Synopsis. The proximal parts of the right ulna and radius of a thyreophoran dinosaur from Bear- reraig Bay, Isle of Skye, represent the first occurrence of this type ...
A thyreophoran dinosaur from the Early Bajocian (Middle Jurassic) of the Isle of Skye, Scotland N. D. L. CLARK
Hunterian Museum, University of Glasgow, Glasgow, G12 8QQ Synopsis The proximal parts of the right ulna and radius of a thyreophoran dinosaur from Bearreraig Bay, Isle of Skye, represent the first occurrence of this type of dinosaur from the Middle Jurassic rocks of Scotland. The incompleteness of the bones and the lack of more diagnostic features does not allow a precise identification although the ulna has more features in common with that of a eurypodan (stegosaurs and ankylosaurs) than a basal thyreophoran. The earliest identifiable stegosaur from Europe that has similar proximal expansion of the ulna is the middle Callovian (Middle Jurassic) Lexovisaurus of England and France. The ulna of the ankylosaur Mymoorapelta from the Upper Jurassic of Western Colorado, also shows close morphological similarity to the Scottish bone. Other ankylosaur fragments have been recorded from as early as the late Bajocian (Middle Jurassic) of England. The stratigraphic horizon from which the new bones come from is the Bearreraig Sandstone Formation (Early Bajocian, Middle Jurassic).
Introduction Dinosaurs are only recently known from the Scottish Jurassic. In 1982 the first evidence that dinosaurs inhabited what is now Scotland came to light. A dinosaur footprint was discovered from near the top of the Lealt Shale Formation (Bathonian, Middle Jurassic). It is thought to have been produced by an ornithopod dinosaur (Delair and Sarjeant 1985) although it was originally described as a possible theropod footprint by Andrews and Hudson (1984). The footprint was collected and is now housed in the Hunterian Museum, University of Glasgow. A theropod bone, thought to be the proximal part of a theropod tibia referable to the Ceratosauria, was recov^et-dfrttm Hett&ngv&n d e p u t e on the \s\e oi Sk^e (Benton et al. 1995). The femur of a cetiosaur-like sauropod was found in the Bathonian sediments of northern Skye (Clark 1995; Clark etal. 1995) as was an undescribed caudal vertebra of a similar sauropod. A small undescribed ceratosaurian caudal vertebra, also from the Isle of Skye and of similar age, was found in 1995, and is held in the collections of the Hunterian Museum, Glasgow. Scotland's first dinosaur trackway, belonging to at least one small animal with tracks about 20-30 cm in length was discovered in 1996 (Clark and Barco-Rodriguez 1998). Another possible dinosaur is the early Norian (Triassic) Saltopus elginensis Huene 1910 from Lossiemouth (Benton and Walker 1985). However, the material is not unequivocally dinosaurian and could represent the remains of an early pterosaur or lagosuchid (Norman 1992). The ulna and radius described here (SM 1977.1997.1 (SM = Staffin Museum)) were found articulated in a single loose block of grey calcareous sandstone. Also associated with these bones was another elongate bone approximately 4-5 cm in diameter and approximately 25 cm in
length. Unfortunately, another unknown collector had removed the bone before this bone could be recovered from the beach (Aitken pers. comm.). It is possible that this missing bone represented another part of the same animal.
Stratigraphical setting The bone was found on the foreshore north of Bearreraig Bay (precise grid reference lodged with the Hunterian Museum, University of Glasgow), in a grey sandstone characteristic of the Bearreraig Sandstone Formation (Bajocian, Middle Jurassic). The block is likely to come ftosn eithei the ^»gg Sandstone Member oi the Hokn Sandstone Member of the Bearreraig Sandstone Formation (Early Bajocian, Middle Jurassic) (Fig. 1) as these horizons are exposed in the cliffs above where the block was found (the Upper Sandstone Group and the Massive Sandstone Group of Morton 1963, 1965, 1975). The Rigg Sandstone Member contains ammonites of the Stephanoceras humphriesianum Zone and the lowest part of the Holm Sandstone Member contains ammonites of the Witchellia laeviuscula Zone providing a clear Early Bajocian age to the rocks (Morton 1963, 1965, 1975; Anderson & Dunham 1966) (Fig. 1). Both the Rigg and Holm Sandstone Members were deposited within a shallow marine near shore environment (Bell & Harris 1986; Harris 1992) although there is some evidence of emergence in the Holm Sandstone Member (Bradshaw & Cripps 1992a). Material The Staffin Museum specimens (SM 1977.1997.1), described here (Figs 2, 3 and Ad), were discovered by Colin Scottish Journal of Geology 37, (1), 19-26, 2001
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N. D. L. CLARK
Lithostratigraphy at Bearreraig Bay
Biozone Late Bajocian
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FIG. 1. Diagram showing the stratigraphic setting (based on Morton 1965 and Harland et al. 1990) from which the ulna and radius were derived. Aitken of Edinburgh in 1997. The specimen was donated to the Staffin Museum and taken to the Hunterian Museum for preparation and study. The bones were separated from the matrix using an engraver. Broken fragments were glued using paraloid B72 with acetone.
to the coronoid process are convex (Fig. 2d). The sea-worn shafts show less than 5 mm thickness of perichondral bone and an approximate shaft width of 3.5 cm. The shaft of the ulna has a hollow centre that is filled with calcite (Fig. 2a). Discussion
Description The shafts of the right ulna (Figs 2a-e and 3a-b) and radius (Figs 2e, f) are incomplete allowing only the proximal ends to be described. The two bones represent part of the right forelimb. In cross-section, the radius is oval and the head of the ulna is triangular. The ulna is 14 cm long with a 4 cm olecranon process and a pronounced 5 cm lateral (or coronoid) process (Figs 2b-e and 3b). The head of the radius rests within a concavity of the coronoid process for about 4 cm along its edge (Fig. 2e). The posterior surface of the ulna is curved outwards (convex) and broadens to 7.5 cm wide at the height of the coronoid process (Figs 2b, c, 3a and 4a). This curvature is accentuated slightly due to a small amount of surface bone still attached to the counterpart. Both sides of the ulna lateral
The sizes of the olecranon and coronoid processes correspond to those seen in the thyreophoran dinosaurs, and more specifically the Eurypoda (Sereno 1986); stegosaurs and ankylosaurs. The Thyreophora are a diverse group of armoured dinosaurs that include the basal thyreophoran Scutellosaurus, from the Kayenta Formation (Hettangian) of the USA (Fig. 4e), and a diverse assemblage of Jurassic and Cretaceous stegosaurs and ankylosaurs (Colbert 1981; Coombs et al. 1992; Galton 1992; Coombs & Maryanska 1992) (Fig. 5). Of the basal thyreophorans, the ulna and radius of Scutellosaurus (Fig. 4e) are incompletely known (Colbert 1981), but are straight and cylindrical and the olecranon of the ulna is poorly developed, unlike specimen SM 1977.1997.1 (Fig. 2d). The ulna and radius of Scelidosaurus
A THYREOPHORAN DINOSAUR FROM THE EARLY BAJOCIAN
FIG. 2. (a) Worn shaft of the ulna (xl.5), (b) Right lateral view and (c) Left lateral view of ulna showing curvature of posterior edge and the extent of the coronoid and olecranon processes (XO.75), (d) Anterior view of ulna showing lateral convexity below coronoid process, (e) Stereographic pair of ulna and radius in oblique anterior view (XO.75), (f) Head of radius showing surface that articulates with the ulna (XO.75).
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N. D. L. CLARK
FIG. 3. Line drawing of the ulna in (a) anterior view and (b) left lateral view. O = olecranon process, C = coronoid process, r - articulating surface with the radius. are undescribed (Coombs et al. 1992), although, according to Steel (1969), the limb bones are generally of slender proportions. The earliest known named stegosaur to date is Huayangosaurus from the Bathonian-Callovian (Middle Jurassic) of China (Dong et al. 1982). Some fragmentary remains from the Lower Bathonian of England (Boneham & Forsey 1992; Galton 1992, 1997), and unsubstantiated Lower to Middle Jurassic stegosaur fragments also exist (Hoffstetter 1957; Lucas 1994). Spines assigned to 'Stegosaurus' from the Inferior Oolite (AalenianBajocian) were recorded by Benton and Spencer (1995) and two dermal plates of a stegosaur were figured by Reynolds (1939) from the Late Bajocian of Gloucestershire. Hoffstetter (1957) suggested that the dermal plates from Gloucestershire have more in common with dermal plates of the Cretaceous ankylosaurs; however Galton (1985) regards the material as stegosaurian. A fragment of a left mandible of Tatisaurus from the Lufeng Formation (Lower Jurassic) of China which was originally referred to the Hypsilophodontidae by Simmons (1965), was placed with the stegosaurs by Dong (1990), and was most recently referred to the basal thyreophoran Scelidosaurus by Lucas (1996). The proximal end of the ulna of Lexovisaurus (middle Callovian-Kimmeridgian) and Dacentrurus (OxfordianKimmeridgian) is more expanded anteroposteriorly than the slender ulna of Huayangosaurus (Galton 1992). The proximal end of the ulna of Stegosaurus (Kimmeridgian-
Tithonian) (Fig. 4d) is massive compared to other stegosaurs (Galton 1992). The Scottish thyreophoran (Fig. 4a) has a massive olecranon similar to that of Stegosaurus ungulatus (Fig. 4d) although the ulna itself is approximately half the size. The radius is also similar to that of stegosaurs in that it has an oval cross-section. In the ankylosaurs, the olecranon process is as large as it is in some stegosaurs and commonly is up to a third of the full length of the ulna (Figs 4b, c). Ankylosaurs are primarily known from the Cretaceous Period and are represented by only a few fragments from the Jurassic (Coombs & Maryanska 1992) and partial skeletons, such as that of Tianchiasaurus nedegoapeferima from the Middle Jurassic Toutunhe Formation of China (Dong 1993), Gargoyleosaurus parkpini and Mymoorapelta maysi from the Upper Jurassic Morrison Formation (Kirkland & Carpenter 1994; Carpenter et al. 1998). Nodosaurid ankylosaurs are now known from the Middle Callovian {Sarcolestes leedsi), the Upper Oxfordian (Cryptodraco sp.) and the Kimmeridgian of England (fragments) and Portugal (Dracopelta zbyszewskii) (Galton 1983). There is a tooth from closer to the Jurassic-Cretaceous boundary described by Galton (1983) although this is now thought to be basal Cretaceous (Berriassian) in age (Allen & Wimbledon 1991). The ankylosaur Priodontognathus phillipsii may also be from the Jurassic, however, as its contextual label is lost, it is unknown whether it originated from the Oxfordian or the Wealden (Galton 1980). In nodosaurids the radius is somewhat circular in cross-section
A THYREOPHORAN DINOSAUR FROM THE EARLY BAJOCIAN
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FIG. 4. Comparative sketches of the lateral views of the right ulna of (a) the Skye thyreophoran, (b) Mymoorapelta maysi (after Kirkland and Carpenter 1994), (c) Sauropelta edwardsi (after Coombs and Maryanska 1992), (d) Stegosaurus ungulatus (after Galton 1992; Gilmore 1914), (e) Scutellosaurus lawleri (after Colbert 1981). Scale bar = 10cm except in (e) where it is 1cm.
whereas that of ankylosaurids is ovate in cross-section (Coombs & Maryanska 1992). The cross-section of the radius of specimen SM 1977.1997.1 is oval like that of an ankylosaurid or stegosaur. The curvature of the posterior edge of the ulna in the Cretaceous nodosaur Sauropelta edwardsi is greater than that of specimen SM 1977.1997.1. The ulna of the possible nodosaurid Mymoorapelta maysi has a broad and massive olecranon process with a rugose surface (Kirkland & Carpenter 1994; Kirkland et al 1998) similar to the ulna from Skye. The height of the olecranon process relative to the length of the coronoid process is very similar in Mymoorapelta although the angle between the two processes is about 140° in Mymoorapelta and about 110° in the Skye bone. Kirkland & Carpenter (1994) remark on the similarity between the ulna of Mymoorapelta (Fig. 4b) and that of the early stegosaurs although it is generally more massive. The Skye specimen is as massive as the ulna of Mymoorapelta, has a similar postero-proximal curvature of the olecranon process, and is almost identical in size. Unfortunately the shaft of the Scottish bone is not complete enough to compare with the sigmoidal shaft of the ulna in Mymoorapelta. Although Mymoorapelta has been tentatively placed in the Nodosauridae (see Fig. 5), Kirkland et al. (1998) found it difficult to place it with any confidence in either the Ankylosauridae or the Nodosauridae and suggested that a new family may be erected to include Mymoorapelta and Polacanthus. Context In the Early Bajocian a shallow seaway may have extended northwards to western Scotland (Bradshaw & Cripps \992a). Tidal currents flowing along the east coast of
an Inner Hebridean Strait were deflected by the offshore banks formed by the mid-Skye High and north Raasay and Rona (Morton 1983, Bradshaw & Cripps 1992a). These offshore highs were periodically emergent as shown by rootlets in sediments of the sauzei Zone at Torvaig (Bradshaw & Cripps 1992a). This emergence at the level of the Holm Sandstone Member of the Bearreraig Sandstone Formation coincides with regression and hardground formation in England (Bradshaw & Cripps 1992a). It may be that the articulated thyreophoran ulna and radius (SM 1977.1997.1) derived from one of these periods of emergence which may have connected the Trotternish area with the Hebrides Platform, or even the Scottish Landmass. During the Late Bajocian the Sea of Hebrides Basin was subjected to a marine transgression probably allowing a seaway to develop again connecting it with the North Minch Basin (Bradshaw & Cripps 1992ft). Later (in the Mid Bathonian) sedimentological and palaeontological evidence suggests that the basin was subject to salinity fluctuations, with the Lealt Shale Formation and the Valtos Sandstone Formation being deposited in a lagoonal setting (Hudson & Harris 1979; Harris 1992). The lagoon was shallow and periodically exposed to produce hypersaline mudflats (Bradshaw & Cripps 1992c). In the Late Bathonian there is a trend to more terrestrial and alluvial sedimentation isolating shallow pools from the sea to the south (Bradshaw & Cripps \992d) to a greater extent than occurred during the Early Bajocian. The Scottish Landmass was connected to the Welsh, Pennine, and AngloBrabant Landmasses as well as the Mid North Sea High during the Late Bajocian, although during the Bathonian the southern landmasses were substantially reduced with the development of a seaway between the Cleveland basin and the Cotswold-Weald Shelf.
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