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THOMAS L. STUBBS AND MICHAEL J. BENTON


history and exponentially decreasing rates through time (Harmon et al. 2010). In EB, rates are constrained to exponential deceleration. BM and EB are the most commonly contrasted models when trying to identify the quantitative signatures of niche filling during adaptive radiation. Pagel’s δ is a time-dependent model that fits the relative contributions of early versus late evolution in a clade’s history by elaborating upon BM with an additional phylogenetic rescaling parameter “δ” (Sallan and Friedman 2012). When Pagel’s δ<1, trait disparity and rapid morphological change are concentrated early in a clade’s history, mimicking early burst (Ingramet al. 2012). The sample size–corrected Akaike’s information criterion (AICc) and Akaike weights were used to identify the best-fitting model. The average and range of AICc values and Akaike weights from each analysis, based on the randomly resolved time-calibrated topologies, are reported, in addition to the relevant para- meter values for favored models.


Results Marine Reptile Morphospace.—Marine reptile


functional morphospace visualizes the phenotypic similarity of the sampled taxa based on the functional data from the jaws and dentition (Fig. 2). Statistical tests measuring the strength of association between the ordination axes and the functional characters are summarized in Supplementary Table 2. Here, discussion and visualization is limited to patterns on the first three multivariate coordinate axes. Together, PC1–PC3 account for 50% of


absolute variance and encapsulate many significant traits (Fig. 2). PC1 subsumes 27.4% of absolute variance and 34.7% of variation when only axes with positive eigenvalues are considered. PC1 is most heavily influenced by dental specializations. Taxa with high nega- tive PC1 scores either possess tooth plates (placodonts), are entirely edentulous (e.g., turtles), or have bulbous crushing dentition (e.g., mixosaurid ichthyosaurs and thalattosaurs). Conversely, taxa with low positive PC1 scores have pointed and recurved tooth crowns, and those at the extreme positive


end of PC1 have specialized cutting dentition (Massare 1987). Most other characters also have strong association with positioning along PC1, with the notable exception of jaw depth, area for muscle attachment, and anterior mechanical advantage, suggesting there is marked variation in robustness and potential bite force along this axis. PC2 accounts for 12.1% of overall absolute variance and 15.3% of variation when only axes with positive eigenvalues are considered. Variation in mandibular robustness and mechanical advantage is strongly expressed on PC2. Taxa with moderate to high negative PC2 scores have very slender jaws, with restricted regions of attachment for the adductor muscles and low anterior mechanical advantage. Taxa with increasingly positive PC2 scores have progressivelymore robust jaws, with increased area for muscle attachment and higher anterior mechanical advantages. PC3 encapsulates a further 10.7% of absolute variance and 13.5% of variation from positive eigenvalues only. This axis appears to specifically isolate taxa with specialized dentition in the anterior part of the dentary, such as enlarged anterior fangs or procumbent chisel-like anterior teeth. A number of noteworthy trends are evident


from the distribution of marine reptiles in functional morphospace. Sauropterygians occupy an extensive area, but many forms cluster centrally (Fig. 2). The sauropterygians shown to be distinct from all other groups in PC1–PC2 morphospace are the highly specialized placodonts. Most ichthyosaur- omorphs have restricted morphospace occupation along PC2, with slender jaws and low mechanical advantages, but show wide distribution along PC1. Thalattosaurs are con- centrated in areas of weakly negative PC1 scores, but a number of disparate forms branch out from this region. Turtles have restricted morphospace occupation, most likely due to lack of dental diversity (all edentulous, with the exception of Odontochelys) and similar geometric configuration of the mandible— generally robust with a small or absent retroarticular process. In both plots, most mosasauroids are concentrated together, potentially constrained by the presence of an intramandibular joint and unfused mandibular


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