Chelodina sp.

Lapparent de Broin, France & Molnar, Ralph, 2001, Eocene chelid turtles from Redbank Plains, Southeast Queensland, Australia, Geodiversitas 23 (1), pp. 41-79 : 60-62

Chelodina sp. c

DESCRIPTION

Material ( Table 2)

The Redbank Plains form 3 consists of specimens QM F18216 and 18215, the impression of a fragment of carapace, comprising some dislocated plates in which we recognize three right bridge peripherals, four or five fragments of pleurals and a plastral bridge element, with the fragmentary left pubis and ischium.

Measurements (in cm)

Right peripheral 7: posterior ventral width: 2.2; lateral length: 2.6; anterior ventral width: 1.5.

Right peripheral 6: posterior ventral width: 1.2; lateral length: 2.6; anterior ventral width: 0.9.

Right peripheral 5: posterior dorsal width: 1.5; lateral length: 2.5; anterior dorsal width: 1.5.

Pleurals anteroposterior length: 2.2 to 2.4.

Length of the bridge: 5.2 (and c. 2 mm more of hyoplastron hidden under the hypoplastron).

The elements belonged to a carapace of 24 or 25 cm long, estimated by comparison with C. longicollis .

Decoration

Size of the polygons (approximate), on pleurals: 3-4 over 1-1.5 mm, on peripherals: 1 over 1 mm, on plastral bridge part: 0.5 over 1 mm.

The decoration is made of small well-marked polygons (strong decoration), most of them rounded and well-distributed, as in Chelodina and Pseudemydura . The carapace polygons are similar to those of the shell of C. oblonga and C. longicollis , for example within the extant species, and to those of the figured specimens attributed to C. insculpta . But the polygons are smaller for a shell of equivalent size to that of C. longicollis , C. oblonga and C. insculpta , although larger than in the two other species of Chelodina from Redbank Plains. The plastral bridge element bears still much smaller polygons than the carapace, arranged in transversal elongate lines and in radiate lines. The carapace decoration is also similar to that of Pseudemydura , but the plastral polygons are smaller than in this extant genus and the other elements prevent the attribution to this genus.

Dermal bones of the carapace

The preserved right peripherals 7 and 6 are seen in ventral and medial views. The seventh is the last of the bridge and, then, less open than the sixth. Besides, the right peripheral 5 is shown only in dorsal view, associated with a fragment of corresponding pleural 3. Two superposed fragments of even pleurals (with the sulcus between two costals) are preserved besides.

The plates have the morphology of the corresponding plates in the extant C. longicollis . The external dorsal border of the fifth peripheral is narrow and upwardly-curled. In medial view of the peripherals 7 and 6, the hole for the free rib is well-visible and the medial width (between dorsal and ventral borders of the plate) is equivalent to that of C. longicollis . But their ventral width is slightly less than that of the corresponding plates in C. longicollis : the peripheral border was less expanded. It is also nearly similar to that of C. oblonga , but less narrowed and more curled.

Plastron

A small part is seen under the peripherals. It consists of the bridge part of the hyoplastron and hypoplastron and does not show any determinant characters except its very fine and granulate unusual decoration, as in the following specimen, form 4, from Redbank Plains.

Pelvis

The left pubis is seen in inferoposterior view. The pubic ventral suture with the xiphiplastron is diamond shaped, nearly oval: anterior and posterior sharp angle, with rounded lateral borders. The type of pelvic scar on xiphiplastra has not been observed in every Australian chelid, owing to lack of the preparation of the material. But this type of pubic scar has been seen in Chelodina oblonga . It is different from that of C. expansa , long, narrow posteriorly and wide anteriorly, from that of C. longicollis , short, triangular, wide anteriorly, and from that of C. steindachneri , short, oval and nearly round. In the Emydura group, some species have an oval shaped suture, very similar but without extreme sharp angles. In Pseudemydura , the suture is similar to that of C. longicollis , a short rounded-triangle, wide anteriorly.

The ischium is represented by the ventral part, in inner and ventral view. The ventral suture for the xiphiplastron is incomplete medially but fortunately complete posterolaterally: its posterior extremity is well-rounded. This is typical of Chelodina , as seen in the C. alanrixi n. sp. section in its differing from the other chelid forms with an angulate posterior part (primitive condition). In the Pleistocene example of Chelodina , fig. 18A of Gaffney 1981, given above, the rounded scar is closer to the lateroposterior border of the plate than in Emydura s.l. If the scar is rounded posteriorly in all the examined species of Chelodina , its position varies. The scar is still closer to the lateroposterior border of the plate in C. oblonga (narrowed form), farther laterally and posteriorly in C. longicollis (form with enlarged xiphiplastra and a reduced anal notch) and farther posteriorly in C. steindachneri (narrow xiphiplastra but short anal notch). In the Pleistocene form, the scar is situated as in C. expansa , within the Chelodina species , but we do not know its position in the Redbank Plains form 3.

DISCUSSION

This specimen belongs clearly to Chelodina by the set of obvious characters:

– the complete cover of polygons on the dorsal shell. When there are polygons on the dorsal carapace in Emydura s.l., there are also ridges or granulations or smooth areas: the polygons are not well-marked and well-distributed over all the dorsal shell, a difference from the plastron which has often a cover of well distributed polygons (although it is probable that the primitive Emydura group members had a more polygonal carapace);

– the curled peripherals: there is a slight upwardlycurled border in some Emydura s.l. such as E. macquarrii , but the width of the bridge peripherals is much larger and the border is not so curled;

– the well-rounded posterior border of the ischiadic suture with the xiphiplastron.

The two first characters are also shared by Pseudemydura , the first one being a primitive character in chelids, the second a derived character but homoplastic in turtles. The third is autapomorphic in the observed material of Chelodina , and the set is diagnostic of the genus. But it is impossible to define the species. It is different from C. alanrixi n. sp., which is much larger and has relatively small (owing to the larger size) and narrower polygons. In the latter, the bridge peripherals are not preserved so that we do not know if they were upwardly-curled. But the relatively wide and flat posterior border does not indicate that the peripherals had to become recurved at the bridge; they had to be flat or scarcely recurved as in C. expansa . The undefined species is probably also different from the form 2, smaller with smaller polygons and probably also without upwardly-curled border. It does not seem a round- ed species of the lineage of C. steindachneri . It might better belong to an elongated species (such as in C. oblonga or C. siebenrocki-rugosa lineages) or to a moderately elongated species (such as in C. longicollis lineage).