Putting a Name to a Face: Using Geometric Morphometrics to Identify Middle Pleistocene Varanid Cranial Fossils From Naracoorte Caves
Monitor lizards (Varanidae) are ecologically important components of the Australian fauna and are abundant in Quaternary fossil deposits like those in Naracoorte Caves (South Australia). However, the narrow range of morphological variation in varanids makes identification of their fossil remains difficult. Here we explore use of geometric morphometrics (GM) to assess fossil affinities of varanid cranial material. Five fossils from Naracoorte’s Fossil Chamber (Victoria Fossil Cave), represented by two parietals and three dentaries, were included in the GM analysis. Fossils were compared to the parietals and dentaries of reconstructed X-ray computed tomography specimens belonging to modern species that are currently found in the area: Varanus gouldii, Varanus rosenbergi, and Varanus varius. Both dentaries and parietals belonging to these species were consistently distinguished using GM analytical methods, across a range of sizes from immature to large adults. Our sample of fossils were all unambiguously classified as V. varius. The presence of V. varius during the Middle Pleistocene is consistent with forest and woodland environments reconstructed from previous paleoecology and geochemical research for the region. Our results highlight the potential of modern GM methods in identifying Pleistocene fossil material based on comparisons with modern species. Consequently, our paleoecology interpretations and understanding of relationships between changing environments and species distributions over time are greatly improved, with strong implications for species conservation.Abstract
A map showing the location of Fossil Chamber from Reed (2008) (left), and photographs of varanid fossils (A and B = P59504; C = P59509; D = P16127, E = P59506) from Fossil Chamber, Victoria Fossil Cave, Naracoorte (right). Scale bars all 1 cm.
Landmark configurations for parietal (P = 54, 12 fixed, 42 sliding) and dentary (P = 30, 7 fixed and 23 sliding). For standardization purposes, dentaries were aligned so that the anterior and posterior ends of the dorsal margin aligned horizontally. Abbreviations: anterolateral processes (Al.P); adductor crests (Add.C); parietal table (PT); supratemporal rami (St.R); parietal foramen (Pa.F); posterior foramen (Po.F).
Principal component analysis plot of parietal fossils P59504 and P59506 against candidate species Varanus gouldii, Varanus rosenbergi, and Varanus varius with 95% confidence ellipses. Datapoints are proportional to centroid size and color-coded as follows: fossil (black), V. gouldii (brown), V. rosenbergi (lavender), and V. varius (red). Dorsal view surface warps (XY) of the average varanid parietal showing shape change at minimum (negative) and maximum (positive) values for principal components one, two, and three along their corresponding axes. See lateral view surface warps (YZ) and in Figure S2. Direct visual comparison of reconstructed computed tomography scans of each of the species are supplied in Figure S3. Abbreviation: PCA (principal component analysis).
Principal component analysis plot of dentary fossils P59504, P16127 and P59509 against candidate species Varanus gouldii, Varanus rosenbergi, and Varanus varius with 95% confidence ellipses. Datapoints are proportional to centroid size and color-coded as follows: fossil (black), V. gouldii (brown), V. rosenbergi (lavender), and V. varius (red). Surface warps of the average varanid dentary showing shape change at minimum (negative) and maximum (positive) values for principal components one, two, and three along their corresponding axes. See dorsal view surface warps (YZ) of P59504 and P59509 in Figure S3. Direct visual comparisons of reconstructed computed tomography scans of each of the species are supplied in Figure S3. Abbreviation: PCA (principal component analysis).
Linear discriminant analysis plot of parietal fossils P59504 and P59506 against candidate species Varanus gouldii, Varanus rosenbergi, and Varanus varius with 95% confidence ellipses. Key: fossil specimen (black diamond); V. gouldii (brown circle); V. rosenbergi (lavender circle); V. varius (red circle). Abbreviation: LDA (linear discriminant analysis).
Linear discriminant analysis plot of dentary fossils P59504, P16127, and P59509 against candidate species Varanus gouldii, Varanus rosenbergi, and Varanus varius with 95% confidence ellipses. Key: fossil specimen (black diamond); V. gouldii (brown circle); V. rosenbergi (lavender circle); V. varius (red circle). Abbreviation: LDA (linear discriminant analysis).
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