A new specimen of Prolacerta broomi from the lower Fremouw Formation (Early Triassic) of Antarctica, its biogeographical implications and a taxonomic revision

Stephan N F Spiekman

doi:10.1038/s41598-018-36499-6

Abstract

Prolacerta broomi is an Early Triassic archosauromorph of particular importance to the early evolution of archosaurs. It is well known from many specimens from South Africa and a few relatively small specimens from Antarctica. Here, a new articulated specimen from the Fremouw Formation of Antarctica is described in detail. It represents the largest specimen of Prolacerta described to date with a nearly fully articulated and complete postcranium in addition to four skull elements. The study of this specimen and the re-evaluation of other Prolacerta specimens from both Antarctica and South Africa reveal several important new insights into its morphology, most notably regarding the premaxilla, manus, and pelvic girdle. Although well-preserved skull material from Antarctica is still lacking for Prolacerta, a detailed comparison of Prolacerta specimens from Antarctica and South Africa corroborates previous findings that there are no characters clearly distinguishing the specimens from these different regions and therefore the Antarctic material is assigned to Prolacerta broomi. The biogeographical implications of these new findings are discussed. Finally, some osteological characters for Prolacerta are revised and an updated diagnosis and phylogenetic analysis are provided.

Highlights

In his description of the first-discovered Antarctic Prolacerta material, Colbert described 17 new specimens[1]
The aim of this study is to present a full description of a newly discovered Prolacerta skeleton, UWBM 95529 (Burke Museum of Natural History and Culture, USA) that constitutes the most complete Antarctic specimen to date (Fig. 1) and to provide an osteological revision of Prolacerta with a focus on a detailed comparison between the Antarctic and South African specimens of the taxon
Prolacerta broomi is a non-archosauriform archosauromorph distinguished from other archosauromorphs by the combined presence of the following characters: presence of septomaxillae; presence of recurved maxillary teeth; long horizontal dorsal margin of the maxilla with a concave posterior margin; well-developed posterolateral process on the frontal; two foramina for the passage of the carotid arteries and parasphenoid crests on the ventral surface of the parasphenoid; presence of medially directed teeth on the medial margin of the pterygoid; conical process on the proximal end of the humerus; ambiens process on the anterior margin of the pubis; ischial symphysis raised on a triangular shaped peduncle

Summary

Introduction

In his description of the first-discovered Antarctic Prolacerta material, Colbert described 17 new specimens[1] He pointed out several small morphological differences, he considered these specimens to be conspecific with Prolacerta broomi specimens from South Africa. This identification supported the view at the time that the Antarctic fauna from the lower Fremouw Formation was nearly indistinguishable from that of the Lystrosaurus AZ of the Karroo Basin of South Africa. The aim of this study is to present a full description of a newly discovered Prolacerta skeleton, UWBM 95529 (Burke Museum of Natural History and Culture, USA) that constitutes the most complete Antarctic specimen to date (Fig. 1) and to provide an osteological revision of Prolacerta with a focus on a detailed comparison between the Antarctic and South African specimens of the taxon. Prolacerta broomi is a non-archosauriform archosauromorph distinguished from other archosauromorphs by the combined presence of the following characters: presence of septomaxillae; presence of recurved maxillary teeth; long horizontal dorsal margin of the maxilla with a concave posterior margin; well-developed posterolateral process on the frontal; two foramina for the passage of the carotid arteries and parasphenoid crests on the ventral surface of the parasphenoid; presence of medially directed teeth on the medial margin of the pterygoid; conical process on the proximal end of the humerus; ambiens process on the anterior margin of the pubis; ischial symphysis raised on a triangular shaped peduncle

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