Throughout this paper, we refer to a generic group as a group of species that are sufficiently distinct collectively to warrant recognition at the level of genus, though this has not yet been formally established. These groups were first identified by Legler (1981), have a foundation in electrophoretic studies (Georges ∓ Adams, 1996), and have been referred to since several times in the literature. In contrast, a species complex is a group of species, all but one of which are undescribed, which together presumably represent a distinct clade but which are not considered distinctive enough to warrant recognition at the level of genus.
    We refer to the Elseya dentata species complex as comprising the distinctive forms of Elseya from coastal Queensland currently assigned to Elseya dentata, and the Northern Territory forms including Elseya dentata (sensu stricto) and Elseya sp. aff. E. dentata from the Alligator Rivers region (Georges & Adams, 1996). The Elseya dentata generic group (sensu Legler, 1981) comprises the Elseya dentata species complex plus Elseya novaeguineae and Elseya branderhorsti from New Guinea. The Elseya latisternum generic group comprises Elseya latisternum (sensu stricto), a related form from the headwaters of the Darling River drainage and a sibling species pair from coastal New South Wales (Georges & Adams, 1996; Thomson & Georges, 1996). The later three are currently undescribed. It is not the purpose of this paper to describe new genera, so for consistency, we use the nomenclature of Georges & Adams (1992) and Legler (1981) and recognise six groups of Australian short-necked chelid at generic level: Elusor, Emydura, Rheodytes, Pseudemydura, the Elseya latisternum generic group and the Elseya dentata generic group.
    Throughout this paper, names of the bony elements of the shell and the overlying scutes follow those of Zangerl (1969). A complete list of the specimens examined in this study will be found in Appendix A.

     Five characters were identified as diagnostic at generic level. Where polarity is indicated, it was determined by comparison with South American chelids and African pelomedusids in a cladistic analysis (Thomson & Georges, unpublished data). Only those characters relevant to the identification of the fossil specimen are presented.

     Anterior Bridge Struts

Character A. Contact with Pleural 1.

• A0:In the primitive state, the posterior edge of the bridge-carapace suture runs parallel and adjacent to the rib/gomophosisis of pleural 1 (Figures 1a-c).
• A1:In the derived state, the posterior edge of this suture contacts the rib/gomophosisis at its anterior end, but is set at a forward divergent angle of between 15 and 50 degrees. This angle is most pronounced in Emydura, least in Rheodytes (Figures 1d-h).

Character B. Bridge suture shape.

• B1:The anterior and posterior edges of the bridge-carapace suture diverge from their point of congruence closest to the vertebral column. The widest extent of the suture is distal to the vertebral column and there is no medial constriction (fig. 1a-c)
• B2:The anterior and posterior edges of the bridge-carapace suture are parallel or closely so with a prominent suture surface between them. There is no medial constriction (fig 1d,f-g)
• B3:The bridge-carapace suture is expanded for its full length, but more so at extremes, there being an obvious medial constriction (fig 1e).
• B4:The bridge-carapace suture narrows from its widest point proximal to the vertebral column, and constricts completely to form a ridge confluent with the edge formed by the ventral suture of the peripheral bones (fig 1h). Rib/Gomophosisis of Pleural 1

Character C: Rotation of the Rib/Gomophosisis.

• C0: The ventral surface of the distal extent of the rib/gomophosisis is rotated obliquely, to face ventrally but with posterior inflection (fig 1a-d).
• C1: The rib/gomophosisis shows no such torsion distally (fig. 1e-h).

     Dorsal Characters

Character D: Relative width of Vertebral 1.

• D1: First three vertebral scutes equal or sub-equal in width (fig 2a-d,h).
• D2: First vertebral scute wider than second and third (fig 2e-g).

Character E: Cervical Scute

• E0: Cervical scute typically present (fig 2h).
• E1: Cervical scute typically absent (fig 2e-g).

     The distribution of the character states for each taxon is provided in Table 1. The holotype Emydura lavarackorum had a combination of a widely divergent angle (45°) between the anterior bridge suture and the rib/gomophosisis of pleural one; parallel anterior and posterior edges of the bridge-carapace suture throughout their length, widely spaced, with no medial constriction; no distal rotation of the gomophosisis of pleural one; a first vertebral scute that was markedly wider than vertebrals 2 and 3; and no cervical scute.