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Horns and Beaks
Ceratopsian and Ornithopod Dinosaurs
By Kenneth Carpenter Indiana University Press
Copyright © 2007 Indiana University Press
All rights reserved.
ISBN: 978-0-253-02795-5
CHAPTER 1
Callovosaurus leedsi, the Earliest Dryosaurid Dinosaur (Ornithischia: Euornithopoda) from the Middle Jurassic of England
José Ignacio Ruiz-Omeñaca, Xabier Pereda Suberbiola, and Peter M. Galton
Abstract
Callovosaurus leedsi (Lydekker 1889), based on an isolated femur from the Oxford Clay (Middle Jurassic, Callovian) of Peterborough, England, is reinterpreted as a dryosaurid. It represents the oldest record of this poorly known group of ornithopods. Callovosaurus was previously regarded variously as a hypsilophodontid, camptosaurid, or iguanodontid, but the femur shows a combination of characters typical of dryosaurids: bowed shaft; proximally placed pendant fourth trochanter; pit for insertion of the M. caudifemoralis longus well developed and separated from the fourth trochanter; and anterior intercondylar groove. Further, the concave excavation posteriorly proximal to medial condyle meets the medial surface of the distal end at a sharp edge, and the lateral condyle is transversely reduced with a rounded ledge lateral to it. Callovosaurus differs from Dryosaurus and Valdosaurus in the more expanded, transversely flattened anterior trochanter. Moreover, it differs from Valdosaurus in the shallow anterior intercondylar groove and the very slightly concave internal surface of the distal end.
Introduction
Lydekker (1889) described a left femur from the Oxford Clay near Peterborough, England as a new species of Camptosaurus, C. leedsi. In addition to the original (BMNH R1993; collection of Mr. A. N. Leeds, purchased in 1892), Lydekker (1890) also mentioned a cast of the femur (BMNH R1608, made in 1888). Gilmore (1909: 290) noted that C. leedsi is similar to Camptosaurus, but "if referable at all to an American genus, its closest affinities, as indicated by the femur, are with Dryosaurus." Galton (1972, 1974) placed C. leedsi in the Hypsilophodontidae as being closely related to Dryosaurus Marsh 1894 and the "Wealden hypsilophodont" (subsequently named Valdosaurus Galton 1977a). Later, Galton (1975) regarded C. leedsi as closer to Camptosaurus than to Dryosaurus, and assigned it to the Iguanodontidae as Camptosaurus (?) leedsi. Galton and Powell (1980: 437) listed several differences between the femur of C. leedsi and that of C. dispar and noted that "this femur should not be referred to the genus Camptosaurus and it probably represents a new genus." Finally, Galton (1980a) made Camptosaurus leedsi Lydekker 1889 the type species of Callovosaurus Galton 1980a, and referred it to the Camptosauridae. Since then, Callovosaurus leedsi has been regarded as Iguanodontidae? indet. by Weishampel (1992) and as Iguanodontia nomen dubium by Norman and Weishampel (1992), but Norman (1998) listed it as a valid species of Camptosauridae. More recently, Mateus and Antunes (2001) considered Callovosaurus a nomen dubium.
Age and Provenance of Callovosaurus leedsi
Lydekker (1889, 1890) recorded the type locality of C. leedsi as near Peterborough, Cambridgeshire (formerly Northamptonshire). According to Leeds (1956; see Galton 1980a), the femur was found in a brick pit near Fletton. The horizon is from the Oxford Clay, Middle Jurassic in age. It was referred to the Oxfordian (Upper Jurassic) by Galton (1975; note that the age is Callovian in the abstract), but the bone-bearing layer is from the lower Oxford Clay, which is middle Callovian (probably from the Jason Zone; see Galton 1980a). C. leedsi has also been listed as coming from the upper Callovian (Galton 1977b, 1980b; Galton and Powell 1980) or the middle-upper Callovian (Weishampel 1992). Cox et al. (1992) renamed the lower Oxford Clay the Peterborough Member of the Oxford Clay Formation. In addition to Callovosaurus leedsi, the reptilian fauna of the Peterborough Member (mainly the Jason Zone) of the Peterborough district includes other dinosaurs and reptiles: stegosaur Lexovisaurus durobrivensis (Galton 1985), ankylosaur Sarcolestes leedsi (Galton 1983a), sauropod Cetiosauriscus leedsi (Woodward 1905), pterosaurs (Unwin 1996), plesiosaurs, ichthyosaurs, and crocodilians (see Benton and Spencer 1995; Martill 1988).
Institutional Abbreviations. BMNH, Natural History Museum [formerly the British Museum (Natural History)], London; MB, Museum für Naturkunde (formerly the Humboldt Museum für Naturkunde), Berlin; MNHN, Museum National d'Histoire Naturelle, Paris; SMC, Sedgwick Museum, University of Cambridge, Cambridge; YPM, Peabody Museum of Natural History, Yale University, New Haven, Connecticut.
Systematic Paleontology
Ornithischia Seeley 1888
Ornithopoda Marsh 1881
Iguanodontia Dollo 1888
Dryosauridae Milner and Norman 1984
Callovosaurus Galton 1980a
Type Species. Camptosaurus leedsi Lydekker 1889.
Diagnosis. As for the only species known.
Callovosaurus leedsi (Lydekker 1889)
Synonymy. For a summary, see Martill (1988).
Holotype. BMNH R1993, a left femur (cast, BMNH R1608).
Type Locality and Horizon. Fletton, near Peterborough, Cambridgeshire, England; Oxford Clay Formation, Peterborough Member (=lower Oxford Clay), Middle Jurassic, middle Callovian (Jason Zone).
Revised Diagnosis. Broad, transversely flattened anterior trochanter (unexpanded, oval to roughly triangular in transverse section in Dryosaurus and Valdosaurus; closely pressed against the greater trochanter in Kangnasaurus); shallow anterior intercondylar groove (deep in Valdosaurus and some specimens of Dryosaurus); slightly concave medial surface to the distal end (flat in Valdosaurus and Kangnasaurus; variable in Dryosaurus).
Referred Material. Galton (1977a) referred an incomplete left tibia (SMC J.46889, collected in 1902) from Fletton to Dryosaurus (see Galton 1980a: 76, figs. 1g–i). The specimen, which came from the same or a nearly locality to that of C. leedsi, has since been assigned to an indeterminate hypsilophodontid (Galton 1977b; Weishampel 1992), but Galton (1980b) pointed out that it may be referable to C. leedsi.
Description
The femur of Callovosaurus leedsi (Figs. 1.1–1.2) has been briefly described and illustrated in several papers (Galton 1975, 1980a; Galton and Powell 1980; Gilmore 1909; Lydekker 1889, 1890), but it has never been described in detail. The specimen is 280 mm long, with maximum proximal and distal widths of 85 and 73 mm, and it belongs to a small ornithopod with an estimated total body length of about 2.5 m (Galton 1980a: table 1). The femur, originally complete, is now in three pieces that still fit together to give the whole bone, as illustrated (Fig. 1.1) by Galton (1980a). As noted by Lydekker (1889), the specimen is damaged and the middle portion of the shaft has been lateromedially crushed, but the proximal and distal ends are well preserved. The shaft is bowed in lateral and medial views. The neck of the femur is perpendicular to the shaft. On the posterior surface of the head, there is a well-developed depression for the antitrochanter (Sereno 1991: 193; "ischial peduncle" of Galton 1980a). The anterior trochanter (i.e., "lesser" trochanter; see Carpenter and Kirkland 1998) is separated from the greater trochanter by a deep and wide cleft (Figs. 1.1E, 1.2E), which extends distally to the level of the base of the head in medial view (Figs. 1.1C, 1.2C), but it is slightly shallower laterally (Figs. 1.1B, 1.2B). The anterior trochanter is broad, being flattened transversely, and its proximal end is below the top of the greater trochanter. The pendant fourth trochanter is proximally placed (the value of the fourth trochanter index is probably close to 0.48 according to Galton 1980a: table 1), but it is distally incomplete. The pit for the attachment of the M. caudifemoralis longus is a big (38 mm high, 20 mm wide) and shallow oval depression, located anteriorly on the medial surface of the shaft (see Discussion). It is separated from the fourth trochanter and is well above the base of this process. The shallowness of this depression could be an artefact due to crushing of the shaft. Distally, the femur has well-developed anterior (extensor) and posterior (flexor) intercondylar grooves. The base of the incomplete posterior lateral (fibular) condyle is transversely reduced in width and internally offset with a rounded shelf external to it. Proximal to the posterior medial condyle, there is a prominent, concave striated excavation, which meets the adjacent medial surface as a sharp sloping edge (area probably for part of M. femorotibialis). The medial surface of the distal end is anteroposteriorly very slightly concave (Figs. 1.1F, 1.2F).
Discussion
Callovosaurus as a Dryosaurid Ornithopod
Dryosaurids were small to medium sized (about 2–6 m in total length; see Heinrich et al. 1993), bipedal, cursorial ornithopods. They resemble Hypsilophodon and closely related forms in many respects and were for a long time placed within the Hypsilophodontidae (see Cooper 1985; Galton 1972, 1977a, 1981; Galton and Taquet 1982; Sternberg 1940). Following Milner and Norman (1984), Sues and Norman (1992) placed Dryosaurus (including Dysalotosaurus) and Valdosaurus in a separate family, the Dryosauridae, as the sister group of all other iguanodontians (Sereno 1986). Sues and Norman (1992) diagnosed the Dryosauridae on the basis of five characters, including two femoral ones: deep extensor groove in distal articular end, and deep pit for insertion of M. caudifemoralis longus developed at base of fourth trochanter. Moreover, Ryan (1997) mentioned additional femoral features: anteriorly bowing shaft as seen in all small ornithopods; deep separation of the greater and anterior trochanters; and very well developed fourth trochanter.
The femur of Callovosaurus exhibits all of the above-mentioned characters and several others that are seen in both Dryosaurus and Valdosaurus (Galton 1977a, 1980b, 1981; Galton and Taquet 1982), i.e., proximally placed pendant fourth trochanter, and posteriorly a reduced lateral condyle with the internally placed condylid bordered by a shelf laterally, and a prominent excavation proximal to the medial condyle that meets the medial surface of the distal end as a sharp edge (Blows 1998; Galton 1981; BMNH specimens). The deep cleft between the greater and anterior trochanters is absent in Kangnasaurus (see Cooper 1985). The deep intertrochanteric cleft, which is present in Dryosaurus, Valdosaurus, and Callovosaurus, could be related to the more cursorial abilities of these forms. The cleft increases the surface for the insertion of the M. iliofemoralis (which connects the anterior trochanter to the middle part of the ilium; see Galton 1969) and probably allows for a faster response of this muscle. In the same way, the development of a large pit in all dryosaurids (although its depth is variable) for the M. caudifemoralis longus, which connects the femur to the anterior caudal vertebrae (Galton 1969), is probably another cursorial adaptation that provides a bigger area for muscular insertion.
The presence of a distinct anterior intercondylar groove on the distal end of the femur is a synapomorphy of Iguanodontia (Sereno 1986; Weishampel and Heinrich 1992) (=Dryomorpha of Norman 1998). This clade includes Tenontosaurus, dryosaurids, camptosaurids, iguanodontids, and hadrosauroids. Gasparinisaura cincosaltensis from the Late Cretaceous of Argentina has been regarded as a basal member of the Euiguanodontia (Coria and Salgado 1996a; Salgado et al. 1997), but its phylogenetic relationships are controversial. The femur does not possess an anterior intercondylar groove, and the fibular condylid is not internally offset but is continuous with the lateral surface of the distal end, so it is more similar to hypsilophodontids than to iguanodontians (see Norman 1998). However, following Coria and Salgado (1996a), Gasparinisaura shares a number of characters with the Dryosauridae and evolved iguanodontians, i.e., a well-developed brevis shelf on the ilium and a metatarsal I that is reduced or absent.
The femur of Callovosaurus differs significantly from those of "hypsilophodontids." In Hypsilophodon foxii from the Upper Wealden (Barremian) of the Isle of Wight, England (BMNH collection, including BMNH R192a, R193, R195, R196, R2487, R8352; see Galton 1974), the anterior trochanter is slender and separated from the greater trochanter by a shallow cleft in medial view. However, some "hypsilophodontids" (a pectinate grade; see Scheetz 1998; Winkler et al. 1998), such as Othnielia rex, have a deep cleft (see Galton and Jensen 1973). The depression for the M. caudifemoralis longus is shallow in some femora of Hypsilophodon and deep in others, but is generally located close to the base of the fourth trochanter. Distally, the lateral and medial posterior condyles are almost equal in size, lacking the anterior intercondylar groove, the medial excavation, and its associated edge, and the medial surface of the distal end is convex (Galton 1974; Sues and Norman 1992).
Among the Iguanodontia, only dryosaurids have a femur with a proximally placed fourth trochanter. The fourth trochanter is located in the middle or on the distal part of the shaft in Tenontosaurus (Forster 1990), camptosaurids such as Camptosaurus (Gilmore 1909) and Draconyx (Mateus and Antunes 2001), iguanodontids such as Iguanodon (Norman 1980, 1986), Lurdusaurus (Taquet and Russell 1999), and Ouranosaurus (Taquet 1976), and all hadrosauroids (Godefroit et al. 1998; Weishampel and Horner 1992). This character may be related to large size.
On the other hand, Callovosaurus has a bowed femur in lateral view, as occurs in Dryosaurus, Valdosaurus, Kangnasaurus, Camptosaurus, and "hypsilophodontids," but also in Lesothosaurus (Sereno 1991) and Heterodontosaurus (Santa Luca 1980). The shaft is straight or slightly curved in iguanodontids and nearly straight in hadrosauroids (Godefroit et al. 1998; Norman and Weishampel 1992).
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