Taxonomic survivorship and morphologic complexity in Paleozoic bryozoan genera

Abstract

The shape of bryozoan taxonomic survivorship curves is strongly influenced both by grade of morphologic complexity and by mass extinction. Paleozoic bryozoan genera that are morphologically simple have linear taxonomic survivorship; morphologically intermediate taxa have slightly concave survivorship, and complex forms have very concave survivorship. Increasing morphologic complexity, and by inference, increasing specialization of adaptation appear to accompany a systematic departure from a stochastically constant extinction rate. However, the extinctions of the complex taxa are entirely concentrated during three mass extinction events, whereas the extinctions of the simple taxa are more uniformly distributed throughout the Paleozoic; the extinction pattern of the morphologically intermediate taxa is intermediate to those of the simple and complex groups. Exclusion of the genera affected by mass extinction increases the convexity of the survivorship curves, and reverses the apparent correlation of extinction rate with morphologic complexity. The macroevolutionary pattern of the complex genera resembles an r-strategy, whereas that of the simple taxa resembles a K-strategy.

Extinction and the spatial dynamics of biodiversity

“…43 and 44) and during mass extinctions. For example, marine bryozoan genera with complex colonies suffer more severely during mass extinctions than simple genera, but colony complexity is also inversely related to genus-level geographic range (45,46), which may well be the ultimate basis for differential survival during the end-Ordovician mass extinction. The end-Ordovician extinction also preferentially removed snails with broad selenizones providing access to the mantle cavity, and planktonic graptolites with multiple stipes creating complex pendant colonies; the K-T extinction also preferentially removed bivalves with schizodont hinges (trigonioids), echinoids with elongate rostra (a clade of holasteroids), cephalopods with complex sutures (ammonites), and a major clade of birds with foot bones that fused from the ankles to the toes (Enantiornithes).…”

The continuity of microevolution and macroevolution

“…The mechanisms underlying the generation of individual‐level diversification strategies, such as polymorphism in seed germination behaviour (Cohen, 1966), are poorly known (Cooper & Kaplan, 1982; Simons & Johnston, 1997). Although some clades seem to be characterized by consistently high rates of speciation and extinction (Anstey, 1978; Jablonski, 1986; Dennis et al ., 1995) – suggestive of clade‐level diversification – it is unknown whether mechanisms exist at the clade level that would allow diversification bet hedging to evolve. In contrast, conservative traits at the clade level may be individual‐level traits.…”

Cladistic analysis of the Paleozoic bryozoan families Monticuliporidae and Mesotrypidae

“…Characters useful in cladistic studies of Paleozoic stenolaemate bryozoans were developed, in part, from those used in several earlier studies Perry 1970, 1973;Corneliussen and Perry 1973;McKinney 1977McKinney , 2000Anstey 1978;Prezbindowski and Anstey 1978;Pachut and Anstey 1984;Blake and Snyder 1987;Hickey 1988;Anstey 1990;Key 1990;Cuffey and Blake 1991;Hageman 1991;Pachut, Anstey and Horowitz 1994;Anstey and Pachut 1995;Spearing 1998;Tang and Cuffey 1998;Taylor and Weedon 2000) and merged into a comprehensive listing of 317 characters that include 701 derived states (Paquette 2008;Appendix B). A subset of 267 of these characters was appropriate for coding the trepostome genera analyzed in this study.…”