Article: Small temnospondyl amphibians from the Middle Pennsylvanian of Illinois
Andrew R. Milner
The small temnospondyl amphibians from the Middle Pennsylvanian Francis Creek Shale of Mazon Creek near Morris, Illinois, are redescribed together with recently collected Francis Creek Shale specimens from Pit 11 near Braidwood, Illinois. A well-preserved larval specimen of the trimerorhachoid Saurerpeton cf. obtusum represents a new record for the 'Mazon Creek' fauna. It demonstrates the presence of three pairs of external gills and rows of ceratobranchial dental ossicles modified as gill rakers in a larval saurerpetontid. Amphibamus grandiceps Cope from Mazon Creek and the contemporary A. lyelli (Wyman) from Linton, Ohio, are shown to be distinct species which can be diagnosed on several size-independent and size-linked characters. Three of the specimens of small larval temnospondyls from Mazon Creek are identified as the larvae of A. grandiceps whilst two others are referred to the genus Branchiosaurus and, as such, constitute another new record for the 'Mazon Creek' fauna. The shared similarities of Amphibamus and Branchiosaurus support a hypothesis that the branchiosaurids are a monophyletic or polyphyletic group of neotenous dissorophoids most closely related to the Dissorophidae.The Francis Creek Shale fossil assemblages contain very few tetrapods, all of which appear to be transported erratics. They appear to have been mostly small terrestrial and pool-dwelling forms living on the levees of the delta-fan and having been inundated by a flood and washed into an offshore depositional area.