Emydura, Bonaparte, 1836
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publication ID |
https://doi.org/ 10.5281/zenodo.5372741 |
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publication LSID |
lsid:zoobank.org:pub:DE7BA85F-4CD1-4B77-BD6F-B5697F6D6CF2 |
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persistent identifier |
https://treatment.plazi.org/id/03CB87F5-255F-AD31-F011-FA98FBF5FA41 |
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treatment provided by |
Marcus |
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scientific name |
Emydura |
| status |
s.l. |
Emydura s.l. sp. a or b?
DESCRIPTION
Material ( Fig. 9A, B View FIG )
The specimen of the Redbank Plains form 6 is represented by QM F37913. It consists of a rounded nodule with the ventral view of a partial shell, with the natural mould and some bone fragments of pleurals 4-8 and the suprapygal and, on the other side, the impression of the medial part of the plastron, without the most anterior part (epiplastra, entoplastron) and the most posterior part (xiphiplastra).
Measurements (in cm)
Maximum preserved length and width of the fragment: 10.8 and 15.
Estimated length and width of the carapace: c. 21 and 17.5.
Plastral bridge length: c. 4.2.
Anterior lobe width at its base: 7.6.
Maximum estimated entoplastral length: 2. Medial hyoplastral length: 5.2.
Decoration
It is not preserved.
Shell shape
The plates were relatively thick for their length and there is no indication of fontanelles. It was fully adult. Based on the preserved parts, i.e. the scars of the thoracic ribs 1 and 2, the axillary process, the entoplastral size, the distance between the entoplastron and the hyo-hypoplastral suture and the bridge proportions, the species belongs to the Emydura group. It is therefore possible to reconstruct a carapace of Emydura s.l. shape. This is not much variable in the group, i.e. there is always a carapace particularly elongated relative to the primitive length and expanded posteriorly and slightly narrowed anteriorly, with a narrow plastron a little smaller than the carapace, with a open U-shaped anterior lobe, a relatively long bridge and a long posterior lobe.
In the Emydura group, the posterior border of the carapace is more expanded in some species than in others, some are more rounded and some are more elongated. The anterior lobe may be more narrowed and the posterior lobe may be more or less narrowed posteriorly at the femoroanal sulcus, depending on the species to which it belonged.
Dermal bones of the shell
The nuchal is moderately wide (the anterior part is missing) and the pleurals 1 are moderately long. There are no neurals between the preserved pleurals 1 to 5 which are wide for their length. Between the two pleurals 1, the neural arch of the dorsal vertebra 1 is preserved in its medial part (reduced anteriorly and posteriorly) which means that dorsal vertebra 1 was still intercalated between the plates while between pleurals 2 to 5 there are no more intercalated vertebrae and no more neurals at all. The axillary process enters pleural 1 at the mid posterior part of peripheral 2 and at the beginning of peripheral 3, anterior to rib 2. It covers c. 2/3 of the width of pleural 1, being well-rounded and well-overlapping thoracic rib 2 at that place. The lateral part of the impression of the right pleural 5 bears the short rounded extremity of the inguinal process.
Plastron
The impression of a good part of the entoplastron is preserved and easy to reconstruct in its length. It was small and largely separated from the hypoplastra (slightly more than two times its length) as in Emydura s.l. This is the difference from Chelodina where the entoplastron is large in the anterior lobe and closer to the hypoplastra (distance of a little more than its length in C. expansa and C. oblonga , less than its length in C. longicollis or in C. steindachneri ). The situation in Pseudemydura is intermediate; the entoplastron is small, but the anterior lobe is wide and short and the entoplastron is separated from the hypoplastra by a distance of a little more than its length. As in Emydura s.l., the bridge is long relative to the width of the plastral lobes at their base, its length being clearly more than the half the plastral width, different from Chelodina and Pseudemydura where it is as long or shorter.
DISCUSSION
As seen above, the specimen belongs clearly to the Emydura group in light of these features: the characters of the pleurals 1, the entoplastral size, the distance between the entoplastron and the hyo-hypoplastral suture and the bridge length relative to the width of the lobes. In the Emydura group, the axillary processes generally enter the pleural 1 from forward, at peripherals 1 to 3 depending on the taxa (it is more derived where it is more forwardly positioned). Within the Chelidae , Emydura s.l. is particularly derived in its long bridge and more forwardly prolongated axillary processes. The axillary process enters pleural 1 and goes obliquely backwards to join thoracic rib 2 that it covers, up to half the width of pleural l. It is forward positioned relative to the primitive condition which is still found in Chelodina and Pseudemydura , where the process enters at peripheral 3 and directly covers thoracic rib 2. In the Emydura group, the more primitive condition is that of Elseya latisternum , Rheodytes leucops and Elusor macrurus (see Thomson et al. 1997; QM and BMNH specimens) where the process enters at peripheral 3 just forward but close to thoracic rib 2, that it covers up to the mid-width of the plate, i.e. more widely than in the primitive condition seen in most of Chelodina species and in Pseudemydura . In Chelodina and Pseudemydura , the process, entering at peripheral 3, either is still short laterally but, directed obliquely backwards, may cover a small part of pleural 2 (for example C. steindachneri , C. longicollis , Pseudemydura ), or it is also prolongated up to pleural 2 and furthermore it is wide up to the mid-width of pleural 1 ( C. expansa , with the more derived condition in Chelodina ).
In the Redbank Plains form, the position of the axillary process which enters pleural 1 at the posterior part of peripheral 2 and beginning of peripheral 3, anterior to rib 2, is consistent with that of various species of the Emydura group moderately derived on that point: that of Elseya dentata (BMNH 765-19-77, Thomson et al. 1997), Emydura macquarrii (BMNH 86-9-26- 5, MNHN 1897-814), specimens attributed to E. kreftii ( White & Archer, 1994) , E. novaeguineae (BMNH 1835-5-10 -177), extant Elseya lavarackorum in Thomson et al. 1997, Emydura sp. , fossil from Lake Tarkarooloo, Miocene, Burke et al. 1983. In these forms, the process is more or less thin where it contacts with pleural 1 and either situated in front of the posterior part of peripheral 2 or just backwards, at the limit of both peripherals 2 and 3. It is clearly situated more backwards than in the fossil Elseya lavarackorum White & Archer, 1994 , Pleistocene from Riversleigh, figured in Thomson et al. 1997, Emydura subglobosa ( Thomson et al. 1997) and young (MNHN P 1880-467), where it is positioned at mid-length of peripheral 2. It is still more backwards situated than in “ E. australis ” (AM, R26581), where it enters pleural 1 at peripheral 1, which is the more derived condition in the Emydura group. But it is more forward positioned than in Rheodytes leukops (see Thomson et al. 1997) and Elseya latisternum ( White & Archer 1994; BMNH 71-9-25-8) where the process enters at peripheral 3.
The specimen, adult, is approximately the same size as the other Emydura s.l. specimen from Redbank Plains described above and the doubtful specimen of Chelodina sp. c? (possibly a specimen of Emydura ?), also adults: c. 20 cm in sp. a, c. 23 cm in “ Chelodina sp. c?”, and c. 21 cm in form 6. The presence of the remains of the neural arch 1 between the pleurals 1 can be compared to the presence of the remains of neurals 6 to 8 between the pleurals 6 to 8 in Emydura s.l. sp. a, thus showing that both forms could belong to the same species without a complete neural reduction. The possible shape of the plastron of form 6 conforms to the plastron of form 5. It is possible that the carapace was slightly wider than in form 5. This species cannot be of the same group as the species from Proserpine nor the same as the species of the form 4, if it is a Emydura s.l. instead of a Chelodina .
Even if only one species of Emydura s.l. existed in Redbank Plains, two species of two distinct groups existed in the Palaeogene of Queensland. In fact many more extant species exist than were known in the last Century ( Cann 1978; Wells & Wellington 1985; Iverson 1992) and a rediagnosis of each species ought to be given, based on the anatomy of the skeleton, allowing better definition of the new subgroups of Emydura s.l.
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