Effects of the Mesozoic Marine Revolution on the taxonomic, morphologic, and biogeographic evolution of a group: aporrhaid gastropods during the Mesozoic

-The mid-Mesozoic time was characterized by massive reorganization of the ecological structure of benthic marine communities. Although the general pattern of this "Mesozoic Marine Revolution" has been extensively documented, little is known about how it affected the taxonomic and morphologic diversities of individual lineages of organisms. Here I document group-level dynamics of the Mesozoic change using the fossil record of aporrhaid gastropods, a diverse and widespread family during that time. Using data gathered through field and museum work and extensive literature search, I compare and contrast patterns of taxonomic and morphologic change during the Jurassic and Cretaceous within a geographic framework. I also examine how the changes were manifested at different ecological and geographic scales. The Mesozoic history of the Aporrhaidae includes two major radiations separated by a period of overall stability. While both radiations increased the taxonomic diversity of the family, they had very different morphologic consequences, resulting in a striking discordance between morphologic and taxonomic diversity patterns. The initial radiation during Bajocian-Bathonian times established two large morphologic groups within the aporrhaids based mainly on differences in the shape of the apertural margin. The second, post-Albian, radiation saw higher origination rates and increased taxonomic and morphologic diversity among genera having simpler apertural margins, while genera with elaborate multidigitate apertures declined in taxonomic diversity but not in morphologic diversity. During post-Albian times the group with simpler apertural margins also tended to be more widespread compared to the group with multidigitate apertures. Comparison of regional and global taxonomic diversities reveals a discordance between regional and global taxonomic patterns: while diversities within certain geographic areas increased in concert with the global radiation, those for other areas decreased substantially, resulting in a remarkably constant average taxonomic diversity within geographic areas. Kaustuv Roy. Department of Geophysical Sciences, University of Chicago, 5734 South Ellis Avenue, Chicago, Illinois 60637 Accepted: December 14, 1993 Introduction clear, little is known about the dynamics of the Mesozoic Revolution at the level of inOne of the most significant changes in the dividual clades or within individual geoecological structure of benthic marine comgraphic regions. The lack of such data has munities began during the mid-Mesozoic. prevented a better understanding of the naThis reorganization, termed the "Mesozoic ture of this change and the underlying proMarine Revolution," included a number of cesses. In fact, the few studies that have atimportant events such as the diversification tempted to document this change for of major groups of durophagous predators and individual groups suggest that effects can be predatory mollusks, significant increase in rehighly clade-specific. For example, Budd and paired shell damage among gastropods, inCoates (1992) examined the morphologic crease in antipredatory morphologies in varchanges exhibited by Montastraea-like corals ious molluscan groups, disappearance of during the Cretaceous, but failed to detect stalked crinoids and brachiopods from shalany long-term evolutionary trends. Ward low-water habitats, and a substantial increase (1986), on the other hand, documented imin levels of bioturbation (Meyer and Macurda portant temporal changes in shell morphol1977; Vermeij 1977,1983,1987; Steneck 1983; ogy of various groups of ammonoids during Thayer 1983; Lidgard et al. 1993). the same time period. Although the general pattern of change is While most studies of the Mesozoic RevoO 1994 The Paleontological Society. All rights reserved. 275 MACROEVOLUTION IN APORRHAID GASTROPODS lution have documented temporal patterns of either taxonomic or morphologic change for various groups of marine organisms, almost none has explicitly attempted to compare and contrast these two types of patterns. Recent work has shown that a comparison of temporal patterns of taxonomic and morphologic diversity of a group can often provide insights into the macroevolutionary processes underlying the changes (see Foote 1993). Hence, such a comparative approach, especially in a geographic context, should greatly aid in understanding the dynamics of the hlesozoic change. Similarly, attempts to examine the nature of the Mesozoic change at different ecological and geographic scales have been lacking despite the observation that a comparison of patterns at different ecological levels can provide important information about evolutionary radiations (Bambach 1977; Sepkoski 1988; Lidgard et al. 1993). In this paper, I examine the clade-level dynamics of the Mesozoic Marine Revolution using a multifaceted approach. I first document global patterns of taxonomic and morphologic diversity exhibited by Jurassic and Cretaceous genera of aporrhaid gastropods, a group that was an important component of the shallow-water molluscan fauna during the Mesozoic Revolution. I then document the patterns of geographic distribution exhibited by the Mesozoic aporrhaids and use this geographic framework to (1) compare and contrast the temporal patterns of taxonomic and morphologic diversity, and (2) examine temporal patterns of taxonomic diversity at a global as well as a local scale. In addition, I use the preliminary results from a cladistic analysis of the Jurassic aporrhaid genera to discuss the phylogenetic aspects of the morphologic radiations documented here.