Climatic drivers of latitudinal variation in Late Triassic tetrapod diversity

Abstract

The latitudinal biodiversity gradient (LBG), the increase in biodiversity from the poles to the equator, is one of the most widely recognized global macroecological patterns, yet its deep time evolution and drivers remain uncertain. The Late Triassic (237–201 Ma), a critical interval for the early evolution and radiation of modern tetrapod groups (e.g. crocodylomorphs, dinosaurs, mammaliamorphs), offers a unique opportunity to explore the palaeolatitudinal patterns of tetrapod diversity since it is extensively sampled spatially when compared with other pre‐Cenozoic intervals, particularly at lower palaeolatitudes. Here, we explore palaeolatitudinal patterns of Late Triassic tetrapod diversity by applying sampling standardization to comprehensive occurrence data from the Paleobiology Database (PBDB). We then use palaeoclimatic model simulations to explore the palaeoclimatic ranges occupied by major tetrapod groups, allowing insight into the influence of palaeoclimate on the palaeolatitudinal distribution of these groups. Our results show that Late Triassic tetrapods generally do not conform to a modern‐type LBG; instead, sampling‐standardized species richness is highest at mid‐palaeolatitudes. In contrast, the richness of pseudosuchians (crocodylians and their relatives) is highest at the palaeoequator, a pattern that is retained throughout their subsequent evolutionary history. Pseudosuchians generally occupied a more restricted range of palaeoclimatic conditions than other tetrapod groups, a condition analogous to modern day reptilian ectotherms, while avemetatarsalians (the archosaur group containing dinosaurs and pterosaurs) exhibit comparatively wider ranges, which is more similar to modern endotherms, such as birds and mammals, suggesting important implications for the evolution of thermal physiology in dinosaurs.

Body Size Disparity of the Archosauromorph Reptiles during the First 90 Million Years of Their Evolution

“…A recent study on the palaeobiogeographic distribution of Late Triassic tetrapods supported this idea for nonpseudosuchian archosaurs because they had a palaeobiogeographic distribution similar to that of extant endothermic animals (Dunne et al, 2020). By contrast, pseudosuchians were more diverse in warmer environments, as is the case for extant ectothermic reptiles (Dunne et al, 2020). This study also concluded that palaeoclimate could have had a direct influence on the distribution of species during that time span.…”

“…There is evidence for a relationship between body size and palaeolatitudinal variation, where the evolution of body size follows a punctuated model of evolution consistent with periods of environmental change, for crocodylomorphs (Stockdale & Benton, 2021). A recent study on the palaeobiogeographic distribution of Late Triassic tetrapods supported this idea for nonpseudosuchian archosaurs because they had a palaeobiogeographic distribution similar to that of extant endothermic animals (Dunne et al, 2020). By contrast, pseudosuchians were more diverse in warmer environments, as is the case for extant ectothermic reptiles (Dunne et al, 2020).…”

Description of the metoposauridAnaschisma brownifrom the New Oxford Formation of Pennsylvania

“…In this vein, the recent report of metoposaurid “osteoderms” from Zimbabwe (~40°S) by Barrett et al (2020) warrants some discussion here. This is an intriguing novel occurrence, not yet addressed by temnospondyl workers, that has already been propagated by several studies (Dunne et al, 2020; Sciscio et al, 2020) and has been utilized as a line of evidence for broader claims, such as biome analogy with geographically disparate deposits (Barrett et al, 2020). However, there are several reasons to exercise caution with this purported occurrence.…”

“…Consequently, the mechanisms that contributed to the development of a metoposaurid zone of exclusivity by the latest Carnian (Otischalkian LVF) in North America remain unclear. Climatic variation along paleolatitudinal gradients (e.g., Sues and Olsen, 2015) and climatic instability (e.g., Whiteside et al, 2015) are popular hypotheses invoked for explaining the distribution of amniotes (e.g., Dunne et al, 2020), such as the absence of the herbivorous sauropodomorph dinosaurs, but hypotheses specifically regarding the distribution of large-bodied stereospondyls have not been formulated. A large body of recent work has emerged examining the role of the Carnian Pluvial Episode (CPE) in shaping terrestrial tetrapod evolution, but few have explicitly addressed temnospondyls.…”

“…13), as demonstrated by the central European and Indian populations, which bracket this range. Other stereospondyls are known from higher paleolatitudes in both hemispheres, particularly brachyopoids, although Late Triassic temnospondyls as a whole were more abundant at the lower latitudes where presumably seasonality was less pronounced (Dunne et al, 2020), much like lissamphibians today (Pyron and Wiens, 2013). The North American metoposaurid-bearing basins, specifically those in the southwestern United States, would have been the closest to the equator; the Argana Basin in Morocco lay intermediate between these basins and those of central Europe.…”