Saturday, April 18, 2015

[Paleontology • 2015] Pelagic Neonatal Fossils support Viviparity and Precocial Life History of Cretaceous Mosasaurs

Figure 4. Clidastes liodontus giving live birth in an open pelagic setting ~85 million years ago,
 beneath the shadow of the toothed stem bird Ichthyornis.
Artist's interpretation of this study's principal conclusions || Illustration by Julius Csotonyi.

Mosasaurs were large marine squamates that inhabited all of the world's oceans during the Late Cretaceous. Their success as apex predators has been attributed to their rapid acquisition of aquatic adaptations, which allowed them to become fully pelagic. However, little is known about the breeding biology of derived, flipper-bearing mosasaurs, as the record of neonatal mosasaur fossils is extremely sparse. Here, we report on the fragmentary cranial remains of two neonatal mosasaurs from the Niobrara Formation, referred to Clidastes sp. Comparison with other preliminary reports of neonatal mosasaurs reveals that these specimens are among the smallest individuals ever found and certainly represent the smallest known Clidastes specimens. The recovery of these extremely young specimens from a pelagic setting indicates that even neonatal mosasaurs occupied open oceanic habitats and were likely born in this setting. These data shed new light on the ecology of neonatal mosasaurs and illustrate the degree to which size-related taphonomic and collection biases have influenced our understanding of the early life history of these iconic marine reptiles. 

Keywords: mosasaur; viviparity; Niobrara; marine reptile; life history

Figure 1. The smallest mosasaurid remains from the Niobrara Formation, previously misidentified as toothed stem birds.
, lateral and dorsal view of the left dentary of Clidastes, YPM VP.058125; lateral and dorsal view of the left dentary of Clidastes, YPM VP.058126. Scale bar represents 1 cm. B, size comparison of several of the smallest Clidastes dentaries from the Niobrara Formation, in lateral (left) and dorsal (right) views. Scale bar represents 1 cm. The newly discovered specimens reported here (YPM 058125, YPM 058126) are by far the smallest. C, comparison of maximum tooth diameters for juvenile Clidastes from the Niobrara chalk. The newly discovered specimens (YPM VP.058125, YPM VP.058126) represent the smallest mosasaurids known from the Niobrara Formation and are currently the youngest Clidastes specimens ever found. D, estimated body length of neonatal mosasaurs (YPM VP.058125 and YPM VP.058126) relative to an adult specimen of Clidastes liodontus. The adult is 3 m in length; the neonate is 0.66 m in length (see text).
Redrawn from Caldwell and Diedrich (2005, fig. 5) and Lindgren et al. (2011, fig. 12). Scale bar represents 1 m. Colour online. DOI: 10.1111/pala.12165

Daniel J. Field, Aaron LeBlanc, Adrienne Gau and Adam D. Behlke. 2015. Pelagic Neonatal Fossils support Viviparity and Precocial Life History of Cretaceous Mosasaurs. Palaeontology. DOI: 10.1111/pala.12165