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THOMAS L. STUBBS AND MICHAEL J. BENTON


preservation are more likely to provide material of sufficient quality, such as complete skulls and skeletons, that can be included in large-scale studies of disparity, potentially inflating sample size in such intervals. In fact, exceptionally preserved marine reptile biotas that yield many specimens are scattered through the Mesozoic, and it is a moot point which would be termed “lagerstätten,” and which not. A detailed study of the ichthyosaur fossil


record showed that specimen quality is subject to many factors (Cleary et al. 2015). These include geographic location (the Northern Hemisphere is better documented than the Southern), specimen size (medium-sized specimens are more complete than small or


large), and facies (best specimens in fine-grained siliciclastics). Importantly, Cleary et al. (2015) found no relationship between specimen quality and any of the commonly used temporal sampling metrics such as formation counts or map areas, nor were named lagerstätten the unique sources of complete specimens. Disparity is widely considered to be both


conceptually and empirically different from diversity; sampled intervals commonly have high diversity but low morphological disparity, or vice versa (Foote 1997) (Fig. 4). In addition, variance-based measures are generally robust to sample-size discrepancies (Ciampaglio et al. 2001). In this study, tem- poral patterns of functional disparity and skull-size variation cannot be simply attributed to the distribution of lagerstätte deposits. Jurassic marine reptiles together exhibit generally low disparity, despite being dominated by lagerstätten (Figs. 3–7). In contrast, the mid- to Late Cretaceous interval shows higher disparity, despite not being associated with lagerstätte effects (Benson and Butler 2011). Marine reptile diversity trends were driven


by marine transgression and regression (Benson and Butler 2011). Marine reptiles can be broadly divided into shallow-marine or open-ocean habitat groups based on the degree of postcranial specialization and locomotory modes. Benson and Butler (2011) identified the Anisian–Carnian, Bathonian–Tithonian, and


Cenomanian–Maastrichtian as times with elevated diversity of shallow-marine taxa. These intervals broadly correspond to sea-level


highstands and times of greater continental flooding. The strong negative correlation recovered between shallow-marine taxic diversity and nonmarine area was interpreted by Benson and Butler (2011) as representing a species diversity–area relationship, whereby greater continental flooding increases the habitable area for shallow-marine organisms and elevates the deposition of fossiliferous rock, a pattern formalized as the “common cause” hypothesis (Peters 2005). Intriguingly,


times of transgression, increased continental flooding, and higher diversity in shallow- marine taxa also correspond to intervals of greatest disparity in the current study. Functional disparity and the diversity of skull sizes are highest in the Anisian–Carnian, Kimmeridgian–Tithonian and Late Cretaceous (Figs. 3, 7). Intervals in which shallow-marine reptiles are rarer and open-ocean taxa dominated, such as the Early Jurassic and earliest Cretaceous (Benson and Butler 2011), have reduced functional disparity and a less diverse range of skull sizes. Such times occur during or after major regression events (Benson and Butler 2011; Kelley et al. 2014). This study therefore provides tentative evidence that major patterns ofmorphological evolution inMesozoic marine reptiles were driven by changing physical environmental conditions. Shallow-marine reptiles may exhibit greater


functional disparity and a more diverse range of skull sizes because coastal and shallow- shelf environments accommodate a greater diversity of habitats and of prey. Transgression and continental flooding concentrate nutrients in coastal and shallow-shelf environments because there is abundant terrigenous input through sediments and soils (Smith et al. 2001). This is expected to increase productivity and biomass, particularly in benthic invertebrates (Polycn et al. 2014). A diversity of prey is likely to catalyze phenotypic innovation in the jaws and dentition. For example, feeding on hard-shelled benthic invertebrates requires morphofunctional modifications, such as increased musculature, greater jaw robustness, higher mechanical advantages, crushing or


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