Scelidosaurus

Norman, David B, 2021, Scelidosaurus harrisonii (Dinosauria: Ornithischia) from the Early Jurassic of Dorset, England: biology and phylogenetic relationships, Zoological Journal of the Linnean Society 191 (1), pp. 1-86 : 55-57

SCELIDOSAURUS : DERMAL SKELETON

The structure ( Norman, 2020c), histology and biological role of the osteodermal skeleton of Scelidosaurus are topics that will be dealt with elsewhere (Norman & Baker, in preparation).

THE SYSTEMATIC AND PHYLOGENETIC POSITION OF SCELIDOSAURUS

SCELIDOSAURUS, EARLY DINOSAUR CLASSIFICATION AND RELATIONSHIPS

Richard Owen established that Scelidosaurus was a member of his Dinosauria , based on the structure of its hindlimb:

The internal process [4 th trochanter of the femur] is similarly well preserved, repeating the character of the herbivorous Dinosauria which is exemplified in the Iguanodon [italics]…. The medullary or unossified cavity of the shaft of the bone has been more considerable, in relation to the compact shaft, than in the large femora previously described.

From the foregoing characters it may be concluded that the present femur has belonged to a Dinosaur, allied to the Iguanodon [italics], ….

( Owen, 1861: 5) Despite the fact that Owen was describing the characteristics of a theropod femur ( Owen, 1861: tab. I – see Norman, 2020a: fig. 2A), his words were still applicable to other genuine scelidosaur remains that he included in this preliminary description ( Owen, 1861: tab. III – see Fig. 2 ). Owen was notably reticent when it came to expressing views about the relationship of this new dinosaur to any of the other, then known, dinosaurs. His clearest remark was:

Upon the whole, I find the closest agreement to be between Scelido - and Hylaeo-saurus in the characters of the vertebral column; ….

( Owen, 1863: 12)

And yet, despite Owen’s extended description of parts of the dermal skeleton of Scelidosaurus ( Owen, 1863: 20–26) , there is neither a single mention of the osteoderms visible on the slab of Tilgate Stone containing the remains of Hylaeosaurus , nor a more detailed consideration of the potential relationship between these taxa.

Attempts to systematize and classify known taxa of Dinosauria began in the late 1860s, stimulated by the work of Edward Drinker Cope (1866). Cope established two groupings or ‘orders’ of dinosaur based on his interpretation of their supposed ankle and foot structures: Orthopoda (including Scelidosaurus , Hylaeosaurus, Iguanodon and Hadrosaurus ) and the Goniopoda ( Megalosaurus and Laelaps ). A year later, Cope (1867) added a third order: Symphopoda, based upon his interpretation of the shin (crural), ankle (tarsal) and foot (pedal) structures in Compsognathus . The latter, he pointed out, suggested a resemblance between the feet of some dinosaurs and those of living birds. Subsequently, Thomas Henry Huxley (1868) demonstrated that the anatomical (‘intermediate’) similarities shared between the then-known dinosaurs and living reptiles and birds provided evidence that strongly supported Darwin’s theory of evolution. Listing the anatomical differences between living reptiles and birds, Huxley proceeded to fill the anatomical ‘gap’ between such distinctly different organisms by demonstrating that the fossil bird Archaeopteryx exhibits a number of reptilian features. Furthermore, he was able to demonstrate that the larger dinosaurs possessed many bird-like features in their sacra, pelves, hindlimbs and feet. The discovery of the small, lightly built dinosaur Compsognathus compounded the similarities between dinosaurs, Archaeopteryx and living birds, and allowed him to observe that ‘ Dinosauria wonderfully approached … birds in their general structure, and therefore that these extinct reptiles were more closely allied to birds than any which now live’ ( Huxley, 1868: 73). This proposition was subsequently reinforced in a summary article ( Huxley, 1870a) that led him to conclude ‘if the whole hind quarters, from the ilium to the toes, of a half-hatched chicken could be suddenly enlarged, ossified and fossilized as they are, they would furnish us with the last step of the transition between Birds and Reptiles; for there would be nothing in their characters to prevent us from referring them to the Dinosauria ’ ( Huxley, 1870a: 30–31). Huxley also began, in this same article, to dismantle Cope’s classification of dinosaurs by pointing to anomalies in the leg and ankle structure among Cope’s ordinal varieties. Subsequently, Huxley (1870b) reviewed and revised the classification of Dinosauria and, notwithstanding the primacy of Meyer’s (1832) name Pachypoda, grudgingly accepted Owen’s (1842) name Dinosauria :

… it may be well to allow justice to give way to expediency, and to retain the name of Dinosauria for these reptiles.

( Huxley, 1870b: 33)

Huxley, using an approach that strikes us today as singularly prescient, provided a list of 12 diagnostic characters shared by known dinosaurs. The dinosaurs, thus diagnosed, were then divided into three ‘natural groups’ (given familial ranking): Megalosauridae , Scelidosauridae , Iguanodontidae – and he added to these a group differentiated from the previous three by referring to it as Compsognatha. Each of these groups was supported by a list of diagnostic anatomical characters. However, Huxley stressed that the taxon Compsognatha (not given a familial suffix) was distinct anatomically from the three other dinosaurian families by virtue of possessing a slender, elongated neck and cursorial hindlimb proportions. All these features were demonstrable in the nominal taxon Compsognathus . He concluded that it was necessary to create a new order of fossil reptiles that he named Ornithoscelida (because all of them exhibited bird-like limbs) and within this order he identified two suborders that he named Dinosauria (characterized by being short-necked and having stout graviportally adapted limbs) and Compsognatha (with long necks and slender cursorial limbs). Given that Huxley was dealing with just seven recognized dinosaur taxa, and how little of their anatomy was known, this work was remarkable.

HUXLEY’ S (1870B) CLASSIFICATION

Order: ORNITHOSCELIDA

Suborder: DINOSAURIA

Family Megalosauridae

Family Scelidosauridae

Family Iguanodontidae Suborder: COMPSOGNATHA

Nominal taxon Compsognathus

Developing his concept of ornithoscelidans by c o m p a r i n g t h e m t o o t h e r t h e n - k n o w n r e p t i l e groups, Huxley introduced a confusing set of alleged affinities (= relationships) that resulted in his creating further higher level taxonomic groupings based upon vertebral column morphology alone. These superordinal categories – Suchospondylia, Erpetospondylia, Pleurospondylia and Perospondylia – were neither acknowledged nor formally adopted by contemporary taxonomists and systematists and – perhaps fortunately – went the way of Cope’s ill-fated Orthopoda, Goniopoda and Symphopoda.

During subsequent decades, a greater range and variety of better-preserved dinosaur fossils were discovered, largely as a result of the prodigious efforts of Cope and Othniel Charles Marsh in the United States, as well as Louis Dollo in Europe ( Colbert, 1968; Desmond, 1975). A clearer but more complicated picture of the range and variety of dinosaurian anatomy and morphology began slowly to emerge. Marsh (1881, 1884, 1891, 1895) assembled, illustrated and defined groupings of dinosaurs whose names and general attributes are still recognized today: Theropoda, Sauropoda, Ornithopoda , Stegosauria and Ceratopsia. Harry Govier Seeley [1887 (1888)] also developed a utilitarian classification of dinosaurs that was based primarily upon pelvic morphology (using the term ‘ischia’ to mean pelvis in an Aristotelian sense) and the presence of vertebral pneumatism. Seeley’s system divided all known dinosaurs into two fundamentally distinct groups that he named Saurischia and Ornithischia . The clear-cut anatomical differences accommodated Marsh’s groupings consistently and rationally, and were taken by Seeley to be so fundamental as to imply that Dinosauria per se was not a natural group (clade):

Dinosauria has no existence as a natural group of animals, but includes two distinct types of animal structure with technical characters in common, which show their descent from a common ancestry rather than their close affinity.

( Seeley (1887 [1888]: 170)

And a little later (p. 171):

I see no ground for associating these two orders in one group.

SEELEY’ S (1887[1888]) CLASSIFICATION

Order: SAURISCHIA

Suborder: Theropoda

Suborder: Sauropoda

Order: ORNITHISCHIA

Suborder: Ornithopoda

Suborder: Ceratopsia

Suborder: Stegosauria

Although there were fundamental disagreements concerning the monophyly or (Seeley’s preference) diphyly and even polyphyly of dinosaurs with respect to their origin from archosaurian predecessors (e.g. Romer, 1933, 1968; Charig et al., 1965; Bakker & Galton, 1974; Charig, 1982), Seeley’s fundamental classificatory scheme proved robust in the face of continuous discoveries of new dinosaur taxa. In a major cladistic review of the Archosauria, Gauthier (1986) finally established a consensus over the matter of the monophyly of the Dinosauria and one that incorporated Seeley’s two principal taxa as sister-taxa. It is only comparatively recently that the underlying topology of the constituent members of the clade Dinosauria has been challenged ( Baron et al., 2017b, c).

Figures

Figure 2. Scelidosaurus harrisonii Owen, 1861 (LYMPH 1998 6.1-6.7).The small(‘foetal’) specimen from Charmouth. Part of the paratype series belonging to the name S. harrisonii. These specimens belong to a small individual and appear to be associated. They were illustrated in Richard Owen’s first monograph (Owen, 1861: tab. III).