MESOZOIC MARINE REPTILE DISPARITY


bulbous dentition, and specialized anterior dentition for plucking sessile prey. Durophagous and benthic foraging specializa- tions are abundant in the shallow-marine reptiles, including Triassic sauropterygians, thalattosaurs and ichthyosauromorphs, Late Jurassic plesiochelyid turtles, and specialized Cretaceous mosasauroids and turtles. Deep-water facies, inhabited by open-ocean


marine reptiles, receive little terrigenous input and are expected to be more resource poor (Hudson et al. 1991; Benson and Butler 2011). Open-ocean marine reptiles would generally feed on nektonic prey, such as fast-moving cephalopods, fish, and tetrapods (Massare 1987), and therefore the absence of benthic foragers and durophages could explain reduced disparity. Hydrodynamic constraints on structural variation are also heightened in open oceans when feeding on fast-moving nektonic prey occurs (Taylor 1987). Extinction Intensity and Selectivity.—Selective


extinction and the lack of opportunistic ecospace refilling in the Jurassic meant that the Late Triassic extinctions had profound consequences for marine reptile evolution. Major marine regression through the late Carnian and Norian, followed by massive eruptions and ocean anoxia close to the Triassic/Jurassic boundary, resulted in widespread lineage extinction, and the associated decline in functional disparity and skull-size diversity identified here had long-lasting effects, with comparable levels of ecomorphological variation not recovered for another 100Myr. Low sample size in the long Norian bin and lack of material for the Rhaetian makes assessing the timing of the extinctions and loss of disparity difficult. However, the functionally distinct placodonts persisted into the Rhaetian (Rieppel 2002; Nordén et al. 2015), so disparity could have remained high until the Triassic/Jurassic boundary. Extinction victims were concentrated in themoderateto extreme negative regionsof PC1 in functional morphospace, an area associated with durophagous and benthic


foraging specializations. This supports the findings of Kelley et al. (2014), who, using categories, illustrated that the shallow-marine durophagous taxa were vulnerable to the


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geologically rapid regression of the Late Triassic. Our study also points toward size as an ecologically selective trait. Whereas most studies find selectivity against larger taxa (e.g., Friedman 2009), in this case it was small marine reptiles that did not transcend the extinction interval (Fig. 7). Major extinction events are predicted to


give rise to episodes of morphological diversification when ecological space is rapidly refilled during the recovery interval (Droser et al. 1997; Erwin 2008). Jurassic marine reptiles do not conform to this trend but instead appear to have passed through a macroevolutionary bottleneck (Thorne et al. 2011). Although marine reptile groups diversifying in the Early Jurassic achieved high numerical diversity (Thorne et al. 2011; Benson et al. 2012; Cleary et al. 2015) (Fig. 8), they failed to explore the same range of ecomorphological character- istics as extinction victims and were generally concentrated in a reduced range of adaptive zones (Figs. 3, 5, 7). Similar canalization was reported by Thorne et al. (2011) and Dick and Maxwell (2015) for ichthyosaurs based on morphological variation in whole-body skeletal characters and ecospace modeling. As previously noted, this could be attributed to variable diversification patterns in shallow- marine versus open-ocean environments. The diversifications of chondrichthyans and osteichthyans may have also impacted marine reptile macroevolution. For example, there are elevated origination rates in chondrichthyans during the Early Jurassic (Friedman and Sallan 2012), while others have highlighted greater potential competition in the small-bodied, durophagous, and benthic-foraging niches from actinopterygians such as Dapedium (Thorne et al. 2011; Smithwick 2015). There is some evidence for a loss of


ecomorphological diversity through the Jurassic/Cretaceous transition. Marine reptile functional disparity decreased, but statistical support for this decline is generally weak or absent (Fig. 3, Table 3). Representatives of all major groups passed through the extinction interval (Benson and Druckenmiller 2014), along with most functional morphotypes. Adaptive Radiations in the Marine Realm.— When separately diversifying clades are


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