On the Upper Triassic mammals

Abstract

1. The various accounts of the Triassic mammals which have been published since 1941 are summarized. 2. It is claimed that the identification of a tooth ofKuehneotheriumas evidence that a similar tooth ofEozostrodoncannot be identified is fallacious. It is held that for the most part differences in the appearances of the roots of these teeth illustrate growth stages and do not provide useful taxonomic data.Eozostrodon problematicusandMorganucodon watsoniare held to be synonyms ofEozostrodon parvus. 3. The dentition of E.parvusis described and the formula is given as I f C {-PM f M f. 4. The loss of premolars, followed by the plugging of their sockets by bone, is described, and it is shown that in the lower jaw the process usually starts with the first and then the second premolars. Examples of irregular loss and plugging are given and evidence of the same happening taking place in the upper jaw is described. 5. A series of abnormal postcanine teeth are described. Some are of interest in the light they throw on possible relationships. 6. On the evidence of four cases of tooth replacement, and other relevant considerations, it is claimed that the mammalian diphyodont condition had been achieved inEozostrodon. It is suggested that, in view of the great amount of tooth wear found in many teeth, such a reduction could only have been tolerated if the life cycle was short. This is in accordance with the very small size of the animal. 7. Some evidence on the nature of the hind limb is given. 8. Trituberculate teeth of the formKuehneotherium praecursorisare described. It is shown that some are very similar to those of the symmetrodontSpalacotheriumand support the interpretation that the form is a symmetrodont as first recognized by Kühne and later demonstrated by Crompton & Jenkins. 9. It is claimed that the view that the eozostrodonts are triconodonts and represent the basic mammalian stock from which later triconodonts, the trituberculates, the docodonts and possibly the multituberculates evolved, is well founded.