Inframarginal Scutation in Australasian Chelid Turtles (Testudines: Chelidae)

Abstract

Numerous fossil Mesozoic and Cenozoic turtle taxa of the cryptodiran lineages, including Paracryptodira and Eucryptodira, are characterized by series of inframarginal scutation of the lateral plastron. In extant representatives of the subordinal turtle lineages, inframarginal scutes are again most marked in cryptodires among which some retain full series of inframarginals, including sea turtles of the family Cheloniidae, the riverine Dermatemyidae and the semi-terrestrial Platysternidae. Amongst these scutes, two occur in the form of axillary and inguinal scutes at the polar ends of the bridge, which may be reduced in the freshwater families Geoemydidae and Emydidae, and terrestrial Testudinidae. In pleurodires, these two scutes are only present in several genera in both South American and Australasian chelid turtles but are mostly extremely reduced in size. With the exception of a few brief mentions in the published literature on the chelids of Australia and New Guinea, no formal documentation of the presence of axillary and inguinal scutes, or illustrations of these exist, nor any record of which taxa possess them. They are most likely overlooked in descriptions because their positions on the plastral bridges are normally associated with Rathke’s gland (the musk glands). We examined specimens of all extant Australasian taxa and, for the first time, provide an account of those that typically do and do not possess axillary and inguinal scutation, along with illustrations and descriptions of shape, relative size and plastral positioning of them. These scutes may be applicable as diagnostic characters for some taxa.