Journal of Vertebrate Paleontology 26(1):196–199, March 2006 © 2006 by the Society of Vertebrate Paleontology
NOTE
PLESIOSAUR REMAINS FROM CRETACEOUS HIGH-LATITUDE NON-MARINE DEPOSITS IN SOUTHEASTERN AUSTRALIA BENJAMIN P. KEAR, School of Earth and Environmental Sciences, University of Adelaide, Adelaide, South Australia 5005, and South Australian Museum, North Terrace, Adelaide South Australia 5000 (address for correspondence)
Australian plesiosaur remains are common although currently poorly documented. At present, most of the described material is derived from extensive Early Cretaceous epicontinental marine rocks in central and northeastern Australia (Kear, 2003). In recent years, however, a number of fragmentary specimens have been recovered from Lower Cretaceous non-marine sequences in the southeastern part of the continent (Fig. 1). The fossil-producing strata are included within the middle Valanginian– early Albian Wonthaggi (Gippsland Basin) and Eumeralla (Otway Basin) formations, southern Victoria, and early–middle Albian Griman Creek Formation (Surat Basin), Lightning Ridge, New South Wales/ Surat region, Queensland. Interestingly, these units were deposited in an Early Cretaceous high-latitude zone (60–80º S), subject to highly seasonal, cool to cold conditions and months of winter darkness near the southern pole. Fossils recorded include a diverse range of freshwater/ terrestrial vertebrates, non-marine invertebrates, and plants (see Dettmann et al., 1992 for summary). Although several previous reports have mentioned plesiosaur material from southeastern Australia (e.g. Rich et al., 1988; Rich and Rich, 1989; Vickers-Rich, 1996; Smith, 1999; Rich and Vickers-Rich, 2000; Kear, 2003), none of the remains have yet been described. It is therefore, the purpose of this paper to present an up-to-date summary of the existing specimens (Table 1) and assess their taxonomic and paleoecological implications. Abbreviations—AM, Australian Museum, Sydney; NMV, Museum Victoria, Melbourne; QM, Queensland Museum, Brisbane.
DESCRIPTION Most of the Cretaceous plesiosaur material from southeastern Australia consists of isolated teeth. Many of these lack roots and appear to represent shed tooth crowns. A few isolated postcranial elements have also been discovered; these correspond to small-bodied and/or osteologically immature (indicated by incomplete ossification of axial elements, sensu Brown, 1981) individuals of around 2–3 meters total length. Dental Remains All of the plesiosaur teeth (Fig. 2A-G) examined are very similar in morphology and probably derive from one or more closely related taxa. Tooth size is highly variable (ranging from 4.8–53.5 mm total crown height) and may reflect taxonomic/ontogenetic variation and/or positioning along the jaw. Crowns are conical and curved with sub-circular cross sections. All bear coarse, widely spaced striations that extend to the apex (where preserved) and occasionally bifurcate towards the base. Buccal (convex) surfaces are finely striated or smooth. There is no evidence of carinae. Tooth apices frequently show signs of breakage and wear. Indeed, one specimen (AM F112844; Fig. 2F) comprises only the lowermost third of the crown (the upper portion being sheared off) with the remainder worn smooth presumably by abrasion against hard food items and/or other intermeshing teeth. Another unusual isolated tooth (AM F121713; Fig. 2G) bears an elongate depression (5.1 mm long, 2.4 mm wide) in the enamel surface possibly resulting from a developmental abnormality.
GEOLOGICAL AND PALEOENVIRONMENTAL SETTING The Wonthaggi Formation (Strzelecki Group) and middle Eumeralla Formation (Otway Group) comprise finely laminated sandstones and mudstones with locally abundant horizontally stratified fossiliferous claystone/mudstone conglomerates. These were laid down by meandering to braided river systems in a mid-Cretaceous rift valley flood plain formed as a result of the onset of rifting between Australia and Antarctica (Veevers, 1984; Mutter et al., 1985). Age estimates place the Wonthaggi Formation within the middle Valanginian–Aptian Cyclosporites hughesii spore-pollen Subzone, and Eumeralla Formation within the late Aptian– early Albian Crybelosporites striatus spore-pollen Subzone (Wagstaff and McEwan Mason, 1989). Lithic glauconitic sandstones, siltstones and mudstones dominate the stratigraphically younger Griman Creek Formation (Rolling Downs Group). Characteristic molluscan fossils, cross bedding, plant root impressions, and vertebrate taphonomy indicate a coastal-plain, fluviatile/ estuarine depositional setting (Dettmann et al., 1992). Burger (1980) determined the age of the Griman Creek Formation as early-middle Albian, corresponding to the Coptospora paradoxa spore-pollen Zone of Helby et al. (1987). Estimates of paleolatitude place southeastern Australia near the southern polar circle (about 60–80º S; Embleton, 1984) throughout much of the Jurassic–Cretaceous. Palaeoclimatic indicators suggest highly seasonal cool to very cold (possibly with winter freezing) conditions during deposition of Aptian–early Albian sequences (Douglas and Williams, 1982; Frakes and Francis, 1988, 1990; Gregory et al., 1989; Frakes et al., 1995; Constantine et al., 1998; De Lurio and Frakes, 1999). The younger middle-late Albian sediments, however, are thought to correspond to a much warmer temperate climatic period (Dettman et al., 1992).
Postcranial Remains Isolated plesiosaur ribs (NMV P180852, NMV P186328, Fig. 2H) have been recovered from both the Wonthaggi and Eumeralla formations. They are single-headed and apparently derive from the anterior-middorsal region (total length of NMV P180852 is 332 mm; total length of NMV P186328 is 238 mm). A small dorsal centrum (QM L380; Fig. 2I) is the only plesiosaurian axial element currently known from the Griman Creek Formation. The specimen (24.3 mm total length, 47 mm width of anterior surface, 40.1 mm height of anterior surface) is ovoid in anterior outline (dorsal surface damaged) and anteroposteriorly compressed in lateral view. The lateral centrum sides are almost flat and lack an inset waist. Articular surfaces are ovoid in outline and shallowly amphicoelous with convex, rounded rims. The paired nutrient foramina are positioned high on the lateral centrum sides. There is no evidence of fusion with the neural arch, suggesting derivation from an osteologically immature (sensu Brown, 1981) juvenile or sub-adult individual. A badly weathered, incomplete bone (59.5 mm total length, 44 mm height of proximal end) recovered in association with the dorsal centrum (QM L380) is tentatively interpreted as the proximal portion of an ilium. The fragment is short and club-like with slight expansion towards the proximal (articular) end; the shaft is largely broken away. The proximal extremity of the bone bears weakly defined facets for the ischium (posteriorly) and acetabulum (anteriorly); the acetabular facet projects anterolaterally as a narrow lip. A weathered propodial (QM F41328; Fig. 3J) was found in the same area as QM L380 but may belong to a separate individual. It lacks most of the proximal articular head and shaft, with the remaining distal end
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NOTES TABLE 1.
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Specimen numbers and locality information for material used in this study. Unit
Griman Creek Formation (Surat Basin)
Eumeralla Formation (Otway Basin) Wonthaggi Formation (Gippsland Basin)
Material
Locality
AM F43240, AM F68333, AM F68334, AM F68336, AM F68338, AM F68344, AM F72261, AM F105661, AM F106554, AM F112630, AM F112843, AM F112844, AM F112845, AM F112846, AM F112847, AM F112848, AM F112849, AM F121675, AM F121678, AM F121679, AM F121713, AM F121714, AM F121715, AM F121716, AM F121717, AM F121718, AM F121719, AM F121720 QM L380, QM F41328 NMV P180852 NMV P182958, NMV P186256, NMV P186350, NMV P186416 NMV P19124, NMV P185855, NMV P208117, NMV P208306 NMV P186328, NMV P186376 NMV P210088
(122.4 mm total length, 75.5 mm total distal anteroposterior width, 28.5 mm total distal height) having suffered damage from surface exposure. The distal propodial shaft is sub-circular in cross section, becoming dorsoventrally compressed distally. The distal extremity is asymmetrical (being slightly posteriorly offset) and fan-shaped in dorsal outline. The anterior margin is strongly rounded and appears to have supported only two articular facets for the proximal epipodials. Long (1998) described a large (268.82 mm total length) opalized bone (AM F102462) from the Griman Creek Formation, Lightning Ridge, as a plesiosaurian propodial, suggesting similarity to those of Leptocleidus. However, re-examination of the specimen has failed to identify any morphological features that support this conclusion. Consequently, the element is here reinterpreted as belonging to another large tetrapod, perhaps a dinosaur.
Lightning Ridge opal fields (exact mine localities unknown), northern New South Wales
Surat, southern Queensland Cumberland River, southern Victoria Dinosaur Cove, Cape Otway, southern Victoria Inverloch southern Victoria Cape Paterson southern Victoria
and coastal estuary sediments laid down near the Cretaceous southern polar circle. Taxonomic affinities are unclear at present; however, a number of diagnostic characters do support attribution to Pliosauroidea (these include small, slender tooth crowns with coarse, widely spaced striations, anteroposteriorly compressed amphicoelous dorsal vertebral centra, dorsoventrally compressed, fan-shaped distal propodials, and presence of well-developed facets for only two proximal epipodials— supernumerary elements apparently absent). Notably, these features are also evident in the contemporaneous small-bodied pliosauroid Leptocleidus, a taxon common in Aptian–Albian near-shore marine sediments of central Australia (Kear, 2003). Unfortunately, however, until more informative cranial material is discovered there is insufficient evidence to warrant definitive assignment to this taxon. DISCUSSION
Comments Despite being fragmentary, the plesiosaur remains from southeastern Australia are significant because they derive from inland freshwater river
The record of plesiosaurians from freshwater deposits is sparse in comparison to those from marine sediments. Despite this, a number of discoveries have been made from around the world (see Sato et al., 2003
FIGURE 1. Geographic position of source localities for material used in this study (shaded areas show extent of Gippsland, Otway, and Surat depositional basins).
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FIGURE 2. Early Cretaceous plesiosaur remains from non-marine deposits in southeastern Australia. A, MV P186416 tooth crown (Eumeralla Formation) in lateral view. B, MV P186376, tooth crown (Wonthaggi Formation) in lateral and lingual views. Scale bars equal 10 mm. C, AM F105661, large tooth crown (Griman Creek Formation) in lingual and lateral views. Scale bar equals 20 mm. D, AM F121679, E, AM F121720, tooth crowns (Griman Creek Formation) in lateral view. Scale bars equal 10mm. F, AM F112844, sheared tooth crown (Griman Creek Formation) showing secondary wear. Scale bar equals 10mm. G, AM F121713, tooth crown (Griman Creek Formation) in buccal and lateral views showing potential developmental abnormality in enamel (arrowed). Scale bar equals 10 mm. H, MV P180852 (Eumeralla Formation) and MV P186328 (Wonthaggi Formation), isolated ribs. Scale bar equals 50 mm. I, QM L380, vertebral centrum (Griman Creek Formation) in anterior and lateral views. Scale bar equals 20 mm. L, QM F41328, incomplete propodial (Griman Creek Formation). Scale bar equals 20 mm.
for summary). The fact that these range in age from Early–Middle Jurassic (e.g. Thulborn and Warren, 1980; Sato et al., 2003) to Late Cretaceous (e.g. Russell, 1931; Sato et al., 2004) attests to the group’s long history of habitation in non-marine environments. The vast majority of non-marine plesiosaur specimens are fragmentary, and many are taxonomically uninformative. Where they are diagnostic, however, many of the freshwater specimens are referable either to ‘rhomaleosaurid-like’ taxa (see O’Keefe, 2004, for recent reassessment of Rhomaleosauridae), or to the widespread Cretaceous pliosauroid genus Leptocleidus (Sato et al., 2003). Not surprisingly, therefore, the material from southeastern Australia shares similarities with this latter taxon, and lends support to the hypothesis that freshwater and near-shore marine environments may have served as refugia for plesiomorphic pliosauriform plesiosaurs well into the late Early Cretaceous (see Andrews, 1922). The plesiosaur fossils from southeastern Australia constitute one of a number of recognized finds from Cretaceous high-latitude deposits. However, most other occurrences are marine in origin, including examples from central Australia (Kear, 2003), New Zealand (Welles and Gregg, 1971; Wiffen and Moisley, 1986; Cruickshank and Fordyce, 2002), the Chatham Islands (Cruickshank and Fordyce, 2002), Patagonia (see Gasparini et al., 2003 for summary), Antarctica (Chatterjee and Small, 1989) and the Canadian Northwest Territories (Nicholls and Russell, 1990). Amongst the currently documented specimens, those from the Early Cretaceous units of central and southeastern Australia are unusual because they occur in association with paleoclimatic indicators (e.g., cryoturbated sediments, glacial erratics, glendonites, and growth-banded wood) denoting seasonally very cold to near freezing conditions (see Frakes and Francis, 1988, 1990; Dettman et al., 1992; Frakes et al., 1995; Constantine et al., 1998; DeLurio and Frakes, 1999). This contrasts markedly with climatic regimes typically tolerated by modern aquatic reptiles, but suggests that some plesiosaur taxa may have been able to cope with extremely low average water temperatures. Acknowledgments—I am indebted to Tim Flannery and Mike Lee (South Australian Museum) and Tom Rich (Museum Victoria) for their continuous support and enthusiasm for the study of Australian Creta-
ceous faunas, floras, and environments. Robert Jones (Australian Museum), Pat Vickers-Rich and Lesley Kool (Monash University), Tom Rich, Ralph Molnar, and Jo Wilkinson (Queensland Museum), Henk Godthelp (University of New South Wales), the National Opal Collection, and Cody Opal Pty. Ltd generously assisted in making information and specimens available. Robert Hamilton-Bruce (South Australian Museum) contributed to production of the figures. The Australian Research Council (LP0453550 to BPK and M.Y.S. Lee: University of Adelaide), South Australian Museum, Umoona Opal Mine and Museum, Outback@Isa, Origin Energy, The Advertiser, The Waterhouse Club, the Coober Pedy Tourism Association, Commercial and General Capital Ltd and Kenneth J. Herman Inc. gave financial support for this research. The Australian Research Council (DP0209280 to P. Vickers-Rich, T. H. Rich, and T. F. Flannery) and the Committee for Research and Exploration of the National Geographic Society (T. H. Rich and P. VickersRich) provided funding for collection of the specimens from the Wonthaggi and Eumeralla formations.
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