An atlas of the dorsal thalamus of the marsupial brush-tailed possum, Trichosurus vulpecula.

In several respects the dorsal thalamus of Trichosurus presents a level of organizational complexity considerably in advance of that seen in Didelphis. In particular, such features as the lamination of the dorsal lateral geniculate, the distinct subdivisions of the ventroposterior and lateroposterior complexes, and the more pronounced separation of the ventroanterior from the ventrolateral nucleus, are similar to conditions found in many eutherians. On the other hand, some features which Trichosurus shares with Didelphis, such as the lack of a median central nucleus, and the well developed midline nuclei, are usually said to be indicative of a more 'primitive' level of neural organization. In most instances the thalamic nuclei in Trichosurus are easily homologized with those of Didelphis and other mammals. The difficult areas are the lateral and posterior groups of nuclei, regions which have generally been troublesome in most mammals thus far examined. What does become apparent after this examination of the Trichosurus thalamus is that this representative of the diprotodontid radiation has produced some neurological features which strikingly parallel those of certain advanced eutherians while retaining other features generally considered to be generalized and primitive. Within the metatheria Trichosurus represents a considerable advance over Didelphis. However, as will be seen in later papers in this series, there are other features, particularly those of thalamocortical fibre distribution, which serve to set Trichosurus quite markedly apart from both Didelphis and at least certain eutherian mammals (Haight & Neylon, 1977b, c, 1978).