Permo--Carboniferous Paleoecology and Morphotypic Series

The genera of primitive and synapsid reptiles which form theusual morphotypic series leading to cynodonts are not membersof a phyletic series. Ecological analyses show three developingevolutionary lines during the Permo-Carboniferous—onelowland, living on deltas and in swamps; a second somewhat moreupland; and a third distinctly upland. The members of thesethree lines are the source of the morphotypic genera, but mostof them come from deposits formed in lowlands where they cameby successive invasions from moreupland habitats. Evidence ofthe upland lines comes from these lowland sites, from invadinganimals, and from those introduced by mechanical transport fromtheir upland habitats. The brain cases and masticatory structures ofthe members ofthe morphotypic series—Hylonomus, Haptodus, Varanosaurus,Ophiacodon, Dimetrodon, Eotitanosuchus, Scymnognathus, Lycosuchus,Thrinaxodon—are examined in light of the ecological interpretations.Deformed coordinates applied to the lateral aspects of the skullsof the genera show clearly that the morphotypes do not providea coherent evolutionary series. The "trends" of evolutionarychange from Hylonomus to Thrinaxodon can best be seen if thesetwo and Haptodus are used as an evolutionary series, the stagesthat are missing interpolated, and the roles ofthe other generaevaluated on the basis of this more or less idealized, conceptualseries.