Abstract
Susisuchus anatoceps is a neosuchian crocodylomorph lying outside the clade Eusuchia, and associated with the transition between basal and advanced neosuchians and the rise of early eusuchians. The specimen MPSC R1136 comprises a partially articulated postcranial skeleton and is only the third fossil assigned to this relevant taxon. Thin sections of a right rib and right ulna of this specimen have been cut for histological studies and provide the first paleohistological information of an advanced non-eusuchian neosuchian from South America. The cross-section of the ulna shows a thick cortex with 17 lines of arrested growth (LAGs), a few scattered vascular canals, and primary and secondary osteons. This bone has a free medullary cavity and a spongiosa is completely absent. Thin sections of the rib show that remodeling process was active when the animal died, with a thin cortex and a well-developed spongiosa. In the latter, few secondary osteons and 4 LAGs were identified. According to the observed data, Susisuchus anatoceps had a slow-growing histological microstructure pattern, which is common in crocodylomorphs. The high number of ulnar LAGs and the active remodeling process are indicative that this animal was at least a late subadult, at or past the age of sexual maturity. This contradicts previous studies that interpreted this and other Susisuchus anatoceps specimens as juveniles, and suggests that full-grown adults of this species were relatively small-bodied, comparable in size to modern dwarf crocodiles.
Highlights
Crocodylomorpha is the most common group of Mesozoic tetrapods in Brazil [1, 2]
Fossils described in the past years indicate that the maximum diversity of this group took place during the Cretaceous Period (145–66 Ma), when the Notosuchia dominated the ancient Brazilian terrestrial landscapes (e.g., [3,4,5,6,7,8,9]), along with other less specialized crocodylomorphs (e.g. [10])
MPSC R1136 is recognized as the third specimen of S. anatoceps on the basis of at least three diagnostic features shared with the holotype, SMNK PAL 3804 (Staaliches Museum für Naturkunde Kalsruhe, Germany) [20]
Summary
Crocodylomorpha is the most common group of Mesozoic tetrapods in Brazil [1, 2]. Fossils described in the past years indicate that the maximum diversity of this group took place during the Cretaceous Period (145–66 Ma), when the Notosuchia dominated the ancient Brazilian terrestrial landscapes (e.g., [3,4,5,6,7,8,9]), along with other less specialized crocodylomorphs The fossil record of Neosuchia is relatively poor for Cretaceous deposits of PLOS ONE | DOI:10.1371/journal.pone.0155297. Bone Histology and Lifestyle of Susisuchus anatoceps. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
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