Saturday, October 22, 2011

Scelidosaurus harrisoni, a basal thyreophoran from southern England

Fig 1.- A reconstruction of Scelidosaurus harrisonii.

The Black Ven cliff between Charmouth and Lyme Regis, in Dorset, southern England is world famous for its Early Jurassic fossils of marine animals such as ammonites and ichthyosaurs. It is also known for a single species of dinosaur that has only been found there, Scelidosaurus harrisoni. How this fully terrestrial animal ended up preserved in marine deposits is somewhat a mystery (carcasses were presumably washed to sea after death and quickly buried by layers of sand) but it allows getting a rare glimpse of the fauna that lived during the Early Jurassic period on the islands of Europe (Most of Western Europe was under a shallow sea at that time).

Scelidosaurus was known from quite some time having been described in 1861 by Sir Richard Owen based on a skull and various non-associated postcranial bits discovered near the village of Charmouth. The original material contains a right knee joint, a femur fragment and a phalanx (finger bone) that proved later to belong to one or several type of megalosaurs (theropod dinosaurs) (Newman, 1968). Some other elements described by Owen as belonging to a juvenile Scelidosaurus proved later to be from a hypsilophodont.  In the meantime, the postcranial skeleton associated to the original skull was uncovered and described by Owen in 1862. He named the species Scelidosaurus harrisonii, in honor of its discoverer, James Harrison.

Fig 2.- Original skull of Scelidosaurus in Owen's 1861 description of the genus. This skull is presently the lectotype as the holotype (a femur fragment) turned out to belong to a megalosaur.

These were the only fossils of the animal known for over a century and the exact affinities of the new dinosaur remained uncertain until fairly recently. The characteristic dermal scutes that make a kind of armor covering the body pointed to a close relationship with others armored ornithischians such as the stegosaurs, the ankylosaurs or the fabrosaurs. It was in turn considered a primitive stegosaur (von Zittel, 1902, Romer, 1956), an ankylosaur (Romer, 1968), a basal ornithopod (Thurlborn, 1977) and then generally considered a primitive ornithischian of some sort. A second skull previously described as a juvenile Scelidosaurus (Rixon, 1968) might actually belong to a hypsilophodont. Scutes from the Kayenta formation in Arizona were attributed to Scelidosaurus, which would indicate a large geographical range for the genus (Padian, 1989), but this identification has been questioned.

The uncertainty has dwindled in the last decade or so thanks in part to the fact that the original specimen described by Owen has been acid prepared so that it can be studied more thoroughly (Norman, 1996).  Also, new specimens of Scelidosaurus began to turn up in the 1990s. Dave Martill described two of them in 1991 and 2000. The first is a rather complete articulated skeleton including skull fragments.  It was found in 1985 near Charmouth in the Black Ven marls of Upper Sinemurian age. The second consists of a set of 8 articulated tail vertebrae and was acquired in 1998 from a private collection at the death of his owner, Prof. John Challinor. It unfortunately misses information about its provenance. A palynological analysis however indicates an Early Jurassic age from the same period as the Owen specimen (Late Hettangian- Sinemurian). Interestingly, both specimens show kerogenized traces of soft tissues indicating that the scutes (osteoderms) were covered by a horny sheath.  A third specimen was collected in the same area between Charmouth and Lyme Regis in 2000 by a local fossil collector, David Sole. It has been described as the most complete skeleton of a dinosaur ever found in the UK.

Carpenter (2001) places Scelidosaurus as a basal ankylosaur. However, most of the recent phylogenetical analysis including the latest comprehensive one on the Ankylosauria (Thompson et al., 2011) points toward an ancestral position of Scelidosaurus relatively to both the stegosaurs and ankylosaurs, as a basal member of the thyreophorans. Other genera probably related to it include the gracile Scutellosaurus lawleri from the Kayenta formation in Arizona (Sinemurian-Plienbaschian), and the rather enigmatic Emausaurus ernsti from the Toarcian of Germany. Tatisaurus oeheleri from the Lufeng formation of Yunnan might be another relative, although the remains are so fragmentary that it is difficult to be sure. The same can be said of Bienosaurus lufengensis from the same formation known from fragments of a skull, and Lusitanosaurus liasicus from the Early Jurassic of Portugal, also known from a few skull fragments and teeth.

 Fig 3.- Cast of a nearly complete skeleton of Scelidosaurus harrisonii found in 2000 and on display at the Charmouth Heritage Coast Center (photo by User:Ballista via wikipedia, CC3.0 licensed).

Scelidosaurus was a 4-meter long herbivorous dinosaur that probably walked on all fours although it was possibly able to stand on its hind legs from time to time. The body was covered with bony scutes called osteoderms as a protection against predators. Contrary to earlier depictions of the animal as a bulky quadruped, Scelidosaurus was probably more gracile in appearance with a relatively long neck compared to stegosaurs and ankylosaurs.

Original artworks on Paleoexhibit are copyrighted to Nobu Tamura. Do not use without permission (Email: nobu dot tamura at yahoo dot com)


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