Downloaded from rsbl.royalsocietypublishing.org on March 12, 2010 Biol. Lett. (2010) 6, 265–269 their posterior legs retained for walking, range from doi:10.1098/rsbl.2009.0700 the Late Ordovician to the Late Permian, and also Published online 14 October 2009 attain gargantuan proportions: Pagea, from the Early Palaeontology Devonian, is around 120 cm long (Plotnick & Elliott 1995) and Cyrtoctenus, from the Carboniferous, is 135 cm long (Waterston et al. 1985). A trackway from Cope’s Rule and Romer’s Scotland, attributed to Hibbertopterus, indicates an animal 160 cm long (Whyte 2005). theory: patterns of During the Devonian, the diversity of Eurypterina plummeted, whereas the hibbertopterid radiation diversity and gigantism in during the Late Devonian and Carboniferous rep- eurypterids and Palaeozoic resents the last major radiation of eurypterids. Romer (1933) proposed that eurypterids and early armoured vertebrates fish evolved in an ‘arms race’; while this theory has fallen out of favour in recent years, deemed too James C. Lamsdell* and Simon J. Braddy simplistic, it remains in text books and popular science Department of Earth Sciences, University of Bristol, Wills Memorial writing. Analysis of evolutionary trends in eurypterids Building, Queen’s Road, Bristol BS8 1RJ, UK thus provides an interesting case study to examine *Author for correspondence ( [email protected]). whether causal mechanisms for gigantism in Palaeo- Gigantism is widespread among Palaeozoic zoic arthropods, and their extinction, were primarily arthropods, yet causal mechanisms, particularly owing to abiotic (e.g. environmental) or biotic (e.g. the role of (abiotic) environmental factors versus competition between species) factors. (biotic) competition, remain unknown. The eur- ypterids (Arthropoda: Chelicerata) include the largest arthropods; gigantic predatory pterygotids 2. MATERIAL AND METHODS (Eurypterina) during the Siluro-Devonian and A database of all known eurypterids was compiled from the primary bizarre sweep-feeding hibbertopterids (Stylonur- literature and supplemented with size data derived from the maxi- ina) from the Carboniferous to end-Permian. mum recorded specimen size and environmental occurrence data from Plotnick (1999), supplemented with data from the literature Analysis of family-level originations and extinc- (see the electronic supplementary material). Taxa were assigned to tions among eurypterids and Palaeozoic benthic assemblages following Braddy (2001). Placoderm generic vertebrates show that the diversity of Eurypterina data follows from Carr (1995), while familial occurrence data follows waned during the Devonian, while the Placodermi from Purnell (2001). The phylogenetic topology and family-level radiated, yet Stylonurina remained relatively assignments of Eurypterina are derived from Tetlie (2007); those unaffected; adopting a sweep-feeding strategy of Stylonurina follow Lamsdell et al. (in press). Family- and generic-level eurypterid diversity curves were compiled at series they maintained their large body size by avoiding resolution (31 time bins). competition, and persisted throughout the Late Palaeozoic while the predatory nektonic Euryp- terina (including the giant pterygotids) declined 3. RESULTS during the Devonian, possibly out-competed by Plotting the temporal ranges of the constituent families other predators including jawed vertebrates. of Eurypterida according to their phylogenetic top- Keywords: Eurypterida; Cope’s Rule; Romer’s ology (figure 1) reveals that only two families of the theory; extinction; competition; gigantism Eurypterina (Waeringopteridae and Adelophthalmi- dae) persist through to the Late Devonian, while only one group of Stylonurina (parastylonurids) go extinct during the Early Devonian. Generic-level diversity 1. INTRODUCTION curves show a massive decline in Eurypterina during The fossil record of Palaeozoic arthropods reveals that the Early Devonian, losing just over 50 per cent of gigantism was widespread among aquatic and terres- their diversity in 10 Myr. Stylonurina, by comparison, trial groups; griffenflies, morphologically similar to persist throughout the Devonian with a comparatively dragonflies, attained a wingspan of 70 cm and arthro- consistent diversity, until the Frasnian/Famennian pleurid millipedes were 200 cm long (Dunlop 1995). extinction event, when their diversity dropped margin- Such selection for gigantism is often attributed to elev- ally, coinciding with a changeover to a stylonurine ated oxygen levels during the Late Palaeozoic (Berner fauna consisting of the highly specialised sweep- et al. 2003); however, the presence of gigantism in feeding Mycteropidae and Hibbertopteridae. aquatic arthropods including Ordovician trilobites Palaeoenvironmental data show that most eurypterid (Rudkin et al. 2003), and Early Devonian eurypterids groups originated in marine habitats in the Ordovician (Braddy et al. 2008a) suggests that mechanisms for or Early Silurian, although the only groups that gener- gigantism selection are more complex. ally frequented marine shelf environments are the Eurypterids are extinct chelicerates found in a range Pterygotidae and Megalograptidae, along with some of aquatic habitats throughout the Late Palaeozoic. carcinosomatids; eurypterids were predominantly eur- Most are small-medium nektonic predators (Eurypter- yhaline; however, by the Late Devonian the surviving ina), and include the largest arthropods ever to have eurypterid groups were confined to freshwater- lived; gigantic pterygotids, with lengths of 250 cm esti- dominated settings (Braddy 2001). Analysis of size mated in Jaekelopterus (Braddy et al. 2008a). among eurypterid genera shows that basal taxa in Stylonurine eurypterids (Stylonurina), which have both suborders are relatively small (10–20 cm). The Electronic supplementary material is available at http://dx.doi.org/ taxa in each group tend to increase in size through 10.1098/rsbl.2009.0700 or via http://rsbl.royalsocietypublishing.org. time, with active predators in some Eurypterina Received 27 August 2009 Accepted 18 September 2009 265 This journal is q 2009 The Royal Society Downloaded from rsbl.royalsocietypublishing.org on March 12, 2010 266 J. C. Lamsdell & S. J. Braddy Competition and gigantism in eurypterids 470 460 450 440 430 420 410 400 390 380 370 360 350 340 330 320 310 300 290 280 270 260 250 Ordovician Silurian Devonian Carboniferous Permian Dar Llandov WPL L Em Ei Gi Fras Famenn Tourna Visean Ser Ba Mo A Sak Art K CWC Rhenopteridae ‘parastylonurids’ Stylonuridae Kokomopteridae Hardieopteridae Drepanopteridae Mycteropidae Hibbertopteridae Moselopteridae Onychopterellidae Megalograptidae Dolichopteridae Eurypteridae ‘carcinosomatids’ Mixopteridae Orcanopteridae Waeringopteridae ‘nanahughmillerids’ Adelophthalmidae ‘hughmilleriids’ Pterygotidae 1 m Jaekelopterus Dunkleostus 3 m+ 25 Acutiramus Hibbertopterus Eastmanosteus 2 m 20 Pterygotus 15 Erettopterus Pagea Cyrtoctenus 1 m 10 Mycterops 5 Arctolepis 0 Figure 1. Family-level evolutionary tree of the Eurypterida. Bars represent known temporal ranges, and are shaded according to habitat, indicating maximum salinity tolerance (black indicates open marine, grey indicates shallow marine and brackish environments and white indicates a restriction to fresh water). Silhouettes of various taxa are shown to scale with lines indicat- ing their temporal placement within the family. Diversity curves of Eurypterina (black line), Stylonurina (grey line) and Arthrodira (black dashed line) are at generic level. Maximum sizes of several pterygotids (black circles) and stylonurines (grey circles) are overlain, with three placoderm genera (black squares) shown for comparison. (Megalograptidae, Mixopteridae, carcinosomatids and coincides with the diversification of placoderms in hughmilleriids) reaching sizes of up to a metre. Ptery- Europe and North America. Stylonurina family diver- gotids reach the largest sizes, with individuals reaching sity remains largely unaffected except for a slight 250 cm as their diversity declined during the Devo- decrease in the Early Devonian, until a large reduction nian. Stylonurina also reached large sizes, with coinciding with the end-Devonian extinction event, 120 cm recorded from freshwater-inhabiting Stylonur- accounting for five of the seven existing stylonurine idae and the exclusively freshwater Hibbertopteroidea families. During this time, agnathan diversity approaching 200 cm in length. decreased and placoderms went extinct. With the Comparing the family-level diversity of the euryp- beginning of the Carboniferous, chondrichthyans had terid suborders with those of various fish groups diversified and sarcopterygian fishes such as rhizodonts (figure 2), the decline of the Eurypterina begins had taken over the role of apex predators (Andrews while agnathans are at their acme; however, it 1985). Biol. Lett. (2010) Downloaded from rsbl.royalsocietypublishing.org on March 12, 2010 Competition and gigantism in eurypterids J. C. Lamsdell & S. J. Braddy 267 Cha. Wuc. = one family Cap. Permian Wor. Roa. Kun. Art. Sak. Ass. Carboniferous Gzh. Kas. Mos. Bas. Ser. Vis. Tou. Fam. Devonian Fra. Giv. Eif. Ems. Pra. Loc. Silurian Lud. Wen. Lla. Ordovician Hir. Kat. San. Dar. Eurypterina Stylonurina AgnathaPlacodermi Acanthodii Chondrichthyes Figure 2. Family-level diversity of Eurypterida and various aquatic vertebrate groups throughout the Palaeozoic. Diversity of the suborder Eurypterina begins to decline steadily through the Early Devonian, coinciding