The Fossil Record of Chelodina

Chelodina has a long fossil record, as one might expect from such a deep branch within Chelodininae. However it is much sparser than the fossil record of the short-necked chelodinine clade.

The earliest specimens referred to Chelodina have been found in the Eocene deposits of Redbank Plains in south eastern Queensland. Here there are three forms known from incomplete shells, one of which has been named C. alanrixi (Lapparent de Broin & Molnar, 2001). None of these forms can be easily assigned to the extant subgenera, not least because the material is limited and incomplete. In the case of C. alanrixi there are some similarities with C. (Macrochelodina) but there are also some notable plesiomorphies such as a complete row of exposed neural bones in the carapace and a posterior vertebral scute that is as wide as the suprapygal scute (Lapparent de Broin & Molnar, 2001). These indicate that C. alanrixi could well belong to the Chelodina stem-group, rather than to any of the recognised subgenera but this will need to be tested with more complete fossils and a phylogenetic analysis of Chelodina ingroup relationships that includes fossils.

There is a considerable gap between the Eocene and the next oldest occurrences of Chelodina in the early to middle Miocene carbonate deposits of Riversleigh in Queensland and Bullock Creek in the Northern Territory. Chelodina has been recorded from two sites at Riversleigh, Gag Site and Quentin’s Quarry, both of which have been assigned to system C (Archer et al., 1989). However, constrained seriation analysis indicates that Quentin’s Quarry is referrable to the early Miocene Wipajirian Australian Land Mammal Age, whereas Gag Site belongs to the younger middle Miocene Camfieldian ALMA (Megirian et al., 2010). The Quentin’s Quarry specimen is an intriguing skull fragment that exhibits several distinct autapomorphies and cannot be easily placed in any of the extant subgenera (White, 1997). The specimen from Gag site is a fragmentary shell that would appear to have affinities C. (Macrochelodina) based on the absence of exposed neural bones and the length of its intergular scute which is only slightly longer than the midline contact of the pectoral scutes (Gaffney, Archer & White, 1989).

The Camfield beds of Bullock Creek are Camfieldian in age and have produced two distinct Chelodina species. Both species are only known with certainty from isolated epiplastra (Megirian & Murray, 1999). One of these, Chelodina sp. B, may belong to C. (Chelodina) based on the squared-off profile of the anterior lobe of the plastron.

The record becomes a little better in the Plio-Pleistocene. From Tara Creek in Queensland there is a Chelodina specimen that is likely to be similar in age to the nearby Bluff Downs local fauna (Gaffney, 1981). The Bluff Downs local fauna is Early Pliocene in age and belongs to the Tirarian ALMA (Megirian et al., 2010). The specimen is unusual in that it lacks a finely ornamented surface even though the seams between scutes are well-marked (Gaffney, 1981). Based on the narrow anterior lobe of the plastron, a long inter-pectoral seam which exceeds the length of the intergular scute (Gaffney, 1981, Fig. 8), this specimen would appear to be a member of Chelodina (Macrochelodina).

From the Pliocene Bluff downs local fauna itself there is a single nuchal bone that has been identified as Chelodina based largely on the presence of a broad, square-shaped cervical scute (Thomson & Mackness, 1999). The specimen was tentatively assigned to the nominate subgenus (as the ‘Chelodina longicollis group’) on the basis of its well developed ornamentation. However, as the authors themselves note, this is a variable character, for example Chelodina (Macrochelodina) insculpta also has well-developed surficial ornamentation (Thomson, 2000, Fig. 4).

Chelodina (Macrochelodina) insculpta is a named fossil species based on fragments from the Plio-Pleistocene of the Darling Downs that bear a close resemblance to C. (M.) expansa but differ from it in a having a less flared margin of the carapace (Thomson, 2000).

Lapparent de Broin & Molnar (2001) point out that a posterior plastral fragment figured by Gaffney (1981, Fig. 18A) belongs to Chelodina based on the rounded shape of the posterolateral margin of the ischiadic scar on the dorsal surface of the xiphiplastron and the rounded nature of the fine tubercles that decorate its ventral surface. This fragment comes from the late Pleistocene, or Naracourtean, Katapiri Formation of Lake Kanunka, in the Tirari Desert of central South Australia. A second probable Pleistocene from central South Australia is represented by an isolated cervical vertebra from Cooper Creek (Gaffney, 1981). Unfortunately neither specimen displays diagnostic characters that would allow determination of their subgeneric affinities. Nonetheless these specimens are interesting because they show that Chelodina was surviving in central Australia until quite recently, well outside its present range.

Lastly there is a Naracourtean (late Pleistocene) record of Chelodina from Henshkes Cave, in south eastern South Australia based on isolated plastron elements (Gaffney, 1981). These are similar to the extant C. (Macrochelodina) expansa (Gaffney, 1981).

While it is highly likely that remains of the widespread, common species C. (Chelodina) longicollis are present in some of many Naracourtean vertebrate faunas of south eastern Australia, none have been positively recorded.

Thus C. (Chelodina) has an exceptionally poor fossil record, if any at all, with just a couple of fragmentary possible occurrences reported in the literature. Here I describe a new species of this subgenus from the late Miocene vertebrate fossil locality of Alcoota Station, Northern Territory. This is the first definite occurrence of the subgenus in the fossil record and the first diagnosable extinct species in the subgenus.

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