New transitional fossil from late Jurassic of Chile sheds light on the origin of modern crocodiles

Fernando E Novas,Sebastián Rozadilla,David Rubilar-Rogers,Rita De La Cruz,Manuel Suárez,Ismar De Souza Carvalho,Federico L Agnolin,Gabriel L Lio,Marcelo P Isasi

doi:10.1038/s41598-021-93994-z

Abstract

We describe the basal mesoeucrocodylian Burkesuchus mallingrandensis nov. gen. et sp., from the Upper Jurassic (Tithonian) Toqui Formation of southern Chile. The new taxon constitutes one of the few records of non-pelagic Jurassic crocodyliforms for the entire South American continent. Burkesuchus was found on the same levels that yielded titanosauriform and diplodocoid sauropods and the herbivore theropod Chilesaurus diegosuarezi, thus expanding the taxonomic composition of currently poorly known Jurassic reptilian faunas from Patagonia. Burkesuchus was a small-sized crocodyliform (estimated length 70 cm), with a cranium that is dorsoventrally depressed and transversely wide posteriorly and distinguished by a posteroventrally flexed wing-like squamosal. A well-defined longitudinal groove runs along the lateral edge of the postorbital and squamosal, indicative of a anteroposteriorly extensive upper earlid. Phylogenetic analysis supports Burkesuchus as a basal member of Mesoeucrocodylia. This new discovery expands the meagre record of non-pelagic representatives of this clade for the Jurassic Period, and together with Batrachomimus, from Upper Jurassic beds of Brazil, supports the idea that South America represented a cradle for the evolution of derived crocodyliforms during the Late Jurassic.

Highlights

In contrast to the Cretaceous Period and Cenozoic Era, crocodyliforms from the Jurassic Period are predominantly known from marine forms[1]
Small-sized crocodyliform diagnosed on the following combination of characters: cranial roof bones ornamented by grooves and pits; frontals fused and subtriangular in contour, with strongly convergent lateral margins anteriorly; frontals anteroposteriorly short*; squamosal posteroventrally flexed forming a wide bony wing and delimiting the posterior opening of the meatal chamber, which is reduced to a small duct*; supratemporal foramen small; squamosal and quadrate widely exposed on occipital surface of cranium; paraoccipital processes of otoccipital relatively small; foramina for cranial nerves IX-XII dorsally limited by the paraoccipital process; and dorsal vertebrae with kidney-shaped prezygapophyses
The new taxon was included in a comprehensive phylogenetic analysis including most crocodyliforms[34]

Summary

Introduction

In contrast to the Cretaceous Period and Cenozoic Era, crocodyliforms from the Jurassic Period are predominantly known from marine forms (e.g., thalattosuchians)[1]. SGO.PV 17700 (Colección Paleontología Vertebrados, Museo Nacional de Historia Natural, Santiago, Chile), partial skeleton including partial neurocranium, a cervical neural arch, four dorsal vertebrae, right scapula and coracoid, right humerus and ulna, left ischium, distal end of right femur, and one cervical and two dorsal osteoderms (Fig. 1; Table 1; Supplementary Information).

Results

Conclusion