Paleobiology, 31(4), 2005, pp. 656±675 Correlated trends in the evolution of the plesiosaur locomotor system F. Robin O'Keefe and Matthew T. Carrano Abstract.ÐThis paper investigates trends in the evolution of body size and shape in the Plesiosau- ria, a diverse clade of Mesozoic marine reptiles. Using measures from well-preserved plesiosaur specimens, we document and interpret evolutionary patterns in relative head size, body size, and locomotor variables. Size increase is a signi®cant trend in the clade as a whole, and in constituent clades. The trend in relative head size is of variance increase; observed head sizes are both smaller and larger than ancestral values. In the locomotor system, changes in propodial and girdle pro- portions appear concomitant with body size increase and are interpreted as allometric responses to the physical constraints of large body size. Other trends in the locomotor system are signi®cantly correlated with both body size and relative head size. These locomotor trends evolved convergently in several clades of plesiosaurs, and may have had an ecomorphological basis, although data are lacking to constrain speculation on this point. The evolution of the locomotor system in plesiosaurs sheds new light on the response of aquatic tetrapods to the physical constraints of foraging at large body size. F.Robin O'Keefe. Department of Anatomy, NYCOM II, Room 326, New York College of Osteopathic Med- icine, New York Institute of Technology, Old Westbury, New York 11568. E-mail: [email protected] Matthew Carrano. Department of Paleobiology, Post Of®ce Box 37012, MRC 121, Smithsonian Institution, Washington, D.C. 20013-7012. E-mail: [email protected] Accepted: 19 January 2005 Introduction Background. The Plesiosauria (de Blainville The goal of this paper is to test several hy- 1835) is a diverse, entirely extinct clade of Me- potheses concerning the evolution of the lo- sozoic marine reptiles. Along with a paraphy- comotor system in the Plesiosauria. This goal letic series of ancestral ``nothosaurs,'' plesio- is timely, because recent work has demonstrat- saurs are members of the reptilian clade Sau- ed that the evolution of shape within the ropterygia (see Rieppel 2000 for review). Ple- group is more complex than previously sup- siosaurs were ®rst discovered in the 1820s in posed (O'Keefe 2002). We aim to investigate Mesozoic marine rocks of England (Taylor the in¯uence of two factorsÐbody size and 1997). Since that time, plesiosaurs have been trophic specializationÐon the locomotor sys- unearthed on every continent, in strata rang- tem of plesiosaurs. This investigation is dif®- ing from the Late Triassic through to the K/T cult because body size and trophic speciali- boundary. Richard Owen (1860: p. 230) once zation have complex histories within the compared a plesiosaur's long, graceful neck, clade; they are in fact trends themselves. Our stout body, and four ¯ippers to ``a snake ®rst goal, therefore, is to characterize and threaded through the trunk of a turtle.'' Ow- quantify these two trends. Once these trends ing to this distinctive shape, plesiosaurs are are understood, we then test for correlation among the more familiar of Mesozoic reptiles. between each trend and a suite of locomotor This plesiosaur bauplan is derived relative variables. These variables are the lengths of plesiosaur appendicular bones. We establish to that of nothosaur-grade sauropterygians that the geometry of the locomotor system via the acquisition of a characteristic suite of does covary with body size and trophic spe- morphological features. First, the limb girdles cialization. Last, we make some suppositions are elaborated ventrally but reduced dorsolat- about why the observed covariances occur, erally (Storrs 1991, 1993) (Fig. 1). Along with while acknowledging that inferring form from a tightly meshed ``basket'' formed by the ab- function in extinct organisms may be a peril- dominal ribs, these girdles form a platelike ous exercise (Lauder 1995). structure reminiscent of a turtle plastron q 2005 The Paleontological Society. All rights reserved. 0094-8373/05/3104-0008/$1.00 PLESIOSAUR LOCOMOTOR TRENDS 657 son 1963; Brown 1981; Carroll 1988; Brown and Cruickshank 1994). These two groups were the large-headed, short-necked ``plio- saurs'' and the small-headed, long-necked ``plesiosaurs.'' Recently, however, several au- thors have challenged the phylogenetic basis of this view (Carpenter 1997; Bardet 1998; Druckenmiller 2002; O'Keefe 2001a, 2004), and a growing body of opinion now holds that the evolution of body proportions among ple- siosaurs is quite complex. O'Keefe (2001a; 2002) found that ``pliosaurs'' had actually evolved independently in three different clades, whereas ``plesiosaurs'' evolved twice (for cladogram and taxonomy, see Fig. 2). These studies separated monophyletic plesio- saur clades from the more familiar but con- vergently evolved grades. To highlight this distinction, O'Keefe (2001a) re-named the grades ``pliosaur'' and ``plesiosaur'' as ``plio- sauromorph'' and ``plesiosauromorph,'' re- FIGURE 1. Measurements (top) and body schematic spectively; the terms Pliosauria and Plesiosau- (bottom) used in this study. Top, skeleton of the primi- ria were restricted to monophyletic groups. tive plesiosaur Thalassiodracon hawkinsi (BMNH 2020); coracoids and pubes slightly reconstructed. Both left Most recent phylogenetic and morphomet- limbs are present in the specimen but have been omitted ric studies agree that the evolution of body for clarity. Bottom, the same taxon represented sche- shape in the Plesiosauria was much more com- matically. Abbreviations: cor 5 coracoid; isch 5 ischi- um; pub 5 pubis; scap 5 scapula. plex than previously believed, yet this com- plexity remains largely unexplored, particu- larly with regard to intercorrelations among (Robinson 1977; Taylor 1981). This contrasts different trends. The construction of well-sup- with the ancestral nothosaurian condition, ported phylogenetic hypotheses and the pos- where the girdles are proportioned more like session of comprehensive morphometric data those of generalized primitive diapsids. Ple- now permits detailed analyses of trends in siosaur limbs also differ from those of notho- plesiosaur evolution, and these are the focus of saurs in several respects (Carroll 1985; Storrs our study. 1993; Caldwell 1997a,b). The humerus and fe- Trends in Plesiosaur Evolution. Work on mur (propodials) are relatively short and bear trends of size and shape change in the Plesio- a round proximal articular facet, but become sauria is at a nascent stage; the lack of a for- broad and ¯at distally. The distal end of the mal, comprehensive phylogeny was prohibi- propodial has a cambered cross-section, a ge- tive, and plesiosaur shape was not treated ometry shared by the foreshortened distal el- quantitatively until recently (O'Keefe 2001b, ements. The limb skeleton distal to the pro- 2002). This latter work quanti®ed variation in podial is dorsoventrally ¯attened, hyperphal- shape but did not address the subject of phy- angeal, and tapers distally to give a ¯ipperlike logenetic trends directly. In this paper we con- shape that contrasts markedly with the more cern ourselves with two trends in plesiosaur ``hand''- and ``foot''-like epipodia of ancestral evolution, body size and trophic specializa- nothosaurs (Fig. 2). tion. Understanding these trends is a prereq- Throughout the twentieth century the Ple- uisite for examining the more complex evo- siosauria was conventionally divided into two lutionary trends in the locomotor apparatus. groups differing primarily in body propor- Body size is a frequently cited in¯uence on tions (e.g., Williston 1914; Romer 1956; Pers- organismal biology and evolution. Allometric 658 F. ROBIN O`KEEFE AND MATTHEW T. CARRANO FIGURE 2. Cladogram of the Plesiosauria. Branches in bold are pliosauromorphs, dotted branches are plesiosau- romorphs (Elasmosauridae, Microcleidus,andMuraenosaurus). Topology and taxonomy modi®ed from O'Keefe (2001a, 2004). Ecomorphological assignments based on analysis in O'Keefe (2002) and present study. PLESIOSAUR LOCOMOTOR TRENDS 659 responses to scaling effects are well-known relations between head size, neck length, and phenomena among terrestrial animals (e.g., elements of the locomotor system have been Alexander et al. 1979; Cubo and Casinos 1994; noted qualitatively by several authors Carrano 2001), often associated with the in- (O'Keefe 2002, and others reviewed in Brown creasing supportive demands imposed by 1981). Speci®cally, pliosauromorphs tend to gravity. Aquatic tetrapods have a density very possess relatively long posterior girdle ele- close to that of water and so are released from ments (i.e., coracoid and ischium), but these certain physical constraints thought to act on elements are relatively short in plesiosauro- terrestrial animals, but increasing size may morphs (O'Keefe 2002). The hindlimb is also still in¯uence other aspects of morphology relatively large in pliosauromorphs but rela- (e.g., Vogel 1994). Scaling analyses may be tively small in plesiosauromorphs. Thus, the used to establish whether taxa respond iso- lengths of the girdle and limb elements ap- metrically or allometrically to change in body pear to covary with head and neck length. size. The large size range exhibited by plesi- However, the presence of head and neck var- osaur taxa suggests that scaling may be an im- iables tended to obscure the patterns of loco- portant factor in determining shape changes. motor