MESOZOIC MARINE REPTILE DISPARITY Both sauropterygians and ichthyosaur-


omorphs have bottom-heavy disparity pro- files, with greatest disparity present in the early stages of their history and reduced disparity in later intervals. In ichthyosaur- omorphs, this trend is still recovered when the enigmatic Cartorhynchus and Hupehsuchus are removed (based on Ichthyosauria only). For sauropterygians, this result is consistent when analyses are performed on eosauropterygians only (minus placodonts), but the magnitude of difference between the early and later intervals is far less (Fig. 8). Clades that diver- sified in the Jurassic and Cretaceous, the thalattosuchian crocodylomorphs and mosa- sauroids, have more top-heavy disparity profiles, with reduced disparity in earlier intervals and greatest disparity later in their durations (Fig. 8). Temporal trends of skull-size evolution also


show early bursts of evolution in sauropter- ygians and ichthyosauromorphs (Fig. 8). In both clades, an explosive diversification gave rise to a great range of sizes in the early stages of their evolutionary history (note the logarithmic scale). Ichthyosauromorphs show a marked canalization of sizes through time, while sauropterygian evolution generally becomes concentrated around exploring larger forms, spread over a greater time span. In contrast to disparity trends, a relatively high range of sizes is shown by thalattosuchians during their initial diversification; however, maximum range is not seen until the Late Jurassic, ~30Myr after the clade’s origin. Closely corresponding with morphological disparity trends, a limited range of generally


smaller forms is seen in mosasauroids for the first half of their history, before reaching a stable higher range for the second half of their evolution (Fig. 8). Clade-Specific Evolutionary Models.—Early-


burst maximum-likelihood models receive overwhelming support for sauropterygians, eosauropterygians, and ichthyosauromorphs, but not for mosasauroids and thalattosuchians (Fig. 9, Table 4). In the three clades that diversified during the Triassic biotic recovery, the early-burst model and/or delta model (with early high rates: δ<1) have significantly greater AICc weights for all phenotypic


563


variables (morphospace axes and skull size) (Fig. 9, Table 4). In contrast,BMmodels receive little support in these three clades. In mosasauroids, models associated with rate heterogeneity have generally low AICc weights and are poorly supported, while the BMmodel is favored. ForPC axis 2, the favored delta model is consistent with high rates of evolution later in the clade’s history (δ>1); the opposite of early burst (Ingram et al. 2012). The EB model is best supported for skull-size evolution in mosasauroids, but this is not decisive. In Thalattosuchia, EB model support is mixed. Early burst and/or delta are clearly favored for skull-size evolution, but a BM model is best supported for functional evolution in the jaws and dentition (Fig. 9, Table 4). When dating trees with 1Myr minimum branch lengths enforced, the results are the same (Supplementary Table 4).


Discussion Early EcomorphologicalDiversification in Triassic


Marine Reptiles.—As an adaptive assemblage, Triassic marine reptiles were one of the true success stories to arise in the devastated postextinction oceans. The first wave of marine invasion by reptiles in the Triassic was associated with an exceptional burst of ecomorphological diversity (Figs. 3, 5, 7). Taken together, patterns of functional disparity, morphospace occupation and skull- size evolution, show that the Middle to early Late Triassic was not just a time of marked proliferation in terms of species numbers (e.g., Benson et al. 2010; Benson and Butler 2011), but also a time of explosive phenotypic evolution— something that had only previously been noted speculatively based on qualitative observations (Fröbisch et al. 2013; Liu et al. 2014). In the first 10–20Myr of the Triassic, diversifying lineages explored the greatest breadth of functional morphospace (Fig. 5), and the accumulated functional disparity represented the maximum in the Mesozoic (Fig. 3). Similarly, the Triassic witnessed the greatest disparity of skull sizes, with the full range of potential forms realized by one clade or another (Fig. 7).


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