Tortoises (Chelonii, Testudinidae)

Abstract

Two species of tortoises are known from Pliocene sites on the Eyasi Plateau, including Laetoli. The most common species is a medium-sized tortoise, Stigmochelys brachygularis, which is well represented in the Laetolil Beds (∼3.6–4.4 Ma) and Upper Ndolanya Beds (∼2.66 Ma). The giant tortoise, “Geochelone” laetoliensis, is known only from the Laetolil Beds, and is much less common than S. brachygularis. Stigmochelys brachygularis is represented by a number of relatively complete and partial shells, as well as numerous isolated and associated shell fragments, some postcranial remains and eggs. It is generally similar in size and overall morphology to the extant leopard tortoise, S. pardalis, but a number of features serve to distinguish the two species. Stigmochelys brachygularis and S. pardalis are inferred to be closely related, and are most likely sister taxa that represent time-successive species of a single lineage. The age structure of the fossil sample, in conjunction with evidence of carnivore damage on the shells, indicates that S. brachygularis was subject to relatively high levels of predation in comparison with modern-day S. pardalis. If S. brachygularis is presumed to have been ecologically similar to modern S. pardalis, it would have been capable of living in a wide range of habitats from semi-desert and savanna to open woodland. The material attributed to “Geochelone” laetoliensis is more fragmentary, and as a consequence it is not possible to determine its precise phylogenetic or taxonomic relationships. However, it likely represents a distinct genus, possibly with affinities to Astrochelys from Madagascar. A more thorough assessment of the relationships of “G.” laetoliensis with Miocene and Pliocene giant tortoises from Africa and with extant genera will have to await the recovery of more complete material from Laetoli. Giant tortoises, such as “G.” laetoliensis, became extinct on mainland Africa during the late Pliocene, possibly associated with the appearance of early Homo and stone tool using behaviors at 2.6 Ma.