The Mastodonsauroidea, previously and more widely known as the Capitosauroidea, ranks among the most common and best known Mesozoic temnospondyls. Mastodonsauroids exhibit wide morphological variation and significant homoplasy and their taxonomy and relationships have been in a state of flux. Since the first description of a mastodonsauroid there have been 56 genera and 118 species proposed. A comprehensive review of all these taxa has reduced the content of the group t o 14 genera and 44 species. Most species previously included in the Mastodonsauroidea are undiagnostic at the generic level and designated incertae sedis within the Mastodonsauridae, Heylerosauridae, Mastodonsauroidea or Stereospondyli. Where applicable, definitions, diagnoses and a list of synonyms of all fully diagnosable taxa within the Mastodonsauroidea are given. A computer-based phylogenetic analysis of mastodonsauroid temnospondyls based on 47 cranial characters and 25 terminal taxa was performed. The analysis included the 14 genera of mastodonsauroids as well as selected well-known members of the Lydekkerinidae and Trematosaurrdae. The results supported a monophyletic Mastodonsauroidea with Benthosuchus as the sister group to a clade consisting of the Heylerosauridae and the Mastodonsauridae. The name Mastodonsauridae replaces the name 'Capitosauridae', because Mastodonsaurus was found t o fall within the clade that includes most of the traditional 'capitosaurid genera. The composition of the Mastodonsauroidea was shown t o be similar to that traditionally assumed for the 'Capitosauroidea', and to exclude the Rhinesuchidae, a group often placed within the 'Capitosauroidea'. The Mastodonsauroidea must have a ghost lineage extending back to at least the Late Permian, when the common ancestor of the Mastodonsauroidea and the Trematosauroidea diverged from rhinesuchid ancestors. We can therefore expect to find Late Permian representatives of the Mastodonsauroidea, perhaps present as inconspicuous elements of non-stereospondyl dominated Late Permian faunas. C 2001 The Linnean Society of London ADDITIONAL KEY WORDS: Temnospondyls -Capitosauroidea -Capitosauridaetaxonomy -Mastodonsauridae -Heylerosauridae -Benthosuchusphylogeny. 382 R. J. DAMIANI Table 1. Previous classifications of the 'Capitosauridae'. Taxa in inverted commas were considered paraphyletic. The list is not exhaustive but gives some indication of the lack of consensus in 'Capitosauroid classification

New fossils from the Upper Permian Moradi Formation of northern Niger provide an insight into the faunas that inhabited low-latitude, xeric environments near the end of the Palaeozoic era (approximately 251 million years ago). We describe here two new temnospondyl amphibians, the cochleosaurid Nigerpeton ricqlesi gen. et sp. nov. and the stem edopoid Saharastega moradiensis gen. et sp. nov., as relicts of Carboniferous lineages that diverged 40-90 million years earlier. Coupled with a scarcity of therapsids, the new finds suggest that faunas from the poorly sampled xeric belt that straddled the Equator during the Permian period differed markedly from well-sampled faunas that dominated tropical-to-temperate zones to the north and south. Our results show that long-standing theories of Late Permian faunal homogeneity are probably oversimplified as the result of uneven latitudinal sampling.

A 251 million year old partial burrow cast containing an articulated skeleton of the mammal-like carnivore Thrinaxodon liorhinus is the oldest evidence for burrowing by a cynodont synapsid. The burrow cast comes from terrestrial flood plain sediments close to the Permian-Triassic boundary in the Karoo of South Africa. Together with those of the later cynodont Trirachodon, the Thrinaxodon burrow cast indicates that burrow-making was broadly distributed in basal synapsids and has a long history in non-mammalian synapsid evolution. A reconstruction of its appendicular skeleton in cross-section indicates that Thrinaxodon was able to adopt a facultatively mammalian stance within its burrow shaft. Burrows of cynodont design are more common in Triassic rocks than previously realized, and suggest that burrowing may represent an adaptive response by cynodonts to the environmental conditions associated with the mass extinction event that punctuated the end of the preceding Permian period. The widespread occurrence of burrowing among extant mammals implies that the ancient synapsid ability to burrow conferred a strong adaptive value in the evolution of Mammalia.

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