Scelidosaurus
Name:
Scelidosaurus
(limb lizard).
Phonetic: Skel-is-doe-sore-us.
Named By: Richard Owen - 1859.
Classification: Chordata, Reptilia, Dinosauria,
Ornithischia, Thyreophora, Thyreophoroidea, Scelidosauridae.
Species: S. harrisonii
(type).
Diet: Herbivore.
Size: Roughly about 4 meters long.
Known locations: England, Dorset.
Time period: Sinemurian to Pliensbachian of the
Jurassic.
Fossil representation: Several individuals.
With
study of Scelidosaurus going all of the way back
until the
mid-nineteenth century, Scelidosaurus is one of
the oldest genera of
dinosaur known. In addition to that, out of all the dinosaurs
that are known from the British Isles, Scelidosaurus
is represented
by some of the most complete fossil material.
Scelidosaurus
was once thought to have also lived in what is now the south western
United States. This was based around the discovery of osteoderms in
the Kayenta formation of Arizona, however, later analysis of these
osteoderms has deemed them to be different from the known osteoderms of
Scelidosaurus, and therefore Scelidosaurus
remains only known from
the British Isles. It is perhaps not impossible though that the
osteoderms found in the Kayenta Formation may actually represent a
close relative of Scelidosaurus.
The
key significance of Scelidosaurus is that aside
from being one of the
first dinosaurs known to science, the genus represents one of the
most primitive thyreophoran dinosaurs so far discovered, even though
over one hundred and fifty years have passed since it was first
described. However the true significance of Scelidosaurus
was lost on
the describer of the genus Richard Owen who perceived Scelidosaurus
to
have been semi aquatic and a piscivore, an eater of fish!
The
truth about Scelidosaurus is that it was an
ornithischian dinosaur that
ate plants. Also, while the rear limbs are longer than the fore
limbs, the fore limbs are still formed and long enough to suggest a
weight bearing function, meaning that Scelidosaurus
was mostly if not
entirely quadrupedal. This also means that when given a choice,
Scelidosaurus would have preferred to roam about on
land.
Most
of the study of Scelidosaurus has been focused on
the osteoderms
(scutes), plates of bone that fixed in place within the skin of
Scelidosaurus rather than attaching directly to the
skeleton. These
osteoderms were arranged in rows that ran down the length of the body
from head to tail. Individually the osteoderms were formed into ovals
with a keel (ridge down the centre) on the outer side. The keel
was also slightly higher on the posterior (end facing the tail)
part of the osteoderm. The largest osteoderms were arranged in rows
on the more exposed flanks (sides), while the osteoderms towards
the underside were smaller and somewhat more conical.
It
is most probable that the function of these osteoderms was to provide
defence from the predators of the time such as theropod dinosaurs.
The main killing weapons of these dinosaurs would have been the sharp
teeth in the mouths as well as claws on their hands and feet. These
body parts work by puncturing the skin and outer tissues to damage
internal organs and bones. However, by having round plates of bone
in the skin, it becomes significantly more difficult for a predator
to only puncture the skin of its prey, but get a grip since the hard
round surface of the osteoderm would cause the tooth/ claw to slide,
potentially to even break. It has even been surmised that smaller
granules of bone may have been present in between the spaces of the
regular osteoderms to make the skin and underlying tissue even more
resilient to the attacks of predators.
By
having such a tough skin Scelidosaurus would have
been impervious to
the attacks of all but the largest and most powerful of predators of
the time, and even then these predators may have preferred to hunt
‘softer’ prey. In time though predators adapt to hunt either a
greater variety or specialise in just one type, and this could even
include armoured dinosaurs. This would explain the forms of later
thyreophoran dinosaurs such as the nodosaurs
and ankylosaurs that took
osteoderm armour to the extreme during the Cretaceous period,
probably as an adaptation to increasingly powerful predatory dinosaurs
appearing.
As
already mentioned, Scelidosaurus represents one
of the earliest
appearances of a thyreophoran dinosaur. More famous thyreophorans
include the stegosaurs
which became common by the Late Jurassic,
nodosaurs that are better known from the late Jurassic to the late
Cretaceous and ankylosaurs that were most common in the late
Cretaceous. We are fairly certain that the ankylosaurs evolved from
the nodosaurs, with the main (but not only) difference being that
ankylosaurs have tail clubs. We know that the stegosaurs are related
to the nodosaurs and ankylosaurs with a more ancient and slightly murky
ancestry that probably goes back to the early Jurassic when the two
lines began to diverge from a common group of ancestors.
Unfortunately
we still don’t know exactly how Scelidosaurus was
related to these
later groups. It would be no great surprise to learn that the
osteoderm arrangement of Scelidosaurus and similar
dinosaurs was the
precursor to the thicker and more complete armour of the nodosaurs.
Care should be taken before assuming a link though since the
similarity may simply be the result of two groups facing the same
ecological survival conditions. Only greater study, as well as the
discovery of more transitional forms that show a steady adaptation from
one form into another could help to determine the truth.
In
the past there has been more than one species of Scelidosaurus
named,
though currently the only species considered to be valid is the type
species S. harrisonii. One former species S.
oehleri was
established from the fragmentary remains of Tatisaurus,
however,
many palaeontologists have rejected this in favour of keeping
Tatisaurus a distinct genus.
Further reading
- A monograph of a fossil dinosaur (Scelidosaurus
harrisonii,
Owen) of the Lower Lias, part I. - Monographs on the British
fossil Reptilia from the Oolitic Formations 1 pp 14 - Richard
Owen - 1861.
- A monograph of the fossil Reptilia of the Liassic Formations.
Part 2. A monograph of a fossil dinosaur (Scelidosaurus
harrisonii Owen) of the Lower Lias. - Palaeontographical Society
Monographs. Part 2. pp. 1-26 - Richard Owen - 1863.
- The Mesozoic reptiles of Dorset: Part Two - Proceedings of
the Dorset Natural History and Archaeological Society, for 1958
80: 52-90 - J. B. Delair - 1959.
- The Jurassic dinosaur Scelidosaurus harrisoni,
Owen. -
Palaeontology 11 (1), 40-3 - B. H. Norman - 1968.
- The development of the remains of a small Scelidosaurus
from a Lias
nodule - Museums Journal, 67: 315–321 - A. E. Rixon
- 1968.
- Relationships of the lower Jurassic dinosaur Scelidosaurus
harrisonii - Journal of Paleontology. July 1977;
v. 51;
no. 4; p. 725-739 - R. A. Thulborn - 1977.
- New scelidosaur remains from the Lower Lias of Dorset -
Proceedings of the Dorset Natural History and Archaeological Society
110: 165 & 167 - P. C. Ensom - 1989.
- Presence of the dinosaur Scelidosaurus
indicates Jurassic age for
the Kayenta Formation (Glen Canyon Group, northern Arizona) -
Geology. May 1989, v. 17; no. 5; p. 438-441 - K.
Padian - 1989.
- Scelidosaurus harrisonii Owen, 1861
(Reptilia,
Ornithischia): proposed replacement in inappropriate lectotype
- Bulletin of Zoological Nomenclature, 49: 280–283 - A.
J. Charig & B. H. Newman - 1992.
- Scelidosaurus harrisonii Owen, 1861 (Reptilia,
Ornithischia): lectotype replaced - Bulletin of Zoological
Nomenclature 51: 288 - International Commission on Zoological
Nomenclature - 1994.
- The Thyreophoran Dinosaur Scelidosaurus from
the Lower Jurassic
Lufeng Formation, Yunnan, China. pp. 81-85, in M. Morales
(ed.) The Continental Jurassic. Museum of Northern Arizona
Bulletin 60. - S. G Lucas - 1996.
- Professor Richard Owen and the important but neglected dinosaur
Scelidosaurus harrisonii - Historical Biology,
14: 235–253
- D. B. Norman - 2000.
- A New Specimen of the Thyreophoran Dinosaur cf. Scelidosaurus
with Soft Tissue Preservation. - Palaeontology, Vol. 43,
Part 3, 2000, pp. 549-559 - D. M. Martill, D. J.
Batten & D. K. Loydell - 2000.
- Dinosaurs of Great Britain and the role of the Geological Society
of London in their discovery: basal Dinosauria and Saurischia, -
Journal of the Geological Society, London, 164: 493–510 -
D. Naish & D. M. Martill - 2007.
- Reconsidering the status and affinities of the ornithischian
dinosaur Tatisaurus oehleri Simmons, 1965 -
Zoological Journal of
the Linnean Society 150 (4): 865–874 - D. B. Norman,
R. J. Butler & S. C. R. Maidment - 2008.
- Scelidosaurus harrisonii from the Early Jurassic of Dorset, England:
cranial anatomy. - Zoological Journal of the Linnean Society. 188 (1):
1–81. - David B. Norman - 2020.
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