MESOZOIC MARINE REPTILE DISPARITY


trophic food webs present in the Induan or Olenekian (Scheyer et al. 2014; Motani et al. 2015a)? Although this study was not designed to test these competing hypotheses, the results can still be used to inform this debate. Analyses show that marine reptile


functional disparity was already relatively high in the Olenekian, particularly in terms of var- iance,when ichthyosauromorphs were dominant contributors to overall disparity (Fig. 3). This result is intriguing, given low species diversity (Kelley et al. 2014), but it is not entirely unexpected when surveying the global diversity of forms. Olenekian marine reptiles had already broadly diversified into an array of feeding strategies, including suction-feeding and lunge-feeding ichthyosauromorphs such as Cartorhynchus and Hupehsuchus, potentially durophagous ichthyosaurs like Grippia,and early presumed fish-eating eosauropterygians like Corosaurus (Motani 1997; Rieppel 1998; Motani et al. 2015a,b). Disparity increased into the Anisian, when sauropterygians became dominant contributors to overall disparity (Figs. 3, 5), although the magnitude of increase was not statistically significant. A more substantial increase in the diversity of forms between the Olenekian andAnisian is seen in the skull-size data. The Carnian bin represents the disparity maximum, marginally greater than in the Anisian and latest Cretaceous bins (Fig. 3). No new higher clades diversified during the Carnian, nor was the disparity peak associated with a considerable expansion of morphospace occupation or shift in partial disparity contributions (Figs. 3, 5). It is likely that the high Carnian disparity resulted from the accu- mulated sample of functional extremes, such as large edentulous ichthyosaurs, shell-crushing placodonts (including the enigmatic Henodus), and bizarre heterodont thalattosaurs (e.g., Xinpusaurus). Overall, these patterns cannot exclude either hypothesis regarding the tempo of biotic recovery following the PTME, but there is tentative support for a delay until the Middle– early Late Triassic, when marine reptiles reached their full potential of ecomorphological diversity. Late Burst in Cretaceous Marine Reptiles.—This


study highlights the Late Cretaceous as a second exceptional interval in the trophic diversifications of Mesozoic marine reptiles.


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Functional disparity in the jaws and dentition of Campanian and Maastrichtian taxa equaled that of theMiddleand earlyLate Triassic (Fig. 3). This resulted from the diversification of disparate mosasauroids and turtles, coupled with the persistence of elasmosaurid and polycotylid plesiosaurs. Mosasaurs became increasingly ecologically disparate through their geological history, culminating in broad ecospace occupation in the Campanian and Maastrichtian (Ross 2009). The substantial depth distribution of mosasaurs in the water column meant they could exploit diverse benthic and pelagic prey (Polcyn et al. 2014). Turtles also achieved great taxonomic diversity in the Late Cretaceous and possessed functionally distinct robust jaws without teeth (Brinkman et al. 2006). Massare (1987) suggested that there was a reduction in marine reptile ecomorphological diversity between the Late Jurassic and Late Cretaceous, by examining dental morphotypes in the Kimmeridge Clay, Pierre Shale, and Niobrara Chalk faunas. However, this trend was not recovered here, most likely because the current study incorporates a greater diversity of taxa from global samples (including turtles) and also considers structural variation in the jaw. There is no substantive evidence for a


large-scale decline in marine reptile ecomorphological diversity prior to the end- Cretaceous. Therefore, any destruction brought about by the end-Cretaceous mass extinction was likely to have been geologically abrupt. To investigate further, future studies could focus on narrower sampling intervals and regional trends in the last 10Myr of the Mesozoic. Disparity, Lagerstätte Effects, and Shallow-


Marine Environments.—Exceptionally rich fossil lagerstätten can be overwhelming determinants in temporal patterns of taxonomic diversity, because atypically high preservation can generate artificial peaks (e.g., Butler et al. 2013). In Mesozoic marine reptiles, diversity in the Jurassic, and to some extent the Middle Triassic, is heavily affected by lagerstätte-dominated sampling (Benson et al. 2010; Benson and Butler 2011). The influences of “lagerstätte effects” on patterns of disparity are not well understood. Theoretically, formations with exceptional


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