Olson's Extinction and the latitudinal biodiversity gradient of tetrapods in the Permian

Abstract

The terrestrial vertebrate fauna underwent a substantial change in composition between the lower and middle Permian. The lower Permian fauna was characterized by diverse and abundant amphibians and pelycosaurian-grade synapsids. During the middle Permian, a therapsid-dominated fauna, containing a diverse array of parareptiles and a considerably reduced richness of amphibians, replaced this. However, it is debated whether the transition is a genuine event, accompanied by a mass extinction, or whether it is merely an artefact of the shift in sampling from the palaeoequatorial latitudes to the palaeotemperate latitudes. Here we use an up-to-date biostratigraphy and incorporate recent discoveries to thoroughly review the Permian tetrapod fossil record. We suggest that the faunal transition represents a genuine event; the lower Permian temperate faunas are more similar to lower Permian equatorial faunas than middle Permian temperate faunas. The transition was not consistent across latitudes; the turnover occurred more rapidly in Russia, but was delayed in North America. The argument that the mass extinction is an artefact of a latitudinal biodiversity gradient and a shift in sampling localities is rejected: sampling correction demonstrates an inverse latitudinal biodiversity gradient was prevalent during the Permian, with peak diversity in the temperate latitudes.

Keywords: Olson's Extinction; Permian; Tetrapoda; latitudinal biodiversity gradient; sampling bias.

Figure 1.

Rank-abundance distributions of the tetrapods from (a) late Kungurian equatorial Laurasian bin, (b) early Roadian equatorial Laurasian bin and (c) early Roadian temperate Laurasian bin. Legend in (a) also applies to (b).

Figure 2.

Subsampled diversity curves. (a) Comparing latitudinal bins and (b) comparison through time.