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THOMAS L. STUBBS AND MICHAEL J. BENTON


presented with the same ecological opportunity, such as invading ocean ecosystems, evolution is expected to be strongly deterministic, leading to replicate adaptive radiations and repeated bursts of evolution (Schluter 2000). However, this study shows that invasion of a new habitat does not lead to identical macro- evolutionary trajectories in the most diverse Mesozoic marine reptile groups. The signatures of adaptive radiation are seen in sauropterygians, eosauropterygians, and ichthyosauromorphs, as shown universally by the early accumulation of lineage diversity, early high disparity, rapid proliferation of skull-size diversity, and strong support for early-burst maximum-likelihood models (Figs. 8, 9, Table 4). In contrast, an adaptive radiation model does not adequately describe patterns of evolution in thalattosuchians and mosasauroids. This suggests that the colonization of a new environment alone does not always serve as a catalyst for adaptive radiation and spectacular speciation events. Timing of diversification and biotic


conditions in the marine realm were likely key factors in the contrasting patterns of evolution in Mesozoic marine reptiles. As stated by Simpson (1944: p. 212) “The availability of a new adaptive zone does not depend alone on its physical existence … , but also on its being open to other occupants or so sparsely or marginally occupied that it involves no great competition.” When sauropterygians and ich- thyosauromorphs diversified in the aftermath of the PTME, the marine realm was largely devoid of competitors, leading to numerical and morphological diversifications along disparate ecological axes (Fig. 5) (Chen and Benton 2012). In contrast, thalattosuchians and mosasauroids originated free from any major biotic crises. Thalattosuchians initially diversified in the Early Jurassic (Young et al. 2010), ~20Myr after secondary Jurassic radiations gave rise to a diverse assemblage of plesiosaurian sauropterygians and neoichthyosaurs (Thorne et al. 2011; Benson et al. 2012). Mosasauroids diversified in the Cenomanian and Turonian, when plesiosaurs were represented by three ecologically distinct groups—the elasmosaurids, polycotylids, and


pliosaurids—and large predatory teleosts and sharks were important components of marine ecosystems (Massare 1987; Schwimmer et al. 1997; Everhart et al. 2010). Therefore, for thalattosuchians and mosasauroids, diversifying selection during the initial phase of evolution may not have been as strong, because there was less unoccupied niche space to fill. These divergent trends agree with the growing consensus on how ecological opportunity may operate to drive adaptive radiation, through both “niche availability” (e.g., marine-environment prey resource) and then “niche discordance,” which promotes phenotypic diversification into increased var- iance of niche-related traits (diverse diets and feeding ecologies) (Schluter 2001; Burbrink et al. 2012; Wellborn and Langerhans 2015). Clade duration and large-scale external


perturbations may also explain the dichot- omous macroevolutionary trends in Mesozoic marine reptiles. Sauropterygians and ichthyo- sauromorphs differ from mosasauroids and thalattosuchians in terms of evolutionary duration and terrestrial phylogenetic heritage. Thalattosuchians and mosasauroids represent shorter-lived and specialized offshoots from large, ancestrally terrestrial, clades (Bardet et al. 2014). On the other hand, ichthyosauromorphs and sauropterygians were long-lived marine clades, and their evolution was punctuated by major phylogenetic bottlenecks in the Late Triassic (Thorne et al. 2011). Mosasaur extinc- tion was particularly abrupt and seemingly premature, given that they maintained high diversity and disparity in the later parts of their history (Polcyn et al. 2014) (Fig. 8). Had mosasaurs passed through the Cretaceous– Paleogene extinction event and continued to exist with reduced diversity and disparity for a long interval, their macroevolutionary trends may have closely mirrored those of sauropterygians and ichthyosauromorphs. Direct tests for bursts of evolution character-


istic of adaptive radiation are relatively rare for other secondarily marine tetrapod groups. The Cenozoic witnessed the marine diversifications of sphenisciforms, hydrophiin snakes, cetaceans, sirenians, and pinnipedimorphs (Kelley and Pyenson 2015). These independent invasions of ocean ecosystems were broadly analogous to


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