Integrating fossils, morphology and molecules to understand the diversification of egerniine skinks

Author: Kailah Thorn

Thorn, Kailah, 2020 Integrating fossils, morphology and molecules to understand the diversification of egerniine skinks, Flinders University, College of Science and Engineering

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Squamate reptiles (lizards and snakes) are uncommon in the published Australasian fossil record, despite being a component of most fossil Cenozoic collections. This thesis aimed to address the underrepresentation, of one particular group of lizards, the ‘social skinks’ of the subfamily Egerniinae (formerly the ‘Egernia group’). The Egerniinae represent Australasia’s largest extant scincid lizards, including the iconic Blue-tongued Lizard, Spiny-tailed Skinks, and Shinglebacks. Various degrees of sociality occur across the group, ranging from long-term monogamous bonds to family living. Their presence in palaeoecological projects has thus far not been extensively recorded due to a lack of known species-level osteological apomorphic traits. Mapping when and where these apomorphies occur across their phylogenetic history in combination with a molecular dataset allows us to trace the evolution of phenotypes, species and clades.

The oldest confirmed scincid fossil in Australia Proegernia mikebulli, is described from a near-complete reconstructed mandible, maxilla, premaxilla, pterygoid, and referred vertebra and proximal femur. A tip-dated total-evidence phylogeny inclusive of this new taxon and middle Miocene Egernia and “Tiliqua” from Riversleigh, produced an estimated age of 50 Ma for the subfamily Egerniinae.

The oldest fossil taxa recognised as a tentative member of “Tiliqua” and Egernia are described from the middle Miocene of Riversleigh World Heritage area in north-western Queensland. Our tip-dated tree combining these extinct taxa with molecular and morphological data suggests that the modern Australian egerniine radiation dates to the end of the Eocene (34.1 Ma). Both fossils are within the Australian crown clade Egerniinae: Egernia gillespieae sp. nov. is placed within the extant genus Egernia, while “Tiliqua” pusilla likely sits basal to the divergence of the clade inclusive of Tiliqua and Cyclodomorphus. The fossils thus provide direct evidence that the Australian radiation of the Egerniinae was well underway by the mid-Miocene.

Australasia is home to the world’s largest scincid lizards, with multiple taxa regularly exceeding 1 kg in body mass and 30 cm in snout-vent length. We reinvestigated two Australian Plio-Pleistocene giants: Aethesia frangens Hutchinson and Scanlon, 2009 and Tiliqua laticephala Čerňanský and Hutchinson, 2013 from the Wellington Caves in New South Wales. New material from an excavation of Cathedral Cave, and a combing of museum collections in eastern Australia, links the two previously known taxa into one: Tiliqua frangens (Hutchinson and Scanlon, 2009). This is the world’s largest known scincid lizard with an estimated 2.38 kg body mass and a snout-vent length approaching 60 cm, an armoured giant that might have been an ecological analogue of land tortoises. The disappearance of this animal from the fossil record at the same time as many of Australia’s giant marsupials and birds demonstrates that scincids also lost their largest representatives during the late Pleistocene ‘megafauna’ extinctions.

This body of work encompasses three significant contributions to our knowledge of Australian herpetofauna and its fossil record: Australasia’s iconic Blue-tongued lizards have existed since at least the middle-Miocene; and reached phenotypic extremes beyond that of any other lizard, before succumbing to the late Pleistocene ‘megafauna’ extinction event. This work has expanded our knowledge of Australia's past biodiversity beyond mammals, birds, and the largest monitor lizards—to include our presently most-speciose lizard clade, the Scincidae. By incorporating all of these newly described fossil taxa into tip-dated phylogenetic trees we now have the means to accurately date the appearance and phenotypic plasticity of extant egerniines and shed light on their susceptibility to the looming anthropogenic extinction crisis.

Keywords: Vertebrate Palaeontology, Egerniinae, Scincidae, Total-evidence analysis, Egernia, Tiliqua, Fossil, Phylogeny

Subject: Earth Sciences thesis

Thesis type: Doctor of Philosophy
Completed: 2020
School: College of Science and Engineering
Supervisor: Professor Mike Lee