Climate, competition, and the rise of mosasauroid ecomorphological disparity

Abstract

Mosasauroidea, prominent marine lizards (Squamata, Toxicofera) of the final 30 million years of the Cretaceous, have been extensively studied for their morphology, ecology and systematics in the past two centuries. However, the relative roles of biological and physical processes as drivers of their morphological diversification remain uncertain. Here we investigate the macroevolution of mosasauroid feeding and locomotory disparity using continuous characters measured from the mandible and forelimb as proxies. Patterns of morphospace occupation demonstrate important roles for innovation and niche partitioning in driving morphological disparity. The early evolution of Mosasauroidea is characterized by large shifts in morphology, especially elongation of the mandibular biting area and hydropedality. The later diversification of derived Mosasaurinae and Plioplatecarpinae is associated with a great expansion of morphospace, attributed to the acquisition of novel feeding and locomotory strategies. Temporally, disparity follows a top‐heavy profile, possibly reflecting opportunism in the wake of the Cenomanian–Turonian anoxic event. The highest levels of disparity are found in the latest Cretaceous, associated with the radiation of derived mosasaurids alongside the persistence of more basal forms. Major morphological innovations are not associated with evolutionary rate shifts, which differentiates them from earlier marine reptiles, and may reflect constant and greater niche occupation in Late Cretaceous oceans. Linear modelling of potential physical drivers indicates a minor role for these processes, suggesting that biological drivers were the primary sculptors of mosasauroid morphological disparity.

Global ecomorphological restructuring of dominant marine reptiles prior to the Cretaceous–Palaeogene mass extinction

“…This time interval witnessed the expansion of ecomorphospace occupation by longirostrine and brevirostrine mosasaurid ecomorphologies, in addition to the diversification of ‘grasping’ halisaurines and plioplatecarpines (figure 2A + D). In studies using coarser temporal and geographic frameworks [7], this would result in high mosasaurid disparity up to the K/Pg extinction. Our time-binning approach recovers a within-Maastrichtian decrease in global γ-disparity and α-disparity, in nearly all regions and across all clades.…”

“…Understanding patterns such as these are vital for the accurate interpretation of faunal dynamics and functional variation before and after extinction events. If only γ-disparity of mosasaurids were considered, then this group could be simply interpreted as experiencing a global ecomorphological decline just prior to their ultimate demise at the K/Pg boundary (or not in decline at all if a single and global ‘Maastrichtian’ bin was used; see [7]). However, it is apparent that, when both α- and β-disparities are taken into account, some regional communities were most certainly declining in taxonomic diversity and ecomorphological disparity, whereas others were only minimally affected on both counts.…”

“…Ichthyosaurians and pliosaurids were long gone by the Maastrichtian [4,5,52], restricting the predominant marine reptile groups to xenopsarian plesiosaurians, mosasaurids, chelonioids and crocodylomorphs [8]. These clades do not seem to follow a disparity pattern similar to that recovered here or elsewhere [7] for mosasaurids. Indeed, polycotylid plesiosaurians were already in decline in both phylogenetic diversity and ecomorphological disparity during the Campanian-Maastrichtian interval [22], with only two species potentially being present during the Maastrichtian [22,53].…”

“…Skull and jaw material from 93 mosasaurid specimens were collected, representing 56 species and all subfamilies and tribes [6,18]. Previous studies have focussed on mosasaurid mandible form and function [3,7]; here we chose an approach quantifying function across the entire skull. Due to the current lack of a phylogeny resolving the placement of all taxa represented in this study, and because our analyses focus on global to regional patterns, we opted against the inclusion of a composite phylogenetic tree in favour of broad-scale clade assignments.…”

“…Marine ecosystems were dominated by reptiles during the entire Mesozoic [1][2][3]. Despite important turnovers at its base [4,5], the Late Cretaceous is no exception, as mosasaurid squamates rapidly diversified [6,7], achieving a cosmopolitan distribution prior to the Campanian (ca 83.5 Mya) [6,8], and colonized several ecological guilds until their global extinction at the Cretaceous-Palaeogene (K/Pg) boundary mass extinction (66 Mya) [7,9]. Prior to the Campanian, mosasaurid taxonomic richness saw a steep increase [6], with speciation in the Western Interior Seaway (WIS) in central North America triggering a diversification during the socalled 'Niobraran Age' [10,11].…”