Caldwell, 1996 by Felipe Elias

Zoological Journal of the Linnean Society (1996), 116: 407–436. With 15 figures

Ontogeny and phylogeny of the mesopodial skeleton in mosasauroid reptiles MICHAEL W. CALDWELL Redpath Museum and Department of Biology, McGill University, 859 Sherbrooke St. West, Montr´eal, Qu´ebec, Canada, H3A 2K6

Received January 1995, accepted for publication April 1995

Current phylogenies of mosasauroid reptiles are reviewed and a new phylogeny examining aigialosaur interrelationships presented. Patterns of mesopodial ossification and overall limb morphology are described for adult mosasauroids. Ossification sequences are mapped onto a phylogeny in order to assess the distribution of ontogenetic characters. Consistent and ordered distributions are found. Based on the phylogenetic distribution of ossification patterns, an overall mesopodial ossification sequence for mosasaurs is proposed. Carpal sequence: ulnare – distal carpal four (dc4) – intermedium – dc3 – radiale or dc2 – dc1 or pisiform and dc5. Tarsal sequence: astragalus – distal tarsal four or calcaneum. Skeletal paedomorphosis is recognized as a dominant pattern in the evolution of mosasauroid limbs. Apomorphic characters of skeletal paedomorphosis, apparent in most taxa, reach extremes in tylosaurs. Arguments for the presence of a single proximal cartilage in the tarsus of mosasaurs are made. This cartilage is presumed to include ossification centres from which both the astragalus and calcaneum will ossify. ©1996 The Linnean Society of London

ADDITIONAL KEY WORDS: — paedomorphosis – squamates – mosasaurs – aigialosaurs. CONTENTS Introduction . . . . . . . . . . . . . . . Material and methods . . . . . . . . . . . . Phylogenetic analysis . . . . . . . . . . . Ontogenetic analysis . . . . . . . . . . . Mosasauroid phylogeny . . . . . . . . . . . Review: recent phylogenies . . . . . . . . . Results: phylogenetic analysis . . . . . . . . Osteology and ossification patterns . . . . . . . . Aigialosaurs . . . . . . . . . . . . . . Halisaurines . . . . . . . . . . . . . ‘Russellosaurines’ (Tylosaurines and Plioplatecarpines) Mosasaurines . . . . . . . . . . . . . Discussion and conclusions . . . . . . . . . . Ossification sequences and phylogeny . . . . . Skeletal paedomorphosis . . . . . . . . . Mosasaur astragalus . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . .

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Present address: Department of Biological Sciences, Biological Sciences Center, University of Alberta, Edmonton, Alberta, Canada, T6G 2E9 0024–4082/96/040407 + 30 $18.00/0

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M. W. CALDWELL References . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix I: data matrix and character descriptions from Bell (1993) . . . . . . . .

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INTRODUCTION

Mosasauroid reptiles (aigialosaurs and mosasaurs) are a large group of extinct squamates, united by a number of cranial synapomorphies, that achieved their greatest adaptive and taxonomic diversity in the Upper Cretaceous (Russell, 1967; Bell, 1993; deBraga & Carroll, 1993). Aigialosaurs are thought to have been facultatively aquatic while mosasaurs are believed to have been obligatory aquatic. These interpretations are based on characters of the appendicular and axial skeleton considered indicative of specialized aquatic adaptations (deBraga & Carroll, 1993). The appendicular skeleton of aigialosaurs closely resembles that of a terrestrial squamate, and shows no modifications that might be considered aquatic adaptations. Aigialosaurs do show some reduction of limb size relative to trunk length, but this condition has been shown to be equivocal as a diagnosis of aquatic lifestyles because similar limb to body ratios are also found in fossorial squamates (Caldwell, Carroll, & Kaiser, 1995). However, modification of intervertebral articulations, general shortening and loss of some transverse processes, and lateral compression of caudal centra are interpreted as aquatic adaptations. In contrast, the limbs of mosasaurs are paddle-like and differ significantly from the limbs of aigialosaurs and other limbed squamates. Mosasaur limbs are characterized by incomplete ossification of elements in the carpus and tarsus. Limb to body ratios are significantly different from those of fossorial, terrestrial, or facultatively aquatic squamates such as aigialosaurs (Caldwell et al., 1995). Aquatic adaptations are also observed in the axial skeleton. The cauda central are laterally compressed and the neural spines and haemal arches are elongate. Further modifications include the reduction of zygosphene-zygantral articulations in a caudal to rostral direction, and an increase in the horizontal inflection, throughout the vertebral column, of the plane of articulation for the pre- and postzygapophyses. This paper presents data on patterns of limb skeleton formation in mosasaurs. The immediate difficulty of such a study is that few juveniles of any mosasaur taxon are known (Russell, 1967). Fortunately, for mosasaurs, as for other limbed squamates, ossification of carpal and tarsal bones (mesopodials) is significantly delayed relative to ossification in the remainder of the appendicular skeleton (Rieppel, 1992 a,b,c; Caldwell, 1994). This phenomenon allows analysis of mesopodial ontogenetic patterns in surprisingly late ontogenetic stages of large, adult mosasaurs. Despite the rarity of juveniles, articulated adult mosasaurs are fairly common. Carpal and tarsal (mesopodial) ossification patterns described for extant squamates (Rieppel 1992 a,b,c) and fossil stemgroup lepidosauromorphs (Caldwell, 1994) can therefore be compared to ontogenetic patterns in mosasaurs. To understand the relationship of limb ontogeny to limb phylogeny, a prior understanding of mosasauroid phylogeny, free of assumptions regarding the ontogenetic patterns under investigation, is a prerequisite. Previously proposed mosasauroid phylogenies are reviewed and discussed (Bell, 1993; deBraga & Carroll, 1993; Caldwell et al., 1995) to establish a basis of comparison for ossification patterns. No current phylogeny provides satisfactory resolution regarding the relationships of aigialosaurs. In an attempt to address this problem, the results of a phylogenetic

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analysis, focusing on basal mosasauroid relationships, and based on characters from the data sets of Bell (1993) and Caldwell et al. (1995), are presented. This phylogeny, and those of Bell (1993), deBraga & Carroll (1993), and Caldwell et al. (1995), were derived without the use of the ontogenetic data presented below. The intention is to test for the most congruent distribution of mesopodial ossification patterns among mosasauroid reptiles by comparison with phylogenetic hypotheses presented below. The discovery of character congruence, based on hierarchies of synapomorphies, produces a pattern from which phylogeny may be deduced (Patterson, 1982). A cladogram provides a starting point for analysing and interpreting phylogeny. It is within this context that developmental and evolutionary patterns of skeletal paedomorphosis can be recognized through evolution of a clade such as mosasauroid reptiles. Particular attention is given to the nature of the mosasaur astragalus and calcaneum. The general problem is whether or not these two bones ossified endochrondrally within a single cartilaginous precursor (the typical lizard condition) or within separate cartilaginous precursors (the primitive diapsid condition [Caldwell, 1994]). Much debate has centered on the phylogeny and ontogeny of the amniote astragalus with the focus being the number of elements in the proximal tarsal row (Rieppel, 1993). Understanding the mosasaur astragalus assists in understanding the phylogeny of this bone in diapsids generally and in squamates specifically.

MATERIALS AND METHODS

Phylogenetic analysis Phylogenetic analysis of mosasauroids was conducted using PAUP Version 3.1.1 for the Macintosh (Swofford, 1993). Characters and taxa from Bell’s (1993) analysis were reduced in number (37 taxa to 15; 151 characters to 91) and a new matrix constructed (Appendix I); some new taxa were added based on the analysis of aigialosaurs by Caldwell et al. (1995). All multistate character transformations were unordered and characters were optimized using assumptions of accelerated transformation (ACCTRAN). Polarity for all characters is according to Bell’s (1993) assessment of outgroup character states. Mosasaur taxa included are the basal-most taxa of clades identified by Bell (1993). All aigialosaurs and halisaurs of Bell (1993) were included in this analysis. A number of characters (Bell’s Characters 1, 13, 16, 40, 59, 70, 100, and 125) were recoded for Aigialosaurus ( = Opetiosaurus) buccichi, Aigialosaurus dalmaticus, and the Trieste aigialosaur, referred to here as Carsosaurus marchesetti based on Caldwell et al. (1995). These recodings are based on study of latex peels of the holotypes (see Appendix I for specific changes). Sixty characters from Bell’s (1993) matrix were found to be autapomorphic or invariant within the reduced matrix and were therefore excluded (Appendix I). Characters 74 and 96 of Bell (1993) were recoded to accommodate changes in the distribution of states due to the reduced number of terminal taxa.

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Ontogenetic analysis Ossification patterns for most mosasaur taxa were obtained from original specimens. Data for several European mosasaur taxa were obtained from the primary literature. For aigialosaurs, and a number of mosasaur taxa, limb ontogenies are not available. Articulated limbs from forty-one individual mosasaurs and two aigialosaurs are illustrated. Many specimens were either still in the matrix or were accompanied by field photos or drawings showing the original position of the bones. For ease of comparison some illustrations are reversed so that for a particular limb series all figures are oriented in the same direction. Materials examined or referred to in this study are housed in institutions bearing the following abbreviations: American Museum of Natural History, New York (AMNH); Bayerische Staatssammlung für Paläontologie und historische Geologie, München (BSP); California Institute of Technology (CIT); Canterbury Museum, New Zealand (CM); Dominion Museum, Wellington, New Zealand (DM); Fick Museum, Oakley, Kansas (FM); Fort Hays Museum — Vertebrate Paleontology, Fort Hays State University, Fort Hays, Kansas (FHM — VP); Institut Royal des Sciences Naturelles de Belgique, Brussels (IRSNB); Museum of Natural History, University of Kansas, Lawrence (KU); Los Angeles County Museum of Natural History, Los Angeles, California (LACM); Museo Civico di Storia Naturale, Trieste, Italy (MCSNT); Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts (MCZ); Morden Museum, Morden, Manitoba (MDM); National Museum of Canada, Ottawa, Ontario (NMC); Naturhistorisches Museum, Wien, Austria (NMW); Princeton University, Yale Peabody Museum, New Haven, Connecticut (PU); Saskatchewan Museum of Natural History, Regina (SMNH); South Dakota School of Mines and Technology, Rapid City (SDSM); Museum of Paleontology, University of California, Berkeley (UCBMP); Yale Peabody Museum, New Haven, Connecticut (YPM).

MOSASAUROID PHYLOGENY

Review: recent phylogenies (Fig. 1A–C) Bell (1993), deBraga & Carroll (1993), and Caldwell et al. (1995) examined the phylogenetic relationships of mosasauroid reptiles, drawing heavily from McDowell & Bogert (1954) and Russell (1967). Bell’s (1993) analysis is a taxonomic and phylogenetic revision of mosasauroid reptiles at the level of individual species. DeBraga & Carroll (1993) examined family or genus level relationships and were more interested in macroevolutionary trends. Caldwell et al. (1995) focused on aigialosaur and mosasaur characters in the context of putative varanoid sistergroups. The key problem in these phylogenies, critical to this study as it affects the polarity of morphological transformations, is the relationship of mosasauroids to varanoid lizards. DeBraga & Carroll (1993) assumed mosasauroids to be the sistergroup of the Varanidae (Fig. 1A). Caldwell et al. (1995) found no support for a varanidmosasauroid clade, nor could mosasauroids be placed within the Varanoidea (Fig. 1B). Bell (1993) was unable to falsify or support mosasauroid relationships either with

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or within the Varanidae (Fig. 1C). Neither Bell (1993) nor Caldwell et al. (1995) were able to clarify a Varanoid-Mosasauroid sistergroup relationship. A further important difference between the phylogenies of deBraga & Carroll (1993) (Fig. 1A) and Bell (1993) (Fig. 1C) concerns the ingroup relationships of the mosasaurs Prognathodon and Plesiotylosaurus. DeBraga & Carroll (1993) followed Russell (1967) and reconstruct Prognathodon and Plesiotylosaurus within the Plioplatecarpinae (Fig. 1A) forming the basal plioplatecarpine clade. Tylosaurines are placed below the Plioplatecarpinae, separating plioplatecarpines from the Mosasaurinae. In contrast, Bell (1993) hypothesized the two genera as the sister-taxa to Globidens; together, these three taxa compromise the clade Globidensini within Mosasaurinae.

Results: Phylogenetic analysis Analysis of the matrix given in Appendix I produced 70 most parsimonious trees, each of 189 steps, with a CI of 0.582. A Strict Consensus Tree of all trees (Fig. 2A) supports the monophyly of halisaurines, and all other more derived mosasaurs (‘russellosaurines’ and mosasaurines of Bell [1993]). The position of Ectenosaurus clidastoides is unstable, resulting in a polychotomy within derived mosasaurs. This appears to be a result of the reduced data set and the absence of conclusive synapomorphies for ectenosaurs with either ‘russellosaurines’ or mosasaurines; this is reinforced by the consensus tree presented below. Bell’s (1993) ‘Taxon Novum’, an undescribed mosasaur, is unequivocally the most basal ‘mosasauroid’ taxon, other than Aigialosaurus buccichi. Aigialosaur monophyly, disputed by Bell (1993), is neither supported nor falsified in this analysis as all aigialosaur terminal taxa are within the conventional Mosasauridae, and all aigialosaur branches collapse to a single polychotomous node. It is important to note that this polychotomy includes neither halisaurines nor Bell’s (1993) ‘Taxon Novum’. Halisaurines are resolved as a distinct clade within Bell’s (1993) Mosasauridae but his ‘Taxon Novum’ is not. A Majority-Rule Consensus Tree (Fig. 2B), for all compatible groupings including those below 50%, provides some structure within the Strict Consensus aigialosaur polychotomy. In 43% of the trees, the Dallas aigialosaur was the sister-taxon of the Mosasauridae. The remaining aigialosaurs are reconstructed as successive branches along the main stem in the respective frequencies as illustrated. It appears that the absence of data on many characters for the Dallas aigialosaur, and aigialosaurs in general, may be responsible, in part, for the polychotomies present in this analysis and in Bell (1993). Among derived mosasaurs (‘russellosaurines” and mosasaurines) the position of Ectenosaurus clidastoides is resolved in 50% of the trees as the siston-taxon of tylosaurs and platecarpines. To address the instability of Ectenosaurus clidastoides, a second analysis was conducted in which the ingroup data set was enlarged by one taxon. The addition of Tylosaurus proriger to the character matrix increased the stability of the matrix and in all trees E. clidastoides was reconstructed as the sister taxon of both tylosaur species and Platecarpus planifrons.

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Figure 1. Mosasauroid phylogeny. A, redrawn from deBraga & Carroll (1993). B, strict consensus tree derived from nine most-parsimonious trees of 107 steps; redrawn from Caldwell et al. (1995). C, Bellâ&#x20AC;&#x2122;s (1993) preferred hypothesis, an Adamâ&#x20AC;&#x2122;s Consensus Tree derived from seven most-parsimonious tree of 375 steps; redrawn from Bell (1993).

GENERAL OSTEOLOGY AND OSSIFICATION PATTERNS

Aigialosaurs Carsosaurus marchesetti (Fig. 3A) Forelimb. The carpus contains ten ossified elements identified as the radiale, lateral centrale, intermedium, ulnare, pisiform, medial centrale, and distal carpals two through five. The radiale is an irregularly shaped element, compressed proximodistally, and expanded into small lobes on its medial and lateral margins. The medial expansion of the elements contributes to the distal margin of the antebrachial space. The lateral centrale is an irregular hexagon and articulates proximally with the intermedium, medially with the radiale and medial centrale, laterally with the ulnare, and distally with distal carpals two to four. The ulnare is the largest element in the carpus, articulating with the lateral centrale and intermedium, as well as with distal carpals four and five. The ulnare likely articulated with the pisiform but the pisiform appears to have rotated proximally out of articulation, making its exact morphology and position difficult to determine. Distal carpal four is the largest element in the distal carpal row. In order of largest to smallest in size, the remaining distal carpals are respectively three, two, and five.

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Figure 2. Consensus trees of mosasauroid phylogeny derived from seventy most-parsimonious trees (tree length 189, CI 0.582), derived from data modified from Bell (1993) and Caldwell et al. (1995). A, strict consensus tree. B, majority-rule consensus tree with option requests for all compatible groupings including those below 50% (numbers refer to percentage of clade consistency of arrangement in seventy trees).

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Aigialosaurus buccichi ( = Opetiosaurus) (Fig. 3B) Forelimb. The carpus contains nine ossified elements identified as the radiale, lateral centrale, ulnare, pisiform, medial centrale, and distal carpals two through five. Unlike Carsosaurus, there is no intermedium preserved in proximal articulation with the lateral centrale and the pisiform is in contact with the ulnare. All other elements and their articulations are similar to Carsosaurus.

Halisaurines Halisaurus sternbergi (Fig. 3C) Forelimb. H. sternbergi (unnumbered specimen, Palaeontological Museum, Uppsala, Sweden) has four ossified carpal elements identified as the ulnare, intermedium, and distal carpals four and three. The first metacarpal is the largest in the series and is a unique characteristic of mosasaurid metacarpal rows.

Figure 3. Forelimbs of two aigialosaurs and the mosasaur Halisaurus sternbergi. A, Carsosaurus marchesetti MCSNT unnumbered specimen. B, Aigialosaurus buccichi ( = Opetiosaurus) NMW unnumbered specimen. C, Halisaurus sternbergi (unnumbered specimen, Palaeontological Museum, Uppsala, Sweden; redrawn from Wiman [1920] and Bell [1993]). Abbreviations: a, astragalus; c, calcaneum; Co, coracoid; cS, cartilaginous sternum; F, femur; f, fibula; H, humerus; in, intermedium; lc, lateral centrale; mc, medial centrale; p, pisiform; r1-5, sternal cartilages of presacral ribs 1-5; r, radius; rd, radiale; Sc, scapula; t, tibia; u, ulna; ul, ulnare; 2-5, distal carpals/tarsals; i-v, metacarpals/metatarsals.

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‘Russellosaurines’ (Tylosaurines and Plioplatecarpines) Tylosaurus and Hainosaurus Forelimb (Fig. 4). The smallest specimens (Fig. 4A–C) have only a single ossified element identified as the ulnare. Larger individuals (Fig. 4D,E) have two ossified elements identified as the ulnare and distal carpal four. In the largest individuals known, the tylosaurine carpus never exceeds two ossified carpal elements. Mesopodial elements are all poorly finished, sub-rounded bones. When found in articulation, the spatial relationships and poor ossification of these elements suggests the presence of large cartilaginous margins; only a small portion of the element ossifies. The first metacarpal is the largest element in the metacarpal series. Rearlimb (Fig. 5). The least ossified tarsi (Fig. 5A–C) have only a single ossified element identified as the astragalus. More highly ossified tarsi (Fig. 5D–F) have two ossified elements identified as the astragalus and distal tarsal four. The ossified state of tarsal elements is similar to that described for carpals. The divergent fifth metatarsal is short and proximally enlarged in both Tylosaurus and Hainosaurus. The fifth metatarsal is divergent, short, proximally enlarged, but not hooked. Platecarpus Forelimb (Fig. 6). Three stages of ossification are observed among the available specimens of Platecarpus. The smallest individual (Fig. 6A) has three ossified elements identified as the ulnare, intermedium, and distal carpal four. Intermediate sized individuals (Fig. 6B–D) have four ossified elements: ulnare, intermedium, and distal carpals four and three. Larger individuals (Fig. 6E–H) have five ossified elements, but show considerable variation regarding the identity of the fifth. The conventional four are the ulnare, intermedium, and distal carpals four and three. The individual shown in Figure 6E bears a small element fused to the lateral margin of the fourth distal carpal identified as the fifth distal carpal. This feature was consistent in both the right and left carpus. In two specimens (Fig. 6F,G) the fifth ossification centre is identified as the radiale. This element is never very large and seldom bears finished periosteal bone. A fifth ossification centre (Fig. 6H) is identified as distal carpal two. Unlike tylosaurines, the early ossifying mesopodials are all moderately finished, sub-angular bones. Cartilaginous margins appear to be much smaller and a large portion of the element surface bears finished bone. However, the fifth element to ossify, regardless of its identity, is usually small and poorly ossified. As in other mosasaurs, the first metacarpal is the largest element in the metacarpal series. Rearlimb (Fig. 7). Only a single stage of ossification is observed among the available specimens of Platecarpus. The smallest to largest individuals (Fig. 7A–C) always have three ossified elements: astragalus, calcaneum, and distal tarsal four. There is a large open space distal to the tibia and anterior to the astragalus. The fifth metatarsal is divergent, short, proximally enlarged, and is not hooked; this is similar to Tylosaurus and Hainosaurus (Fig. 5). The fifth metatarsal presumably articulates with both the fourth distal tarsal and the calcaneum. Plioplatecarpus Forelimb. (Fig. 8A). One specimen of Plioplatecarpus has five ossified carpals identified as the ulnare, intermedium, and distal carpals four, three, and two. The first metacarpal is the largest in the series and the fifth metacarpal is small and articulates at an acute angle with distal carpal four. A second specimen (Fig. 8B) has seven

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Figure 4. Forelimbs of tylosaurines. A, Tylosaurus sp. PU, unnumbered specimen. B, Tylosaurus proriger KU specimen, (redrawn from Williston, 1897). C, Hainosaurus bernardi IRSNB R23 (redrawn from Nicholls, 1988). D, AMNH specimen (redrawn from Osborn, 1899). E, Tylosaurus proriger. YPM 4002. For abbreviations see Figure 3.

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ossified carpals identified as the ulnare, intermedium, radiale, and distal carpals four, three, and two. The two small elements, adjacent to the base of the radius, are tentatively identified as the radiale and first distal carpal based on the topological relations of these elements in other squamates. The first metacarpal is equal in size to the second and third. The fifth metacarpal is divergent, small, and articulates with distal carpal four. Ectenosaurus clidastoides Forelimb (Fig. 9). Estenosaurus clidastoides (FHM-VP 401) has six ossified elements in the right limb identified as the ulnare, intermedium, radiale, and distal carpals four, three, and two. The first metacarpal is the largest in the series; the fifth metacarpal

Figure 5. Rearlimbs of tylosaurines. (Aâ&#x20AC;&#x201C;E) Tylosaurus. A, FHM-VP 393. B, AMNH 126. C, YPM 24919. D, AMNH specimen (redrawn from Osborn, 1899). E, FHM-VP 3. F, Hainosaurus pembinensis, MDM M74.06.06 (redrawn from Nicholls, 1988). For abbreviations see Figure 3.

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is small and bears no ossified phalanges (this limb appears to be in natural articulation). Seven elements are present in the left limb, but this pattern is suspect due to obvious reconstruction. The relationships and identity of these elements cannot be relied upon. Mosasaurines Clidastes Forelimb (Fig. 10A–C). Two stages of ossification are observed among the available specimens of Clidastes. There are usually seven ossified elements in the carpus (Fig. 10C). These are identified as the ulnare, intermedium, radiale, distal carpals four, three, two, and the pisiform. A small, but obvious space is always present distal to the radius, proximal to metacarpal one, and anterior to the radiale. In the second stage available, bearing eight elements (Fig. 10A), a small, rounded, bony element is found in this space, possibly distal carpal one. Rearlimb (Fig. 10D,E). Only a single stage of ossification is observed among the available specimens of Clidastes. There are three ossified elements: astragalus, calcaneum, and distal tarsal four. Unlike the condition in ‘russellosaurine’ mosasaurs

Figure 6. Forelimbs of Platecarpus. A, YPM 1254. B, NMC 40911. C, YPM 1426. D, YPM 1269. E, (reversed) FHM-VP 322. F, (reversed) YPM 1430. G, (reversed) YPM 40691. H, (reversed) KU 1001. For abbreviations see Figure 3.

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(Figs 4, 9) there is no open space distal to the tibia and anterior to the astragalus. The first and second metatarsals appear to articulate directly with the tibia and astragalus respectively; this is similar to the condition observed in extant squamates. The fifth metatarsal is divergent, short, proximally enlarged, articulates with the calcaneum and distal tarsal four, and is not hooked; this is similar to ‘russellosaurines’. Mosasaurus Forelimb (Fig. 11A,B). Only the adult stage of seven ossified elements is known for Mosasaurus and these are identified as the ulnare, intermedium, radiale, distal carpals four, three, two, and the pisiform. Unlike Clidastes, there is no obvious space between the radius, metacarpal one, and radiale. The latter element is exceptionally large. Rearlimb (Fig. 11C–E). As in Clidastes, there is only a single stage of ossification preserved for Mosasaurus. The smallest to largest individuals always have three ossified elements: astragalus, calcaneum, and distal tarsal four. The structure and articular relationships of the meso- and metapodials are similar to those described for Clidastes regarding the articulation of the first and second metatarsals. Prognathodon and Plesiotylosaurus Forelimb (Fig. 12A–C). Only the adult stage of ossification is known for Prognathodon and Plesiotylosaurus and is based on a small number of articulated limbs (Yang, 1983).

Figure 7. Rearlimbs of Platecarpus. A, YPM 1430. B, FHM-VP 322. C, KU 1004. For abbreviations see Figure 3.

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From these specimens it is observed that there are at least six ossified elements in the carpus and possibly seven depending on the presence or absence of a pisiform. The six commonly present are identified as the ulnare, intermedium, radiale, and distal carpals four, three, and two. Plotosaurus Forelimb (Fig. 12D). Only the adult stage is known for Plotosaurus. Six ossified elements are preserved in the carpus and are identified as the ulnare, intermedium, radiale, and distal carpals four, three, and two. However, Bell (1993) found a pisiform to be present in the left manus of LACM (CIT) 2750. As in Mosasaurus, there is no obvious space between the radius, metacarpal one, and radiale. With the reconstruction of a pisiform this limb becomes very similar in structure to that of Mosasaurus. The most apparent differences are the morphology of the intermedium (there does not appear to be an antebrachial embayment) and the increased number of phalangeal elements (cf. Fig. 11). There is no evidence of a fifth digit. Rearlimb (Fig. 12E). There is very little preserved of the rear limb of Plotosaurus. The only specimen is represented by the first digit, the tibia, femur and astragalus. While posterior structures and relationships are unknown, the articulation and fit of the tibia, astragalus, and first metatarsal are similar to those described for Clidastes (Fig. 10D,E) and Mosasaurus (Fig. 11Câ&#x20AC;&#x201C;F).

Figure 8. Forelimbs of Plioplatecarpus sp. A, NMC 21853. B, AMNH 14470. For abbreviations see Figure 3.

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M. W. CALDWELL DISCUSSION AND CONCLUSIONS

Ossification sequences and phylogeny A complete ontogenetic sequence of mesopodial ossification cannot be ascertained for any one of the mosasaur species examined. However, it is possible to hypothesize a ‘mosasaur’ ossification sequence for the both the carpus and tarsus from the adult morphologies and partial ontogenies described above. This is accomplished by considering the phylogeny of mesopodial patterns in reverse, i.e. patterns of phylogenetic reduction as an ontogenetic sequence of mesopodial ossification (Fig. 13A,B), in association with the actual ontogenetic sequences. ‘Russellosaurines’ and the Mosasaurinae show differences in the number of terminally deleted elements that are both orderly and predictable; this allows assessment of ontogenetic states within and between taxa. Sequences are also based on the character states possessed by terminal taxa, by the available sequences within several taxa (specifically Platecarpus and Tylosaurus), from reconstructed states for internal nodes, and by the symplesiomorphy of these stages and sequences with stages of ossification and ossification sequences observed in three Permian diapsids (Caldwell, 1994) and in some extant squamates (Rieppel, 1992a,b,c). The carpal ossification sequence for the Permian diapsids Thadeosaurus, Hovasaurus,

Figure 9. Forelimbs and pectoral girdle of Ectenosaurus clidastoides FHM-VP 401. For abbreviations see Figure 3.

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and Claudiosaurus, is ulnare – distal carpal four – intermedium – lateral centrale – and distal carpal three or one (ossification sequence of the remaining carpals is highly variable); the tarsal ossification sequence is astragalus/calcaneum – distal tarsal four

Figure 10. Fore- and rearlimbs of Clidastes sp. A, forelimb FM 1972.119.5F. B, forelimb YPM 1333. C, forelimb KU 1022. D, rearlimb KU 1022. F, rearlimb KU 1026. For abbreviations see Figure 3.

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– distal tarsal three – centrale – and distal tarsals two, one, or five (see Caldwell, 1994). The sequences reported by Rieppel (1992a,b,c) for extant squamates are consistent with those given above for the Permian diapsids with the exception that terminal elements in the sequence are absent in many lizards. The inferred sequence of mosasaur carpal ossification is: ulnare – distal carpal four (dc4) – intermedium – dc3 – radiale or dc2 – dc1 or pisiform and dc5. For the

Figure 11. A, forelimb of Mosasaurus conodon. SDSM 452. B, forelimb of Mosasaurus mokoroa, DM R1535 (from Welles & Gregg, 1971). C, rearlimb of ?Mosasaurus sp. UCBMP 137246 (from Yang, 1983). D, rearlimb of Mosasaurus lemmonieri IRSNB 3098 (from Dollo, 1892). E, rearlimb of Mosasaurus conodon. SDSM 452. For abbreviations see Figure 3.

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mosasaur tarsus the inferred sequence is: astragalus â&#x20AC;&#x201C; distal tarsal four and/or calcaneum. Tylosaurines show one and two element stages for both the carpus and tarsus. Platecarpines show three, four and five element stages for the carpus and a three element stage in the tarsus. Five and seven element stages are known for the carpus of plioplatecarpines, and Ectenosaurus shows a six element stage of carpal ossification.

Figure 12. A, forelimb of Prognathodon sp. UCBMP 126715 (from Yang, 1983). B, forelimb of Prognathodon waipariensis CM Zfr 108 (from Welles & Gregg, 1971). C, forelimb of Plesiotylosaurus sp. UCBMP 126716 (from Yang, 1983). D, forelimb of Plotosaurus bennisoni CIT 2750 (from Camp, 1942). E, rearlimb of Plotosaurus bennisoni CIT 2750 (from Camp, 1942). For abbreviations see Figure 3.

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Figure 13. Hypothetical carpal and tarsal ossification sequences of Mosasaurs. A, carpal ossification sequences derived from partial sequences available for Tylosaurus (first and second figures) and Platecarpus (third, fourth and fifth figures), followed by Ectenosaurus and Plioplatecarpus, and finally the seventh and eighth element stages of Clidastes. Most of the direct sequence data are available for â&#x20AC;&#x2DC;russellosaurinesâ&#x20AC;&#x2122;; mosasaurine sequences are almost completely hypothetical. B, tarsal ossification sequences are derived from Tylosaurus (first and second figures) and Mosasaurus (third figure); in this case any other mosasaur with an articulated rearlimb shows the three element stage, presumed here to be the final stage of tarsal ossification. For abbreviations see Figure 3.

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Seven to eight elements in the carpus and three in the tarus characterize all mosasaurines (Clidastes, Mosasaurus etc.). If tylosaurines have the least ossified mesopodia, and platecarpines, plioplatecarpines, ectenosaurs and mosasaurines have mesopodia where ossified elements increase by consistent, sequential addition from a tylosaurine-like pattern, then the logical conclusion is that mesopodial ossification in mosasaurs proceeded from one to seven carpals and one to three tarsals (Fig. 14) in manner consistent with the sequences observed in other diapsids (Rieppel, 1992a,b,c; Caldwell, 1994). There is no reason to assume that ontogeny proceeded in any other direction than absence to presence of ossified elements; in other words, three ossified elements in the adult were not preceded by four ossified elements in earlier ontogenetic stages where the fourth element is lost by a process other than late stage fusion. The distribution of adult ossification patterns, optimized on MacClade (Maddison & Maddison, 1992) and mapped onto a tree topology (Fig. 14) constructed from the consensus trees presented above (Fig. 2A,B), and from Bell (1993; Fig. 1C), shows that skeletal paedomorphosis was a dominant heterochronic pattern in mosasaurid limb evolution. Continued reduction of mesopodial ossification resulted in the

Figure 14. Ossified carpals and tarsals of adult mosasauroid reptiles optimized against a phylogeny constructed from consensus trees given in Figure 2A,B, and from Bellâ&#x20AC;&#x2122;s (1993) preferred phylogeny (Fig. 1C). â&#x20AC;&#x2DC;Russellosaurinesâ&#x20AC;&#x2122; and halisaurines show extreme paedomorphosis of mesopodial ossification. Mosasaurines show very limited paedomorphosis. Characters were optimized using MacClade 3.04 (Maddison & Maddison, 1991) and ancestral states for internal nodes reconstructed. Numbers indicate the number of ossified carpals and tarsals, respectively; values in parentheses indicate reconstructed states for internal nodes. Missing data are indicated by question marks. Values in parentheses, such as (10/7/4, 5/3), indicate equivocal state reconstructions for those internal nodes; this is due to missing data.

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deletion of ossified elements from the ossification sequence; this pattern characterizes more exclusive groups within the Mosasauroidea (Fig. 14). The exception is the eight and three combination seen in the manus and pes of the most highly ossified specimen of Clidastes (Fig. 10A, Fig. 14). The optimized adult character state distributions support this condition as a reversal; heterochronically, this would be a peramorphic pattern via terminal addition. It is important to note that the eight and three states are seen in only one individual; in all other Clidastes specimens examined, the eighth carpal element (distal carpal one) was never found to be ossified. It should also be noted that this specimen (Fig. 10A) is assigned to Clidastes liodontus, a smaller, stratigraphically lower occurring taxon than Clidastes propython, a taxon for which only the seven and three combination is known. Therefore, the eight and three combination may be plesiomorphic for clidastine mosasaurines and is again a reversal from the natantid condition. It is also possible that all natantids are derived from either an eight and three combination at the node Mosasauridae (Fig. 14), or from an unknown condition found within the equivocal reconstructed states ten/seven and five/three at the node below Mosasauridae. Even though ontogenetic sequences are not known for aigialosaurs, the morphology of the adult carpus is found to be symplesiomorphic with that of other anguimorphs (Caldwell et al., 1995) and more primitive diapsids (Caldwell, 1994). The existence of symplesiomorphic morphologies suggests the retention of plesiomorphic diapsid ossification sequences (Caldwell, 1994), and, though phylogenetically uninformative, the retention of plesiomorphic sequences is also observed in highly derived mosasauroids such as Platecarpus (Figs 6 and 7) and Tylosaurus (Figs 4 and 5). Based on the symplesiomorphies of conserved lepidosauromorph sequences (Caldwell, 1994), mosasaur apomorphies include variations on the degree of ossification, the number of ossified elements, and the number of terminal deletions in the digital arch sequences. The derived limbs of halisaurines, as described for the only known specimen with an articulated forelimb (Fig. 3), can only be considered relative to the Natantia, (Fig. 14), not aigialosaurs. The reduction of ossified elements in the carpus of Halisaurus sternbergi, from ten to four (the tarsus is unknown), defines the halisaurine condition at the node Mosasauridae. This reduction is supported as a synapomorphy of halisaurines in the same way that the reduction from ten to seven carpals and five to three tarsals is supported as a synapomorphy of the Natantia. The halisaurine condition should not be interpreted as a rapid loss of carpal ossifications in a stemgroup mosasaur. This character (four ossified carpals), despite missing information for other OTUs, can only be considered a synapomorphy of halisaurines until further evidence is collected. The halisaurine carpus of four elements is found to correspond to the four element stage of Platecarpus and the four element stage of the hypothetical sequence (Fig. 13A). The difficulty in interpreting the condition of the halisaurine carpus is that there is no comparative basis for knowing the relative stage of ossification this individual has achieved. It is a large specimen and is doubtless an adult, as are all other mosasaurs described in this study. However, there is no comparison possible with other halisaurines, nor even with the hindlimb, to determine whether four ossified mesopodials was the terminal stage or a midpoint as seen in platecarpines (Fig. 6). Reduction of ossified mesopodials, producing apparent terminal deletions along the digital arch sequence, are apomorphic for various clades within the Mosasauridae (Fig. 14). The character states differ between halisaurines and natantids

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(mosasaurines and ‘russellosaurines’) in the degree of reduced ossification. More exclusive clades within ‘russelosaurines’, specifically the tylosaurines, are characterized by highly reduced sequences. It is interesting to note that mosasaurines show little change from the hypothesized mosasaurid or mosasauroid plesiomorphic state (ten to eight or seven carpals, and five to three tarsals). The mesopodial ossification patterns described above are equally ‘congruent’ with both the phylogenetic hypothesis of Bell (1993) and that of deBraga & Carroll (1993). However, character analysis of individual elements (see above descriptions and Fig. 12A,B) require homoloplasious distributions for all limb characters of Prognathodon and Plesiotylosaurus if the reconstruction of this clade within plioplatecarpines is accepted, as has been proposed by deBraga & Carroll (1993). The ingroup relationships of aigialosaurs are still poorly resolved (Figs 1C, 2A,B) but this does not affect the distribution of ontogenetic limb characters with current mosasauroid phylogeny. Aigialosaurs retain the plesiomorphic squamate sequence, while mosasaur clades show derived states regarding the degree of ossification and termination of ossification sequences. Skeletal paedomorphosis The distribution of mesopodial ossification patterns among the Mosasauridae, as compared to the sistergroup condition of aigialosaurs, supports the conclusion that the evolution of mosasaur limbs was significantly influenced by skeletal paedomorphosis. Paedomorphic reductions of limb ossification take several forms in mosasaurs: reduction in the number of elements (spatial); reduction in degree of ossification (temporal); retention of the paedomorphic state of perichondral ossification vs. endochondral ossifications; retention of juvenile chondrocyte to osteocyte ratios (Sheldon, 1994; Sheldon et al., 1994). Compared to aigialosaurs or varanoid anguimorphs, mosasaurs have fewer ossified carpal and tarsal elements. Unfortunately, it is not possible to determine whether cartilage precursors of the ‘missing’ bones have been lost or simply did not ossify. Comparisons between ‘russelosaurines’ and mosasaurines, or between tylosaurs and platecarpines-plioplatecarpines (Fig. 14), show that in those taxa possessing a larger complement of ossified elements, the cartilage precursors for 7 to 8 elements are present; this would suggest that in those clades with fewer elements, the cartilaginous precursors for at least the 7 to 8 elements were present. However, arguments in favour of the retention of the cartilage precursors can only be made as arguments based on the phylogenetic distribution of ossified elements. For some elements, such as those never represented as ossified structures in mosasaurs, i.e. the centrale series, plesiomorphically present in basal diapsids (Caldwell, 1994), it is possible that evolutionary loss of the cartilage element has occurred. Such loss would be a synapomorphy of the Mosasauridae. Ossified centrale elements present in aigialosaurs and other lizards indicates plesiomorphic retention of the cartilage precursors to at least these branching points in mosasauroid phylogeny. Assuming that mosasaurs had similar patterns of chondrogenesis as have been reported for many tetrapods (Shubin & Alberch, 1986), at the chondrogenic stage of development, the centrale element(s) either did not form, were resorbed in some manner, or else were fused to proximal or distal mesopodials prior to ossification.

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Sheldon (1994) and Sheldon et al. (1994), provide histological data indicative of large scale heterochronic changes in the mode and degree of ossification of ribs and some long bones in mosasaurs. These histological features include both an increased percentage of cartilaginous matrix, as well as differences in the type of perichondral and endochondral bone, present in adult bone. These modifications are most pronounced in â&#x20AC;&#x2DC;russellosaurinesâ&#x20AC;&#x2122;, and more specifically tylosaurs, and were interpreted as skeletal paedomorphosis. Endochondral osteogenesis involves complex temporal and spatial controls over the replacement and differentiation of perichondral and endochondral tissues (Wolpert & Tickle, 1993; Wozney, Capparella, & Rosen, 1993). Paedomorphosis, in the form of neoteny, might involve the truncation of endochondral osteogenesis resulting in the retention of chondrocytes in endochondral bone matrix; the proportion of chondrocytes would be similar to a developmental stage of osteogenesis in juveniles of a sistergroup. Mosasaur astragalus In younginiforms and claudiosaurids the astragalus and calcaneum are distinct structures in both juveniles and adults; fusion into a single proximal tarsal bone, as seen in extant lizards, never occurs (Caldwell, 1994). The absence of fusion of these elements, at any of the known stages in ontogeny, in association with the presence of a large foramen between them that permits passage of the perforating artery, suggests that the astragalus and calcaneum of early diapsids developed from independent cartilaginous elements. In extant lizards, a single ossified element, referred to as the astragalocalcaneum, occupies the proximal tarsal row (Rieppel, 1993). This single element results from the fusion of two centres of ossification, referred to the astragalar and calcaneal centres, that form within a single proximal tarsal cartilage. The astragalus ossifies endochondrally from a single centre located in the intermedium position of this proximal tarsal cartilage (Sewertzoff, 1908; Rieppel, 1992a,b,c), while the calcaneum begins endochondral ossification within the cartilage distal to the fibula and lateral to the astragalus. As these independent ossification centres increase in size they eventually come into contact. Later in ontogeny these elements fuse so that only minor traces of the fusion line can be determined (pers. observ.). The perforating artery does not pass through either the single proximal cartilage or the astragalocalcaneum, but rather follows a groove over the proximal portion of the astragalus and through the antebrachial space. There are two elements in the proximal tarsal row of mosasaurs: the astragalus and calcaneum (Figs 7, 10 and 12). In only a few platecarpines was it apparent that very limited fusion of these two tarsals had occurred and sutures were very pronounced in the few showing fusion (Fig. 15). The degree of completion of endochondral and perichondral ossification in the astragalus and calcaneum was very poor with surface bone often present as a small disc restricted to the middle of the element, and with poorly finished endochondral margins. These poorly finished margins suggest contact with a large cartilage border. Comparison of the mosasaur astragalus and calcaneum with the ossified elements found in the single proximal cartilage of extant squamates at various ontogenetic stages (Rieppel, 1992a,b), shows remarkable similarities in the shape of the element and the position of ossification (Fig. 15). Articulated mosasaur hindlimbs show very

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Figure 15. Morphology of the astragalus in mosasaurs, and the calcaneum of Platecarpus, A&B, Platecarpus sp., YPM 55712; A, flexor view, right astragalo-calcaneum; B, proximal view, right astragalus. C&D, Tylosaurus sp., YPM 24908; C, extensor view; D, flexor view. E, Clidastes unnumbered KU specimen. F, Mosasaurus conodon SDSM 452. Arrows indicate the position of the groove for the perforating artery (gr pa). Scale bar (1 cm) applies Aâ&#x20AC;&#x201C;E; 5 cm scale bar applies to F. For abbreviations see Figure 3.

clearly that the astragalus ossified from a single centre in the intermedium position of the proximal tarsus, while the calcaneum also ossified from a single centre beneath the fibula. In even the most poorly ossified astragali there is a groove on the proximal edge, where the element forms the distal margin of the antebrachium, for passage of the perforating artery. This feature is present in all mosasaur taxa (Fig. 15) and in all extant lizards. Though cartilage elements are not preserved, it is reasonable to conclude, by congruence of characters, that a single proximal cartilage was present in mosasaurs and that the astragalus and calcaneum ossified from single centres within that cartilage. The existence of fused proximal tarsal bones, though limited, and the fact that the perforating artery passes through the antebrachium, not between the astragalus and calcaneum as seen in more primitive diapsids (Caldwell, 1994), lends support to this conclusion. Skeletal paedomorphosis in mosasaurs does not appear to have altered the earlier ontogenetic condition of the single proximal tarsal cartilage. There is no â&#x20AC;&#x2DC;recapitulationâ&#x20AC;&#x2122; of the independent cartilaginous centres found in more primitive diapsids. The heterochronic effects observed in mosasaurs appear to have altered the timing and duration of ossification in the mesopodium. There is no evidence, at least in terms of the astragalus, for assuming that there were alterations of earlier pattern formation involving cartilaginous elements.

ACKNOWLEDGEMENTS

For assistance while gathering of data on mosasauroids reptiles I want to thank C. Holton, D. Brinkman, S. Chapman, J. Chorn, A. Currant, F. Crompton, J. Martin,

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L. Martin, A. Neuman, B. Nicholls, V. Sowiak, J. Storer, T. Tokaryk, M. Turner, and R. Zachevsky. I thank R. Carroll, and M. Wilson for extremely useful criticisms of various drafts of this paper. I thank O. Rieppel, G. Bell, and an anonymous reviewer for valuable criticisms. In particular I wish to thank M.V.H. Wilson for the use of his laboratory space while I finished the writing of this paper and of my thesis. The research presented here was supported by a Natural Sciences and Engineering Research Council of Canada (NSERC) Post Graduate Scholarship to M.W. Caldwell, and by NSERC operating grants to R. Carroll.

REFERENCES Bell GL. 1993. A phylogenetic revision of Mosasauroidea (Squamata). Ph.D. dissertation, University of Texas, Austin, Texas, 293 pp. Caldwell MW. 1994. Developmental constraints and limb evolution in Permian and modern lepidosauromorph diapsids. Journal of Vertebrate Paleontology 14: 459–471. Caldwell MW, Carroll RL, Kaiser H, 1995. The pectoral girdle and forelimb of Carsosaurus marchesetti (Aigialosauridae), with a preliminary phylogenetic analysis of mosasauroids and varanoids. Journal of Vertebrate Paleontology 15: 516–531. Camp CL. 1942. California Mosasaurs. Memoirs of the University of California 13: 1–68. Carroll RL, deBraga M. 1992. Aigialosaurs: Mid-Cretaceous varanoid lizards. Journal of Vertebrate Paleontology 12: 66–86. DeBraga M, Carroll RL. 1993. The origin of mosasaurs as a model of macroevolutionary patterns and processes. Evolutionary Biology 27: 245–322. Dollo L. 1892. Nouvelle note sur L’Osteologie des Mosasauriens. Bulletin de la Société Belge de Géologie de Paléontologie et d’Hydrologie (Bruxelles) 7: 219–259. Kornhuber AG. 1893. Carsosaurus marchesetti, ein neuer fossiler Lacertilier aus den Kreideschichten des Karstes bei Komen. Abhandlungen der geologischen Reichsanstalt Wien 17: 1–15. Kornhuber AG. 1901. Opetiosaurus bucchichi, eine neue fossile Eidechse aus der unteren Kreide von Lesina in Dalamtien. Abhandlungen der geologischen Reichsanstalt Wien 17: 1–24. Kramberger KG. 1892. Aigialosaurus, eine neue Eidechse aus den Kreideschiefern der Insel Lesina mit Rücksicht auf die bereits beschriebenen Lacertiden von Comen und Lesina. Glasnik Hrvatskoga Naravoslovnoga Drustva (Societas Historico-Naturalis Croatica) u Zagrebu 7: 74–106. Maddison WP, Maddison DR. 1992. MacClade: Analysis of phylogeny and character evolution. Version 3. Sunderland, Massachusetts: Sinauer Associates. McDowell SB, Bogert CM. 1954. The systematic position of Lanthanotus and the affinities of the anguimorph lizards. Bulletin of the American Museum of Natural History 105: 1–142. Nicholls EL. 1988. The first record of the mosasaur Hainosaurus (Reptilia: Lacertilia) from North America. Canadian Journal of Earth Sciences 25: 1564–1570. Osborn HF. 1899. A complete Mosasaur skeleton, osseous and cartilaginous. Memoirs of the American Museum of Natural History 1: 167–188. Patterson C. 1982. Morphological characters and homology. In: Joysey KA, Friday AE, eds. Problems of phylogenetic reconstruction Vol. 21. New York: Academic Press, 21–74. Rieppel O. 1992a. Studies on skeleton formation in reptiles. I. The postembryonic development of the skeleton in Cyrtodactylus pubisulcus (Reptilia, Gekkonidae). Journal of Zoology, London 227: 87–100. Rieppel O. 1992b. Studies on skeleton formation in reptiles. III. Patterns of ossification in the skeleton of Lacerta vivipara Jacquin (Reptilia, Squamata). Fieldiana Zoology, New Series 68: 1–25. Rieppel O. 1992c. The skeleton of a juvenile Lanthanotus (Varanoidea). Amphibia-Reptilia 13: 27–34. Rieppel O. 1993. Studies on skeleton formation in reptiles. IV. The homology of the reptilian (amniote) astragulus revisited. Journal of Vertebrate Paleontology 13: 31–47. Russell DA. 1967. Systematics and morphology of North American Mosasaurs. Bulletin of the Peabody Museum of Natural History, Yale University 23: 1–241. Sewertzoff AN. 1908. Studien u¨ ber die Entwicklung der Muskeln, Nerven und des Skeletts der Extremitäten der niederen Tetrapoda. Bulletin de la Société Imperiale des Naturalistes de Moscou, Année 1907. 21: 1–430. Sheldon AS. 1994. Ecological implications of mosasaur bone microstructure. Abstracts of Papers, 54th Annual Meeting of the Society of Vertebrate Paleontology, Journal of Vertebrate Paleontology 14: 45A. Sheldon AS, Williams M, Bell GL, Donachy J. 1994. Microstructural and biochemical analysis of mosasaur bone. Abstracts of Papers, 52nd Annual Meeting of the Society of Vertebrate Paleontology, Journal of Vertebrate Paleontology 14: 45A.

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Shubin NS, Alberch P. 1986. A morphogenetic approach to the origin and basic organization of the tetrapod limb. In: Hecht MK, Wallace B, Prance GT, eds. Evolutionary Biology, Volume 20. New York and London: Plenum Press, 319–387. Swofford DL. 1993. PAUP: Phylogenetic Analysis Using Parsimony, Version 3.1.1. Computer program distributed by the Illinois Natural History Survey, Champaign, Illinois. Welles SP, Gregg DR. 1971. Late Cretaceous marine reptiles of New Zealand. Records of the Canterbury Museum 9: 1–111. Williston SW. 1897. On the extremities of Tylosaurus. Kansas University Quarterly 6: 99–102. Wiman C. 1920. Some reptiles from the Niobrara Group of Kansas. Bulletin of the Geological Institute of Uppsala 18: 11–18. Wolpert L, Tickle C. 1993. 14. Pattern formation and limb morphogenesis. In: Bernfield M, ed. Molecular Basis of Morphogenesis. New York: Wiley-Liss, Inc., 207–219. Wozney JM, Capparella J, Rosen V. 1993. 15. The bone morphogenetic proteins in cartilage and bone development. In: Bernfield M, ed. Molecular Basis of Morphogenesis. New York: Wiley-Liss, Inc., 221–230. Yang D. 1983. A study of the pectoral and pelvic appendages of California Mosasaurs. Unpublished Masters Thesis, California State University, Fresno, 65 pp.

APPENDIX I Data matrix and character descriptions for Mosasauroidea as revised from Bell (1993). Sixty of Bell’s 151 characters were deleted from this analysis as they were either invariant or uninformative (Characters 3, 6, 10, 12, 18–20, 22, 29, 32, 35, 36, 45, 48, 51, 53, 55, 56, 60, 61, 64, 65, 68, 72, 73, 76, 78–80, 84, 87–89, 92, 95, 97, 102–105, 107, 112–114, 117, 119, 120, 127, 128, 131, 134, 135, 143–149, and 151). The character states for characters 74 and 96 were recoded from Bell (1993) as some states did not apply to the reduced terminal taxa. Specific characters and character states were recoded based on examination of latex peels and photographs of the type specimens of Aigialosaurus buccichi (NMW specimen and Kornhuber [1901]), Aigialosaurus dalmaticus (BSP 1902II501 and Kramberger [1892]), and Carsosaurus marchesetti (MCSNT unnumbered specimen and MCSNT 11430, 11431, 11432 [in three parts], and Kornhuber [1893]). The character number in brackets refer to the character number as listed in Bell (1993). Altered states are as follows:

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Character descriptions 1(1). Bony predental rostrum on premaxilla: absent (0); present (1). 2(2). Size of bony predental rostrum on premaxilla: short and obtuse (0); distinctly protruding (1). 3(4). Premaxillary rostral foramen size: small (0); large (1). 4(5). Width of premaxillary internarial bar: narrow (0); wide (1). 5(7). Dorsal keel of premaxillary internarial bar: absent (0); present (1). 6(8). Entrance of Vth cranial nerve on internarial bar of premaxilla: close to rostrum (0); far removed from rostrum (1). 7(9). Premaxillary tooth count: more than four (0); only four (1). 8(11). Nasal bones: present (0); absent (1). 9(13). Width of internarial process of frontal: not constricted (0); very constricted (1). 10(14). Frontal width: broad and short (0); long and narrow (1). 11(15). Narial emargination of frontal: not invaded by posterior nares (0); embayment of frontal present (1). 12(16). Sagittal dorsal keel on frontal: absent (0); low and inconspicuous (1); high, thin and well developed (2). 13(17). Shape of frontal ala: sharply accuminate (0); broadly pointed and rounded (1). 14(21). Fronto-parietal suture: low interlocking ridges from each (0); overlapping flanges (1). 15(23). Frontal invasion of parietal: posteriorly extended lateral sutural flange (0); posteriorly extended median frontal sutural flange (1); both present (2). 16(24). Frontal medial invasion of parietal: posteriorly extended median sutural ridge short (0); same but long (1). 17(25). Length of parietal: dorsal surface short (0); dorsal surface elongate (1). 18(26). Shape of parietal table: rectangular to trapezoidal with convergent sides (0); triangular with straight sides contacting anterior to suspensorial rami (1). 19(27). Parietal foramen size: small (0); large (1). 20(28). Position of parietal foramen: near to center of parietal table (0); close to suture (1); touching suture (2); straddles suture invading frontal (3). 21(30). Parietal posterior shelf: distinct shelf projecting posteriorly between suspensorial rami (0); shelf absent (1). 22(31). Parietal suspensorial ramus greatest width: vertical or oblique (0); horizontal (1). 23(33). Prefrontal suborbital process:process absent or a small knob (0); large overhanging wing (1). 24(34). Prefrontal contact with postorbital frontal: no contact at edge of frontal (0); elements in contact at edge of frontal (1). 25(37). Postorbitofrontal: without low, rounded, transverse dorsal ridge (0); with dorsal ridge (1). 26(38). Postorbitofrontal squamosal ramus: does not reach end of supratemporal fenestra (0); does reach end of ramus (1). 27(39). Maxillary tooth count: 20 to 24 (00; 17â&#x20AC;&#x201C;19 (1); 15â&#x20AC;&#x201C;16 (2); 14 (3); 13 (4); 12 (5). 28(40). Posterior terminus of maxillo-premax suture: anterior or even with midline of fourth maxillary tooth (0); between fourth and ninth tooth (1); even with or posterior to ninth tooth (2). 29(41). Posterodorsal process of maxilla: recurved wing of maxilla dorsolaterally overlaps a portion of the anterior end of prefrontal (0); no recurved posterodorsal process present (1). 30(42). Posterodorsal extent of maxilla: recurved wing of maxilla prevents prefrontal emargination on lateral edge of narial opening (0); does not prevent emargination (1). 31(43). Angle of posteroventral margin of jugal: very obtuse or curvilinear (0); near 120 degrees (1); approximately 90 degrees (2). 32(44). Posteroventral process of jugal: absent (0); present (1). 33(46). Pterygoid tooth row: teeth arise from main shaft of pterygoid (0); teeth arise from thin pronounced ridge (1). 34(47). Pterygoid tooth number: 12 or less (0); more than 12 (1). 35(49). Length of quadrate stapedial process: short (0); moderate length (1); long (2). 36(50). Constriction of quadrate stapedial process: distinct (0); none (1). 37(52). Fusion of quadrate stapedial process to ventral process: absent (0); present (1). 38(54). Quadrate stapedial pit shape: oval to circular (0); narrow oval (1); elongate with constricted middle (2). 39(57). Thickness of quadrate ala: thin (0); thick (1). 40(58). Quadrate conch: ala and main shaft describe a deep bowl (0); alar concavity shallow (1). 41(59). Quadrate ala shape: anterodorsal segment of tympanic rim more tightly curved than rest of rim (0); or rim curve uniformly circular (1). 42(62). Quadrate ala groove: no groove in anterolateral edge of ala (0); long distinct groove (1). 43(63). Quadrate tympanic rim size: large, almost as high as quadrate (0); smaller, 50â&#x20AC;&#x201C;65% of quadrate height (1). 44(66). Quadrate ventral median ridge: single thin ridge (0); thin ridge diverging ventrally (1). 45(67). Quadrate ventral condyle: condyle saddle-shaped, concave anteroposterior view (0); condyle gently domed, convex in any view (1).

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M. W. CALDWELL 46(69). Quadrate anteroventral condyle modification: no upward deflection of anterior edge of condyle (0); distinct deflection (1). 47(70). Basisphenoid pterygoid process shape: process relatively narrow with articular surface facing mostly anterolaterally (0); process thinner, more fan-shaped with posterior extension of articular surface (1). 48(71). Basioccipital tubera size: short (0); long (1). 49(74). Dentary tooth number: 20 to 24 or more (0); 15 to 16 (1); 14 (2); 13 (3). 50(75). Anterior projection of dentary: projection of bone anterior to first tooth present (0); absent (1). 51(77). Medial parapet of dentary: median subdental shelf low (0); shelf elevated and straplike (1); equal in height to lateral wall (2). 52(81). Coronoid shape: with slight dorsal curvature, posterior wing not wide fan-shape (0); very concave above, posterior wing expanded (1). 53(82). Coronoid posteromedial process: present (0); absent (1). 54(83). Coronoid medial wing: does not reach angular (0); contacts angular (1). 55(85). Surangular-coronoid buttress: low, thick, and parallel to lower edge of mandible (0); high, thin, rapidly rising anteriorly (1). 56(86). Surangular-articular suture position: behind condyle in lateral view (0); at middle of glenoid on lateral edge (1). 57(90). Foramina on lateral aspect of retroarticular process: none (0); one to three (1). 58(91). Tooth surfaces: teeth finely striate medially (0); not striated medially (1). 59(93). Tooth facets: no facets (0); with facets (1). 60(94). Tooth fluting: no fluting (0); numerous broad flutes (1). 61(96). Tooth carinae: present but weak (0); strong and elevated (1). 62(98). Tooth replacement mode: in shallow excavations (0); in subdental crypts (1). 63(99). Atlas neural arch: notch in anterior border (0); no notch (1). 64(100). Atlas synapophysis: extremely reduced (0); large and elongate (1). 65(101). Zyosphenes and zygantra: absent (0); present (1). 66(106). Cervical synapophysis ventral extension: extend slightly or not all below ventral margin of centrum (0); far below ventral margin (1). 67(108). Trunk vertebrae condyle inclination: inclined (0); vertical (1). 68(109). Condyle shape: extremely dorsoventrally depressed (0); slightly depressed (1); rounded (2). 69(110). Posterior trunk condyle shape: not higher than wide (0); slightly compressed (1). 70(111). Dorsal ridge of posterior vertebral synapophysis connecting with zygapophysis: no sharp ridge making connection (0); sharp ridge makes connection (1). 71(115). Number of sacral vertebrae: two (0); one (1). 72(116). Neural spines of caudal vertebrae: uniformly shortened posteriorly (0); several spines dorsally elongated in mid-tail region (1). 73(118). Haemal arch articulation: articulate (0); fused (1). 74(121). Scapula-Coracoid size: bone approximately equal in proximo-distal length (0); scapula about half length of coracoid (1). 75(122). Scapula width: no anteroposterior widening (0); distinct fan-shaped expansion (1); extreme widening (2). 76(123). Scapula dorsal margin convexity, if widened: very convex (0); or broadly convex (1). 77(124). Scapula posterior emargination: gently concave (0); deeply concave (1). 78(125). Scapula-coracoid fusion: bones fused (0); not fused (1). 79(126). Scapula-coracoid suture in unfused state: interdigitate suture (0); flat, no interdigitation (1). 80(129). Humerus length relative to distal width: elongate, 3 to 4 (0); shortened, 1.5 to 2 (1); length and width equal (2); distal width greater (3). 81(130). Humerus postglenoid process: absent or very small (0); distinctly enlarged (1). 82(132). Humerus deltopectoral crest: single ridge (0); two separate insertion areas (1). 83(133). Humerus pectoral crest: located anteriorly (0); located medially (1). 84(136). Humerus entepicondyle: absent (0); present as a prominence (1). 85(137). Radius shape: radius not expanded (0); slightly expanded (1); broadly expanded (2). 86(138). Ulna contact with centrale: excluded by broad ulnare (0); contacts centrale (1). 87(139). Radiale size: large and broad (0); reduced or absent (1). 88(140). Carpal reduction: size or more (0); five or less (1). 89(141). Pisiform: present (0); absent (1). 90(142). Metacarpal I expansion: spindle shaped, elongate (0); broadly expanded (1). 91(150). Appendicular epiphyses: formed from ossified cartilages (0); from thick unossified cartilage (1); missing or extremely thin (2).