A rebbachisaurid sauropod from the Lower Cretaceous of the Isle of Wight, England

Abstract

Rebbachisauridae is one of the most enigmatic and poorly understood clades of sauropod dinosaurs. They are considered to be basal diplodocoids, are known solely from the Cretaceous (Hauterivian-Coniacian), and have only been recovered from Africa, South America, and Europe. As a result of their extreme skeletal reduction, rebbachisaurid material is highly susceptible to destructive taphonomic processes and thus most remains are highly incomplete and fragmentary. Previous remains attributed to rebbachisaurids from England are restricted to isolated teeth. Here a sauropod scapula from the Lower Cretaceous (Barremian) Wessex Formation of the Isle of Wight, England, is described. Although incomplete, this scapula possesses both the extreme dorsoventral expansion of the scapular blade and the “hook”-like acromial process that are characteristic of rebbachisaurids. This study has also enabled the recognition of a putative local synapomorphy of Rebbachisauridae, with the highest point on the dorsal margin of the scapula blade equal to or exceeding that of the dorsal margin of the proximal plate. This scapula is one of the oldest known examples of a rebbachisaurid and represents the first postcranial remains of this clade to be described from the United Kingdom. In addition, it provides further support for the presence of rebbachisaurids in the Early-mid Cretaceous of Europe.

Introduction

Rebbachisaurids are a relatively poorly understood clade of basal diplodocoid sauropods (Wilson, 2002, Upchurch et al., 2004, Sereno et al., 2007), known entirely from the Cretaceous (Hauterivian-Coniacian). Remains have been described from Africa (Lavocat, 1954, Sereno et al., 1999), South America (Calvo and Salgado, 1995, Bonaparte, 1996, Gallina and Apesteguía, 2005, Salgado et al., 2004, Salgado et al., 2006, Apesteguía, 2007), and Europe (Dalla Vecchia, 1998, Pereda-Suberbiola et al., 2003). Table 1 lists all named taxa.

The most complete rebbachisaurid, Nigersaurus Sereno et al., 1999, is known from several skeletons from the middle Cretaceous of Niger, Africa, and includes cranial material. Nigersaurus displays several unusual characteristics not seen in other sauropods, including possession of a dental battery (Sereno et al., 1999, Sereno et al., 2007, Sereno and Wilson, 2005). However, Nigersaurus, as well as the Argentinean taxon Limaysaurus Salgado et al., 2004, are the exceptions; most rebbachisaurid material is extremely fragmentary, and the paucity of remains may be a result of the extremely thin and open nature of their skeletons, which would make them particularly susceptible to destructive taphonomic processes.

The Barremian (Lower Cretaceous) Wessex Formation of the Isle of Wight, off the southern coast of England, possesses a diverse sauropod fauna, including titanosauriforms (with both brachiosaurids and titanosaurs present) as well as putative camarasauromorphs and diplodocoids (Blows, 1995, Upchurch, 1995, Naish and Martill, 2001, Naish and Martill, 2007, Canudo et al., 2002, Upchurch et al., 2004). Teeth illustrated by Naish and Martill (2001, pl. 36) were referred to the Rebbachisauridae by Sereno and Wilson (2005). Most remains from the Isle of Wight, however, are fragmentary and cannot be diagnosed at the generic level, although they can often be referred to higher clades.

This paper describes a sauropod scapula (MIWG 6544) collected by Nick Chase in 1989 from a red-brown clay bed on the southwest coast of the Isle of Wight (Fig. 1), England (S. Hutt pers. comm., 2008). Another specimen (an associated tibia and fibula; MIWG 5308) was also collected (by different collectors) at this locality in 1983 and a possible association with the scapula has been suggested (Martill and Naish, 2001, appendix). However, there is no strong evidence for this association (S. Hutt pers. comm., 2008) and consequently MIWG 5308 will not be discussed further.

Institutional abbreviations: CM, Carnegie Museum of Natural History Museum, Pittsburgh, Pennsylvania, USA; MIWG, Museum of Isle of Wight Geology (now Dinosaur Isle Visitor Centre), Sandown, Isle of Wight, England; MNN, Musée National du Niger, Niamey, Republic of Niger; USNM, Smithsonian National Museum of Natural History, Washington D.C., USA; UWGM, University of Wyoming Geological Museum, Laramie, Wyoming, USA; YPM, Peabody Museum of Natural History, Yale University, New Haven, Connecticut, USA.