Giant dinosaur (theropod) eggs of the Oogenus macroelongatoolithus (Elongatoolithidae) from southeastern Idaho : taxonomic, paleobiogeographic, and reproductive implications
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Date
2014
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Montana State University - Bozeman, College of Letters & Science
Abstract
Macroelongatoolithus eggs are best known for their gigantic size and elongate shape, ranging from 34 - 61 cm in length with an elongation index of 2-3.1. Ootaxonomic placement of this oogenus, and the oospecies within it, has varied widely among authors, necessitating a re-evaluation of the group. This study represents the first comprehensive comparison of gross morphology, microstructural variation, and gas conductance within a Macroelongatoolithus oospecies. Four specimens, egg pair IMNH 2428\49608 and additional eggshell from the Albian-Cenomanian Wayan Formation of Idaho, eggshell fragment ES305 from the Albian-Cenomanian Blackleaf Formation of Montana, eggshell fragment thinsections ES301 and ES305-306 from the early Late Cretaceous Thomas Fork Formation of Wyoming, and egg pair ZMNH M8705 from the Cenomanian-Turonian Liangtoutang Formation of China, are examined, described, and assigned to the oospecies Macroelongatoolithus carlylei. Eggs are 39.8 - 41.65 cm long and 10.8 - 14.28 cm wide, with an eggshell thickness ranges from 1.01 - 2.17 mm, and CL:ML ratio ranging from 2.05:1 - 7.68:1. Eggshell consists of two structural layers of calcite separated by a distinct, undulating boundary. Gas conductance of IMNH 2428\49608-2 and ZMNH M8705-1 is 1119 - 1597 mg H2O torr-1 day-1, 2.9 - 5.6 times higher than values predicted for open nest mass-equivalent eggs. These values suggest a humid or enclosed nesting environment. The lower value calculated for the Liangtoutang Formation specimen (ZMNH M8705-1) may reflect differences in nesting environment between the semi-arid floodplain mudstones of the Wayan Formation in Idaho and the shallow laucustrine red siltstones and sandstones of the Liangtoutang Formation of Zhejiang, China. The observed microstructural variation within and between IMNH 2428/49608 and ZMNH M86705 expands the range of expected variation for an oospecies, and calls into question the over splitting of ootaxa on the basis of traits that may reflect individual variation or growth stage of the laying animal. The discovery and description of an intact M. carlylei specimen from North American confirms the synonymy of M. xixiaensis into the previously named M. carlylei suggested by Zelenitsky et al. 2000, and expands the geographic and temporal distribution of the oogenus and parent animal to include the Albian - Cenomanian of North America.