Within Elseya, this species is most similar to E. irwini (Cann, 1998) from the Burdekin River, but can be distinguished by the close encroachment of the ileum suture to the seventh pleural. In E. irwini the suture is widely spaced as is typical of Elseya but in E. nadibajagu they are extremely close, almost approaching the Emydura condition in this character.

     Carapace consists of a complete nuchal bone with no cervical scute present. The left pleural one is more complete than the right and the anterior bridge strut has a wide suture throughout its length, which is preserved. The suture is deep and angled sharply away from the rib/gomphosis. The sulci preserved in this region indicate that the first vertebral scute was wider than the second and third.
     Pleurals two through six are partially preserved on either side but without their peripheral contacts. Also preserved as an unarticulated unit is the left eighth peripheral. The anterior sutural surface for the ileum is clearly constrained to this unit and does not extend on to, or make sutural contact with, the seventh pleural. It does however, continue on to the pygal in the posterior.
     All the units are represented in the plastron except the epiplastra, which are either both missing or not identifiable among the fragments. Included here also are both bridge struts. The bridge struts are wide throughout the length at the sutural surface where they contact the carapace. The plastral elements both in sulci and bony elements are similar in form to any extant member of the Elseya lavarackorum group.

     The specific epithet is from the Gugu-Yalanji dialect phrase nadi bajagu, meaning 'very long time ago' (Oates et al. 1964) and is used to denote the significant age of the fossil. It is of neuter gender.

Material examined: QM F30578, an isolated nuchal bone from a long-necked turtle of the Chelodina longicollis group.

     This specimen can be diagnosed by the extreme widening of the posterior half of the nuchal bone as well as the wide square cervical scute. There are also a large series of muscle attachments for the muscles at the base of the neck which, by necessity, are enlarged in the long-necked turtles (Thomson & Georges 1996).

     The placement within the Chelodina longicollis group is done on the sculptured surface of the shell, a feature more prevalent in species such as Chelodina longicollis and Chelodina novaeguineae. This is, however, a highly variable character and probably of poor taxonomic value (Thomson, in prep., Gaffney, 1981).

Material examined: QM F 30579, a series of pleurals all diagnostic of the genus Emydura using the bridge strut characters of Thomson et al. (1997).

     None of the pleurals are diagnosable from the extant species in the area Emydura macquarii (=Em. krefftii, Georges and Adams, 1996) and we therefore take the most parsimonious view and assign the fossil to the living species.

     The closest living species to Elseya nadibajagu is E. irwini described by Cann (1997) on the basis of its external morphology. Georges & Adams (1996) have confirmed the validity of E. irwini on the basis of electrophoretic studies. Both of these taxonomic indicators have not been preserved in the fossil. Whilst the use of osteological characters has enabled the separation of E. nadibajagu from the rest of the genus, the possibility exists that those characters may be subject to a lot more variation than can be seen in the limited sample of both E. irwini and E. nadibajagu. Reptiles have a lower rate of turnover than their mammalian counterparts with many extant species having fossil records stretching back millions of years (Mackness & Hutchinson in press). White & Archer (1994) described the fossil chelid Emydura lavarackorum from the Pleistocene deposits of Riversleigh with living examples being described just three years later (Thomson et al.1977).

     The occurrence of three different chelid taxa from Bluff Downs is not unusual with tropical river systems having four or more different genera in the one region (Legler & Georges 1993). There have been five different turtles recorded for the Burdekin (Cann 1997) including three short necked and two long necked taxa.

     The palaeoenvironment of the Bluff Downs local fauna has been interpreted as being similar to that found in present day Kakadu (Boles & Mackness 1994) with avian species like darters and pygmy-geese indicating permanent water bodies (Mackness 1995, in press). There may have also been riparian rainforest or vine thickets (Mackness in press, Mackness & Archer in press). Fossils of short-necked chelids dominate the Bluff Downs fauna indicating well developed rivers, creeks and lagoons with abundant aquatic fauna (Cann 1978, Legler 1985). The long-necked turtles indicate that there must have been shallow turbid lagoons that suited these ambush predators (White 1997) [No].

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