MESOZOIC MARINE REPTILE DISPARITY


Kelley et al. 2014). A second, less severe extinction interval has been posited for the Jurassic/Cretaceous boundary (Bardet 1994), although recent analyses show weak support for an exceptional loss of diversity, but point rather to a “thinning out” of major clades, such as thalattosuchians, pliosaurids, and cryptoclidid plesiosauroids (Benson and Druckenmiller 2014). While these proposed extinction intervals are defined by losses of taxonomic diversity, they could also have had profound impacts on ranges ofmorphology and ecology exhibited by surviving taxa (Bapst et al. 2012; Ruta et al. 2013). Extinctions can be nonrandom and highly selective, with particular clades, ecologies, morphotypes, or adaptive zones being susceptible to biotic perturbations (Friedman 2009; Korn et al. 2013). By dividing Triassic and Early Jurassic marine reptiles into broad ecotypes, Kelley et al. (2014) demonstrated that shallow-marine durophages were most susceptible to sea-level fluctuations during the Late Triassic, while pelagic forms were more resilient and able to avoid widespread extinction. Quantitative studies have focused on individual marine reptile clades. For example, Thorne et al. (2011) revealed a massive loss of ichthyosaur disparity during the Late Triassic/Early Jurassic transition, and showed that surviving taxa occupied different morphospace than Triassic forms. By expanding on this numerical approach and considering marine reptiles as an adaptive assemblage, we assess what ecomorphological traits are characteristic of extinction victims through the Mesozoic and test for evidence of infilling in vacant adaptive zones by different marine reptile groups.


Materials and Methods Taxon Sampling.—This study represents


the largest comparative and quantitative investigation of phenotypic evolution in Mesozoic marine reptiles. In total, 206 marine reptile species are used to investigate trends of functional disparity in the jaws and dentition (Supplementary Data), and 354 species are included in the analyses of skull-size trends


549


(Supplementary Data). Individual specimens represent each species, and taxa range in age from the Olenekian to the end Maastrichtian. Five monophyletic clades are included: the sauropterygians, ichthyosauromorphs, thalattosaurs, thalattosuchian crocodylomorphs, and mosasauroids. Additionally, we sample marine representatives from clade Testudinata, incorporating the stem turtle Odontochelys; Jurassic plesiochelyids; and the Cretaceous clade Chelonioidea. In the primary data set we also include a number ofsmaller clades andindividual generaknowntohave inhabited Mesozoicmarine environments: tanystropheids, saurosphargids, Helveticosaurus and Qianosuchus from the Triassic, and nonmosasauroid squamates from the Cretaceous. Late Jurassic pleurosaurids and Cretaceous marine snakes, dyrosaurids, and pholidosaurids were not included in the data sets, due to a lack of material. When testing for adaptive radiation based on time-series trends and evolutionary rates, investigation is focused on the five most diverse clades: Sauropterygia, Eosauropterygia (Sauropterygia minus Placodontia), Ichthyosauromorpha, Thalattosuchia, and Mosasauroidea. Separate analyses were performed for Eosauropterygia to account for the aberrant morphology of placodonts. Each of these clades could be effectively sampled on an individual basis in terms of both phylogenetic and strati- graphic coverage. The thalattosaurs, turtles, tanystropheids, and saurosphargids could not be incorporated into these comparisons because of smaller sample sizes and sporadic stratigraphic occurrences. Functional Disparity.—When examining


functional disparity, focus is placed on quantifying the diversity of forms and innovations that have known ecomorphological and/or biomechanical consequences and are therefore directly associated with resource use and the acquisition and processing of prey (e.g., Anderson et al. 2011, 2013; Stubbs et al. 2013; Button et al. 2014). Functional and/or biomechanical traits are primarily derived from themandible and dentition. Figure 1 illustrates a range of marine reptile jaws, highlighting the great diversity of forms. Here, functional disparity was assessed using a combination of continuous measured


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