Skeletochronology and Paleohistology of Hyposaurus rogersii (Crocodyliformes, Dyrosauridae) from the Early Paleogene of New Jersey, USA.

Rodrigo A Pellegrini,Alexander K Hastings,David C Parris,Wayne R Callahan,John D Mccauley

doi:10.3390/ani11113067

Rodrigo A Pellegrini, Alexander K Hastings + Show 3 more

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https://doi.org/10.3390/ani11113067

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Abstract

Simple SummaryThe way of life and biology of extinct animals can be inferred from growth marks and microstructures in their fossil bones. Hyposaurus is a genus representative of dyrosaurids, a group of crocodile-like animals that survived the extinction event that killed the dinosaurs but nonetheless has long become extinct. The current study focuses on bone sections from a single individual, extracted from the optimal location in the best bones for such studies. It complements and corrects previous works based on less optimal material, and concludes that the genus was a marine ambush predator rather than a fast-swimming pursuit predator, that the males of the genus grew larger than the females, and that the individual studied was a fully mature 17–18-year-old female.The paleohistology of dyrosaurids is known from a small sample, despite being common fossils and representing a rare lineage of crocodylomorphs that survived the Cretaceous–Paleogene extinction. Their lifestyle has been inferred only from sections of the snout, vertebrae, partial femur, and tibia. To improve this, we conducted a skeletochronological and paleohistological study of midshaft cross-sections of both femora and humeri of a nearly complete Hyposaurus rogersii skeleton. We found lamellar-zonal bone that underwent remodeling, evidenced by resorption cavities and abundant secondary osteons within the primary periosteal cortex. The osteons, mostly longitudinally oriented and arranged in circular rows, often anastomose radially along a linear path, resembling radial rows. The medullary cavity is completely open, lacking trabeculae: endosteal deposition is limited to thin lamellae surrounding the cavity. Analysis of cyclical growth marks and the presence of an external fundamental system indicate the specimen was a fully mature adult 17–18 years of age. Comparison of the skeleton to others suggests sexual dimorphism and that it was female. The open medullary cavity, and no evidence for pachyosteosclerosis, osteosclerosis, osteoporosis, or pachyostosis indicate H. rogersii was not a deep diver or a fast swimmer in the open ocean but a near-shore marine ambush predator.

Highlights

The longirostrine crocodylomorph Hyposaurus is the only dyrosaurid genus known from North America, except perhaps for dyrosaurid remains not identified to the generic level
In the autumn of 2011, a team from the New Jersey State Museum (NJSM) collected the most complete skeleton of H. rogersii recovered to date
The bone microstructure and histology of the midshaft transverse cross-sections of the humeri and femora of Hyposaurus rogersii NJSM 23368 show none of the adaptations normally found in tetrapods secondarily adapted to an aquatic lifestyle

Summary

Introduction

The longirostrine crocodylomorph Hyposaurus is the only dyrosaurid genus known from North America, except perhaps for dyrosaurid remains not identified to the generic level. These include most notably a possible dyrosaurid partial skull, with an additional mandible and partial braincase from the Maastrichtian Escondido Formation of Mexico [1], and a single distal caudal vertebral centrum from the late Eocene Yazoo Clay of Alabama [2]. Partial remains of Hyposaurus have been recovered from the Danian Hornerstown Formation of New Jersey, and the early Danian basal Clayton Formation of Alabama, in addition to a questionable vertebral centrum from spoil piles of the late Paleocene Williamsburg Formation of South Carolina [3]. Designated NJSM 23368, it was excavated from the basal Hornerstown

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