Triassic fishes from the Cassange depression (R. P. de ANGOLA)

Abstract

Key-words.' Fishes - Triassic - Baixa do Cassange - Angola. Following an historical summary of the field invesrigations and later research on the srratigraphy and paleontology of the Cassange series in Northern Angola (and to a lesser extent of correlated units from Lunda Province, Eastern Angola), the stratigraphic succession at the Baixa do Cassange is reviewed mainly on the basis of field observations and published conclusions of F. Mouta and M. M. Marques. The fish assemblage from two levels in the "Beds with fishes" that comprise the lower part of the Cassange series, formerly studied by Teixeira, have been critically redescribed. This includes (1) a freshwater hybodont shark now assigned to the genus Lissodlls. also known from the Cynognathus Zone of the Beaufort series in South Africa; (2) a new "paleoniscoid", Marquesia (previously assigned to Elonichthys), now regarded as a canobiid, a group with an apparent history extending from the Carboniferous ro the Triassic; (3) two species, one new, of the genus Perleidus which has otherwise an extensive distribution in lower Triassic marine sediments, although it occurs in the nonmarine ones at the Baixa do Cassange and Lunda; (4) a generalized neopterygian Angolaichthys. which is apparently unique to Baixa do Cassange and Lunda. It is the sole taxon in the somewhat higher, friable greenish shales level at Baixa do Cassange; (5) an apparently distinct lungfish Microceratodm. which does not show a demonstrably close relationship ro other Triassic ceratodonrids. The most common of these taxa are Marquesia. Perleidus lutoensis and Angolaichthys. All other taxa ate very scarce and are only known from the first localiry exploited by Mouta (earlier 1930 collecrion). In regard to the chronology of the Cassange series, the evidence of the fishes points clearly rowards a Triassic and possibly early Triassic age for the "Beds with fishes". The lithic and paleontological evidence suggests that the environment of preservation was a shallow, widely fluctuaring marginal freshwater lake area with shifring shorelines. The climate was presumably characterized by alternating wet and dry seasons, with the fishes dying during the dry phases, subsequently covered by sediment and so being preserved. The dispersal routes involving the Cassange series organisms remain unknown, as do most freshwater assemblages that inhabited the continent of Pangea during the late Paleozoic and early Triassic times.