MESOZOIC MARINE REPTILE DISPARITY
and evolution is concentrated in a narrower adaptive zone (Fig. 5). Ancillary Disparity Results.—Disparity trends
are consistent when alternative protocols and data subsets are used (Fig. 6). Based on mean pairwise dissimilarity from the Gower intertaxon distance matrix, temporal patterns follow those fromthe primary sumof variances results (Fig. 6A). Disparity trends based on continuous characters only also match those of theprimary analyses.Onceagain disparity peaks in the Middle and early Late Triassic. This trend is recovered in bothmetrics andwith the inclusion/exclusion of total mandibular length (Fig. 6B–E). Disparity dynamics through the Triassic/Jurassic transition and Jurassic/ Cretaceous transition follow those described from the primary data set; the only exception is a lack of overlap between the confidence intervals associated with the Jurassic/ Cretaceous transition in the sum of variances (Fig. 6B,C). The only marked difference between the primary results and ancillary results regards the extent to which disparity increased in the Late
Cretaceous.Based on continuous characters only, levels of disparity reach a moderately high plateau from the Aptian–Albian bin until the end-Cretaceous; there is no apparent increase in disparity seen in the last three sampled intervals of the Cretaceous. This result indicates that discrete characters may be inflating disparity in these latter bins. This is unsurprising, given that they are dominated by mosasaurs, which possess a unique intramandibular joint and have great dental diversity. Patterns of Skull-Size Evolution.—The first
40Myr of marine reptile evolution witnessed an exceptional range of skull sizes. Collectively, the marine reptiles of the Triassic explored the full range of forms seen in the entire Mesozoic (Fig. 7). Only a limited range of small forms is present in the Olenekian. However, the Anisian witnessed a great burst of skull-size evolution, including the diversification of both smaller taxa, such as pachypleurosaurs, and larger forms, including gigantic nothosaurs and the ichthyosaurs Cymbospondylus and Thalattoarchon. This disparate array of cranial sizes is present just 5–10Myr after the PTME. Despite a considerable reduction in diversity during the Norian, the overall range of skull
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sizes remains high, includingsmaller forms such as Endennasaurus and the largest marine reptile of the Mesozoic, the ichthyosaur Shonisaurus sikanniensis (skull length ~3m). The diversification of marine reptiles in the
Early Jurassic did not produce the same array of forms as in the post-PTME radiation (Fig. 7). Compared to the Triassic, the overall range of sizes seen through the Jurassic is greatly reduced. There is a lack of smaller forms but an abundance of large taxa, including ichthyosaurs, pliosaurids, and thalattosuchians. The range of skull sizes is larger in the Late Jurassic, owing to the diversification of plesiochelyid turtles and the presence of large thalassophonean pliosaurs (e.g., Pliosaurus kevani, skull length ~2m). A great diversity of skull sizes is found in the
mid-Cretaceous (Aptian–Turonian). During this interval, the range of sizes equals that of the Middle and Late Triassic (Fig. 7). Both large-skulled pliosaurs (e.g., Kronosaurus, skull length ~2.3m) and small marine turtles are found in the Aptian–Albian. Similarly, the Cenomanian–Turonian witnesses the diversification of marine squamates, some of which (e.g., dolichosaurs) are very small, comparable to pachypleurosaurs of the Triassic (skull lengths of <10 cm). Large pliosaurs persist into the Turonian (e.g.,Megacephalosaurus,skull
FIGURE 7. Temporal trends of marine reptile skull-size evolution. In the upper plot, log10 skull length for 354 marine reptile species is plotted at the midpoint of their stratigraphic range. Symbols are used to differentiate the major groups. Lower plot represents the same data expressed as box-and-whisker diagrams plotted at the midpoint of each time bin. Group symbols correspond to Figures 2 and 5.
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