Functional Morphology, Ecology, and Evolution of the Scaphitaceae Gill, 1871 (Cephalopoda)
Total Page：16
File Type：pdf, Size：1020Kb
J. Moll. Stud. (2000), 66, 205–216 © The Malacological Society of London 2000 FUNCTIONAL MORPHOLOGY, ECOLOGY, AND EVOLUTION OF THE SCAPHITACEAE GILL, 1871 (CEPHALOPODA) NEALE MONKS Department of Palaeontology, Natural History Museum, Cromwell Road, South Kensington, London SW7 5BD e-mail: [email protected] ABSTRACT Scaphitids were widespread during the Late Cretaceous, and in some areas have consider- Scaphitids are heteromorph ammonites exhibiting able importance in biostratigraphy, for example morphological trends counter to of those of the other in the Chalk of Europe and the Western main heteromorph ammonite groups. These trends Interior Seaway deposits of North America. include the shortening of the body chamber, lateral compression of the whorl, closure of the coil, and They were also persistent, ranging from the more regular, spiral coiling. Scaphitid-like morpholo- Late Albian to Late Maastrichtian, and are gies may have appeared in other heteromorphs, but among the youngest of all ammonite fossils the Scaphitaceae are monophyletic. The most primi- known (Kaplan et al. 1987; Landman & Waage, tive scaphitids are known from the Albian, but an 1993; Emerson et al. 1994; Kennedy & Jagt, important radiation occurred in the Western Interior 1998). The systematics of the group has been Seaway of North America from the Santonian to subject to many studies, and they are generally Maastrichtian, some of which spread to the Old considered to fall into two main groups, the World. The scaphitid morphology is consistent with Otoscaphitinae Wright, 1953 and the Scaphi- improved swimming abilities, but scaphitids remained associated with the seafloor and are best considered tinae Gill, 1871 both placed within a single to have been nektobenthos similar to modern family, the Scaphitidae Gill, 1871 (Wright et al. nautiluses. 1996). These subfamilies are taken to represent a divergence early in the evolution of the group, probably during the Late Albian (Wright, 1953). INTRODUCTION Cooper (1994) revised the systematics of the group using a cladogram and the morphological The Scaphitaceae were a diverse group of suites of characters which support it. In that heteromorph ammonites which in many ways study, the subfamilies Scaphitinae and Oto- reversed the trends characteristic of hetero- scaphitinae were elevated to family status but morph ammonites on the whole, such as the further divided into subfamilies corresponding approximately regular spiral coiling of many to assumed evolutionary lineages. Cooper (1994) species, and the lack of an open or helical initial removed the most primitive species into a new coil. They do, however, retain some typically family of their own, the Eoscaphitidae Cooper, heteromorph features, most importantly the 1994. However, this family was not defined on quadrilobate septum, at least during the early the basis of any unique, shared derived charac- stages of ontogeny (Wiedmann, 1964). The ters but rather as the ‘grade group’ leading to Scaphitaceae is believed to be monophyletic, the ‘higher’ scaphitids. derived from the Hamitidae Gill, 1871, a prob- ably paraphyletic group of primitive hetero- Palaeobiology of scaphitids morphs with simple septal walls, open, often crozier or paper-clip shaped shells and long Like ammonites in general, the palaeoecology narrow body chambers, but lacking any orna- of the scaphitids is less well studied than their ment more complex than simple annular ribs distribution and systematics. Particularly well (Monks, 1999a). The group may also be ances- studied are the scaphitids of the North Ameri- tral to certain extant cephalopods, though this is can Western Interior Seaway, which ran from controversial to say the least (Lewy, 1996). A the Gulf of Mexico up to Arctic Canada typical scaphitid is illustrated in Fig. 1, Scaphites through the Late Cretaceous (Landman & hugardianus, from the Late Albian (Lower Waage, 1993). Although this basin was open to Cretaceous). the Atlantic (and to the North Pacific through 206 N. MONKS Figure 1. Reconstructed shell of a generalised scaphitid, with the major morphological characteristics labelled. Cenomanian to Campanian times) movement deposited during oxygen-poor bottom water of animals in and out of the basin seems to have conditions, or else occur are in such numbers as been limited, and it developed its own endemic to suggest a mass mortality (Batt, 1989, Land- ammonite fauna of which heteromorphs such as man & Waage, 1993). This contrasts with some the scaphitids were an important constituent heteromorphs like the straight-shelled bacu- (Kennedy & Cobban, 1976). The waters of the litids, whose fossils are widely distributed in Seaway appeared to have been stratified, and normal and oxygen-poor bottom water facies, at times bituminous shale was deposited indi- and have carbon and oxygen isotope values cating dysaerobic bottom waters (Gill & Cob- consistent with having lived higher up the water ban, 1966; Sageman et al. 1997). Differences or column (Fatherree et al., 1998). Such ammonites similarities in the isotopic compositions of the probably occupied a midwater, vertically- shells of benthic molluscs (like inoceramids and migrating niche similar to modern cranchid gastropods) and cephalopods allow some infer- squids (Klinger, 1980). ences of the ecology of different cephalopods to However, heteromorph ammonites with open be made. Scaphitids appear to have lived near shells and hook-shaped body chambers such as the sea floor and have carbon and oxygen iso- the scaphitids but also others such as the hami- tope values similar to those of the benthic mol- tids which are the presumed ancestors of the luscs (Whittaker et al. 1986). Furthermore, fossil scaphitids, are widely believed to have been scaphitids frequently absent from the sediments more or less planktonic. This is mostly because EVOLUTION OF THE SCAPHITACEAE 207 such shells would have produced a lot of drag, Scaphitaceae are a monophyletic clade derived being covered in ribs and spines; and the long from a group of ‘scaphitiform’ hamitids which body chambers would make swimming ineffi- appeared late in the Early Albian (Monks, cient, assuming propulsion was generated in a 1999). These scaphitiform hamitids show a similar way to modern cephalopods. In addi- tendency towards shorter body chambers, invo- tion, the hook shaped body chamber, if entirely lute and more regular spiral coiling, characteris- filled with the soft body of the ammonite would tics exemplified by the Scaphitaceae (Fig. 2). hang beneath the phragmocone, as is the case Eoscaphites circularis (J. de C. Sowerby in with the living nautiluses. In such an orienta- Fitton, 1836) is generally taken as the earliest tion, the aperture would point upwards, away genus of scaphitids, known from the Upper from the sea floor, and it is difficult to visualise Albian; but while having close, involute coiling such an ammonite being benthic (Trueman, still retains an open umbilicus and more or less 1941; Klinger, 1980; Westermann, 1996). Hami- hamitid septal walls (Cooper, 1994). It is tids are certainly widely distributed, with many believed that there were two main radiations of species having near-global distributions despite scaphitids: the medium to large tuberculate and being obviously poor swimmers, but are absent generally close-coiling Scaphitinae; and the more from sediments deposited during anoxic bottom weakly ornamented, rather loosely coiling and water conditions (Batt, 1989, 1991). As noted diminutive, perhaps paedomorphic, Otoscaphi- above with the scaphitids, this would seem to tinae (Wright, 1953). indicate a benthic lifestyle. This study attempts to tackle the twin prob- lems of ecology and evolution, using a cladistic Analytical methods analysis to help define the likely phylogenetic To investigate the relationships between scaphitid trends, and then apply new theories about the genera, a phylogenetic analysis was accomplished using functional morphology of ammonites to it to try PAUP (Swofford, 1993). This program performs a and deduce the basic themes which characterise cladistic analysis using parsimony. The analysis was the radiation of the group. based on a data matrix comprising 16 taxa and 21 characters. The characters are listed in Table 1. Terminology is essentially that of the recently revised Cretaceous ammonite volume of the Treatise on PHYLOGENY invertebrate paleontology (Wright et al. 1996) and the more detailed revisions of scaphitid morphology of I have elsewhere described a cladistic analysis Wiedmann (1965). They were based upon aspects of supporting the view of Wiedmann (1965) and suture line, coiling mode, and shell morphology. A Wiedmann & Marcinowski (1985) that the few characters were ordered; these are indicated in Figure 2. Abbreviated phylogeny of the Albian heteromorphs as described by Monks (1999). The Scaphitaceae are a clade within the ‘scaphitiform’ heteromorphs, which show a tendency toward closed, regular spiral coiling. 208 N. MONKS Table 1. Characters used for the phylogenetic analysis Suture line 11. Trifid lateral lobe (0) or bifid (1). 12. Trifid umbilical lobe (0) or bifid (1). 13*. Umbilical lobe narrower than adjacent saddles (0), similar in breadth (1) or broader (2). Ordering justified on assumption that in going from narrow to broad morphologies, or vice versa, an intermediate stage must be passed through. 14. Internal lobe comparable in size with umbilical lobe (0) or much smaller (1). 15*. Suture quadrilobate throughout ontogeny (0), initially quadrilobate but with a single