrolaterally. Medially, they are linked by the transversely concave ventral edge of the body of the basisphenoid. The articular surface of each process faces ventrally and slightly medially. Just anterior to the sella turcica, the anterodorsally projecting cultriform process arises from the bases of the basipterygoid processes. It bears a blunt, slightly recurved dorsomedial projection; Currie (1985) interpreted a similar structure in Troodon as marking the posteroventral extent of the (otherwise unossified) interorbital septum. Anteroventral to the foramen for N. facialis (VII) and the otosphenoidal crest, the anteroposteriorly gently concave lateral surface of the basisphenoid is marked by a prominent, dorsoventrally oval recess. This depression contains two large foramina (which are fully exposed on the right side of the braincase) and represents the anterior (lateral) tympanic recess (Witmer, 1997b). The larger opening is situated on the anterodorsal margin of the depression and probably represents the posterior entrance of the canal for A. carotis interna (cerebralis), which would have passed anterodorsally to the sella turcica. The other foramen is located posteroventral to the former; it opens into the body of the basisphenoid and may have been pneumatic in origin. Exoccipital–Opisthotic As in most dinosaurs, the exoccipital and opisthotic appear to be indistinguishably fused into a single element (otoccipital). The compound bones do not meet along the midline but are separated by a narrow ventral process of the supraoccipital dorsal to the foramen magnum and by the basioccipital below that opening. The lateral surface of the condylar portion of the exoccipital is pierced by a single foramen for the passage of N. hypoglossus (XII) just anterior to the occipital condyle. (This opening is currently exposed only on the left side of the braincase.) The robust, short paroccipital process projects posterolaterally and ventrally so that its distal tip is situated at about mid-height of the occipital condyle. Its proximal end is thick, but the process gradually tapers posterolaterally. The distal end is neither expanded nor extended downward as, for example, in Allosaurus (Gilmore, 1920; Madsen, 1976). In lateral view, the lower half of the distal end of the process bears a concavity for possible contact with the quadrate. The concave dorsolateral surface of the bone is flattened by the articular surface for the squamosal. The ventral margin of the latter extends anterodorsally to the proximal end of the contact. Thus the medial process of the squamosal fitted into a broad groove formed by the otoccipital and parietal. Stapes On the right side of the skull, a slender, rod-like bone is preserved lying across the infratemporal fenestra and the lateral surface of postorbital process of the jugal. It is 55 mm long and has slightly expanded and flattened proximal and distal ends. The bone is identified as a stapes because it closely resembles the stapes of Allosaurus (Madsen, 1976) and Dromaeosaurus (Colbert and Ostrom, 1958). The presence of this


SUES ET AL.—SKULL OF IRRITATOR element in SMNS 58022 is noteworthy because only a few examples of a complete stapes in non-avian dinosaurs have been reported to date. Mandible The left mandibular ramus is represented by the articulated surangular, angular, articular, and prearticular, and the right by the articulated surangular and articular. The coronoid, dentary, and splenial are not preserved for either lower jaw. A possible exception may be the posterior end of the left dentary, but there is no clear sutural separation from the surangular posteriorly and the fragment in question shows no trace of alveoli. Surangular The large surangular is a more or less vertical and dorsoventrally deep bone, which comprises most of the dorsolateral portion of the mandibular ramus behind the tooth row. A prominent shelf extends along the lateral surface of the surangular from the level of the posterior end of the external mandibular fenestra back to the region anterior to the glenoid facet of the jaw joint. The dorsal surface of this shelf faces obliquely dorsolaterally and is concave anteroposteriorly as well as transversely. Anteriorly, the shelf fades into the dorsoventrally gently convex lateral surface of the surangular. The posterior surangular foramen is small; it is clearly visible on the medial surface of the right element. The medial surface of the surangular is broadly concave dorsoventrally except for a prominent ridge along the dorsal margin of the extensive adductor fossa. Anteromedially, this ridge bears a distinct articular facet, possibly for contact with the coronoid; the latter passes onto the dorsal surface of the surangular. Angular The angular, which is incompletely preserved on the left mandibular ramus, forms the distinctly concave ventral margin of the external mandibular fenestra. Behind this opening, it forms a thin sheet of bone. There is no clearly discernable sutural separation from the surangular, and it is not clear whether the angular contacted the surangular anterior to the external mandibular fenestra. Prearticular The prearticular is exposed in medial view on the left mandibular ramus. It is a curved, long, and thin bone, which is dorsoventrally expanded at either end and borders the adductor fossa ventromedially. The prearticular is most robust at its posterior end, which covers the ventromedial aspect of the articular. Its dorsal edge is concave. The anterior portion of the prearticular forms a vertical, dorsoventrally expanded plate of bone, which presumably contacted the splenial and the medial surface of the dentary. A narrow surface along the dorsal margin of the anterior end may have been for contact with the coronoid. Articular The articular contacts the prearticular anteromedially and the surangular laterally. Ventrally, it is plate-like and wedged between the surangular laterally and the prearticular medially. The articular facet for the mandibular condyle of the quadrate is concave anteroposteriorly and delimited by transverse bony ridges anteriorly and posteriorly. Immediately posterior to the posterior transverse ridge, the dorsomedial edge of the articular is distinctly concave anteroposteriorly. A small foramen, which may have served for passage of the chorda tympani, is situated just posteromedial to the posterior transverse ridge. The articular forms an anteroposteriorly short retroarticular process, which turns posteromedially behind the facet for the mandibular condyle of the quadrate. The process is dorsolaterally-ventromedially flattened so that its anteroposteriorly gently convex lateral surface faces dorsolaterally. RELATIONSHIPS OF IRRITATOR CHALLENGERI Irritator shares with Baryonyx and the probably congeneric Suchomimus (see below) the following derived character-states in the skull and dentition that are absent in most or all other

543

known non-avian theropod dinosaurs: (1) The skull is remarkably narrow throughout its entire length, especially in its rostral region. (2) The long but low external nares are situated far back on the sides of the elongated snout rather than near the rostral end of the snout. (3) The maxillae broadly contact each other medially, forming an extensive secondary bony palate. (4) The maxilla forms a greatly elongated subnarial ramus, which separates the premaxilla and nasal below the external naris and is longer anteroposteriorly than deep dorsoventrally. (5) The maxillary teeth have straight or slightly recurved crowns that are round to subcircular in transverse section, rather than labiolingually flattened. (6) If the holotype of ‘‘Angaturama limai’’ indeed represents the anterior end of the snout of Irritator challengeri, the latter also shares with Baryonyx the apomorphic presence of seven premaxillary teeth. (7) The fused posterior portions of the nasals terminate in a knob-like median projection posteriorly. (8) A narrow fenestra (postnasal fenestra; Charig and Milner, 1997) appears to be present between the frontal, posterior end of the conjoined nasals, and prefrontal on either side of the skull roof; this feature is convergently present in the coelophysoid theropod Syntarsus (Rowe, 1989). (9) The anterior and ventral processes of the lacrimal enclose a much more acute angle (about 358–408) than in other theropod taxa (about 758–908). (10) The braincase is short anteroposteriorly but deep dorsoventrally, extending ventrally far below the occipital condyle. (11) The basipterygoid processes of the basisphenoid are elongate and diverge only slightly ventrolaterally. These features support recognition of the family-level taxon Spinosauridae Stromer, 1915, as defined and diagnosed by Sereno et al. (1998). Combining information from the cranial bones of Baryonyx walkeri and ‘‘Suchomimus’’ tenerensis, Sereno et al. (1998:fig. 2) reconstructed the skull of spinosaurid theropod dinosaurs as long, low, and narrow throughout its entire length. They criticized the reconstruction of the skull of B. walkeri by Charig and Milner (1997:fig. 1) as exaggerating the vertical height of the cranium in the occipital region by ‘‘unnatural ventral displacement’’ of the quadrate to the paroccipital process (Sereno et al., 1998:1301). However, as the more distal portions of the paroccipital processes in the holotype of B. walkeri (BMNH R9951) are not preserved (Charig and Milner, 1997:fig. 9), it is not obvious which reference point was used by Sereno et al. (1998) as the basis for their assertion. The skull of I. challengeri demonstrates that the more posterior region of the skull was indeed deeper dorsoventrally than the snout (Figs. 1, 6). The Spinosauridae had a wide, apparently mainly Gondwanan distribution during the Early and early Late Cretaceous (Charig and Milner, 1997; Sereno et al., 1998). Stromer (1915) named Spinosaurus aegyptiacus on the basis of associated skeletal remains from the Upper Cretaceous (Cenomanian) Bahariya Formation of the Bahariya Oasis in the Western Desert of Egypt. The holotype consists of an edentulous fragment of a maxilla, a partial mandible with four teeth, isolated teeth, vertebrae from all parts of the column, incomplete thoracic ribs, and gastralia elements (Stromer, 1915, 1936). With the possible exception of a caudal vertebra, Stromer interpreted this material as representing the skeletal remains of a single individual. Unfortunately, the holotype and only known specimen was lost during the destruction of the Pala¨ontologische Staatssammlung in Munich by a British air raid in 1944. Stromer (1934a) identified additional postcranial bones from Bahariya as ‘‘Spinosaurus B’’ based on their proportionately smaller size and some morphological differences. However, Sereno et al. (1998) reassigned this material (which was destroyed along with the holotype of S. aegyptiacus) to the allosauroid Carcharodontosaurus saharicus. Buffetaut (1989) identified two jaw fragments and an isolated tooth from the ?Cenomanian-age Kem Kem beds of southern Morocco as Spinosaurus. Russell (1996) re-


544

JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 22, NO. 3, 2002

FIGURE 6. Partial reconstruction of the skull and mandible of Irritator challengeri in left lateral view, based primarily on the better preserved right side of SMNS 58022. Scale bar equals 5 cm. Broken lines indicated restored portions. The height of the nasal crest and the depth of the postdentary portion of the mandibular ramus are uncertain. The supralabial foramina on the lateral surface of the maxilla are based on the condition in Baryonyx tenerensis (Sereno et al., 1998).


SUES ET AL.—SKULL OF IRRITATOR ferred several vertebrae from the same region to this genus, and Taquet and Russell (1998) figured and briefly reported on the anterior portion of a snout. Additional cranial material from the Kem Kem beds will be described by Milner (in prep.). Russell used proportional differences in the cervical vertebrae to distinguish a new species, Spinosaurus maroccanus, for the Moroccan material, but Sereno et al. (1998) considered these differences taxonomically insignificant and synonymized S. maroccanus with S. aegyptiacus. To date, the most completely known spinosaurid theropod is Baryonyx walkeri from the Lower Cretaceous (Barremian) Upper Weald Clay of Ockley, Surrey (England; Charig and Milner, 1986, 1997). Buffetaut (1989, 1992) first recognized its close relationship to Spinosaurus. The holotype of B. walkeri is an incomplete but well-preserved skeleton (BMNH R9951), which includes largely disarticulated and fragmentary bones of the skull and mandible and was described in detail by Charig and Milner (1997). Sereno et al. (1998:1302) diagnosed B. walkeri by ‘‘fused nasals with a median crest terminating posteriorly in a cruciate process, a solid subrectangular lacrimal horn, a marked transverse constriction of the sacral or anterior caudal centra, a well-formed peg-and-notch articulation between the scapula and coracoid, an everted distal margin of the pubic blade, and a very shallow fibular fossa.’’ Charig and Milner (1997) and Martill and Naish (2001) listed additional occurrences of isolated teeth as well as isolated postcranial bones referable to B. walkeri (or related taxa) from the Lower Cretaceous (mainly Barremian) of England and the Isle of Wight. A maxilla fragment from the Barremian-age Enciso Group of La Rioja, Spain has also been referred to Baryonyx (Viera and Torres, 1995). Taquet (1984), Taquet and Russell (1998), and Sereno et al. (1998) reported on a series of specimens of a Baryonyx-like spinosaurid from continental strata of the Lower Cretaceous (Aptian–Albian) Tegama Group at Gadoufaoua (locality GAD 5), Niger. Taquet and Russell (1998) named Cristatusaurus lapparenti on the basis of conjoined premaxillae and associated fragments of a maxilla and dentary. They distinguished this taxon from Baryonyx walkeri solely on the basis of the ‘‘brevirostrine condition’’ of the premaxilla. Sereno et al. (1998) considered this difference uninformative, and Charig and Milner (1997) identified the material discovered by Taquet as Baryonyx sp. indet. Shortly after the publication of the paper by Taquet and Russell, Sereno et al. (1998) briefly announced the discovery of additional skeletal remains, including a snout and a partial postcranial skeleton, from the same locality. They assigned these specimens to a new genus and species, Suchomimus tenerensis, which they distinguished from Baryonyx walkeri by the broader and taller neural spines of the dorsal, sacral, and anterior caudal vertebrae, robust humeral tuberosities, much enlarged olecranon that is offset from the humeral articulation, and hook-shaped radial ectepicondyle. There exists at present no evidence to indicate the presence of more than one taxon of spinosaurid in the faunal assemblage from GAD 5. We concur with Milner (in prep.) that the anatomical differences between the material reported by Sereno et al. (1998) and Baryonyx walkeri only warrant recognition of the former as a distinct species of Baryonyx, B. tenerensis. The generic nomina Cristatusaurus and Suchomimus should be considered subjective junior synonyms of Baryonyx. Buffetaut and Ingavat (1986) described some unusual teeth from the Upper Jurassic Sao Khua Formation of northeastern Thailand as Siamosaurus suteethorni and tentatively referred this taxon to the Spinosauridae. However, the currently available material is insufficient for establishing even dinosaurian affinities for Siamosaurus. Sereno et al. (1994, 1998) and Holtz (2000) have established the phylogenetic position of Spinosauridae among basal Tetanurae, obviating the need for a cladistic analysis in this paper.

545

Sereno et al. (1994, 1998) grouped Spinosauridae with Torvosauridae (comprising Eustreptospondylus from the Middle Jurassic [Callovian] Oxford Clay Formation of England and Torvosaurus from the Upper Jurassic [Kimmeridgian–Tithonian] Morrison Formation of the western United States) and Afrovenator from the Lower Cretaceous Tiourare´n beds of Niger in a clade Spinosauroidea (‘‘Torvosauroidea’’ of Sereno et al., 1994). However, the comprehensive phylogenetic analysis of Theropoda by Holtz (2000) placed Spinosauridae as the most basal clade of Tetanurae and considered Eustreptospondylus, Torvosaurus, and Afrovenator as progressively more derived within Tetanurae. Of the apomorphies listed by Sereno et al. (1998) in support of Spinosauroidea, Irritator shares the presence of a subnarial process of the maxilla that is longer than deep and the dorsoventrally narrow anterior ramus of the lacrimal. However, the derived character-state ‘‘lacrimal anterior ramus, length: . . . less (1) than 65% of the ventral ramus’’ cited by Sereno et al. (1998:1302) is absent in Irritator (SMNS 58022, right side). Sereno et al. (1998) distinguished two clades among Spinosauridae: Baryonychinae, comprising Baryonyx and Suchomimus, and Spinosaurinae, for Spinosaurus and Irritator. At present, the latter grouping is united only by two derived features of the teeth (tooth crowns with distinct but non-serrated carinae and fluted enamel on both the labial and lingual surfaces) and possibly the wide spacing of the maxillary teeth. In Baryonyx, the carinae are very finely serrated (Martill and Hutt, 1996; Charig and Milner, 1997; Sereno et al., 1998). The presence of a spinosaurid theropod in the Santana Formation is not surprising in view of the connection between eastern Brazil and West Africa during at least part of the Early Cretaceous, which is reflected by numerous sister-group pairings among continental fish and tetrapod taxa from these regions (e.g., Forey and Grande, 1998). FUNCTIONAL INFERENCES As noted above, Baryonyx and Irritator share a suite of craniodental features that distinguish them from most known nonavian theropod dinosaurs. The straight or at most slightly recurved, conical tooth crowns in these two taxa are round or oval in transverse section, rather than labiolingually flattened, and their mesial and distal carinae have either very fine serrations (Baryonyx) or none at all (Irritator). This type of teeth was less suited for ‘‘grip-and-rip’’ cutting than the teeth with more coarsely serrated carinae in other non-avian theropods (Abler, 1992), and instead may have been used primarily for impaling and holding prey items (Charig and Milner, 1997). The long, narrow snout of Baryonyx forms an expanded anterior end (‘‘terminal rosette’’ sensu Charig and Milner, 1997) with six or seven teeth in each premaxilla, resembling the spatulate tip of the snout in predominantly piscivorous, longirostrine crocodyliforms such as the extant gharial (Gavialis gangeticus) and various extinct taxa. The terminal expansion of the snout of ‘‘Angaturama’’ is less prominent than that of Baryonyx (Kellner and Campos, 1996). The external nares are situated well back behind the anterior end of the snout in Baryonyx and Irritator. Taquet (1984), Buffetaut (1989), Charig and Milner (1986, 1997), Milner (1996), Martill et al. (1996) and Sereno et al. (1998) interpreted this combination of features as indicating piscivorous habits for spinosaurid dinosaurs. The presumed gastric contents of the holotype of Baryonyx walkeri (BMNH R9951) contain etched scales of the holostean fish Lepidotes and thus are consistent with this dietary inference (Charig and Milner, 1997). However, they also included abraded and(or) etched bones of a small individual of the ornithopod dinosaur Iguanodon, suggesting that B. walkeri fed on terrestrial vertebrates as well as fish. Most extant faunivorous tetra-


546

JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 22, NO. 3, 2002

pods are opportunistic feeders, and there exists no reason not to assume similarly broad dietary habits for non-avian theropod dinosaurs. The deeply implanted, vertically oriented teeth of Irritator with their conical, straight or at most slightly recurved crowns are suitable for apical loading parallel to their long axes, which is consistent with their use for impaling and holding prey. The narrow snout with its nearly vertical sides and more or less convex dorsal surface would have facilitated forceful biting down on prey because this type of rostral shape (oreinirostral condition) is less susceptible to dorsoventral bending stresses (Busbey, 1995). It clearly differs from the long, tubular snout (platyrostral condition) in Gavialis and various extinct crocodyliforms that have been interpreted as piscivorous (Busbey, 1995). The extensive secondary bony palate in spinosaurid theropods would also have served to reduce bending stresses acting on the snout (Thomason and Russell, 1986). We hypothesize transmission of the vertically directed forces applied along the tooth row along the roots of the teeth to the occipital segment of the skull through the dorsal strut formed by the nasals, which was probably further strengthened by the median crest. The posterodorsal inclination of the postorbital region of the skull suggests a reorientation of M. adductor mandibulae externus. Barghusen (1973) interpreted a comparable feature in basal therapsids as reflecting a posterodorsal line of action for the adductor jaw muscle to resist anteroventral displacement of the mandible by prey seized with the enlarged anterior teeth. This functional interpretation is equally plausible for spinosaurid theropods, which have a premaxillary dentition suitable for seizing prey (Charig and Milner, 1997). The skull of Irritator does not appear to be well-suited for catching and processing large, resistant prey. Its structure differs from that in other large theropod dinosaurs such as Allosaurus (Rayfield et al., 2001) and Tyrannosaurus (Erickson et al., 1996), presumably reflecting different modes of feeding. Most likely spinosaurid theropods rapidly and forcefully seized smaller prey, which was then processed by dorsoventral motion of the head facilitated by the powerful neck musculature. (Extensive side-to-side striking movements of the head, as employed by extant crocodylians, appear unlikely in view of the narrow occiput as well as the weak development of the basal tubera.) Whereas fish formed part of the diet in at least B. walkeri, there is nothing to suggest that spinosaurids were exclusively or even predominantly piscivorous. Previous anatomical comparisons between the feeding apparatus of crocodylians and spinosaurid theropods were based only on superficial resemblances. The postcranial skeleton of Baryonyx lacks any obvious specializations suggestive of an aquatic or semiaquatic mode of life (Charig and Milner, 1997). Charig and Milner (1986, 1997) interpreted the greatly enlarged and strongly curved ungual of manual digit I as a ‘‘gaffing’’ device for catching fish, but this intriguing hypothesis remains untestable in the absence of a close analogue among extant tetrapods. ACKNOWLEDGMENTS We are greatly indebted to R. Wild (Staatliches Museum fu¨r Naturkunde Stuttgart) for the extended loan of the material and his continuing interest in this study. D. Pulera` (Toronto) prepared the drawings of the reconstructed skull reproduced in Fig. 6. The remaining illustrations are the work of D. M. S. We thank T. D. Carr (Royal Ontario Museum) who assisted our work through a thorough critique of the manuscript. A. C. Milner (The Natural History Museum, London) and T. R. Holtz, Jr. (University of Maryland, College Park) provided constructive reviews. H.-D. S. gratefully acknowledges support from the Royal Ontario Museum Foundation.

LITERATURE CITED Abler, W. L. 1992. The serrated teeth of tyrannosaurid dinosaurs, and biting structures in other animals. Paleobiology 18:161–183. Bakker, R. T., M. Williams, and P. Currie. 1988. Nanotyrannus, a new genus of pygmy tyrannosaur from the latest Cretaceous of Montana. Hunteria 1(5):1–30. Barghusen, H. R. 1973. The adductor jaw musculature of Dimetrodon (Reptilia, Pelycosauria). Journal of Paleontology 47:823–834. Benton, M. J., S. Bouaziz, E. Buffetaut, D. Martill, M. Ouaja, M. Soussi, and C. Trueman. 2000. Dinosaurs and other fossil vertebrates from fluvial deposits in the Lower Cretaceous of southern Tunisia. Palaeogeography, Palaeoclimatology, Palaeoecology 157:227–246. Berthou, P.-Y. 1990. Le bassin d’Araripe et les petits bassins intracontinentaux voisins (NE de Bre´sil): formation et e´volution dans le cadre de l’ouverte de l’Atlantique equatorial. Comparison avec les bassins ouest-africains situe´s dans le meˆme contexte; pp. 113–134 in D. de A. Campos, M. S. S. Viana, P. M. Brito, and G. Beurlen (eds.), Atas do I Simpo´sio sobre a Bacia do Araripe e Bacias Interiores do Nordeste, Crato, 14–16 de Junho de 1990, Anais. Crato, Ceara´. Bouaziz, S., E. Buffetaut, M. Ghanmi, J.-J. Jaeger, M. Martin, J.-M. Mazin, and H. Tong. 1988. Nouvelles de´couvertes de verte´bre´s fossiles dans l’Albien du Sud tunisien. Bulletin de la Socie´te´ ge´ologique de France (8) 4:335–339. Braun, O. P. G. 1966. Estratigra´fia dos sedimentos da parte interior da regia˜o Nordeste do Brasil (bacias de Tucano-Jatoba´, Mircundiba e Araripe). Boletim da Divisa˜o de Geologia e Mineralogia do Departamento Nacional de Produc¸a˜o Mineral, Rio de Janeiro 263:1– 75. Buffetaut, E. 1989. New remains of the enigmatic dinosaur Spinosaurus from the Cretaceous of Morocco and the affinities between Spinosaurus and Baryonyx. Neues Jahrbuch fu¨r Geologie und Pala¨ontologie, Monatshefte 1989:79–87. ———. 1992. Remarks on the Cretaceous theropod dinosaurs Spinosaurus and Baryonyx. Neues Jahrbuch fu¨r Geologie und Pala¨ontologie, Monatshefte 1992:88–96. ———, and R. Ingavat. 1986. Unusual theropod dinosaur teeth from the Upper Jurassic of Phu Wiang, northeastern Thailand. Revue de Pale´obiologie 5:217–220. Busbey, A. B. 1995. The structural consequences of skull flattening in crocodilians; pp. 173–192 in J. J. Thomason (ed.), Functional Morphology in Vertebrate Paleontology. Cambridge University Press, Cambridge and New York. Charig, A. J., and A. C. Milner. 1986. Baryonyx, a remarkable new theropod dinosaur. Nature 324:359–361. ———, and ———. 1997. Baryonyx walkeri, a fish-eating dinosaur from the Wealden of Surrey. Bulletin of the Natural History Museum, Geology Series 53:11–70. Colbert, E. H., and J. H. Ostrom. 1958. Dinosaur stapes. American Museum Novitates 1900:1–20. Currie, P. J. 1985. Cranial anatomy of Stenonychosaurus inequalis (Saurischia, Theropoda) and its bearing on the origin of birds. Canadian Journal of Earth Sciences 22:1643–1658. ———, and X.-J. Zhao. 1994. A new carnosaur (Dinosauria, Theropoda) from the Jurassic of Xinjiang, People’s Republic of China. Canadian Journal of Earth Sciences 30:2037–2081. [Listed as ‘‘October–November 1993,’’ but actually published on February 10, 1994.] Erickson, G. F., S. D. Van Kirk, J. Su, M. E. Levenston, W. E. Caler, and D. R. Carter. 1996. Bite-force estimation for Tyrannosaurus rex from tooth-marked bones. Nature 382:706–708. Forey, P. L., and L. Grande. 1998. An African twin to the Brazilian Calamopleurus (Actinopterygii: Amiidae). Zoological Journal of the Linnean Society 123:179–195. Frey, E., and D. M. Martill. 1995. A possible oviraptorosaurian theropod from the Santana Formation (Lower Cretaceous,?Albian) of Brazil. Neues Jahrbuch fu¨r Geologie und Pala¨ontologie, Monatshefte 1995: 397–412. Gardner, G. 1846. Travels in the Interior of Brazil, Principally through the Northern Provinces, and the Gold and Diamond Districts, during the Years 1836–1841. Reeve, Benham and Reeve, London, XVI 1 562 pp. Gauthier, J. A. 1986. Saurischian monophyly and the origin of birds;


SUES ET AL.—SKULL OF IRRITATOR pp. 1–55 in K. Padian (ed.), The Origin of Birds and the Evolution of Flight. Memoirs of the California Academy of Sciences 8. Gilmore, C. W. 1920. Osteology of the carnivorous Dinosauria in the United States National Museum, with special reference to the genera Antrodemus (Allosaurus) and Ceratosaurus. United States National Museum Bulletin 110:1–159. Holtz, T. R., Jr. 2000. A new phylogeny of the carnivorous dinosaurs. Gaia 15:5–61. Kellner, A. W. A. 1996. Remarks on Brazilian dinosaurs. Memoirs of the Queensland Museum 39:611–626. ———. 1999. Short note on a new dinosaur (Theropoda, Coelurosauria) from the Santana Formation (Romualdo Member, Albian), northeastern Brazil. Boletim do Museu Nacional, Nova Se´rie, Rio de Janeiro, Geologia 49:1–8. ———, and D. de A. Campos. 1996. First Early Cretaceous theropod dinosaur from Brazil with comments on Spinosauridae. Neues Jahrbuch fu¨r Geologie und Pala¨ontologie, Abhandlungen 199:151–166. ———, and B. J. Mader. 1997. Archosaur teeth from the Cretaceous of Morocco. Journal of Paleontology 71:525–527. Lima, H. R. de. 1978. Considerac¸o˜es sobre a subdivisa˜o estratigra´fica da Formac¸a˜o Santana, Creta´ceo do Nordesto do Brasil. Revista Brasiliera de Geocieˆncias 9:116–121. Madsen, J. H., Jr. 1976. Allosaurus fragilis: a revised osteology. Utah Geological and Mineral Survey Bulletin 106:1–163. Maisey, J. G. (ed.) 1991. Santana Fossils: An Illustrated Atlas. T.F.H. Publications Inc., Neptune, New Jersey, 459 pp. ———. 2000. Continental break up and the distribution of fishes of Western Gondwana during the Early Cretaceous. Cretaceous Research 21:281–314. Marsh, O. C. 1881. Principal characters of American Jurassic dinosaurs. Part V. American Journal of Science 21:417–423. Martill, D. M. 1993. Fossils of the Santana and Crato Formations, Brazil. Field Guides to Fossils, no. 5. The Palaeontological Association, Oxford, 159 pp. ———, A. R. I. Cruickshank, E. Frey, P. G. Small, and M. Clarke. 1996. A new crested maniraptoran dinosaur from the Santana Formation (Lower Cretaceous) of Brazil. Journal of the Geological Society, London 153:5–8. ———, E. Frey, H.-D. Sues, and A. R. I. Cruickshank. 2000. Skeletal remains of a small theropod dinosaur with associated soft structures from the Lower Cretaceous Santana Formation of northeastern Brazil. Canadian Journal of Earth Sciences 37:891–900. ———, and S. Hutt. 1996. Possible baryonychid dinosaur teeth from the Wessex Formation (Lower Cretaceous, Barremian) of the Isle of Wight, England. Proceedings of the Geologists’ Association 107: 81–84. ———, and D. Naish (eds.). 2001. Dinosaurs of the Isle of Wight. Field Guides to Fossils, no. 10. The Palaeontological Association, London, 433 pp. Moody, J. M., and J. G. Maisey. 1994. New Cretaceous marine vertebrate assemblages from north-western Venezuela and their significance. Journal of Vertebrate Paleontology 14:1–8. Pons, D., P.-Y. Berthou, and D. de A. Campos. 1990. Quelques observations sur la palynologie de l’Aptien supe´rieur et de l’Albien du Bassin d’Araripe (NE du Bre´sil); pp. 241–252 in D. de A. Campos, M. S. S. Viana, P. M. Brito, and G. Beurlen (eds.), Atas do I Simpo´sio sobre a Bacia do Araripe e Bacias Interiores do Nordeste, Crato, 14–16 de Junho de 1990, Anais. Crato, Ceara´. Rayfield, E. J., D. B. Norman, C. C. Horner, J. R. Horner, P. M. Smith, J. J. Thomason, and P. Upchurch. 2001. Cranial design and function in a large theropod dinosaur. Nature 409:1033–1037. Rowe, T. 1989. A new species of the theropod dinosaur Syntarsus from the Early Jurassic Kayenta Formation of Arizona. Journal of Vertebrate Paleontology 9:125–136. Russell, D. A. 1996. Isolated dinosaur bones from the Middle Cretaceous of the Tafilalt, Morocco. Bulletin du Muse´um national d’Histoire naturelle, Paris, se´rie 4, 18, section 4, 2–3:349–402. Schultze, H.-P., and D. Sto¨hr. 1996. Vinctifer (Pisces, Aspidorhynchi-

547

dae) aus der Unterkreide (oberes Aptium) von Kolumbien. Neues Jahrbuch fu¨r Geologie und Pala¨ontologie, Abhandlungen 199:395– 415. Sereno, P. C., A. L. Beck, D. B. Dutheuil, B. Gado, H. C. E. Larsson, G. H. Lyon, J. D. Marcot, O. W. M. Rauhut, R. W. Sadleir, C. A. Sidor, D. Varricchio, G. P. Wilson, and J. A. Wilson. 1998. A longsnouted predatory dinosaur from Africa and the evolution of spinosaurids. Science 282:1,298–1,302. ———, J. A. Wilson, H. C. E. Larsson, D. B. Dutheuil, and H.-D. Sues. 1994. Early Cretaceous dinosaurs from the Sahara. Science 265:267–271. Silva, M. A. M. da. 1983. The Araripe Basin, north-eastern Brazil: regional geology and facies analysis of a Lower Cretaceous evaporitic depositional complex. Ph.D. dissertation, Columbia University, New York, 290 pp. Stromer, E. 1915. Ergebnisse der Forschungsreisen Prof. E. Stromers in ¨ gyptens. II. Wirbeltier-Reste der Baharıˆje-Stufe (unden Wu¨sten A terstes Cenoman). 3. Das Original des Theropoden Spinosaurus aegyptiacus nov. gen., nov. spec. Abhandlungen der Ko¨niglich Bayerischen Akademie der Wissenschaften, Mathematisch-Physikalische Klasse 28(3):1–32. ———. 1934a. Ergebnisse der Forschungsreisen Prof. E. Stromers in ¨ gyptens. II. Wirbeltier-Reste der Baharıˆje-Stufe (unden Wu¨sten A terstes Cenoman). 13. Dinosauria. Abhandlungen der Bayerischen Akademie der Wissenschaften, Mathematisch-naturwissenschaftliche Abteilung, Neue Folge 22:1–79. ———. 1934b. Die Za¨hne des Compsognathus und Bemerkungen u¨ber das Gebiss der Theropoda. Centralblatt fu¨r Mineralogie, Geologie und Pala¨ontologie, Abteilung B, 1934:74–85. ———. 1936. Ergebnisse der Forschungsreisen Prof. E. Stromers in ¨ gyptens. VII. Baharıˆje-Kessel und -Stufe mit deren den Wu¨sten A Fauna und Flora. Eine erga¨nzende Zusammenfassung. Abhandlungen der Bayerischen Akademie der Wissenschaften, Mathematischnaturwissenschaftliche Abteilung, Neue Folge 33:1–102. Taquet, P. 1984. Une curieuse spe´cialisation du craˆne de certains Dinosaures carnivores du Cre´tace´: le museau long et e´troit des Spinosauride´s. Comptes Rendus de l’Academie des Sciences, Paris, se´rie II, 299:217–222. ———, and D. A. Russell. 1998. New data on spinosaurid dinosaurs from the Early Cretaceous of the Sahara. Comptes Rendus de l’Academie des Sciences, Paris, Sciences de la terre et des plane`tes 327:347–353. Thomason, J. J., and A. P. Russell. 1986. Mechanical factors in the evolution of the mammalian secondary palate: a theoretical analysis. Journal of Morphology 189:199–213. Viera, L. I., and J. A. Torres. 1995. Presencia de Baryonyx walkeri (Saurischia, Theropoda) en el Weald de La Rioja (Espan˜a). Nota previa. Munibe, Ciencias Naturales 47:57–61. Walker, A. D. 1990. A revision of Sphenosuchus acutus Haughton, a crocodylomorph reptile from the Elliot Formation (late Triassic or early Jurassic) of South Africa. Philosophical Transactions of the Royal Society of London, B, 330:1–120. Wenz, S. 1981. Un Coelacanthe ge´ant, Mawsonia lavocati Tabaste, de l’Albien-base du Ce´nomanien du Sud Marocain. Annales de Pale´ontologie (Verte´bre´s) 67:1–20. Witmer, L. M. 1997a. The evolution of the antorbital cavity of archosaurs: a study in soft-tissue reconstruction in the fossil record with an analysis of the function of pneumaticity. Society of Vertebrate Paleontology Memoir 3:1–73. ———. 1997b. Craniofacial air sinus systems; pp. 151–159 in P. J. Currie and K. Padian (eds.), Encyclopedia of Dinosaurs. Academic Press, San Diego. Zhao, X., and P. J. Currie. 1994. A large crested theropod from the Jurassic of Xinjiang, People’s Republic of China. Canadian Journal of Earth Sciences 30:2027–2036. [Listed as ‘‘October–November 1993,’’ but actually published on February 10, 1994.] Received 7 May 2001; accepted 13 November 2001.


Turn static files into dynamic content formats.

Create a flipbook
Issuu converts static files into: digital portfolios, online yearbooks, online catalogs, digital photo albums and more. Sign up and create your flipbook.