Hylaeosaurus
Hylaeosaurus (hi-LAY-ee-oh-SORE-us) is the most
obscure of the three animals used by Sir Richard Owen to first
define the new group Dinosauria, in 1842. The original specimen,
recovered by Gideon Mantell in 1832 from the Tilgate Forest in the
south of England in 1832, now resides in the Natural History Museum
of London, where it is still encased in the limestone block in
which it was found. Despite never having been prepared, it is still
the best specimen that exists of this primitive, armored
ankylosaurian.
Hylaeosaurus
Description and environment
Hylaeosaurus lived about 135 million years ago, in the Valanginian
to Berriasian ages of the early Cretaceous. Gideon Mantell
originally estimated that the Hylaeosaurus was about 25 feet (7.5
meters) long, or about half the size of the other two original
dinosaurs, the Iguanodon and the Megalosaurus. Modern estimates
range from 3 to 6 meters (10 to 20 feet) in length.
It is a fairly typical armored dinosaur, with three long spines on
its shoulder, two at the hips, and three rows of armor running down
its back. It may also have had a row of plates down its tail. It
has a long head, more like the head of a Nodosaurus than an
Ankylosaurus; and a beak, which it probably used to crop low-lying
vegetation.
Classification
Hylaeosaurus armatus was first named by Gideon Mantell in 1833, and
is currently considered the only species in the genus. It is known
from only two partial skeletons, some horny (dermal) spines, armor,
and various other minor pieces. The best specimen (the original) is
composed of the front end of a skeleton minus most of the head,
though only the parts on the face of the stone block are easily
studied.
Polacanthoides ponderosus, Hylaeosaurus conybearei, and
Hylaeosaurus oweni have all been considered distinct dinosaurs in
the past, but are now considered to be alternate names for this
species (junior synonyms), along with Hyaelosaurus. It has also
been suggested that Polacanthus is same species, but there are a
number of differences in their bone structure (osteology).
Hylaeosaurus is traditionally considered to be a primitive
nodosaurid, in the Polcanthinae subfamily, like Gastonia and
Polcanthus. In the 1990s, the polcanthines were reclassified as
primitive ankylosaurids, because they were mistakenly believed to
have small tail-clubs; they are probably primitive ankylosaurids,
but as a whole the polcanthines are poorly known. The group peaked
in the Barremian age in North America and Europe, and then vanished
shortly after, replaced by more advanced ankylosaurians.
History
The first Hylaeosaurus fossils were discovered in Sussex.
Additional remains have been discovered on the Isle of Wight (also
part of Great Britain), and in Ardennes, France, though the remains
from France may actually belong to a Polacanthus. Mantell published
a lithograph of his find in The Geology of the South-east of
England in 1833; and another drawing in the fourth edition of The
Wonders of Geology, in 1840.
Gideon Mantell originally claimed the name Hylaeosaurus meant
"forest lizard", after the Tilgate Forest in which it was
discovered. Later, he claimed that it meant "Wealden lizard"
("wealden" being another word for forest), in reference to the
Wealden Group, the name for the early Cretaceous geological
formation in which the dinosaur was first found.
In any case, the species epithet armatus, or "armored", was chosen
by Mantell because there
"appears every reason to conclude that either its back was armed
with a formidable row of spines, constituting a dermal fringe, or
that its tail possessed the same appendage".
Conservation status: Fossil
Issue #396
