Abstract
The Permian–Triassic mass extinction (PTME) had an enormous impact on life in three ways: by substantially reducing diversity, by reshuffling the composition of ecosystems and by expelling life from the tropics following episodes of intense global warming. But was there really an ‘equatorial tetrapod gap', and how long did it last? Here, we consider both skeletal and footprint data, and find a more complex pattern: (i) tetrapods were distributed both at high and low latitudes during this time; (ii) there was a clear geographic disjunction through the PTME, with tetrapod distribution shifting 10–15° poleward; and (iii) there was a rapid expansion phase across the whole of Pangea following the PTME. These changes are consistent with a model of generalized migration of tetrapods to higher latitudinal, cooler regions, to escape from the superhot equatorial climate in the earliest Triassic, but the effect was shorter in time scale, and not as pronounced as had been proposed. In the recovery phase following the PTME, this episode of forced range expansion also appears to have promoted the emergence and radiation of entirely new groups, such as the archosaurs, including the dinosaurs.
Highlights
The Permian–Triassic mass extinction event (PTME) was the most dramatic crisis experienced by life on Earth [1,2,3], and its devastating effects were felt on land and in the sea (e.g. [4,5,6,7,8,9,10,11])
The PTME was expressed in three ways in its effects on tetrapods: first by the sharp extinction itself, and the slow recovery thereafter; second by a deep reshuffling in the composition of ecosystems [8]; and third by the so-called ‘equatorial tetrapod gap’ [12], whereby most fossil occurrences are at high latitudes, and fishes and tetrapods had apparently been driven away from the overheated tropics
The biological impact of these poleward migrations has not been explored. These large-scale forced migrations could have played a crucial role in the recovery of life after the PTME, but such hypotheses require clarity on the timing and nature of the geographic upheavals: were the forced migrations equal to north and south? Was it one event or many? How long did the tropical expulsions last? And how did they contribute to the major biotic transitions occurring at the time?
Summary
The Permian–Triassic mass extinction event (PTME) was the most dramatic crisis experienced by life on Earth [1,2,3], and its devastating effects were felt on land and in the sea (e.g. [4,5,6,7,8,9,10,11]). The PTME was expressed in three ways in its effects on tetrapods: first by the sharp extinction itself, and the slow recovery thereafter; second by a deep reshuffling in the composition of ecosystems [8]; and third by the so-called ‘equatorial tetrapod gap’ [12], whereby most fossil occurrences are at high latitudes, and fishes and tetrapods had apparently been driven away from the overheated tropics. We test the hypothesis that major warming episodes associated with the PTME and with several Early Triassic isotopic spikes drove land life away from the tropics. We consider all skeletal and footprint records of tetrapods, and analyse their latitudinal distribution across the Permian –Triassic boundary (PTB) from the Middle Permian (Guadalupian) to the Middle Triassic with the aim of exploring the signature left by the PTME on the palaeobiogeography of land vertebrates. Taking into consideration the possible biases involved, we demonstrate a pronounced latitudinal shift in tetrapod distributions across the PTB, and that there was no long-term equatorial tetrapod gap in the Early Triassic
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