An alternative interpretation of small-bodied turtles from the “Middle Purbeck” of England as a new species of compsemydid turtle

Abstract

A series of small-sized fossil turtles were collected from Beckles’ Pit, Durlston Bay, Dorset, United Kingdom in 1856 from a sediment package referable to the Early Cretaceous (Berriasian) Purbeck Group. The two primary accounts that previously documented these turtles concluded that they represent the juveniles of the coeval early pleurosternid Pleurosternon bullockii. A brief, third account, however, suggested that these may represent a new species of compsemydid turtle. We here highlight a series of discrete morphological characters that consistently distinguish the small-bodied turtles from Beckles’ Pit from large-bodied Pleurosternon bullockii, in particular the arrangement of the bones and scutes along the anterior margin of the shell. As these characters are otherwise used to diagnose new species of turtles, in particular compsemydids, and to establish the phylogeny of fossil turtles, we side with the latter interpretation and name a new taxon of early compsemydid, Tongemys enigmaticagen. et sp. nov. The early record of compsemydid is restricted to the Early Cretaceous of Europe, but is extremely fragmentary. We suggest that this may be a bias towards the collection and identification of small turtle remains, but also that a re-study of Early Cretaceous continental turtle faunas is likely to yield further material.