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THOMAS L. STUBBS AND MICHAEL J. BENTON


sum of ranges is rarefied to the average sample size of the 16 bins used (n=17). As the Norian bin has onlyfive samples, it is unsurprising that a large reduction in disparity is recovered when compared with the Carnian bin (rarefied sample size is 17), because the sum of ranges metrics is susceptible to sample-size bias. If all bins are rarefied to a minimum sample size (n=5), there is no significant decline in disparity between the Carnian and Norian (Supplementary Fig. 6). Marine reptile diversity was already massively depleted in the Norian (Kelley et al. 2014). Therefore, greater insights into the loss of disparity during the Late Triassic/Early Jurassic transition can be gained by comparing the Carnian bin with the Hettangian–Sinemurian bin. In this case, both variance- and range-based metrics show a statistically significant loss of disparity (Fig. 3A,B), and statistical tests confirm that this disparity decline is significant, based on ratios of marginal likelihoods for variance (LR: 10.37) and paired-sample t-tests (p=0.011) (Table 3). In conclusion, these analyses confirm that the progressive and widespread lineage extinctions during the Late Triassic resulted in a significant loss of disparity in Mesozoic marine reptiles. Marine reptile disparity also declined


during the Jurassic/Cretaceous transition. Both metrics show that disparity was higher in the Late Jurassic bin (Kimmeridgian–Tithonian) than at any other interval in the Jurassic, but disparity was reduced by the Early Cretaceous (Berriasian–Barremian) (Fig. 3A,B). Marginal overlap of the confidence intervals in the sum of variances metric suggests that the disparity decline was nonsignificant. Variance-based statistical tests also fail to identify a significant change in disparity between these two bins (LR: 2.27, paired-sample t-tests p=0.153). In contrast, confidence intervals associated with the sum of ranges metric do not overlap. However, it is important to acknow- ledge sample-size discrepancies, because the Berriasian–Barremian bin has eight samples, compared with a rarefied sample size of 17 for the preceding Late Jurassic bin. Overall, there is a disparity reduction resulting from faunal turnover and a putative extinction during the Jurassic/Cretaceous transition, but the


extinctions, diversifications, and faunal turnover, the relative contribution of each


magnitude and statistical significance of this decline is uncertain. Partial Disparity Trends.—As a result of


marine reptile group to overall disparity trends fluctuates markedly throughout the Mesozoic. Examining partial disparities dissects the contributions of each of the sixmajor taxonomic assemblages (Fig. 3C). In the earliest sampled interval, the Olenekian, ichthyosauromorphs are by far the greatest contributors to overall functional disparity, but during the Middle Triassic to early Late Triassic, sauropterygians become the major contributors (approximately 60–70%). Thalattosaurs remain consistently low contributors throughout the Triassic. Following the Late Triassic extinctions, ichthyosauromorphs (neoichthyosaurians) and sauropterygians (plesiosaurs) make equal contri- butions to disparity in the Early Jurassic. While the sauropterygian contribution remains relatively stable throughout theMiddle and Late Jurassic, the proportional contribution of the ichthyosaurs becomes increasingly diminished. This is associated with a greater contribution to disparity by thalattosuchian crocodylomorphs and the diversification of plesiochelyid turtles in the Late Jurassic. In the Early Cretaceous, ichthyosaurs are significant contributors to disparity, but their relative importance wanes substantially before eventual extinction by the end-Cenomanian. Trends of partial disparity from the mid-Cretaceous onward are driven by the diversification of the highly disparate turtles and mosasauroids. The high levels of overall variationinthe Late Cretaceous coincide with the dominance of these two clades. The proportional disparity of the remaining Late Cretaceous sauropterygians (polycotylids and elasmosaurs) is much reduced. Temporal Morphospace Trends and


Selectivity.—High disparity in the Middle Triassic results from an early proliferation of morphospace occupation during the initial diversifications of sauropterygians and ichthyo- sauromorphs, resulting from significant excursions along the first two axes of variation (Fig. 5; see also Supplementary Fig. 7). This gross pattern of morphospace occupation is maintained into the Carnian, and some


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