The ecology and geography of temnospondyl recovery after the Permian-Triassic mass extinction.

Abstract

One of the mysteries of the Permian-Triassic mass extinction was the subsequent success of temnospondyls. Temnospondyls were key early tetrapods in the Carboniferous and Permian and hardly seem to be ideal pioneers in a tough post-extinction world. Did they survive because of some unusual adaptations or by occupying some limited part of the world? We explore temnospondyl success in the Triassic by comparing their functional ecomorphology and palaeogeographic distributions. We find that Early Triassic temnospondyls exhibited all skull sizes and shapes, reflecting a wide diversity of feeding modes: abundant parabolic-snouted forms, and less common longirostrine (long-snouted) and insectivorous (short-skulled) forms. In fact, morphospace occupation by temnospondyls increased dramatically from Late Permian to Early Triassic, and then decreased in the Middle Triassic, but without emphasis on one feeding mode or another. Nor is there any evidence for unusual patterns of evolution: Temnospondyli and subclade Trematosauria follow an Ornstein-Uhlenbeck evolutionary model, suggesting evolution towards a common skull shape. Metoposauroidea, Brachyopoidea and basal Stereospondyli evolved by the stasis model. Further, these Early Triassic temnospondyls did not occupy a limited part of the world; they show temperate distributions, but with some specimens in equatorial regions, contradicting the idea of a permanently impermeable tropical dead zone.