Chelodina alanrixi, Lapparent de Broin & Molnar, 2001

Lapparent de Broin, France & Molnar, Ralph, 2001, Eocene chelid turtles from Redbank Plains, Southeast Queensland, Australia, Geodiversitas 23 (1), pp. 41-79 : 46-58

Chelodina alanrixi n. sp.

HOLOTYPE. — QM F18344 , a nodule with a part of the external and internal impressions of the carapace (Figs 1-3; Table 2).

ETYMOLOGY. — In honour of Prof. Alan Rix, of the University of Queensland, in recognition of his contributions to both invertebrate and vertebrate palaeontology of the Early Cenozoic in Australia.

LOCALITY AND STRATIGRAPHIC HORIZON. — The specimen was collected on the slopes East of Jones Road, Redbank Plains, near Brisbane, Queensland, Australia. Redbank Plains Formation, Early Cenozoic, at least Eocene if not Palaeocene time.

DIAGNOSIS. — A new large species of Chelodina without particular widening or narrowing of the carapace and narrowing of the vertebral 5, but with very narrowed vertebrals 2 to 4. Similar to the extant C. expansa Gray, 1857 , by its large size and shell proportions, the high degree of pygal-peripheral overlap by the costals and vertebral 5, the nearly flattened posterior border slightly pointed and not medially raised, the iliac scar shape and the presence of a strong iliac scar crest. More primitive than C. expansa by the smaller polygons, the wider vertebral 5, still completely wider than the suprapygal, and by the complete series of neurals, although not contacting the suprapygal; more derived than C. expansa by the relatively more narrowed vertebrals 2 to 4 as in the extant elongated C. oblonga Gray, 1841 and in C. longicollis (Shaw, 1802) . Different from the Redbank Plains form 2 of Chelodina which is a small rounded form. Different from C. insculpta De Vis, 1897 , in the greater size of the shell and the relatively smaller size and morphology of the polygons and the greater width of vertebral 5. Presumably distinct from form 3 of Chelodina from Redbank Plains, in the greater size of the shell, the relatively smaller polygons and the presumed absence of an upwardly-curled border.

DESCRIPTION

Material (Figs 1-3; 10; Tables 2; 3)

Form 1 of Chelodina from Redbank Plains, C. alanrixi n. sp., is represented by QM F18344 . It consists of a single nodule, broken in several parts by weathering. The principal elements are the external impression of the dorsal face of the right lateral and posterior part of a shell (Fig. 1B) in reversed relief, the impression of the ventral face of the posterior part of the shell, superposed on the external impression of some shell plates, in reversed relief (Fig. 1A) and a fragment with the ventral aspect of the medial part of the shell.

Measurements (in cm)

Preserved longitudinal length of shell: 37.5. Estimated total longitudinal length of shell: c. 40- 41.

Preserved posterior width of shell: 26.

Estimated posterior width of shell: 30.

Vertebral 4, maximal width: 4.9.

Vertebral 5, maximal width: 10.2.

Suprapygal, maximal width: 8.3.

Peripheral 10, anterior length/external width: c. 4.8/5.1 = c. 94%.

Decoration

This consists of very well-marked, small, salient, elongate or rounded polygons. These appear here, owing to the reversal of relief, as elongate and rounded pits separated by sharp crests. The polygons, narrow, vary from c. 9- 2 mm long over c. 2.5- 1 mm wide. Medially, in the area of the vertebrals, they are longitudinally elongate; on the lateral part of the shell, in the area of the costals, they are transversally elongate; on the last pleurals they are obliquely oriented and their elongation follows the external margin on the peripherals. The polygons are uniformly distributed on the carapace. As in Chelodina and in Pseudemydura , they are more marked and more evenly distributed than in those Emydura s.l. with a partly polygonal decoration. In those forms, the polygons of the carapace are weakly developed and poorly formed, a derived condition relative to the primitive decoration (see below); there are dichotomous sulci, smooth areas, granulations, pits, sharp crests or rounded ridges characteristic of the Emydura s.l. lineage, whether or not polygons are present. Examples of the decoration of the Emydura group can be seen in Gaffney (1981: figs 10-12) and in the figures of De Vis (1897: pls I-IV), showing the polygons weakly developed, crests and ridges; figures of finely ridged specimens of Emydura s.l. from the Miocene of South Australia are given by Gaffney (1979, 1991). Another part of the fossil material of De Vis (1897: pl. V) attributed to Chelodina insculpta De Vis, 1897 shows a typical decoration of extant species of Chelodina . But some of the carapace fragments referred to the species (pl. V, H, a suprapygal) show polygons smaller than usual in extant Chelodina and than in the other referred fragments, including the lectotype designed by Gaffney (1981) and more similar in size to those of the Redbank Plains material of Chelodina . The small, marked, well-distributed dorsal polygons of Pseudemydura are all rounded (not elongate) in form and, as a whole on the shell, smaller than in the other living species.

The plastral polygons are less different in Chelodina and Pseudemydura than in the Emydura group, when present in the latter. The polygons are, in general, more weakly developed and more elongate in Emydura s.l. Comparison may be made using Gaffney (1981), specimens from the Miocene of South Australia, fig. 15, with the more elongate polygons, and fig. 18B-C, with the more elongate polygons and fine ridges, compared to fig. 18A (Pleistocene of South Australia), all being given as Emydura . The latter is in fact a Chelodina specimen, as shown by the rounded posterior form of the ischiadic scar on the xiphiplastron (derived character state found in extant Chelodina ), as opposed to the posteriorlypointed form in Emydura s.l. and Pseudemydura (plesiomorphic character state). The decoration of the plastron of C. insculpta De Vis, 1897 (pl. VI), with rounded polygons, is also typical of Chelodina , with polygons intermediate in size between old specimens of C. expansa and C. longicollis . The lectotype (designed by Gaffney 1981) of C. insculpta De Vis, 1897 , 5 cm long, shell around 27 cm long, bears around 30 polygons along the medial line, the entoplastron of C. longicollis (MNHN AC 1911-181), 3.7 cm long, shell 20 cm long, bears around 35 narrower polygons while the entoplastron of the specimen of C. expansa (MNHN AC 1914-178), 6.2 cm long, shell 37.5 cm long, bears only 22 larger polygons along the medial line. Granulate and well-marked plastral polygons are also found in a specimen from Redbank Plains, possibly belonging to Chelodina sp. c (see below), but the polygons are more clearly longitudinally oriented and smaller than in C. insculpta .

Confusion of isolated parts (those with rounded polygons) of the shells of Chelodina for those of Pseudemydura is possible. In the present case, the other associated parts of the shell indicate that the Redbank Plains specimens do not pertain to Pseudemydura . A polygonal or granulate decoration is primitive for turtles, found in Triassic specimens from Germany ( Proganochelys , Proterochersis ). It is retained in Notoemys (weakly developed polygons), a pleurodiran from the Late Jurassic of Argentina. The decoration consisting of dorsal and ventral polygons that radiate from a center is probably primitive for chelids, as polygons so oriented are found in nearly all chelids, from the Late Cretaceous (of Argentina) to the present. The decoration of polygons is however very little developed in the first known chelids (Aptian-Albian of Argentina) ( Lapparent de Broin, Fuente & Calvo 1997). They, however, represent only a part of the chelid groups and the polygonal decoration is more widely distributed in the various species of the Late Cretaceous that belong to all the chelid groups. The decoration of polygons is particularly developed in the groups including Hydromedusa , Chelodina and Pseudemydura , it is variably developed in the Phrynops and Emydura groups and is nearly completely absent in some fossil forms: the primitive arrangement of the polygons evolved independently in the lineages, either weakening or developing and, as shown above, it is possible to distinguish the three Australian groups. The shell on the whole has narrower and more marked polygons in C. alanrixi n. sp than in C. expansa , the living species of Chelodina of equivalent size. They are distinct from the fragments attributed to Chelodina insculpta De Vis, 1897 . In this species, as previously noted, De Vis (1897) included material from the Eocene of Eight Mile Plains, Queensland, as well as from the Plio-Pleistocene of Darling Downs, Queensland, and of the Warburton River area, South Australia. Some of the material therefore could be contemporaneous with C. alanrixi n. sp. and could be conspecific with it. However, the figured fragments of C. insculpta are relatively smaller, from a carapace about 27 cm long for the entoplastron (lectotype) of 5 cm. The shell of the reconstructed plastron in De Vis (1897) has 30 cm (Thomson pers. comm.). It is a little longer than, for example C. longicollis , with shells 18-22 cm long, within the limits of C. expansa , with shells up to 42.3 cm long ( Goode 1967) and C. alanrixi n. sp., shell around 40-41 cm long. The decoration of the posterior elements of the carapace of C. insculpta which are comparable to those of C. alanrixi n. sp. has relatively more rounded and wider polygons (pleurals, peripherals) and more rounded and smaller polygons (suprapygal), more similar to those of the living C. longicollis than to C. expansa (see Fig. 10D ) and C. alanrixi n. sp. (Fig. 1B).

Carapace shape

The shell is that of a fully adult animal, wellgrown with well-sutured plates. It was probably moderately elongate ( Fig. 10A ), probably showing the primitive condition for shell elongation in Chelodina , the ratio of width to (presumed) length estimated as 73-75%. This percentage ( Table 3) is that of moderately elongate living Chelodina , such as the largest, C. expansa and the smaller C. longicollis ; it also enters in the range of the still shorter species, C. reimanni Philippen & Grossmann, 1990 , C. pritchardi Rhodin, 1994 and C. mccordi Rhodin, 1994 (the distinguishing osteological shell characters of these species remain unknown). The percentage is in the lower range for the small C. novaeguineae Boulenger, 1888 , and of the rounded C. steindachneri Siebenrock, 1914 . It is in the higher range for the medium-size, more elongated or narrowed species, the C. rugosa Ogilby, 1890 – C. siebenrocki Werner, 1901 group and C. oblonga (see shells figured in Burbidge et al. 1974; Goode 1967; Philippen & Grossmann 1990; Rhodin 1994a, b; Rhodin & Mittermeier 1976). Individual variations, including sexual and ontogenetic variation, and overlap between species are all possible. The estimated ratio of C. alanrixi n. sp. indicates that it was not a short rounded form nor a very elongate or narrow form. It, therefore, has a relatively undifferentiated shell form within the genus in this respect. The posterior border is short, round- ed and slightly medially pointed as C. expansa (borders of both not as short as in C. oblonga ) (pointed as in males of C. expansa , more than in females), not convex and raised at the pygal as in C. longicollis and not particularly convex, curved and narrowed ventrally as in Pseudemydura . It is also not elongated and posterolaterally expanded as in the Emydura group.

Dermal bones

From the right side of the shell – that is on the left side in Figs 1B and 2, a partial impression of the eight pleurals is preserved. Only the posterior border of pleural 1 may be seen, but pleurals 2-4 are nearly complete, 5-6 lack the medial border and 7-8 are complete. On the left, pleurals 6 (incomplete) and 7-8 are preserved. These pleurals were connected to the medial neurals. The neurals 1 (incomplete), 2-4, 7 (also incomplete) and 8 are preserved. It is evident that there was a complete series of eight neurals, the first quadrangular, the following hexagonal-short-sided in front, and the last pentagonal: the series does not contact the suprapygal, instead the pleurals 8 meet behind the small pentagonal neural 8.

The presence of a complete neural series is primitive and, furthermore, also plesiomorphic for each South American chelid genus, as demonstrated by the early fossil record (from the Albian and Senonian-Maastrichtian of Argentina, Broin & Fuente 1993; Lapparent de Broin et al. 1997; Fuente et al. 2001). In the primitive chelonian series, known since Late Jurassic, neural 1 or neurals 1 and 2 are hexagonal short sided behind and the first quadrangular neural is 2 or 3. Neural 8 contacts the suprapygal. In Patagonian chelids, reduction of the neural series is already shown by the Albian in a form similar to the small species of the group Phrynops Wagler, 1830 – Acanthochelys Gray, 1873 ( Lapparent de Broin et al. 1997). In Late Cretaceous times, Senonian- Maastrichtian, it appears in forms without living relatives and in forms related to Phrynops and Hydromedusa Wagler, 1830 , at least ( Broin & Fuente 1993). In South American chelids, complete reduction appears only in the Phrynops complex: in Platemys Wagler, 1830 , Acanthochelys and in some species of Phrynops s.l. ( Bour & Pauler 1987; Broin & Fuente 1993; Fuente 1988, 1992; Fuente & Ledesma 1985; Kischlat 1991; Perea et al. 1996; Pritchard & Trebbau 1984; Sanchez-Villagra et al. 1995a, b; Wood 1976; Wood & Moody 1976; specimens of visit- ed Museums). In Australian chelids ( Boulenger 1889; Burbidge et al. 1974; Cann & Legler 1994, Legler & Cann 1980; Rhodin & Mittermeier 1976, 1977; Thomson & Georges 1996; specimens of visited Museums), the tendency for neural reduction is common to all the genera, but each genus has done so at a different rate. In Australia, reduction of the neural series is more marked (complete) in Pseudemydura . It is complete or nearly complete in the members of the Emydura complex, as early as Eocene. Neurals are lost in Emydura s.l. sp. from the Condor oil shale near Proserpine (QM F13687). Neurals are rare, and often incomplete, in specimens given as Emydura sp. from Tasmania ( Warren 1969a) and in some specimens of the extant Elseya latisternum Gray, 1867 and E. novaeguineae (Meyer, 1874) (see Rhodin & Mittermeier 1977), Elseya sp. aff. latisternum from Manning River and Elseya sp. from South Alligator River ( Thomson & Georges 1996; a new determination is given in Thomson in prep., pers. comm.) and only the ventral part of the posterior neurals remains in Emydura s.l. sp. a (Redbank Plains form 5). In the specimens of Chelodina , examined, the reduction is either always complete and not submitted to individual variations ( C. expansa , see Fig. 10B, C . rugosa, C. steindachneri ), much pronounced ( C. siebenrocki , C. novaeguineae ) or slightly pronounced ( C. oblonga ). In the former, the ventral layer of the neurals is preserved below the pleurals which meet over them as in Emydura s.l. sp. a (Redbank Plains form 5) and this may be present in many South Australian chelids ( Thomson & Georges 1996). This shows that the reduction occurred independently in each lineage of Emydura s.l. as well as of Chelodina and that C. alanrixi n. sp. has a primitive character state with a complete series of neurals, but already with the first neural quadrangular and neural 8 not contacting the suprapygal. This morphology indicates that it may be at the base of one of the lineages leading to extant Chelodina species.

The posterior border is represented by the peripherals 8-11, on the right side and the pygal and the peripherals 9-11 on the left. The pleurals are well-sutured with the peripherals and the dorsal rib extremities well-fused in the dermal plates, without any salient ventral part and any free rib extremity, appropriate for a large adult without permanent fontanelles with their intercalated cartilaginous tissue. The peripherals are relatively wide, for example for peripheral 10 the ratio of anterior length to external width is 94%. Among extant forms, this ratio is comparable to that of Chelodina (84% in C. oblonga , 106% in C. steindachneri , 107% in C. expansa , 116% in C. longicollis ) and Pseudemydura (93% and 121%, from two specimens). The value is higher in the Emydura group although a slight overlap between the two groups exists: 105% in Elseya latisternum ; 115% and 141.5% in E. novaeguineae ; 137% in E. dentata (Gray, 1863) ; 120%, 129% (holotype, MNHN H 9409) and 137% in Emydura macquarrii (Gray, 1831) ; 140% in the specimen QM J31703 attributed to Rheodytes leucops Legler & Cann, 1980 ; 171.5% in a specimen from the MNHN (H) coll. attributed to E. australis (Gray, 1841) , a specimen apparently conform to the holotype, BMNH 47-3-4-36; see Goode 1967 (Thomson pers. comm. suggests that the specimens identified here as E. australis are of E. victoriae [Gray, 1842] that he separates from the former with which it was synonymized). The posterior peripherals have the most derived condition, for Australian chelids in the Emydura group – narrower and more elongated – although they are as wide as in some Chelodina in the less derived specimens such as in Elseya latisternum and in some specimens of E. novaeguineae . However, in all the Emydura group species, they are more elongate than in C. alanrixi n. sp. The measurements of the peripherals of C. alanrixi n. sp. are fully concordant only with those of Chelodina and Pseudemydura specimens. Actually, the primitive turtle form of the border, and the original form in the pleurodires in particular, is not expanded anteriorly or posteriorly. These include Proterochersis Fraas, 1913 , from the Triassic of Germany (specimens in the Staatliches Museum für Naturkunde- Stuttgart), Notoemys Cattoi & Freiberg, 1961 , from the Late Jurassic of Argentina (specimens from Museo de Ciencias Naturales de La Plata, Museo de Ciencias Naturales de Buenos Aires; Fuente & Fernandez 1989; Fernandez & Fuente 1994), and Platychelys Wagner, 1853 , from the Late Jurassic of Germany and Switzerland (specimens in Naturmuseum Solothurn; Bräm 1965). The shell border is slightly elongated anteriorly and posteriorly in Pseudemydura , much more elongated anteriorly and slightly posteriorly in Chelodina and slightly elongated anteriorly and much more elongated and expanded posteriorly in Emydura s.l. Parallel evolution produced similar forms in each of the South American chelid groups as well as in the northern Gondwanian Pelomedusoides but, linked to other characters, these still represent important diagnostic features for each genus or group of species.

The suprapygal and pygal are similar to those of C. expansa : the suprapygal is pentagonal but nearly rhomboid and the pygal is more narrowed anteriorly. The suprapygal shape is also more anteriorly triangular in C. alanrixi n. sp. than in C. insculpta De Vis, 1897 . That suprapygal of that species, rather similarly to that of C. longicollis , bears the borders of the vertebral 5, this scute being narrowed as in the extant relatively elongated forms, including C. longicollis (see below) and different from that of C. alanrixi n. sp. The figures 1A and 3 show the scars for the sutural attachment of both ilia below the posterior carapace. This is a typically pleurodiran character in the suture of the pelvis to the shell: in the Pleurodira , relative to the primitive state in turtles, the pelvis is reduced in width at least to the size of the posterior lobe (or even narrower). This has become bifid to receive the points of the ischia ( Lapparent de Broin & Fuente 1996). The dorsal surface of the iliac blade is widened and shortened and sutures to the carapace. The pubis and ischia become vertical and their ventral extremities suture with the plastron. The shape of the iliac scar of C. alanrixi n. sp. is a trapezoid with rounded angles, the anterior side directed medially, as long as wide and occupying most of the surface of pleural 8 and overlapping the suprapygal by a wide, rounded triangular area. The shape of the iliac scar is variable in chelids. It is primitively triangular in chelids and it remains so or rather triangular in Emydura s.l., becomes more elongate and rounded-oval in Pseudemydura and it is most often oval or triangular (but much round- ed) in Chelodina . There is parallel evolution in each South American chelid group. The length of the scar below pleurals 7 and 8 and the suprapygal varies much. If the scar is wider or both short- er and wider, it is more derived: the primitive pleurodiran scar being narrow and prolonged from pleural 7 to the peripherals. In chelids the scar has a tendency to be reduced anteriorly so that it does not contact pleural 7 any more. The ilium also tends to maintain – but eventually to expand in width – its contact with the suprapygal (the pelvis becoming more posteriorly sutured to the shell). This condition is different from that of the Pelomedusoides where the tendency is to reduce the contact with the suprapygal (the contact most often disappears, with the pelvis more anteriorly sutured to the shell) ( Lapparent de Broin & Fuente 1996; Lapparent de Broin & Murelaga 1999). The anterior contact of the scar under pleurals 7-8 is not preserved in C. alanrixi n. sp., but, judging from the part preserved, the scar ought to be little developed in pleural 7 or limited to the suture between the two pleurals, as in C. expansa .

The scar is a depressed region on the pleurals, well-delimited and nearly smooth. But dorsal rib 9, which is enveloped in pleural 8, within the scar, develops in a ventral, strong and salient crest in C. alanrixi n. sp., as in every other observed Chelodina , strong as in C. expansa and particularly wide. The crest is also developed in Emydura s.l. (for example in E. macquarrii ), although less developed than in Chelodina expansa and doubled in some specimens, Pseudemydura (shallow rib) and South American chelids, such as many species of Phrynops and Hydromedusa (narrow or thin, sharp or rounded in the scar). The character is subdued (shallow with a short or thin crest) or absent in many other species of Emydura s.l. and of Phrynops . In the Emydura group, instead of a crest of rib 9, there are often rugosities, for example in Elseya novaeguineae and E. latisternum , or a triangular zone of sutural tubercles, as for example in specimens attributed, in collections, to Emydura australis . A zone of tubercles around the crested rib is present in the South American Chelus Duméril, 1806 . In the present case, the scar and rib conform those of C. expansa .

Scutes

The costals and vertebral 5 much overlap the peripherals and pygal as in Chelodina , dorsally (Figs 1B; 2; 10) particularly as in C. expansa ( Fig. 10B ); ventrally ( Fig. 3 ), the marginals overlap the peripherals more than in that species. Vertebrals 1 (at least in the preserved posterior part), 2, 3 (certainly, although lacking) and 4 are strongly narrowed. Although the vertebrals are incomplete, the narrowing is shown by the close proximity of the lateral vertebral border to the lateral neural border (a narrow pleural covered by the vertebrals). Vertebral 5 is narrow anteriorly and wide posteriorly. It is wider than the suprapygal, which it fully covers as well as covering a part of pleurals 8 and of peripherals 10, 11 and pygal. This particularly strong narrowing of the vertebrals from the posterior part of the first to the anterior part of the fifth is derived and particular in chelids to the relatively elongated extant species of Chelodina (shell moderately elongate to elongate), particularly C. oblonga and C. longicollis , although a regular narrowing of the series is common in all chelid forms, including all Chelodina species. But in the extant C. expansa , with less narrowed vertebrals ( Fig. 10B ), and at least C. siebenrocki , C. parkeri Rhodin & Mittermeier, 1976 , C. rugosa , C. oblonga and C. longicollis , with more narrowed vertebrals, vertebral 5 is so much narrowed anteriorly that it is narrower than the suprapygal (in most examined specimens), or as wide as the suprapygal only posterior to the suprapygal (at least some specimens of C. longicollis and C. oblonga ). In the fossil suprapygal which is part of the specimens attributed to C. insculpta De Vis, 1897 , vertebral 5 is also narrowed relatively to the plate. On the pleurals, which are also parts of those specimens, the preserved parts of the vertebrals are also narrowed. They are not as narrow as in C. alanrixi n. sp., C. longicollis and C. oblonga , but as in C. expansa for example ( Fig. 10B ). In the short rounded C. steindachneri , the vertebrals also look narrow compared to the costals, but the costals are widened, owing to the widening of the shell, thus giving a narrowed aspect to the vertebrals. In the other forms not showing narrowing, the vertebrals are, primitively, little differentiated on the whole, although they tend to be narrowed from the anterior one posteriorly; sometimes there are subdivisions of the second or of the fourth in a short and narrow supplementary scute as in the other group of elongate forms. In these forms that are not elongated as in C. alanrixi n. sp. and unlike all the other fossil and extant species of Chelodina , the vertebral 5 is wider than the pygal, which is the primitive condition. We think therefore that, for the character of narrowed vertebrals, C. alanrixi n. sp. is also separated from the group with rounded shells such as C. steindachneri and probably also from Redbank Plains form 2. C. alanrixi n. sp. may be just at the base of a group including extant Chelodina forms with vertebral 5 narrower, at least anteriorly, than the suprapygal, the character of narrowed vertebrals 2 to 4 being shared in common. If this character, of vertebral 5 narrowed relative to the suprapygal, developed only once, then C. alanrixi n. sp. is the sister group of the latter and the group with rounded shells such as C. steindachneri their common sister group. In this case, the shortening of the sulcus between the pectoral was produced twice, once in C. longicollis and once in the C. steindachneri group (see Discussion below). But the character of vertebral 5 narrow relative to the suprapygal may have developed several times and we lack too much of C. alanrixi n. sp. (plastron, skull, neck) to detect the possible homoplasies and define exactly the phylogenetic relationships of the fossil form.

In chelids, it sometimes happens that instead of covering the peripherals and pygal, vertebral 5 is posteriorly reduced in length and marginals 12 developed, at least medially, over the suprapygal. This happens in Emydura sp. aff. E. macquarrii from the Oligo-Miocene of Tasmania ( Warren 1969a) as well as in some South American extant species of Phrynops and the Cretaceous chelids from the Area Loma de la Lata and Ocho Hermanos ( Broin & Fuente 1993; Lapparent de Broin & Fuente in prep). Sometimes also, marginals 12 meet vertebral 5 just at the pygal-suprapygal suture, which is more like the primitive condition in turtles. But the more frequent state in chelids is that of a moderate overlapping for the peripheral border by the costo-vertebral scutes. C. alanrixi n. sp. is therefore well-developed in this state, as is C. expansa . It differs, on this point, from C. oblonga which has a relatively short posterior border (apomorphic) with a short scute overlap, as compared to C. expansa . On the other hand, C. oblonga is much more elongate anteriorly (longer nuchal relative to shell length) than C. expansa and probably C. alanrixi n. sp.

We note that vertebral 5 is partly longitudinally divided anteriorly, in C. alanrixi n. sp.: this is an individual variation sometimes present in specimens of all extant turtles.

DISCUSSION

Chelodina alanrixi n. sp., clearly pleurodiran as shown by the pelvis sutured to the shell, is a Chelodina species because of the shape of the shell; its decoration; its posterior border, elongate but not too much; its iliac scar morphology and extension; its narrowed vertebrals 2 to 4, as in some extant species of Chelodina .

The genus Chelodina is well-defined, in extant species, principally on two excellent derived characters that are not preserved here:

1) on the anatomy of the skull, with a complete dorsal skull fenestration, beginning from the lateral emargination and prolonged backwards, cutting the supraoccipital-parietal contact with the squamosal; 2) on the intergular scute, withdrawn backwards from the epiplastral border, allowing the gular scutes to meet anteriorly to the intergular, or to nearly meet in the holotype of Chelodina intergularis Fry, 1915 (a deformed C. rugosa fide Thomson pers. comm.) ( Wermuth & Mertens 1961; Goode 1967). Posteriorly, the enlarged intergular completely separates the humeral scutes and a part of the pectorals. For this character, the more derived forms are the shorter forms (including the rounded C. steindachneri ) where the intergular is more backwards prolonged and the suture between the pectorals shorter, but C. longicollis , a relatively elongated form judging by the skull (see below) and without a widened carapace, also approximates them (short hyoplastral suture and interpectoral sulcus). For the intergular position, the less derived are the elongated-narrowed forms and among them the C. siebenrocki-rugosa group, including C. intergularis , and C. oblonga .

A short skull is the primitive condition and an elongation of the palate, correlated with a flattening of the skull, is produced in the elongatednarrowed forms. The elongation can be measured by the ratio of the lateral ventral length of the pterygoid, between its suture with the jugal and the maxillary and its suture with the quadrate, on the skull width, at the quadrate ventral border below the tympanic meatus. The ratio is the shortest in Pseudemydura . Then, the ratio elevates a little in the Emydura group. According to the figures of the authors, the forms of Chelodina with a medium-sized and short, more or less rounded carapace, including C. reimanni , C. pritchardi and C. mccordi , have the same proportions for the skull characters: all of them have the lowest percentage in Chelodina , which in part ( C. steindachneri ) covers that of some Emydura s.l. species. Their higher percentage ( C. novaeguineae ) is below that of C. longicollis and that of the other forms with a moderately elongated or clearly elongated shell: within this group, the skull elongation is visually noticeable from C. longicollis and it is maximum in C. expansa . A less transverse position of the trochlear pterygoid processes is partly correlated with the skull elongation. The neck is also longer in the longer skull forms than in those with short skull forms. There are overlaps between the species.

The living species of Chelodina are often classified in two groups, following a dichotomous system which is very useful, although fully phenetic: it is principally based on the length of the plastron relative to the width (plastral length generally about one time and a half maximum width, rarely exceeding one time and three-fourth the maximum plastral width, or usually greater than 1.9 time the maximum width of plastron ( Cogger 1992; Goode 1967). This system does not take into account: 1) the way the various parts of the plastron (anterior lobe or all the posterior lobe or xiphiplastral points only or all the plastron as a whole) are widened and enlarged, relative to the primitive chelid narrow short state with narrow xiphiplastral points; 2) the way the plastron is widened, relative to the way the shell is elongated or rounded (widened). Also are considered other characters such as the skull (broad or narrow), the contact between the pectorals (short in forms with broad plastron or long in forms with long plastron), some other characters (barbels, head and neck skin surface, odour, etc.) and some relative proportions which are variable and with overlap between the two groups. The primitive and the derived states and the intergradations are not taken into account. The plastral size and the evolution of the intergular posteriorly do not coincide with the other characters of the skull (elongation and flattening, position of the trochlear pterygoid processes, etc.), neck elongation, changes in plastral proportions, neural reduction, border proportions, vertebrals narrowing (see above), etc.

The characters of the two phenetic groups do not coincide with those used here to distinguish the fossil species. For example, in the phenetic dichotomous system, C. longicollis is placed by the authors with the short forms of the type of C. steindachneri and C. novaeguineae . However, it has a relatively elongate skull as C. siebenrocki , narrowed vertebrals as C. oblonga , vertebral 5 narrower than suprapygal as the large C. expansa and as the long forms, and it has not a widened or narrowed-elongated shell although it has a widened plastron. Cladograms of Chelodina species are given ( Rhodin 1994a, b) supporting the phenetic system, but they are not verifiable or refutable because they are given without the matrix and list of the characters. Other phylogenetic studies are based on electrophoretic data or on mitochondrial and nuclear gene sequence variation ( Georges & Adams 1992, 1996; Georges et al. 1999), not useful in our study of fossils (see also White 1997). Their results, not including fossils, do not present the evolution in all the forms from Triassic to present times. They are useful overall in separating paraphyletic taxa at the generic level, according with the osteological results: for example Elseya . As far as Chelodina is concerned, these analysis correspond together and with the phenetic system, grouping species mostly on synplesiomorphic charaters, but not with our observations: in one analysis, C. longicollis is placed with C. steindachneri and C. novaeguineae (two rounded short forms), with C. oblonga as sister group, and the whole with C. rugosa and C. expansa as outgroup ( Georges & Adams 1992). In another paper, C. oblonga and C. longicollis have C. rugosa as outgroup, C. steindachneri not being integrated ( Georges et al. 1999). Thomson (pers. comm.) supports an independent acquisition of skull lenghtening and a possible reversion to a short skull in various extant species and the separation in the two Chelodina groups of the authors, A and B, on osteological characters. It will be interesting to see if the characters shared by each group are synapomorphic or synplesiomorphic, if we can detect the homoplasies and conciliate the contradictions with the present statements.

In fact, the distribution of the derived characters preserved here appears as a mosaic type. In the absence of the skull, neck and plastron, we cannot establish a cladogram to show the phyletic position of C. alanrixi n. sp.

We just can take the obvious characters in C. alanrixi n. sp., which are principally those of the evolutionary states of neurals, of width of vertebrals 2-4 and of anterior width of vertebral 5. The combination of these characters shows that the species is new, compared to the fossil and living species, and that it is rather primitive.

The diagnosis shows it is similar (same derived state of characters) to C. expansa (see Fig. 10 ) by its large size, the posterior slightly pointed flattened border (overall in males), the strength of the iliac scar crest and the posterior plate proportions and scute covering, all characters in which it particularly differs from C. oblonga . Its pecularities are never found as individual variations in extant C. expansa . But it is more primitive than C. expansa , C. longicollis and the relatively elongated and elongated-narrowed living forms by the width of the vertebral 5, not anteriorly reduced, differing there as well as from the fossil suprapygal attributed to C. insculpta .

Relative to C. expansa , it differs also by the presence of a prolonged neural series. In this character, it is similar to C. oblonga , that retains one to seven neurals (although the first one is incomplete in front, when present), but this is a primitive character not allowing a privileged phyletic relation between them. On the other hand, the fossil form 1 shares with C. oblonga a very derived character, the higher degree of narrowing of vertebrals 2 to 4. However, it lacks the shortened posterior border of C. oblonga and the vertebrals are also often much narrowed in C. longicollis , which lacks an elongated-narrowed shell: narrowing of the vertebrals, present in a higher degree in C. alanrixi n. sp., is not linked to the carapace elongation and modification of the posterior border.

The dorsal elements are sufficient to separate this species from C. insculpta De Vis, 1897 . In the absence of a preserved plastron for the Redbank Plains form 1, we cannot analyse its differences from the plastron of the living and fossil species. In the former (and composite) species, the anterior and posterior preserved plastral parts are consistent with the belonging to only one species of a single age, owing to the decoration and the proportions of the elements. The preserved part of the anterior plastron is more similar to that of C. longicollis in the relative distances between sutures and scutes than to the elongated forms; in particular, the suture between the two hyoplastra and the sulcus between the pectoral scutes are relatively short, compared to the elongated forms with a long suture, being closer to the rounded forms with a short suture, but the plastral anterior lobe is much narrower than in C. longicollis . The posterior part is also much less widened, anteriorly and posteriorly, so closer to the primitive chelid state: it is still narrow at the hypoplastra and anterior part of xiphiplastra and less widened at the posterior part of the xiphiplastra below the anal scutes. It is different from the extant species although most of them have still narrowed anals.

In summary, the Redbank Plains form 1, C. alanrixi n. sp., is therefore a new species primitive relative to extant and fossil Chelodina species , although much derived in having the vertebrals narrowing as in the moderately elongated and very elongated forms. It is different from the rounded species, including the following

V

Redbank Plains form 2, and from the extant species without vertebrals narrowing. It differs also from the Redbank Plains form 3 by its elongate and smaller (relatively) polygons, its larger size and the presumed absence of an upwardly-curled border.

Figures

FIG. 10. — A, Chelodina alanrixi n. sp., form 1, Redbank Plains, Queensland, Eocene (QM F18344), 40-41 cm long, reconstruction of the carapace, dorsal view, missing parts doted, presenting generic identities and specific differences with C. expansa in B-D; a, flattened, large and wide carapace as in C. expansa; b, narrow vertebrals, the first one narrower than the nuchal bone, the second to fourth narrower than the first and fifth, the third particularly narrow, vertebrals narrower than in C. expansa except the fifth that is wider than the suprapygal bone; c, polygons of the decoration c. 1/3 smaller than in C. expansa (see D and compare with Fig. 1B); d, 8 neurals present, against no one in C. expansa; e, vertebral 5 wider than the suprapygal contrarily to C. expansa; f, long and flat posterior extremity, as in C. expansa, slightly pointed as in males in C. expansa; B, C. expansa, extant (MNHN AC 1914-178), 37.5 cm long; a, large and wide carapace as in C. alanrixi n. sp. (anteriorly narrower in males); b, wide vertebral 1 wider than the nuchal bone, the vertebrals 2 to 4 (the fourth subdivided) progressively narrowing; c, large polygons (see D); d, no neurals (never); e, vertebral 5 narrower than the suprapygal; f, long and flat posterior extremity, slightly pointed as in C. alanrixi n. sp. but more rounded border (a female); C, C. expansa Gray, 1856 (pl. XII), holotype, from Goode (1967) and Wermuth & Mertens (1961), 35 cm long, a male (anteriorly narrower and posteriorly pointed, plastron with a concavity); D, C. expansa, extant (MNHN AC 1914-178), 37.5 cm long, detail of the decoration of the carapace in the area of the vertebral 4 (subdivided here), compare with Fig. 1B. Scale bars: A, B, 10 cm; D, 2 cm.

FIG. 3. — Chelodina alanrixi n. sp., form 1, Redbank Plains, Queensland, Eocene (QM F18344), posterior part of the shell, impression of the internal face of the Fig. 1A. Abbreviations: il, iliac scar below pleural 8 and suprapygal; M12, marginal scute 12; pl8, pleural 8; py, pygal; spp, suprapygal. Scale bar: 5 cm.