TRUCHEOSAURUS MAJOR AND REASSESSMENT OF THE RHYTIDOSTEIDAE 



153 



Trucheosaurus, and the position of the quadrate condyles behind the 

 occipital ones, visible only in Acerastea. 



The lack of resolution of most of the generic relationships in the 

 analysis performed herein is apparently due to both the low number 

 of informative characters and the amount of missing entries, which 

 contribute substantially to the instability and poor resolution of the 

 resultant cladogram (Novacek 1992). This result reflects the lack of 

 consensus among investigators concerning the taxonomic validity 

 and content of Rhytidosteoidea and its included families, a problem 

 which has been debated over the last thirty years (see above). This 

 situation strongly indicates that a redescription and further prepara- 

 tion of some specimens is needed, and in many cases the discovery 

 of new, more complete material would improve the resolution of the 

 analysis. Nevertheless, and as a result of the present analysis, most 

 of the Australian taxa (Arcadia, Rewana, Acerastea, Trucheosaurus, 

 and Derwentia) appear more closely related than they are to other 

 members of the family. The only exception is the Australian taxon 

 Deltasaurus. which appears as the sister-taxon of Rhytidosteus, 

 from the South African Karoo. 



The fact that Trucheosaurus is considered here to be a rhytidosteid 

 taxon rather than a brachyopid, constitutes the first Palaeozoic 

 record of Rhytidosteidae, a family which has long been considered 

 to be restricted to the Early Triassic (Scythian) and because of this 

 used in a biostratigraphic sense (Cosgriff 1969, 1984; Shishkin 

 1994). Conversely, Brachyopidae no longer has a Permian repre- 

 sentative and is thus restricted to the Mesozoic. A second brachyopid, 

 Bothriceps australis, was considered to be Permian but only because 

 of its taxonomic relationship with Trucheosaurus major, and is most 

 likely Triassic (Warren 1997). 



When a phylogenetic hypothesis is combined with the observed 

 fossil record of the terminal taxa, stratigraphic separation between 

 sister-taxa demands substantial range extensions beyond those pre- 

 dicted by the observed stratigraphic record ('ghost lineages' of 

 Norell 1992). Thus, the age of Trucheosaurus and its position on the 

 cladogram extend the rhytidosteid diversification and preceding 

 nodes into the Late Permian. Equally, if a more inclusive cladistic 

 analysis of temnospondyls is considered (e.g. Milner 1990), the 

 resultant calibrated phylogeny ( Norell 1 992 ) suggests that the phyletic 

 diversification of Mesozoic temnospondyls ('stereospondyls') oc- 

 curred earlier than indicated by the fossil record. The implication is 

 that the apparent radiation of taxa in the Early Triassic was an 

 extension of a Late Permian event, which probably took place in 

 Gondwana as the earliest and most diverse of the Triassic 

 temnospondyls faunas occur in that area. Therefore, the seeming 

 abruptness of the Permo-Triassic temnospondyl turnover (Milner, 

 1 990: fig. 15.3) might be both an effect of the lack of recent revisons 

 of the known temnospondyl record and its interpretation relative to 

 testable phylogenetic patterns, and a taphonomic artifact The 

 taphonomic effect may result from the absence of preserved 

 temnospondyl-bearing sequences in the Late Permian, or the preser- 

 vation of fauna from selected sequences only, as appears to have 

 been the case in South Africa. 



Acknowledgements. We are grateful to the following for the loan of 

 material: Dr Angela Milner of The Natural History Museum, London, Mr 

 Robert Jones of the Australian Museum, Sydney, and Dr John Pickett and Dr 

 Ian Percival of the Geological Survey of New South Wales, Sydney. Professor 

 Michael Shishkin of the Paleontological Istitute, Moscow, confirmed some of 

 our interpretations of the sutures on the skull roof. Comments on an earlier 

 version of the manuscript by Dr. Ana Maria Baez of the Universidad de 

 Buenos Aires, Buenos Aires, and Dr. Andrew Milner of Birkbeck College, 

 London, greatly improved its content. The photographs were taken by the La 



Trobe University Photographic Unit. The work was supported by an Austral- 

 ian Department of Industry and Commerce Grant to both authors and an 

 Australian Research Council Grant to A. Warren. 



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