Evolution of ecospace occupancy by Mesozoic marine tetrapods

Abstract

Ecology and morphology are different, and yet in comparative studies of fossil vertebrates the two are often conflated. The macroevolution of Mesozoic marine tetrapods has been explored in terms of morphological disparity, but less commonly using ecological‐functional categories. Here we use ecospace modelling to quantify ecological disparity across all Mesozoic marine tetrapods. We document the explosive radiation of marine tetrapod groups in the Triassic and their rapid attainment of high ecological disparity. Late Triassic extinctions led to a marked decline in ecological disparity, and the recovery of ecospace and ecological disparity was sluggish in the Early Jurassic. High levels of ecological disparity were again achieved by the Late Jurassic and maintained during the Cretaceous, when the ecospace became saturated by the Late Cretaceous. Sauropterygians, turtles and ichthyosauromorphs were the largest contributors to ecological disparity. Throughout the Mesozoic, we find that established groups remained ecologically conservative and did not explore occupied or vacant niches. Several parts of the ecospace remained vacant for long spans of time. Newly evolved, radiating taxa almost exclusively explored unoccupied ecospace, suggesting that abiotic releases are needed to empty niches and initiate diversification. In the balance of evolutionary drivers in Mesozoic marine tetrapods, abiotic factors were key to initiating diversification events, but biotic factors dominated the subsequent generation of ecological diversity.

The dimensionality and structure of species trait spaces

“…The identification of ecological disparity, gaps, distinctiveness or uniqueness in trait spaces is a long‐standing issue in ecology and evolution (Bapst et al, 2012; Foote, 1990; Gauzere et al, 2020; Ricklefs, 2005; Violle et al, 2017; Winemiller, 1991). It contributes, for instance, to estimate the level of functional insurance and vulnerability to species extinction (Mouillot et al, 2014) but also to better understand the influence of trait rarity on ecosystem functioning (Maire et al, 2018), to set conservation priorities targeting unique species (Loiseau et al, 2020) and to illuminate the capacity for innovation in clades (Cornwell et al, 2014; Deline et al, 2018; Reeves et al, 2020). Yet there is no consensus on the way to determine which species are isolated enough in trait spaces to be considered as unique species.…”

“…This result resonates with the saturating link between ecological disparity and species richness across geological periods (Bapst et al, 2012) contrary to predictions from theory on adaptive radiations and ecological speciation (Rundell & Price, 2009). More precisely, some entire lineages remained ecologically conservative throughout the Mesozoic without exploring vacant portions of trait space, and then trait bursts occurred owing to changing abiotic conditions during the Late Jurassic (Reeves et al, 2020). Both adaptive radiations due to species interactions and innovative solutions to face new environments are certainly at play to explain the invariant saturating scaling of ecological uniqueness with species richness.…”

Ecomorphological diversification of squamates in the Cretaceous

“…These suggest similar sizes to those seen from the Late Jurassic and Early Cretaceous and suggest that squamates remained small through all that time span. The expansion of marine squamates, epitomized by mosasauroids, in the early stages of the Late Cretaceous represents a major evolutionary radiation [ 10 , 33 ], driving squamates to sizes not seen before, and not seen since the eventual extinction of mosasauroids at the end-Cretaceous. This, once again, points to a decoupling between diversity and disparity expansions.…”

“…The fossil record [ 2 , 3 , 7 – 9 ] and phylogenomic studies [ 4 – 7 ] point to an increase in diversity of squamates in the Late Cretaceous, some 84 Ma. This diversification also included a major marine group, the mosasauroids, which became ecologically abundant predators in shallow seas around the world, and some reached huge sizes, before their extinction at the end of the Cretaceous [ 10 ]. Modern squamate clades continued to diversify through the Late Cretaceous and in the Palaeogene, after the end-Cretaceous mass extinction 66 Ma.…”

Evolution of the skull shape in extinct and extant turtles

“…left by other marine lineages and access to different sorts of resources (e.g Stubbs & Benton 2016;Reeves et al 2020),. which could have been important triggers of morphological innovation.For instance, the great similarity between the shape regression scores of the protostegid Rhinochelys cantabrigiensis and the spongivorous cheloniid Eretmochelys imbricata suggests the acquisition of a comparable durophagous specialisation early in sea turtle evolution.…”