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THOMAS L. STUBBS AND MICHAEL J. BENTON


long-necked and fanged nothosaurs (Rieppel 2002), bizarre heterodont thalattosaurs (Benton et al. 2013), and enigmatic apparent filter-feeders, such as Atopodentatus (Cheng et al. 2014). These disparate dietary habits were associated with an exceptional array of ecomorphological specializations, most linked to feeding and prey acquisition. Recent research has emphasized that a


broad-based ecological approach is essential for understanding marine reptile evolution (Motani et al. 2015a). Thus far, studies have focused on body-size trends and trophic web establishment in the first 5–10Myr of the Mesozoic (Liu et al. 2014; Scheyer et al. 2014), while others have examined the proportions of categorical ecomorphs and guilds through the whole Triassic (Benton et al. 2013; Kelley et al. 2014; Motani et al. 2015a). However, no study has examined whether the Triassic was truly a time of unusual trophic proliferation or whether the accumulated diversity of forms was any greater than at other points in the Mesozoic – was the first wave of marine reptile evolution in the aftermath of the PTME really exceptional? A robust quantitative approach is required to test this, and trends of marine reptile ecomorphological diversity through the whole Mesozoic must be quantified. In this study, we focus on two elements of phenotypic diversity. We quantify patterns of functional disparity in the jaws and dentition, focusing on the diversity of forms associated with feeding and prey acquisition (Anderson 2009; Anderson et al. 2011; Stubbs et al. 2013), and separately examine the diversity of skull sizes. The transitions from land to sea in Mesozoic


marine reptiles represent excellent case studies for exploring the macroevolutionary consequences of ecological opportunity. Invasion of new habitat opens up previously unexplored niche space and is considered one of the major ecological opportunities in nature (Schluter 2000). Such opportunities are the primary catalysts for bursts of phenotypic evolution and adaptive radiation (Simpson 1953; Yoder et al. 2010). Mesozoic marine reptiles are a polyphyletic assemblage, made up of species from independent evolutionary invasions of the marine realm from different parts of the neodiapsid tree (Bardet et al. 2014).


These individual diversifications provide a chance to contrast patterns of evolution associated with the ascent of several distinct clades in response to the same ecological opportunity. As ecological opportunity drives adaptive radiation, one could hypothesize that niche filling or “early-burst” diversifica- tion patterns should be universal in marine reptiles. However, there are a number of confounding factors. For example, not all marine reptiles diversified simultaneously. Do clades originating in the immediate aftermath of major extinction events show similar trajectories and rates of evolution as those diversifying at times when there were no major biotic perturbations? There are two commonly applied quantita-


tive methods to identify the signatures of adaptive radiation in deep-time data. Most attention has focused on examining time-series trends of lineage diversity and morphological disparity (e.g., Erwin 2007; Ruta et al. 2013; Hughes et al. 2013). When maximum numerical and morphological diversity is seen early in a clade’s history, this indicates early diversification and morphospace expansion and can be used to infer an adaptive radiation (Benton et al. 2014). Recently, focus has switched to explicitly modeling evolutionary rates in ecologically relevant traits in a phylogenetic framework (e.g., Benson et al. 2014). Under a model of adaptive radiation, rapid evolutionary rates are seen early in a clade’s history, and these decelerate through time, a trend formalized as the “early-burst” model (Harmon et al. 2010). Here, we use a comparative approach to explore trends of lineage diversity, functional disparity, skull-size diversity, and rates of evolution in the most diverse marine reptile clades to test the prediction of universal early high diversity and disparity and early burst. Two major turnover events are believed to


have significantly impacted Mesozoic marine reptile macroevolution. The first occurred during the Late Triassic, when species diversity crashed and whole clades and morphotypes disappeared, including placodont sauroptery- gians, nothosaurs, thalattosaurs, and all non- parvipelvian ichthyosaurs (Benson et al. 2010; Benson and Butler 2011; Thorne et al. 2011;


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