FRESHWATER turtles (pleurodires and cryptodires)

Transcription

1 Copeia 2011, No. 1, Terrestrial Turtle Fossils from New Zealand Refloat Moa s Ark Trevor H. Worthy 1,AlanJ.D.Tennyson 2, Suzanne J. Hand 1, Henk Godthelp 1,and R. Paul Scofield 3 Two fossils from the diverse St Bathans Fauna from Early Miocene sediments in New Zealand are described and identified as from a large, probably terrestrial turtle. They are the first freshwater or terrestrial turtles to have been reported from the Cenozoic of New Zealand. Recent authors have used the absence of turtles and species they considered unlikely to raft to New Zealand to debunk the long held theory that an element of the New Zealand fauna was ancient and vicariant and had evolved on what David Bellamy called Moa s Ark. The discovery that large non-marine turtles were once present in New Zealand adds to a growing and diverse list of terrestrial taxa known from Zealandia shortly after its maximum inundation in the Late Oligocene. Many of these taxa, including a diverse herpetofaunal component, represent lineages endemic to New Zealand and had poor dispersal capabilities, supporting the long held view that a part of the Zealandian fauna was vicariant in origin. FRESHWATER turtles (pleurodires and cryptodires) with a Mesozoic-Recent record are diverse in Australia and New Guinea. Terrestrial eucryptodire turtles, in the form of a radiation of the extinct, large bodied, horned meiolaniid turtles (Chelonia: Meiolaniidae), are prominent in the Cenozoic fossil record of Australia and Southwest Pacific region. These bizarre turtles are characterized by cranial horns or frills developed from the squamosals and a tail bearing a heavily ossified club (Owen, 1887; Gaffney, 1996). Meiolaniids were confined to the Cenozoic of South America and the southwest Pacific region including Australia. Their demise is regarded to be part of the Pleistocene megafaunal extinction event that followed human arrival in the Pacific, and temporal overlap with humans has recently been demonstrated in Vanuatu (White et al., 2010). Here we report the presence of a large terrestrial turtle in the Ma St Bathans Fauna of Central Otago, South Island, New Zealand. This turtle co-existed with a diverse range of other animals now endemic to New Zealand (including a sphenodontid reptile [Jones et al., 2009], mystacinid bats [Hand et al., 2009], and an acanthisittid wren [Worthy et al., 2010]). The proponents of a radical resynthesis of New Zealand biogeography argue that an Oligocene submergence event completely drowned what is now New Zealand, exterminating all terrestrial life (Trewick et al., 2007; Landis et al., 2008). The existence of such a diverse fauna only 3 4 million years after the peak of the so-called Oligocene Drowning makes the existence of such a catastrophic event unlikely. MATERIALS AND METHODS Excavations at several Early Miocene sites in Central Otago, on New Zealand s South Island have, since 2001, uncovered a remarkable terrestrial fauna (now known as the St Bathans Fauna) in lacustrine littoral deposits (Worthy et al., 2007). Among the recovered fossils are two turtle bones, a pelvic fragment and a dorsal vertebra fragment, which have been compared with turtle skeletons in the reptile and paleontology collections of the Australian Museum, Sydney, including a series of horned turtle Meiolania platyceps Quaternary fossils from Lord Howe Island. They derive from the Bed HH1b, in the Trench excavation, stratigraphically m above base of Bannockburn Formation, Home Hills Stn, Manuherikia River, Otago, at 44u S, 169u E, which has the New Zealand Fossil Record File Number H41/f90 (see site details in Worthy et al., 2007). They are catalogued in the Museum of New Zealand Te Papa Tongarewa (NMNZ), Wellington, New Zealand. RESULTS A 56 mm long right pelvis fragment (NMNZ S.51026) preserves only the stumps of the ilium, pubis, and ischium around the acetabulum (Fig. 1). The ilial shaft adjacent to the acetabulum is rounded both laterally and medially, and its section is about 30% mediolaterally wider than craniocaudally long. There is complete synostosis of the ischium, ilium, and pubis on all faces and only shallow grooves ventrally and medially mark the ilial pubal junction. The base of the ischium is least well preserved and reveals nothing of its shape beyond the acetabulum. The pubis medially preserves a sharp crest on its anterior margin as typical in turtles. The acetabulum is subtriangular in shape with its corners raised above the shaft of each constituent element. Its greatest diameter is the craniocaudal length from the ischial corner to the pubal corner. Medially, the surface of the acetabulum is slightly convex with rounded margins. The medial surface of the pubis is slightly flattened. A centrum of a thoracic vertebra (NMNZ S.50858) has a maximum length of 29.7 mm, and a maximum anterior width estimated at 21 mm. The specimen is complete on the left side, with large well-preserved costal articulations anteriorly lateral to neural articulations and caudally a single pair of costal articulations (Fig. 2). The size of the pelvic fragment (NMNZ S.51026) indicates a turtle larger than the largest freshwater species presently living in Australia, the Broad-shelled River Turtle Chelodina expansa. A large individual of the latter species, with carapace 400 mm long, has a much smaller diameter ilial shaft and acetabulum (Table 1). The New Zealand fossil is smaller, however, than the same element of large marine turtles, such as adult Green Turtles Chelonia mydas (with 1 School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, New South Wales, Australia; (THW) t.worthy@unsw.edu.au; (SJH) s.hand@unsw.edu.au; and (HG) h.godthelp@unsw.edu.au. Send reprint requests to THW. 2 Museum of New Zealand Te Papa Tongarewa, P.O. Box 467, Wellington, New Zealand; alant@tepapa.govt.nz. 3 Canterbury Museum, Rolleston Avenue, Christchurch, New Zealand; pscofield@canterburymuseum.com. Submitted: 21 July Accepted: 18 November Associate Editor: M. J. Lannoo. F 2011 by the American Society of Ichthyologists and Herpetologists DOI: /CH

2 Worthy et al. Miocene terrestrial turtle from New Zealand 73 Fig. 1. Turtle pelves: right side in lateral view (A, B), dorsocranial aspect (C, D), medial (E, F), and ventrocaudal views (G, H). Meiolania platyceps, AM F.18825, Lord Howe Island (A, C, F, G) and the St Bathans fossil, NMNZ S (B, D, E, H). Abbreviations: l, ilium; Is, ischium; Pu, pubis. Scale bar 5 5 cm. carapace of approx m long), and all adult specimens of the extinct terrestrial Meiolania platyceps, which had carapaces about 1 m long or greater (Table 1). The aquatic taxa examined, both freshwater (Chelodina expansa and common Snake-neck Turtle Chelodina longicollis) and marine (Chelonia mydas and Hawksbill Turtle Eretmochelys imbricata), have a relatively mediolaterally thick acetabulum (Table 1) and have a much more craniocaudally compressed

3 74 Copeia 2011, No. 1 Fig. 2. Turtle thoracic vertebra: NMNZ S from Early Miocene New Zealand, in left lateral (A) and dorsal (B) views; Meiolania platyceps, AM F.57984, anterior thoracic vertebrae 3 (bottom), 4, and 5 (top) in dorsal view (C). Scale bar 1 cm.

4 Worthy et al. Miocene terrestrial turtle from New Zealand 75 Table 1. Turtle Measurements (mm): Chelonia mydas, AM R ; Eretmochelys imbricata, AM R.2045; Chelodina expansa, AM R All A and F numbers are Meiolania platyceps. Ilium measurements were made just above the acetabulum and before the vertebral articular facet: Ilium width is craniocaudal width; Ilium thickness is the mediolateral thickness. Acet. max thickness is acetabulum maximum mediolateral thickness taken on the ventralcaudal side between the acetabular crests and the medial facies. Acet. max length is acetabulum maximum length craniocaudally, i.e., across the ischial pubal corners. Acet. height was taken from the ilial margin to a line drawn between the corners of the acetabulum next to the ischium and pubis. Ilium width Ilium thickness Ilium width/ thickness % Acet. max thickness Acet. max length Acet. height Acet. thickness/ length % Chelonia mydas Eretmochelys imbricata Chelodina expansa Meiolania platyceps A F F F F.18824,23, F F F S ilium, such that the mediolateral thickness is far greater than the craniocaudal thickness. The most significant feature of NMNZ S is the complete synostosis of the acetabular elements, as otherwise seen in all Meiolania pelves of the Australian Museum. These contrast with the same element in aquatic taxa, both marine and freshwater, where symphyses are obvious on all sides of the acetabulum; in smaller freshwater taxa the symphyses are incomplete across the articular surface of the acetabulum itself. The New Zealand fossil is very similar to part of the pelvis of Meiolania platyceps in general shape and proportions, differing mainly in having a relatively shallower mediolateral depth and in absolute size. Because the specimens of Meiolania examined by us are understood to be from animals with carapace lengths of at least 1 m and, given that the New Zealand fossil is larger than a similar fragment from the pelvis of a 400 mm long specimen of Chelodina expansa, S probably derives from a turtle of carapace length mm long. The vertebral centrum (NMNZ S.50858) differs from more anterior thoracic centra (3 5) in Meiolania platyceps (e.g., AM F.57984) by being relatively shorter with a much reduced lateral notch between the cranial and caudal articular surfaces, and may therefore be from a more caudal position. It approaches a thoracic centrum of Meiolania platyceps (AM F.16863) in general proportions, relative robustness, and in that the lateral notch is reduced; this is most like a meiolaniid thoracic vertebra 8 as described by Gaffney (1996). DISCUSSION The two Miocene New Zealand turtle fossils are the only ones derived from a fauna that has revealed several thousand vertebrate fossils, not including abundant fish (Worthy et al., 2007, 2010; Jones et al., 2009; Lee et al., 2009; Scofield et al., 2010). Considering that the fauna was deposited in a lake and is dominated by fish and aquatic birds such as various waterfowl, detection of freshwater turtles, if they were present, might be expected. Freshwater turtle (Chelidae) remains (such as carapace fragments and limb bones) are common in freshwater fossil deposits in Australia (Georges and Thomson, 2006; pers. obs.), for example the aquatic Oligo-Miocene deposits in Riversleigh (Archer et al., 2006), the lacustrine Oligo-Miocene Etadunna and Namba Formations of South Australia (Woodburne et al., 1994), and the fluvial Pleistocene deposits of the Warburton and Cooper river systems (Tedford and Wells, 1990). The absence of turtle carapace fragments in the St Bathans Fauna is especially noteworthy because these are often very common and conspicuous in palaeo-aquatic deposits in Australia. On this basis alone, the two turtle fossils identified here are unlikely to be from an aquatic chelid turtle. Moreover, the New Zealand fossils more closely resemble the meiolaniid Meiolania platyceps than aquatic turtles and meiolaniid shell is comparatively thin and less robust than that of other chelonians (Gaffney, 1996; THW, pers. obs.). For these reasons, we suggest that the fossil remains probably derive from a terrestrial turtle. The rarity of a facultatively terrestrial taxon parallels that of other terrestrial taxa of varying sizes in the St Bathans fauna (such as bats, sphenodontids, the acanthisittid wren, and eagles; Worthy et al., 2007). On the present evidence, little more can be said other than the fauna of Early Miocene New Zealand included a non-marine turtle that was probably terrestrial. Terrestrial turtles, in the form of a radiation of meiolaniids, are prominent in the Australian Southwest Pacific region (Gaffney, 1996). Undescribed meiolanoid turtles are known from million year old deposits in Australia (Smith, 2010), and through the Tertiary a number of meiolaniid species lived in Australia. From the Pleistocene, meiolaniids are known from New Caledonia (Grande Terre, Wapole, Tiga), Viti Levu, and Lord Howe Island (Gaffney, 1996; Worthy et al., 1999). Until now, no freshwater or terrestrial turtles have been reported from the Cenozoic of New Zealand, and this absence has been used by recent authors (Landis et al., 2008) to debunk the long-held theory that an element of the New Zealand Fauna was ancient and vicariant and had evolved on Moa s Ark (Bellamy et al., 1990). We acknowledge that terrestrial turtles do disperse

5 76 Copeia 2011, No. 1 over substantial water bodies, and must have to achieve their southwest Pacific distribution (Gaffney, 1996) or distribution in, for example, the Mascarenes (Cheke and Hume, 2008). However, the long history of meiolaniids in the Australasian region does not preclude a vicariant origin of turtles on Zealandia. Landis et al. (2008), Trewick et al. (2007), and others have argued that an Oligocene submergence event completely drowned proto-new Zealand, exterminating all terrestrial life. Here we report that a large terrestrial turtle existed in New Zealand only 3 4 million years after the peak of the socalled Oligocene Drowning. We expect that future excavations will reveal more diagnostic testudine elements, but in the interim these observations serve to add a significant component and a new order to the terrestrial faunal history of New Zealand. MATERIAL EXAMINED The identity of our specimens as turtles was established following comparison with turtle skeletons in the reptile collection of the Australian Museum. Further comparisons were made with the series of Meiolania platyceps (Owen) (Meiolaniidae) fossils from Lord Howe Island in the Palaeontology Collection, Australian Museum, Sydney, specifically, thoracic centra (AM F.16863, AM F.57984) and pelvic fragments (AM A.3296, F.17735, F.18252, F.18823, F.18824, F.18825, F.81949, F.97549). ACKNOWLEDGMENTS R. Sadlier and Y. Zhen at the Australian Museum granted us access to the collections in their care. The identity of the New Zealand fossils was established through discussions with A. White and S. Salisbury. The authors are grateful to the efforts of many helpers during excavations, particularly J. Worthy, J. Nguyen, J. Louys, P. Creaser, L. Cannell, and M. Archer. Especially, we thank the land owners A. and E. Johnstone for generous support and allowing access to the site. 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