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STRUCTURE AND EVOLUTIONARY IMPLICATIONS OF THE EARLIEST (SINEMURIAN, EARLY JURASSIC) DINOSAUR EGGS AND EGGSHELLS

THURSDAY SEPTEMBER 26 - 4 PM

KOEN STEIN
AMGC, VRIJE UNIVERSITEIT BRUSSEL

ABSTRACT: The absence of preserved eggs or eggshell during the first third of the known 315-million-year history of amniote evolution remains a great challenge for our current understanding of the origin and evolution of calcareous eggshell and amniotic eggs in general. Here I present a study on the oldest known eggs and eggshells, reported from three Early Jurassic (195-192 Ma) sauropodomorph dinosaurs, Massospondylus  from the Elliot Formation of South Africa, Lufengosaurus  from the Lufeng Formation of Yunnan, China, and Mussaurus  from the Laguna Colorada Formation of Argentina.
We performed polarized light microscopic and geochemical analyses (cathodoluminescence, micro X-ray fluorescence) on samples of these eggshells. Different diagenetic settings affected the original microstructure to different degrees, with Lufengosaurus having the best, and Mussaurus the least preserved details. The eggs of these basal sauropodomorph dinosaurs all have a surprisingly thin calcareous layer (≤ 100 µm) with interlocking units of radiating crystals (mammillae) and a thick shell membrane. Phylogenetically informed analyses substantiate that more derived dinosaurs have considerably thicker calcareous shells in proportion to egg size. Ancestral state reconstruction demonstrates that the thin eggshell of basal sauropodomorphs represents a major evolutionary innovation at the base of Dinosauria, and that the much thicker eggshell of sauropods, theropods, and ornithischian dinosaurs evolved independently. Fossil eggshell evidence from other amniotes suggests that these mineralizations occurred not earlier than Middle Jurassic, corresponding with a global trend of atmospheric oxygen increase. Egg physiology and low atmospheric oxygen levels may thus have inhibited eggshell thickening before the end of the Early Jurassic, when atmospheric oxygen levels started to rise again.

BIO: Koen is a vertebrate palaeontologist, specialised in dinosaur palaeobiology. He studies bone, tooth and eggshell tissues of fossil and extant tetrapods to better understand evolutionary processes. During his PhD Koen studied the effects of gigantism on the structure of sauropodomorph bone tissues. Since then, he has developed histomorphometry and comparative histological methods to characterise and compare different mineralised tissues of various tetrapods, including dinosaurs and other reptiles, but also mammals and amphibians.