A Late Cretaceous mammal from Brazil and the first radioisotopic age for the Bauru Group.

Mariela C Castro,Francisco J Goin,Edgardo Ortiz-Jaureguizar,E Carolina Vieytes,Alessandro Batezelli,Max C Langer,Júlio C A Marsola,Kaori Tsukui,Jahandar Ramezani

doi:10.1098/rsos.180482

Abstract

In the last three decades, records of tribosphenidan mammals from India, continental Africa, Madagascar and South America have challenged the notion of a strictly Laurasian distribution of the group during the Cretaceous. Here, we describe a lower premolar from the Late Cretaceous Adamantina Formation, São Paulo State, Brazil. It differs from all known fossil mammals, except for a putative eutherian from the same geologic unity and Deccanolestes hislopi, from the Maastrichtian of India. The incompleteness of the material precludes narrowing down its taxonomic attribution further than Tribosphenida, but it is larger than most coeval mammals and shows a thin layer of parallel crystallite enamel. The new taxon helps filling two major gaps in the fossil record: the paucity of Mesozoic mammals in more northern parts of South America and of tribosphenidans in the Cretaceous of that continent. In addition, high-precision U-Pb geochronology provided a post-Turonian maximal age (≤87.8 Ma) for the type stratum, which is overlain by the dinosaur-bearing Marília Formation, constraining the age of the Adamantina Formation at the site to late Coniacian–late Maastrichtian. This represents the first radioisotopic age for the Bauru Group, a key stratigraphic unit for the study of Cretaceous tetrapods in Gondwana.

Highlights

The mammalian clade Tribosphenida [1] (=Boreosphenida [2]) includes eutherians and metatherians, with records as old as the Early Cretaceous (Berriasian) in the Northern Hemisphere [3]
We provide the first radioisotopic age for Bauru Group deposits, which is an important window to the biodiversity of Western Gondwana in the Late Cretaceous, especially due to its tetrapod records
The EARTHTIME mixed 205Pb–233U–235U (ET535) tracer [21,22] was used in the analyses and isotopic measurements were made either on the VG Sector 54 or on Isotopx X62 multi-collector mass spectrometers equipped with Daly photomultiplier ion-counting systems at MIT

Summary

Introduction

The mammalian clade Tribosphenida [1] (=Boreosphenida [2]) includes eutherians and metatherians, with records as old as the Early Cretaceous (Berriasian) in the Northern Hemisphere [3]. We describe a new mammal species based on an isolated tooth collected during 2015 (in accordance with Brazilian laws) from the Late Cretaceous Adamantina Formation, in the area of General Salgado, São Paulo State, Brazil (figure 1; electronic supplementary material, figures S4–S7). We provide the first radioisotopic age for Bauru Group deposits, which is an important window to the biodiversity of Western Gondwana in the Late Cretaceous, especially due to its tetrapod records

Methods

Results

Discussion

Conclusion