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1 September 2005 THE AMPHIBAMID MICROPHOLIS FROM THE LYSTROSAURUS ASSEMBLAGE ZONE OF SOUTH AFRICA
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Abstract
The small temnospondyl Micropholis stowi from the Lower Triassic Lystrosaurus Assemblage Zone (Karoo Basin, South Africa) is redescribed on the basis of a larger sample of specimens, revealing a range of previously unknown anatomical features. Micropholis is recognized as a member of the Amphibamidae, representing both the last occurrence and the only Gondwanan member of this clade. (1) The largest specimens have an elongated narrow snout, larger otic notches, prominent quadrate processes, and wide cheeks giving the skull a marked lateral curvature. (2) Palpebral ossifications include a fabric of numerous polygonal ossicles contouring the size and position of the eyeballs. (3) The palate houses large fangs with inwardly curving crowns, an elaborate basicranial region resembling closely that of Amphibamus and Doleserpeton, and an extremely reduced rod-like palatine and ectopterygoid approaching the derived condition seen in other amphibamids. (4) In the postcranial skeleton, the greatly extended transverse processes, the morphology of the interclavicle and scapulocoracoid, and the structure of the pelvis and tail are characteristic features of Micropholis some of which are quite similar to the condition in Eoscopus and Platyrhinops. It is evident that two morphs of Micropholis, which differ in skull width and palatal dentition, are present in the Karoo Basin.Phylogenetic analysis suggests Micropholis to be nested within the amphibamid dissorophoids as the basal-most taxon. Within the Amphibamidae, Doleserpeton and Amphibamus are sister groups, nested with successive sister taxa Platyrhinops and Eoscopus. In particular, the miniaturized genera Doleserpeton and Amphibamus share a range of derived character-states with Micropholis, but the latter lacks the pedicellate dentition and has a more plesiomorphic posterior skull table and parasphenoid, suggesting that some of the shared derived states may have been acquired independently during the long isolated evolution of the Micropholis lineage in the Southern Hemisphere. In the present analysis, Micromelerpeton is the most basal offshoot of the dissorophoids, followed by a grade towards amphibamids formed by the following successive sister groups (from the base crownwards): (1) the trematopids, (2) Ecolsonia, and (3) the Dissorophidae, the latter being the sister taxon of all analyzed amphibamids.
RAINER R. SCHOCH and BRUCE S. RUBIDGE "THE AMPHIBAMID MICROPHOLIS FROM THE LYSTROSAURUS ASSEMBLAGE ZONE OF SOUTH AFRICA," Journal of Vertebrate Paleontology 25(3), (1 September 2005). https://doi.org/10.1671/0272-4634(2005)025[0502:TAMFTL]2.0.CO;2
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