Chelodina sp.
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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/03CB87F5-2557-AD09-F07B-FCB8FF51FAE1 |
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treatment provided by |
Marcus |
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scientific name |
Chelodina sp. |
| status |
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Emydura s.l. sp.? Possible synonymy.
DESCRIPTION
Material ( Fig. 6 View FIG ) ( Rix 1991: fig. 2B)
Redbank Plains form 4 is represented by the specimen QM F18212. It consists of a broken nodule with the impression of half of the plastron, posterior part, without the bridge, with an impression (natural mold) of a part of the right pelvis in external view and parts of two or three right posterior peripheral plates.
Measurements (in cm)
Maximal length and width of the fragment: 13.6 and 10.4.
Maximal length and width of the preserved part of the plastron: 12.5 and 7.8.
Maximal length of the half posterior plastron (from hyo-hypoplastral suture): 10.5.
Maximum length of posterior lobe (from abdomino-femoral sulcus): 8.7.
Width of posterior lobe at abdomino-femoral suture: c. 8.
Ratio of width to estimated length of posterior plastral lobe: c. 92%.
Width at the femoro-anal sulcus: 5.5.
Preserved height of the pelvis (dorsal part of ilium incomplete): 3.6.
Anal notch, width and length: 3.4 and 1.
Decoration
All the preserved part of the plastron ( Fig. 6D, E View FIG ) is covered with small polygons, elevated, wellmarked (strong), well-separated, most of them rounded, c. 1 ✕ 1 mm wide, giving a granulate appearance to the positive (mold) of the specimen: but the fossil impression of the fig. 2B of Rix 1991, which is reversed, shows small pits separated by fine crests. As indicated above, a cover of polygons is present in the plastron of Chelodina and Pseudemydura as in various Emydura s.l. The polygons are arranged in lines, oriented from a center at the lateral border of the abdomino-femoral sulcus and quite longitudinally elongate in the hypo-xiphiplastral suture area.
The plastral polygons ( Fig. 6E View FIG ) are relatively smaller than in Pseudemydura . In Chelodina , we have seen relatively as small polygons in the plastron of C. steindachneri and in the preserved part of bridge of form 3, Chelodina sp. c. But, as seen above, the polygons are not as longitudinally elongate in those two genera and this is rather a character of the Emydura group. In the latter, a similar decoration has been observed in Elseya dentata (BMNH 76-5-19-77), although the polygons are slightly smaller in the fossil, for a similar plastral size ( Fig. 6A View FIG ).
The fragmentary peripherals have also well-marked, small and granulate polygons. One of them also shows small polygons ( Fig. 6C View FIG ), high as in Chelodina and particularly as in Chelodina sp. c. They are also similar to the polygons of the plastron of C. oblonga ( Fig. 6B View FIG ) and we consider them as apomorphic for Chelodina .
Plastron ( Fig. 6D, E View FIG )
The plastron impression has a central rhomboid fontanelle, as observed in juveniles, for example in Elseya latisternum (BMNH 71-9-25-8). It is preserved from the pectoro-abdominal sulcus, anteriorly, to the anal notch, posteriorly. The exact shape and proportions of the posterior lobe are not possible to determine. The fragment is cut short, laterally, so that we do not see where the inguinal notch was positioned nor do we know how long the bridge was. If the inguinal notch was situated at the lateral extremity of the abdomino-femoral sulcus ( Fig. 6F, a View FIG ), it was a fragment of Emydura s.l. as in Fig. 6A View FIG . In the two Redbank Plains forms attributed to the Emydura group, the sulcus finishes in the bottom of the inguinal notch and the bridge was relatively long, as in the other Emydura species. But if the bridge was shortened, the abdomino-femoral sulcus would not end in the bottom of the inguinal notch, but backwards on the border of the lobe ( Fig. 6F, b View FIG ) and the fragment would belong to a Chelodina species as in Fig. 6B View FIG .
The posterior lobe is elongate with proportions known in Australian chelids in the Emydura group. The percentage ratio of width to length of the posterior lobe (measured from the extremity of the abdomino-femoral sulcus) is c. 92%, which conforms to the Emydura group, measuring between 84.69 ( Rheodytes leucops ) and 97 to 103% ( E. macquarrii ) as seen above in the study of the Emydura s.l. sp. a: the ratios are higher in Pseudemydura (141%) and Chelodina (103 to 138%) which have relatively shorter posterior lobes. In Pseudemydura , the plastral lobe is short and much rounded with a very small and narrow anal notch. It is very different from the lobe of the Redbank Plains forms 5 and 6. Here, as in Emydura s.l. sp. a, form 5, the two borders of the posterior lobe clearly converge, from the bottom of the inguinal notch, at the abdomino-femoral sulcus, towards the xiphiplastral extremities in a straight line, without strong narrowing at the femoro-anal sulcus. Just a small indentation for a slight posterior anal narrowing is noticeable at this sulcus. The anal notch is very widely round- ed and short, as in most of species of Emydura s.l., except some forms such as Elseya latisternum and E. novaeguineae which have a less rounded anal notch. But this shape of the posterior lobe is also similar to that of C. oblonga ( Fig. 6B View FIG ) with its slightly convergent straight lateral borders and a wide and short anal notch. The anal notch of the fossil is relatively shorter than in Emydura s.l. but longer than in C. oblonga , as shown in the Fig. 6E View FIG . The percentage ratio “width/length of the posterior lobe” is elevated as in some Emydura s.l., known in E. macquarrii , but not different from the lower one in Chelodina , in C. oblonga (103.44%). C. oblonga is the more elongated species of Chelodina , with the relatively more elongate posterior lobe.
Right pelvis
It is visible in external view, the inferior part being anteriorly positioned lateral to the plastron.
The upper part of the ilium is missing. The acetabulum is slightly posteriorly dilated as in most of adult chelids. Anteriorly, the posterior part of the ventral suture of the pubis with the xiphiplastron is preserved: it is oval but apparentlywithout a sharp posterior angle, different from the specimen of Chelodina sp. c described above. The problem is that this extremity of the pubic scar may be eroded. Such an oval suture and xiphiplastral corresponding scar, primitive in its evolutionary stage, is known in some specimens of the Emydura group such as Elseya latisternum and E. novaeguineae . It differs from that known in one specimen of Emydura macquarrii where it is nearly rectangular with a sharp anterior angle, and of one specimen of Elseya dentata where it is very narrow. As in Chelodina there are differences, unfortunately not studied at the specific or group of species level. No Chelodina and no Pseudemydura are known with this oval morphology but it is known in the Emydura group. The ventral face of the ischium is hidden below the xiphiplastron.
DISCUSSION
It is better to attribute the specimen of the Redbank Plains form 4 to Chelodina sp. c than to Emydura s.l., owing to the granulate plastral decoration and polygonal dorsal decoration that is exactly similar to those of the form 4. The small size of the polygons better conforms with those of Chelodina , in the context of the Redbank Plains Eocene forms and, in particular, the apomorphic decoration of the preserved peripheral bone is consistent with that of Chelodina and not with the Emydura group. But the extant Elseya dentata has a similar granulate plastral decoration and the proportions of the posterior lobe are consistent with those of the Emydura group. The oval pubic scar, if correctly preserved, could better be consistent with attribution to the Emydura group, as it is known at present. But the granulate decoration of oriented small polygons and the oval pubic ventral suture may be primitive for both groups. In the absence of the inguinal notch bottom, we cannot be certain.
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