Chelodina sp.

Lapparent de Broin, France & Molnar, Ralph, 2001, Eocene chelid turtles from Redbank Plains, Southeast Queensland, Australia, Geodiversitas 23 (1), pp. 41-79 : 58-60

Chelodina sp. b

DESCRIPTION

Material ( Figs 4 ; 5 ; Table 2) ( Rix 1991: fig. 2A) Redbank Plains form 2 is represented by the specimen QM F18214. It is an external impression of a fragmentary carapace, of the right lateral and posterior parts. It includes the impression of the lateral border from the peripheral 4, the impression of the right peripherals 5 (a part) to peripheral 11, of the pygal and of a part of the left peripheral 11, of the lateroposterior part of the pleurals 3-8 and a part of the suprapygal.

Measurements (in cm)

Oblique anterior preserved border: c. 13.8.

Width at the peripherals 5: 14.

Estimated medial length: c. 16.5.

Maximal suprapygal width: 3.6.

Maximal vertebral 5 width: 3.6.

Peripheral 10 anterior length on external width: 1.9/2 = 95%.

Decoration

The decoration is similar to that of Chelodina alanrixi n. sp. except that the size of the polygons is much reduced, in proportion with the much reduced carapace size. The polygons are c. 0.5- 1 mm wide and 0.1-4 mm long. The polygons are very well-marked and uniformly distributed on the shell part preserved as in all the Chelodina species , including C. alanrixi n. sp. as well as the rounded extant form C. steindachneri . They are very elongate on the lateral extremities of the pleurals and on the posterior extremity of the suprapygal, at the boundary with the peripherals and the pygal, more elongate there than in C. alanrixi n. sp. and in the extant species. The polygons are slightly smaller than in the C. steindachneri and C. novaeguineae (they are not known in the relatively short and medium sized recently described other extant species). As in Chelodina , the polygons of the peripherals are elongate in the direction of the lateral border of the carapace. The polygons of the suprapygal are equivalent in size to those of the suprapygal attributed to C. insculpta De Vis, 1897 (pl. V, H) but those are not drawn obliquely oriented and not posteriorly elongate. However, the other fragments attributed to this species have coarser polygons, as in the extant species of Chelodina . As shown above ( Table 1), these fragments, including the lectotype selected by Gaffney (1981), are not precise as to their origin and age: Plio- Pleistocene of Darling Downs and Warburton River and Eocene of Eight Mile Plains. They could therefore include a specimen equivalent in age to a Redbank Plains form. Thomson (pers. comm.) thinks that all the figured specimens are from Darling Downs.

Shell shape

Of the parts preserved, the width of the posterior preserved part and the shortness of the preserved pleurals, make it possible to reconstruct a round- ed oval carapace, like that of a rounded form of Chelodina such as the extant C. steindachneri Siebenrock, 1914 . Figure 5 shows the reconstruction of the posterior part of the carapace reconstructing the left side symmetrical to the preserved right side. The shell, with well-sutured plates, is that of a young adult with still uncovered rib extremities or of an adult of a species keeping free lateral rib extremities in the peripherals, visible dorsally, as in various extant chelid species and in particular as in many Chelodina . The shell is estimated to have been rather rounded, the width relative to the presumed length being estimated as c. 85%. This percentage is that of C. steindachneri Siebenrock, 1914 (up to 86.79%, see Table 3). The rounded posterior border of the pleural disc is comparable to that of C. steindachneri and C. novaeguineae , not as narrow as in most of the other extant species and not cordiform as in Pseudemydura . The posterior border of the carapace is short, rounded and not slightly medially pointed, as it is in extant C. expansa and in C. alanrixi n. sp. (and still more pointed in C. longicollis and C. mccordi ). As in C. alanrixi n. sp., C. expansa and some other species such as C. steindachneri and C. pritchardi , it is not raised at the pygal, convex from side to side, as in the extant C. longicollis and C. mccordi . It is not so as markedly convex, inwardly recurved and ventrally narrowed as in Pseudemydura . It is not elongated and expanded as in the Emydura group. Laterally, nothing indicates the presence of the beginning of an upwardly-curled border as in C. sp. c, C. longicollis and in some other Chelodina species and as in Pseudemydura .

Dermal bones

The length of the preserved pleurals is short compared to the lateral pleural borders in contact with the peripherals. This allows the reconstruction of a rounded form as C. steindachneri . The free rib extremities are clearly visible in the impression of the external dorsal view, more than in most extant adult forms. It might therefore be a sub-adult of a slightly larger adult form. The plates of the border are not very elongated. The pygal and peripherals 11 and 9 are as wide externally as long anteriorly, the peripheral 10 is wider externally than long and the peripherals 8 and anterior (preserved to peripheral 5) have their external border progressively elongated relative to the shortened anterior and posterior borders. The ratio length/width of the peripheral 10 is 95%, approximately as in C. alanrixi n. sp. As seen above, it is a low ratio in Chelodina , just higher than in C. oblonga and closer to that of C. steindachneri and C. expansa , as well as that of Pseudemydura , and far from the higher ratios of the Emydura group. Also, it indicates a moderate expansion of the posterior border, distinct from the tendency developed in the Emydura group.

The suprapygal is pentagonal, more or less point- ed anteriorly (missing in part).

Scutes

The vertebral 5 and costals 4 overlap the peripherals and pygal less than in C. alanrixi n. sp., C. expansa and C. longicollis , as in C. steindachneri and more than in C. oblonga . More anteriorly, the sulcus of the costal 3-marginals remains a little external to the peripheral-pleural suture and then is nearly united with it. We do not know how it was in C. alanrixi n. sp. In extant Chelodina , the osteology of which is known, either all the costals remain overlapping the peripherals laterally( C. expansa ) or the costal 2 alone is united with the suture ( C.longicollis ) or nearly united to the suture ( C. steindachneri , C. oblonga ). It therefore may have a specific value. In the Redbank Plains form 2, the morphological stage is closer to that of the forms with a rounded posterior border than to that of C. expansa , C. longicollis and other elongate forms.

Part of the medial carapace, anterior to the suprapygal, is missing. We know nothing about presence of neurals and width of vertebrals, except that the vertebral 5 is just as wide as the maximum width of the suprapygal. It is therefore narrower than in C. alanrixi n. sp. and C. steindachneri (not known in C. novaeguineae but probably wide as in C. steindachneri ). But it is still wider than in forms without a widened shell and with narrowed vertebral 5, including C. longicollis , C. expansa and C. oblonga .

DISCUSSION

The posterior part of carapace QM F18214 shows the decoration of Chelodina . The shape of the preserved part agrees with Chelodina and not with Emydura s.l. and Pseudemydura .

Within Chelodina , the specimen better agrees with the rounded form C. steindachneri but also with some extant forms without a pointed and medially raised posterior border (the primitive condition) such as C. reimanni and C. novaeguineae . It differs from C. alanrixi n. sp. in the relatively smaller size of the polygons, owing to the smaller size of the carapace. The size of the polygon is also smaller than in another fragment of turtle from Redbank Plains, studied below and attribut- ed also to Chelodina , undefined species, from a carapace slightly larger than the Redbank Plains form 2. The size of the polygons is also smaller than in the dorsal shell of extant species (smaller polygons exist in plastral part of extant species such as C. steindachneri ). It is similar to that of the suprapygal referred to C. insculpta De Vis, 1897 . But this has a clearly narrowed vertebral 5, narrower than the suprapygal, as in the extant C. longicollis , C. expansa and the elongated species. In return, in this character, the Redbank Plains form 2 is less derived, intermediate between them and the short C. steindachneri which has a vertebral 5 wider and even much wider (enlarged in some specimens) than the suprapygal and C. alanrixi n. sp.: the vertebral 5 is here slightly narrower and shortest on the external border plates.

It differs also from C. alanrixi n. sp. by the more rounded shape of the carapace with relatively shortest pleurals, the smaller size of the carapace and of the polygons.

It seems to differ also from the Redbank Plains form 3, undefined sp. of Chelodina . This is larger (maybe because an older individual) with relatively larger dorsal polygons. Furthermore, the latter has a recurved lateral border, as in many extant species, which does not seems to be present here.

In summary, it is probable that it belongs to the C. steindachneri lineage and to a new species, but the elements are insufficient to define it. Furthermore, as long as the osteology of all the extant species is not known, it is not possible to satisfactorily address this question.

Figures

FIG. 4. — Chelodina sp. b, form 2, Redbank Plains, Queensland, Eocene (QM F18214), right lateral and posterior part of the shell, impression of the external face. Scale bar: 2 cm.

FIG. 5. — Chelodina sp. b, form 2, Redbank Plains, Queensland, Eocene (QM F18214), reconstruction of the lateroposterior part of the shell. Abbreviations: C4, costal scute 4; py, pygal; spp, suprapygal; V4, V5, vertebrals scutes 4 and 5. Scale bar: 5 cm.