560


THOMAS L. STUBBS AND MICHAEL J. BENTON


Cretaceous is overwhelmingly driven by diversifications of the morphologically distinct marine turtles and mosasauroids. The Late Cretaceous is characterized by an abundance of mosasauroids positioned at the positive extreme of PC1 and an absence of taxa with strongly negative scores on PC2 (Fig. 5). Statistical tests highlight two shifts in func-


tional morphospace occupation between suc- cessive sampling intervals (Table 3). The first significant shift is seen between the Norian and Hettangian–Sinemurian bins, namely those associated with the Triassic/Jurassic transition (NPMANOVA, F=4.278, p=0.0009). This resulted from the extinction of placodont sauropterygians and the excursion into regions of morphospace marked by positive PC1 and PC2 scores by plesiosaurs in the Early Jurassic (Fig. 5). A second, less significant shift is found between the Aptian–Albian and Cenomanian– Turonian bins of the mid-Cretaceous (NPMANOVA, F=4.278, p=0.0429, not robust to p-value corrections for multiple comparisons). This second shift could be dri- ven by the diversification of mosasauroids in the Cenomanian and Turonian and the decreasing abundance of ichthyosaurs. There is evidence for ecologically selective


FIGURE 6. Marine reptile functional disparity plotted through time. Based on (A) within-bin mean pairwise dissimilarity calculated from the Gower intertaxon distance matrix using all characters, (B–E) using PC scores from analyses using only continuous characters. Mean disparity values based on pairwise dissimilarity (A), and the sum of variances (B, C) and sum of ranges (D, E) metrics (white circles) are plotted in 16 Mesozoic time intervals. The blue envelopes represent 95% confidence intervals based on 10,000 bootstrap replicates. The sum of ranges is rarefied to the average sample size of the 16 bins (n=17). In (C) and (E) the character total mandibular length was excluded from the data set.


extinctions through the Late Triassic. When morphospace occupation in the Triassic bins is compared with the Early Jurassic, it is evident that extinction victims are concentrated in the left-hand regions of the plots (corresponding to negative PC1 scores) (Fig. 5). Throughout the Triassic, this adaptive zone was occupied by various eosauropterygians, placodonts, thalattosaurs, and ichthyosaurs. These taxa had a range of functional specializations. Those with positive PC2 scores (i.e., placodonts and thalattosaurs) possessed specialized dentition, including crushing dentition and dentary tooth plates, as well as having robust jaws with high mechanical advantages and increased musculature. Taxa with negative PC2 scores had slender jaws with weaker bites and reduced musculature, but all had dental specialization, including bulbous dentition (e.g., mixosaurids and Wumengosaurus) and in extreme cases, no dentition (e.g., Endennasaurus). No marine reptiles in the Early Jurassic possessed this suite of functional traits,


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