Further evidence in support of the recognition of the Freshwater Turtle Elseya oneiros (Testudines: Chelidae) from the Nicholson and Gregory Rivers of Northern Queensland

Abstract

Thomson et al. (1997) proposed that the extant Elseya from the Nicholson-Gregory drainages in northwestern Queensland was conspecific with the holotype of Elseya lavarackorum, which comprised a fossil carapace and associated plastron excavated from the late Pleistocene Terrace Site at Riversleigh. Analysis of additional fossil material, and examination of a suite of 16 scute characters (eight for each of the carapace and plastron) by Joseph-Ouni et al. (2020) concluded that the two species were distinct and represented separate Elseya lineages and proposed the new name Elseya oneiros for the extant lineage. Recently, the Turtle Taxonomy Working Group (TTWG) in their 9th edition Checklist of Turtles of the World (TTWG 2021) questioned the status of E. oneiros and placed it in the synonymy of E. lavarackorum, making the claim, amongst others, that the skeletal characters of Thomson et al. (1997) were not addressed. Here we fully address the claims made and evaluate those skeletal characters. We also assess an additional nine thoracic skeletal characters of the pertinent Elseya species, including E. dentata sensu stricto. The results again do not support the conclusions of Thomson et al. (1997) of the holotype of Elseya lavarackorum being conspecific with the extant Elseya from the Nicholson-Gregory Rivers, reaffirms the proposal offered by Joseph-Ouni et al. (2020) for the placement of E. lavarackorum in the subgenus Elseya, and the status of E. oneiros in the Nicholson-Gregory drainages as a distinct species.

The Australian gulf snapping turtle Elseya lavarackorum (Testudines: Chelidae) revisited—Is the late Pleistocene fossil species extant?

“…We question the taxonomic value of characters used by Joseph-Ouni et al and argue that the evidence they present is not sufficient to establish that Elseya lavarackorum (represented by the focal fossil) and Elseya oneiros (represented by a fossil from the same Riversleigh Terrace Site stratum) are distinct evolutionary species (separate and diagnosable lineages -sensu Futuyma 2005). We further argue that the evidence and argument presented by Joseph-Ouni et al (2022) is not sufficient to establish that the extant Elseya sp. aff.…”

“…3) from the same strata at the Riversleigh Terrace Site referred to Elseya lavarackorum by Thomson et al (1997) by association; and a third near complete fossil plastron (QMF30818) (Fig. 4) also from the same strata at the Riversleigh Terrace Site referred to Elseya lavarackorum by Thomson et al (1997) by association and to Elseya oneiros by Joseph-Ouni et al (2022). Note that specimens QM F24121, QM F30817 and QM F30818 have been accessioned and catalogued by the Queensland Museum but not yet housed there.…”

“…Terminology of the elements of the shell associated with the bridge follows that of Thomson et al (1997) and Thomson and Mackness (1999). For consistency with this earlier work to which we continually reference, we refer to the anterior bridge strut which is also referred to as the axillary buttress or the axillary hyoplastral buttress (Joseph-Ouni et al 2022). Strictly, the anterior bridge strut is a structural complex comprising the axillary buttress of the plastron, the associated pleural bones and peripherals of the carapace, and associated sutures (Thomson et al 1997).…”

“…We also examined high quality photographs of extant specimens provided to us by the co-authors of Joseph- Ouni et al (2022) and others (Supplementary Materials) to assess interindividual variability in characters based on scutellation, where relevant.…”

“…Referring to the recessed nuchal region, Thomson et al (1997) assigned Elseya lavarackorum to the subgenus Pelocomastes. This character state has been variously described as: "the leading edge of the carapace is almost straight and does not curve posteriorly until the suture line between the second and third marginal scutes" (White and Archer 1994); "an indentation of the carapace margin in the area of the cervical cleft and first marginal scutes" (Thomson et al 1997); "anterior carapace expanded, anterior extent of second marginal scute as great or greater than that of first marginal scute in adults" (Georges and Thomson 2010); or nuchal emargination (Joseph-Ouni et al 2022). This character state is not observed in Elseya (Elseya) and is ontogenetically variable in Elseya (Pelocomastes) becoming evident in only the largest, presumably oldest, specimens.…”