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Adaptive Evolution in Paleozoic Coiled Cephalopods

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Adaptive Evolution in Paleozoic Coiled Cephalopods Paleobiology, 31(2), 2005, pp. 253±268 Adaptive evolution in Paleozoic coiled cephalopods BjoÈrn KroÈger Abstract.ÐCoiled cephalopods constitute a major part of the Paleozoic nekton. They emerged in the Early Ordovician but nearly vanished in the Silurian. The Emsian appearance of ammonoids started a story of evolutionary success of coiled cephalopods, which lasted until the end-Permian extinction event. This story is investigated by using a taxonomic database of 1346 species of 253 genera of coiled nautiloids and 1114 genera of ammonoids. The per capita sampling diversities, the Van Valen metrics of origination and extinction, and the probabilities of origination and ex- tinction were calculated at stage intervals. The outcome of these estimations largely re¯ects the known biotic events of the Paleozoic. The polyphyletic, iterative appearance of coiled cephalopods within this time frame is interpreted to be a process of adaptation to shell-crushing predatory pres- sure. The evolution of the diversity of coiled nautiloids and ammonoids is strongly correlated with- in the time intervals. Once established, assemblages of coiled cephalopods are related to changes in sea level. The general trends of decreasing mean (or background) origination and extinction rates during the Paleozoic are interpreted to re¯ect a successive stabilization of the coiled cephalopod assemblages. Different reproduction strategies in ammonoids and nautiloids apparently resulted in different modes of competition and morphological trends. Signi®cant morphological trends to- ward a stronger ornamentation and a centrally positioned siphuncle characterize the evolution of Paleozoic nautiloids. BjoÈrn KroÈger. Department of Geological Sciences, Ohio University, Athens, Ohio 45701 Present address: Museum fuÈr Naturkunde, Invalidenstrasse 43, D-10115 Berlin, Germany. E-mail: [email protected] Accepted: 7 July 2004 Introduction of evolution can be described in terms of tax- onomic rates and trends in morphologic char- Cephalopods were among the most highly acters by using a species-based database. The developed animals and the topmost predators technique of a data-based investigation of tax- in the free water column during long intervals onomic rates has strongly improved in recent of the Paleozoic. Their role in the seas changed years (see Sims 2003, and references therein). with the appearance of jawed ®shes in the late In this paper, I examine the hypothesis of a Paleozoic, but until recent times, cephalopods common niche of coiled cephalopods and ad- remained a major part of the nektonic life of dress several questions. Can we speak of an the seas. Over long periods in the Paleozoic, adaptive evolution of coiled cephalopods? Is cephalopods with a coiled conch constituted there any similarity between the ®rst emer- the most common cephalopods, but they are gence of coiled cephalopods in the Ordovician represented today only by Recent Nautilus. and the emergence of ammonoids in the Em- Their fate, the ecological role of these ancient sian? Do the main representatives of coiled molluscs, and their niche in the water column cephalopodsÐammonoids and coiled nauti- remain poorly understood. loidsÐexhibit any common pattern in their The ecological role and niche of coiled ceph- evolution? Finally, which factors drove the alopods can be studied by considering the evolution of coiled cephalopods during the common morphological characters of these Paleozoic? fossils. The morphospace and the adaptation- al value of the characters of coiled ceph- Development of a Hypothesis alopods are particularly well investigated through the work of Raup (1966, 1967), Raup The word ``coil'' comes from the Latin col- and Chamberlain (1967), Chamberlain (1980, ligere, ``to collect.'' In geometric terms, it com- 1993), and many others. prises a series of loops, a curve or circle of The fate of coiled cephalopods in the course more than 2p. Following Teichert (1964), a q 2005 The Paleontological Society. All rights reserved. 0094-8373/05/3102-0005/$1.00 254 BJOÈ RN KROÈ GER coiled cephalopod is de®ned by a spirally Data Set and Methods curved conch of more than one volution. There Within the current investigation I distin- are both openly coiled cephalopods (gyro- guish between a non-ammonoid database and cones) and closely coiled cephalopods in an ammonoid database. The ®rst data set, which the volutions touch each other. Only the ``coiled nautiloids,'' comprises all non-am- latter are considered here. monoid coiled cephalopods; it represents a The logarithmic curved conch (cyrtocone) is paraphyletic group with the shared characters considered to be the standard realization of ``nautiloid'' and ``coiled.'' The second data set, the growth of a conchiferan with a constant ``ammonoids,'' represents a monophyletic but slightly irregular accelerated increment of group of genera. material at its aperture (Bayer 1978; LoÈvtrup Both data sets are surveyed and treated dif- and LoÈvtrup 1988; Ackerly 1989; Ubukata ferently. The basic version of both data sets 2003). An accretional growth system that used in the current investigation was part of builds the special logarithmic curve of the coiled conch requires a suite of parameters, the database of genera compiled by Sepkoski which are maintained in a very narrow zone (2002). But, coiled nautiloids are only very in- of tolerance (Raup 1961; Raup and Michelson completely treated in Sepkoski's database and 1965). As a consequence of this growth the, thus required a complete revision. To do this, prominent Raup parameters W (whorl expan- I used the partially published Data Retrieval sion rate) and D (position of the curve in re- System Fossil Nautiloidea, of Engeser (2003), lation to the axis) have to be in a relation such followed by an additional search for all avail- that 1/D $ W. (Raup 1967). A coiled conch is able citations of coiled nautiloids in the liter- therefore a case of a logarithmic curved conch, ature. The non-ammonoid database comprises whose chance to evolve only by totally ran- at present 1346 species of 253 genera (see sup- dom processes is very low. Therefore, we have plemental material online, at http://dx.doi. to look for a constraint that causes the evolu- org/10.1666/03079.s1). tion of coiled conchs. Functional aspects that In contrast, the ammonoids, which are rep- control selectional constraints could represent resented by 1114 genera in the database, are in the main factors of the evolution toward a general taxonomically well known and de- coiled conch. scribed, as Sepkoski's (2002) database re¯ects. In fact, the coiling of the conch offers a num- To evaluate the usefulness of the entire data ber of advantages over the ancient cyrtocone set of all coiled cephalopods, I concentrated or the alternative gyrocone and orthocone on the weaker part of the compilation. Thus, shell. It mechanically strengthens the conch in this study I use the data set of coiled nau- (theoretical evidence: e.g., Raup 1967; Saun- tiloids as primary reference. This is a data set ders and Wehmann 1977; Currey 1988; Hewitt that I compiled very thoroughly myself, and Westermann 1990; Hewitt 1996; and em- whereas the ammonoid database is adopted pirical evidences: Zipser and Vermeij 1978; directly from that of Sepkoski (2002). Vermeij 1982; KroÈger 2002), increases the ma- Generally, the sum of the ranges of the spe- neuverability (e.g., Trueman 1941; Raup 1967; cies within a genus is considered to represent Saunders and Shapiro 1986; Crick 1988; Jacobs the time range of the genus. The stage de®- and Chamberlain 1996; Westermann 1996; nitions and their durations, which follow the Westermann and Tsujita 1999), and offers a IUGS-ICS (Remane et al. 1996), serve as the moderate hydrodynamic ef®ciency at the time grid (see Appendix). The estimation of same time (Chamberlain 1980; Jacobs and per capita taxonomic rates follows the time in- Chamberlain 1996). The coiled conch addi- tervals between stage boundaries; the esti- tionally requires a minimum expense of en- mation of probabilities follows the intervals ergy for buoyancy regulation compared with between stages midpoints. other ectocochleate cephalopods (Crick 1988). For all coiled nautiloids I compiled geo- These are strong arguments for an adaptive graphical species occurrences. The sum of the origin of coiling in the cephalopod conch. species occurrences represents the paleogeo- COILED CEPHALOPODS 255 FIGURE 1. Cumulative frequency plot of species of Pa- leozoic coiled nautiloids described since the eighteenth century. The nearly linear increase of described species since the mid-nineteenth century gives evidence for some potential of discovery of new species in the future and for the relative incompleteness of the sampled re- cord. graphical occurrence of a genus. If available, the dimension of the adult conch, the orna- mentation of shell, and the position of the si- phuncle of coiled nautiloids were compiled from the literature. The resulting database was tested for sampling bias. Completeness of the Record. In contrast to other fossil groups, the Paleozoic nautiloids are poorly known. Figure 1 gives a cumulative image of all species of coiled nautiloids de- scribed since the eighteenth century. A nearly complete record of the fauna based on taxo- nomic work through the last centuries would produce a simple logarithmic growth function of all known species with few initial descrip- FIGURE 2. Frequency distribution of described species tions in early times, a strong increase in the of Middle Ordovician, middle Silurian, Early Devonian, Mississippian and late Permian time interval per paleo- nineteenth and twentieth centuries, and only latitude. The shift of frequency peaks from the Southern few new descriptions in recent times. Figure 1 Hemisphere in the early Paleozoic toward the equator is distinguished from such an ideal picture by and back toward the Southern Hemisphere in the late Permian re¯ects a mixture of sampling bias and a real lacking a signi®cant decrease in new descrip- latitude effect with an equatorial frequency peak. tions in recent times, thus demonstrating a strong potential for discovering new species.
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