A fossil egg clutch from the stem turtle Meiolania platyceps: implications for the evolution of turtle reproductive biology

ABSTRACT

A fossil egg clutch from the Pleistocene of Lord Howe Island, Australia that we assign to Testudoolithus lordhowensis, oosp. nov. belongs to the stem turtle Meiolania platyceps. This ootaxon is diagnosed by the following unique combination of characters: large spherical eggs (53.9 mm diameter), 800-µm-thick eggshell, and barrel-shaped shell units with height-to-width ratio of 1.2:1. Thin sections and scanning electron microscopy demonstrate that these eggs are composed of radiating acicular aragonite crystals. This mineral composition first evolved either before the split between Meiolaniformes and crown Testudines or prior to Proterochersis robusta, the earliest known stem turtle. A calculated gas conductance of 170.27 mg H₂O day⁻¹ Torr⁻¹ for Meiolania platyceps eggs compares closely with that of two extant tortoise eggs. This value and the presence of at least two superimposing egg layers within the clutch indicate that Meiolania platyceps deposited its eggs inside an excavated hole nest. This nesting strategy likely evolved no later than the Early to Middle Jurassic.

http://zoobank.org/urn:lsid:zoobank:org:pub:35AC8467-EAFB-4828-B79F-18C98FCBFE2B

Citation for this article: Lawver, D. R., and F. D. Jackson. 2016. A fossil egg clutch from the stem turtle Meiolania platyceps: implications for the evolution of turtle reproductive biology. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2016.1223685.

ACKNOWLEDGMENTS

We thank R. Pogson and the Australian Museum for providing specimens for this study. Additionally, R. Pogson provided photographs and measurements of Meiolania platyceps specimens. Two anonymous reviewers provided helpful comments that greatly improved the paper. Extant tortoise eggs come from the M. A. Ewert Memorial Turtle Egg Collection in the Department of Earth Sciences, Montana State University. J. Horner provided access to the Gabriel Laboratory for Cellular and Molecular Paleontology (Bozeman, Montana), and E. Lamm assisted with histology. A. Moore-Nall assisted with cathodoluminescence and provided feedback on analyses. The Department of Earth Sciences and the Image and Chemical Analysis Laboratory, Montana State University (Bozeman, Montana), provided access to laboratory equipment.