摘要：Permo-Triassic dicynodont (Therapsid) fossils attributed to the genus Lystrosaurus 1 are known from the South African Karoo Basin, including two Triassic taxa referred to as L. (Dicynodon) murrayi 2 and L. (Ptychognathus) declivis 3 . Distinction of the two taxa has been recognised inter alia by Brink 4 , Cluver and King 5 , Botha and Smith 6 , Grine et al. 7 and Botha-Brink et al. 8 However, in this instance, the appropriateness of alpha taxonomy may be questioned, recognising that boundaries between these species of Lystrosaurus are not necessarily clear, especially for specimens close to the time of divergence postdating the Permo-Triassic boundary, 252 million years ago. The question has arisen as to whether only one species occurred. 9 Thackeray 10 proposed the following hypotheses:• H T – that one species existed and• H BB – that there were two taxa, clearly distinct at the species level.Both hypotheses need to be considered in the light of a statement by Camp 11 : ‘traditional data were unable to distinguish between L. murrayi and L. declivis without overlapping measurements in regions where the geometric morphometric analyses indicated they differed most’. Irrespective of which statistical methods are used for taxonomic purposes, it is essential to have some kind of definition of a species that can be applied in palaeontological contexts, to address opposing views held by ‘alpha taxonomists’ who assume clear boundaries between species 12 , and ‘sigma taxonomists’ 13-15 who do not.
