A new species of Proegernia from the Namba Formation in South Australia and the early evolution and environment of Australian egerniine skinks

Abstract

The diverse living Australian lizard fauna contrasts greatly with their limited Oligo-Miocene fossil record. New Oligo-Miocene fossil vertebrates from the Namba Formation (south of Lake Frome, South Australia) were uncovered from multiple expeditions from 2007 to 2018. Abundant disarticulated material of small vertebrates was concentrated in shallow lenses along the palaeolake edges, now exposed on the western of Lake Pinpa also known from Billeroo Creek 2 km northeast. The fossiliferous lens within the Namba Formation hosting the abundant aquatic (such as fish, platypus Obdurodon and waterfowl) and diverse terrestrial (such as possums, dasyuromorphs and scincids) vertebrates and is hereafter recognized as the Fish Lens. The stratigraphic provenance of these deposits in relation to prior finds in the area is also established. A new egerniine scincid taxon Proegernia mikebulli sp. nov. described herein, is based on a near-complete reconstructed mandible, maxilla, premaxilla and pterygoid. Postcranial scincid elements were also recovered with this material, but could not yet be confidently associated with P. mikebulli. This new taxon is recovered as the sister species to P. palankarinnensis, in a tip-dated total-evidence phylogenetic analysis, where both are recovered as stem Australian egerniines. These taxa also help pinpoint the timing of the arrival of scincids to Australia, with egerniines the first radiation to reach the continent.

Keywords: Egerniinae; Miocene; Oligocene; Scincidae; palaeontology.

Figure 1.

Stratigraphic sections through the Namba Formation at Lake Pinpa at Site 12 (figure 2) and Billeroo Creek, Site 2, constructed in SedLog 3.1 from authors' field observations in comparison with R. H. Tedford's sections described in notes and figures in [20,45], height in metres. Grain sizes were determined in the field. Colours are converted from Munsell figures to corresponding RGB values using [46].

Figure 2.

Location maps of the Lake Pinpa and Billeroo Creek sites, Frome Downs Station, South Australia. Stratigraphic columns in figure 1 were compiled from locations Site 12 (= LP12) and Site 2 Fish Lens (=BC2) marked above.

Figure 3.

Results of the soluble mineral analyses of Layers 2–4 and the white (5W) and black (5B) sediments from Layer 5 at Lake Pinpa Site 12 (a); and Billeroo Creek Site 2 Layers 2–5 (b).

Figure 4.

(1a–c) Holotype, a right dentary, SAM P57502, (2a–c) paratype; a left dentary SAM P57499; two right post-dentary compound bones (3a–c) SAM P57543 and (4a–c) SAM P57542 of Proegernia mikebulli sp. nov. from the Fish Lens at BC2; and (5) complete reconstruction in medial view of the right mandible of Proegernia mikebulli, the coronoid and splenial (hatched area) are reconstructed based on modern egerniine specimens, no fossil representatives of these elements are known. adf, adductor fossa; anf, angular facet; art., articular; d., dentary; gl, glenoid fossa; iaf, inferior alveolar foramen; mg, Meckel's groove; psf, posterior surangular foramen; rap, retroarticular process; san., surangular; and sy, symphysis.

Figure 5.

Right maxilla of Proegernia mikebulli sp. nov. reconstructed from SAM P57541 (anterior fragment; top left) and SAM P57503 (near complete; top right). ‘A’ marks the same tooth locus on each specimen. Enclosed by the box is the tooth ‘B’. cl, cuspis labialis; cla, culmen lateralis anterior; cli, cuspis lingualis; clp, culmen lateralis posterior; fp, facial process; psm, palatal shelf of the maxilla; pxp, premaxillary process; sop, suborbital process; and str, striae.

Figure 6.

Left premaxilla (SAM P57542) of Proegernia mikebulli sp. nov. from posterior (left), anterior (centre), and lateral (right) view. A, tooth one, enlarged in box. flc, foramen of longitudinal canal; inp, internasal process; mp, maxillary process; nef, notch of the ethmoidal foramen; and o, osteoderm.

Figure 7.

(a,b) 24th tooth on the dentary SAM P57502, medial view, and (c) the 16th tooth in occlusal view with lateral and medial facies marked. ai, antrum intercristatum; cl, crista lingualis; cm, crista mesialis; cul, cuspis labialis; cula, culmen lateralis anterior; and culp, culmen lateralis posterior.

Figure 8.

Left pterygoid of Proegernia mikebulli sp. nov., SAM P57512 in dorsal (a) and ventral (b) views. en, epipterygoid notch; pp, palatine process; pvc pterygoid ventral concavity; qp, quadrate process; and tp, ectopterygoid process.

Figure 9.

A single vertebra (SAM P57514) potentially referable to Proegernia mikebulli sp. nov., recovered from Fish Lens, BC2. (a) anterior view; (b) posterior view; (c) lateral view, anterior to the left; (d) dorsal view; and (e) ventral view. A proximal fragment of a right femur potentially referable to Proegernia mikebulli sp. nov. SAM P57540. (f) Dorsal; (g) anterior; and (h) ventral views.

Figure 10.

Comparative SEM of Proegernia mikebulli sp. nov. (b,c) and photomicrographs of P. palankarinnensis (a,d) from Martin et al. [16], in medial (above) and occlusal (below) views. Note the anterior extension of the splenial notch (3) in P. palankarinnensis and the increased the number of teeth/loci in P. mikebulli. Features described in text are labelled: 1 medial inflection of the symphysis; 2 dentary depth; 3 position of the inferior alveolar foramen; 4 Meckel's groove extension anterior of the inferior alveolar foramen, present in P. palankarinnensis and absent in P. mikebulli; 5 anterior extent of the internal septum similar in both taxa; 6 overall robustness as dentary width; 7 facet for articulation with anterior ramus of coronoid; 8 splenial notch height and 9 height of the coronoid process relative to the last tooth.

Figure 11.

Comparative medial views of the dentaries of Eutropis multifasciata (SAMA R35693), Proegernia mikebulli sp. nov., Lissolepis coventryi (SAMA R57317) and Liopholis multiscutata (FUR168). 1 Anterior extent of splenial; 2 symphysis shape and robustness; and 3 dentary depth at mid-tooth row length.

Figure 12.

Medial view of left dentary, mid-tooth row, tooth crowns of Plestiodon fasciatus (SAMA R66784), Eutropis multifasciata (SAMA R35693), Proegernia mikebulli sp. nov. (SAM P57502), Lissolepis coventryi (SAMA R57317), Eugongylus rufescens (SAMA R36735) and Sphenomorphus jobiensis (SAMA R6736). Plestiodon represents the basal skink tooth condition as described by Richter [22]. Culmen lateralis posterior is labelled culp.

Figure 13.

Egerniine phylogeny based on the single most parsimonious tree produced by TNT [26]. Bootstrap values greater than 50 shown. ^† denotes extinct taxon.

Figure 14.

Egerniine phylogeny based on consensus total evidence Bayesian inference tree from BEAST. ^†denotes extinct taxon. Small numbers at nodes denote posterior probability. Reconstructed node ages for key clades are in brackets, the green shading denotes the Eocene, after which Australia became isolated from Gondwana [4].