Emydura, Bonaparte, 1836

Lapparent de Broin, France & Molnar, Ralph, 2001, Eocene chelid turtles from Redbank Plains, Southeast Queensland, Australia, Geodiversitas 23 (1), pp. 41-79 : 65-70

Emydura s.l. sp. a

DESCRIPTION

Material ( Figs 7-9C, D ; Table 2)

The single specimen of the Redbank Plains form 5 (QM F31697) consists of a broken nodule an.n

showing two parts of an incomplete carapace: the external impression of the scutes and plates of the partial left lateral border and posterior border of the pleural disc and a part of the peripherals ( Figs 8 ; 9D ); the internal cast showing the impression of the ventral part of the plates of the pleural disc, at the anterior border in an oblique line, from left side at the pleural 2 to right side at the pleural 3 to 5 ( Figs 7 ; 8C ). The anterior part of the pleural disc is hidden by the impression of the external part of the partial left half plastron.

Measurements (in cm)

Preserved length and width of the internal cast (i.e. approximately of the pleural disc): 17.5 and 15.5.

Presumed approximate length and width of the carapace: 20 and 15.8.

Ratio of width to estimated length of posterior plastral lobe: 6.6/6-7, percentage ratio c. 94- 100%.

Decoration

The external impression shows only a very weak decoration of some shallow, thin and short sulci, some of them dichotomous. A small part of the ventral decoration of the right marginal scutes 9 and 10 is preserved on the cast of the fragmentary peripherals 9 and 10: some small dichotomous sulci, shorter and closer together than in the pleurals, are also visible. The decoration could be slightly different at the level of the missing vertebrals.

The impression of the plastron shows a stronger decoration of rather large polygons, not elevated (weakly marked), elongate, oriented from a center near the inguinal notch, longitudinally on the posterior lobe and radially forward on the anterior hypoplastral part. Below the preserved femoral scute, the polygons are 7- 5 mm long on 4- 1 mm wide.

Such an attenuated decoration of the carapace is often seen in the Emydura group, for example in Elseya latisternum and in young E. novaeguineae . In other forms, the carapace may be covered by irregular polygons, not as well-formed and as closed as in Chelodina and Pseudemydura , oriented in transverse or oblique (lateral part) and longitudinal lines (medial part). For example a specimen (MNHN AC 1887 814), attributed to Emydura macquarrii , has some longitudinal fine lines, in the medial part and some lateral pits and short weak ridges. But the holotype of E. macquarrii (MNHN H 9409) and a specimen of the Natural History Museum (BMNH 86-8-26-5) correctly attributed to this species, have strong, wide, flat and long ridges separated by grooves, oblique below the costals and radiating from a medioposterior center below the vertebrals. In most of forms, the carapace is ornamented with granulations, pits, sulci and ridges, more or less wide, rounded or sharp, longitudinally and transversally oriented, also depending on the part of the carapace. The decoration of ridges is strong in the extant specimens of the MNHN collections attributed to E. krefftii and E. australis , the ridges being less oblique and medially finer than in the holotype of E. macquarrii , at least in some specimens of E. krefftii (MNHN H 1883-380 and 381) (the synonymy of E. krefftii with E. macquarrii is admitted by Thomson pers. comm., but the holotype of the former is not conform to the holotype of the latter: the study of the individual and sexual variation of the species, from specimens collected in the area typica of each species could verify this assertion). But the ornamentation seems variable within the species, more or less pronounced, and varies with age. For example there may or may not be granulations and pits on the dorsal carapace of E. krefftii , but there are granulations in young Elseya latisternum (BMNH 71-9-25-8) and pits in adult (MNHN H 7973). The plastron is always more ornament- ed, often more granulate and with well-formed and well-distributed polygons as in Pseudemydura and Chelodina , although more oriented in lines on the posterior lobe. For example Elseya dentata (NHM specimens) has very well marked, granulate and small polygons on the plastron and Emydura macquarrii (holotype) has plastral granulate polygons, longitudinally oriented medially, below the pectorals, the abdominals and the femorals. As seen above, if polygons (the primitive design) are shared by all the chelid forms, only the Emydura group has longitudinal lines, sulci and ridges, sometimes much developed. And some fossil forms with ridges described by De Vis (1897) and considered as Testudines or Chelidae indeterminant by Gaffney (1981) are evidently forms of the group Emydura s.l. (Thomson in prep.). This is important because in the Cape Hillsborough Beds, near Proserpine (between Bowen and Cape Hillsborough), Queensland, an area considered as Palaeogene and probably Eocene, possibly contemporaneous of Redbank Plains, a partial carapace is preserved (Queensland Museum, in prep.). It has a typical Emydura decoration of fine longitudinal ridges on the pleural disc, laterally and medially, and weak polygons on the peripherals. It is much more decorated than the Redbank form 4 which is devoid of longitudinal lines (at least in the preserved parts), different as a species and probably from a different group of species. It is similar to Miocene South Australian Emydura specimens figured in Gaffney 1979 and 1991 (such as the specimen of Emydura sp. from the Ngapakaldi Lake, Miocene), and described in Burke et al. 1983; but fine longitudinal medial and lateral lines are also present in adult Elseya novaeguineae (BMNH 13-6-9-20). Longitudinal lines are also present in the holotype of E. macquarrii , as said above, but they are coarser. A study of extant material would be necessary to precisely diagnose the decoration of the Emydura s.l. forms, as nearly all the specimens in the visited museums are not prepared. But we can hypothesize that, owing to the decoration, the two Eocene (or Palaeogene) fossil forms were representatives of two already separated Emydura s.l. species lineages. The fossil Oligo-Miocene form from Tasmania described by Warren 1969a, and attributed to Emydura sp. aff. E. macquarrii , has a weak decoration of polygons and, besides, some fine longitudinal medial lines under the vertebrals, a decoration differing from that of the holotype of the species. The decoration of its plastron is not given. This species may be possibly closer to the Redbank Plains form than to the Proserpine form.

Carapace: shell shape

The pleural disc was oval, slightly narrowed posteriorly but not cordiform, the posterior border being rounded and rather short, not pointed. Its anterior part is missing, hidden below the plastral part. The shape of the carapace was elongate, long- er than wide, but not much elongated relative to the primitive stage, probably as in the fossil form of Emydura from Tasmania ( Warren 1969a) and less elongate than the Proserpine form. We cannot say if it was dentate or not nor how much its peripheral border was expanded, although possibly more than the minimum given in the reconstructed drawing ( Fig. 8 ).

Scutes

The external impression shows the sulci of the costals 1, 2 (partly), 3 and 4, vertebrals 4 and 5 and a part of some marginals sulci. They are straight and well-marked. The vertebral 4 is just slightly narrower than the vertebral 5, which is much wider than the suprapygal. The vertebral 4 is also much wider than the costal 4: there is no narrowing of the vertebrals. The lateral borders of the vertebral 4 are united in a sharp angle, as in many extant Emydura s.l. (see Goode 1967), particularly in small and young forms, and also as in the fossil Emydura from Tasmania. But at the difference with the latter, the marginals 12 do not overlap the suprapygal medially, the vertebral 5 covering all the suprapygal. The costals 3 and 4 overlap the peripherals, as in Elseya novaeguineae , for example, and less than in Emydura macquarrii and in the Tasmanian form.

Dermal bones

The bones are firmly linked, without fontanelles between pleurals and peripherals, which indicates an adult form.

There are no neurals appearing in dorsal view ( Figs 8 ; 9D ), but the centra of the dorsal vertebrae corresponding to neurals 6, 7 and 8 are preserved and visible in the cast ( Figs 7 ; 9C ), between the pleurals 6, 7 and anterior half of 8 which do not contact on the mid-line. This also happens in some living specimens of Emydura s.l. and in many specimens of living Chelodina (see above). It is the last remnant of the primitive series of the dorsal vertebrae linked to the neurals and still intercalated between the pleurals ventrally. In this case, only the inferior layer of the neural remains, attached to the vertebra. But here we have only the internal cast of the carapace with the inferior part of the vertebrae (centrum). We do not see the upper part with the neural inferior layer which was surely present in the complete specimen. In the other living taxa as in the anterior part of the fossil carapace of Redbank Plains, the neurals have completely disappeared and the dorsal arch of the vertebrae is separated from the carapace, just united at the medial line, the pleurals of each pair meeting in the mid-line dorsally and ventrally. As seen above, rare and often incomplete neurals have been mentioned in the Emydura sp. from Tasmania ( Warren 1969a), in some specimens of extant Elseya latisternum Gray, 1867 and E. novaeguineae (Meyer, 1874) and extant specimens of Elseya sp. aff. E. latisternum from Manning River and Elseya sp. from South Alligator River ( Rhodin & Mittermeier 1977; Thomson & Georges 1996).

The suprapygal is pentagonal, much wider than long.

The internal cast shows the scars of the ilia below the carapace. Each scar is trapezoido-triangular with rounded angles, pear-shaped, as long as wide, similar to that of the specimen (MNHN AC 1887-814) attributed to Emydura macquarrii but not similar to the specimen (BMNH 86-8- 26-5) which is consistent with the holotype in external view (internal characters not known in the holotype): in the second specimen, the scar is triangular trilobate and shorter. The scar of the Emydura s.l. sp. a covers a wide very short part of the pleurals 7, a wide part of the pleurals 8 and a small angular part of the suprapygal (as in Elseya latisternum fide Thomson pers. comm., but not as in NHM specimens attributed to E. latisternum ). It is depressed, well-delimited by a denticulate border, with shallow rugosities inside. In the middle part of the scar, there is a rounded zone of stronger rugosities, on the central part of the rib 9, corresponding to a rugose excavated zone in the iliac surface, as in specimens attributed to E. australis (MNHN) and to E. aff. australis (WA) . Emydura macquarrii (BMNH 86-8-26-5 and MNHN AC 1887- 814 specimens, not known in holotype) has an apparent rib 9, as in Chelodina , thin and sharp. The scar zone is very weakly rugose in a specimen of Elseya novaeguineae (MNHN P 1880-467), variably rugose in Rheodytes leucops and Emydura krefftii . It is not prepared in the Natural History Museum specimens of E. dentata and E. novaeguineae . As we have seen above in the section on Chelodina alanrixi n. sp., the shape of the iliac scar is quite variable in chelids and it derived independently in all the lineages, with some homoplasies. It is triangular with rounded angles to oval in Chelodina , rounded-oval in Pseudemydura and most often triangular, inverted or not, in Emydura s.l., sometimes also with rounded angles. The longer triangular shape is primitive in Eupleurodira and the more rounded or oval shape is derived. In most of observed specimens of the Emydura group ( Emydura and Elseya ), the scar is limited to the pleural 8 and suprapygal and it does not contact the pleural 7 any more. White (1997) noted that “the pelvic scars on pleurals 8 and the suprapygals are more typical of Elseya ”. We observed a scar reduced to pleurals 8 and suprapygal in Emydura macquarrii (BMNH 86-8-26-5, specimen externally consistent with the holotype of the species), Rheodytes leucops (Queensland Museum and Australian Museum specimens) and Emydura krefftii (Queensland Museum specimens and MNHN H 1983-380 and 381). The scar is still on the pleurals 7 in a specimen of the Queensland Museum attributed to Elseya dentata (while not in most other specimens attributed to this species, including QM J47911 , attributed now to E. lavarockorum [Thomson pers. comm.]) as in the specimen attributed to Emydura macquarrii (MNHN AC 1887-814) and observed specimens attributed to E. australis (MNHN) and E. aff. australis (WA) . Actually, the iliac scar, in the Redbank Plains form 5, which clearly covers the suprapygal and hardly the pleural 7, is similar to many chelids and conforms to Emydura s.l. in the rugosities and shape.

Plastron

The part of left plastron superposed to the cast of the internal carapace ( Figs 7 ; 9C ) consists of the posterior medial part of the hyoplastron and of the medial part of the hypoplastron (the axillary and inguinal processes are incomplete laterally) and the xiphiplastron, less its posterior extremity at the anal notch.

The posterior lobe is narrow without widening forward of the anals at the femoro-anal sulcus and without narrowed pointed xiphiplastral points. The lateral border of the femoral scute is slightly incomplete, but the posterior lobe border was nearly straight, only slightly converging towards the anal notch ( Fig. 7 ), less than in the species studied above, form 4, Chelodina sp. c? The anal notch, from the marks on the broken nodule, had to be relatively wide and short as in the above form 4.

The posterior lobe is elongate, relatively long for its width (and may be still longer than reconstructed in the Fig. 7 ), with an estimated ratio width/length of 94-100%. The elongated posterior lobe is a derived character of the Emydura group among the Australian chelids, immediately observable in figures (see Goode 1967), and different from Chelodina and Pseudemydura with short posterior lobes. The posterior lobe ratio, width at abdomino-femoral sulcus to full length, measured in some specimens, is: 84.69% in Rheodytes leucops ; c. 95% in Elseya latisternum , 97% in E. novaeguineae and 102% in E. dentata ; 97%, 97.17% (holotype) and 103% in E. macquarrii and c. 98% in a specimen attributed to E. australis . The ratio is 141% in Pseudemydura . In Chelodina , the ratios vary from 103% (103.44%, 103.17% and 111.76%) in C. oblonga to 138% in C. novaeguineae . The plastron had no reduced bridge length. The abdomino-femoral sulcus finishes in the bottom of the inguinal notch instead of further backwards on the lobe border, as it occurs in Chelodina (see above, Chelodina sp. c?, Fig. 6 ). The posterior lobe is not much rounded and widened relative to the base as it occurs in Pseudemydura (see Burbidge et al. 1974).

DISCUSSION

The Redbank Plains form 5 is clearly a member of the Emydura group. Owing to the very weak decoration (although eventually not completely preserved in the dorsal part of the shell) we can differentiate it from the species from Proserpine (Cape Hillsborough Beds) which has a stronger decoration of longitudinal lines, although fine and not the strongest possible in the group (see the figures in De Vis 1897; Gaffney 1979, 1981; Goode 1967; Burke et al. 1983). The Redbank Plains form is surely different from the extant species E. macquarrii s.s., in being more rounded (shortest behind), less decorated and without a mark of rib 9 in the iliac scar. Because of its rounded shape, its “faded” decoration and its vertebrals wide and with angulate lateral angles, it is similar to the specimen from Tasmania ( Warren 1969a) (a latisternum group member fide Thomson pers. comm.) It specifically differs from the latter by the marginals 12 not overlapping the suprapygal. But, although possible, it is not sure that they both belong to the E. macquarrii group, to which the Tasmanian form is referred. The Redbank Plains Emydura s.l. sp. a is also surely different from the Redbank Plains form 4, described above, doubtfully attributed to Chelodina sp. c?, although possibly belonging to the Emydura sp. group, in the strong difference in the plastral ornamentation of the latter, with smaller granulate polygons like those of Elseya dentata , for example, instead of wider medial polygons like those of Emydura macquarrii , for example.

Figures

FIG. 7. — Emydura s.l. sp. a, form 4, Redbank Plains, Queensland, Eocene (QM F31697), internal cast of the carapace, anteriorly covered by the external impression of the left lateroposterior part of the plastron. Abbreviations: an.n, reconstruction of the anal notch; hyo, partial hyoplastron; hypo, hypoplastron; il, iliac scar; n, ventrally preserved neural; per. 9, partial peripheral 9; pl7, pleural 7; spp, partial suprapygal; Xi, partial xiphiplastron. Scale bar: 5 cm.

FIG. 8. — Emydura s.l. sp. a, form 5, Redbank Plains, Queensland, Eocene (QM F31697), impression of the external left lateral and posterior part of the carapace. Abbreviations: C4, costal scute 4; pl1, pl8, pleurals 1, 8; spp, suprapygal; V4, V5, vertebrals 4 and 5. Scale bar: 5 cm.

FIG. 9. — Redbank Plains, Queensland, Eocene; A-B, Emydura s.l. sp. a or b?, form 6 (QM F37913), internal mold of an anterior part of the shell; A, dorsal part; B, ventral part; C-D, Emydura s.l. sp. a, form 5 (QM F31697); C, internal mold of the left lateral and posterior part of the shell, anteriorly covered by the external impression of the left lateroposterior part of the plastron; D, impression of the external left lateral and posterior part of the shell. Abbreviations:ax.pr, axillary process; ip, inguinal process; n1, remnant of neural 1; nu, nuchal; pl5, pleural 5; r1, r2, thoracic ribs 1 and 2; ento, posterior part of the entoplastron; hyo, hyoplastron; hypo, hypoplastron. Scale bars: 2 cm.

FIG. 6. — A, plastron of Elseya dentata (BMNH 76-5-19-77); B, plastron of Chelodina oblonga (BMNH 64-12-22-6), ventral views; C-E, Chelodina sp. c?, Redbank Plains, Queensland, Eocene (QM F18212), positive of the posterior mid-part of plastron, partial right pelvis and small part of shell; C, enlargement of the shell part, showing the polygons similar to those of Chelodina; D, the specimen, ventral view; E, enlargement of a plastral part showing the small granulate polygons; F, posterior mid-part of the plastron, superposed to that of Chelodina oblonga, with reconstruction of the inguinal notch. Abbreviations: a, left, as in Elseya; b, right, as in Chelodina. Scale bars: A, B, D, 2 cm; C, E, 1 cm.