Hypsilophodon
Name:
Hypsilophodon
(Hypsilophus tooth).
Phonetic: Hip-sil-o-foe-don.
Named By: Thomas Henry Huxley - 1870. *Note
- The year of naming is sometimes credited as 1869 because this
was when the name Hypsilophodon was first presented
by Huxley, but
the description of this dinosaur did not get formally published until
1870.
Classification: Chordata, Reptilia, Dinosauria,
Ornithischia, Ornithopoda, Hypsilophodontidae.
Species: H. foxii (type).
Diet: Herbivore.
Size: Average about 1.5 meters long, but
larger specimens up to 1.8 meters long.
Known locations: England, Isle of Wight -
Wessex Formation.
Time period: Barremian of the Cretaceous.
Fossil representation: Remains of several
individuals.
Hypsilophodon
is one of the more famous dinosaurs that have been discovered in the
Isle of Wight, however its discovery was delayed by the fact that for
the first twenty years of the first fossils discovery, Hypsilophodon
was actually believed to be a juvenile Iguanodon.
This is of course
in reference to the Mantell-Bowerbank block, a collection of post
cranial remains with no skull that was preserved flat on a block that
was then broken in half and sent to two separate palaeontologists
(today these two pieces are catalogued as NHM 28707, 39560-1).
Thomas Henry Huxley first came up with the notion that a new genus of
dinosaur was present when they had the opportunity to study a new
specimen, this time with a skull (NHM R197) in 1868, and in
1869 Huxley presented Hypsilophodon as a new
genus in a
lecture. Just as Gideon Mantell named Iguanodon
for the similarity of
its teeth to the modern Iguana genus of lizard, Huxley named
Hypsilophodon because its teeth were similar to
another genus of lizard
called Hypsilophus that was named in 1830.
However in a quirk of
fate, Hypsilophus is today treated as a synonym
to Iguana, which
means that both Iguanodon and Hypsilophodon
are
actually named after
the same genus of lizard. The type species name H. foxii
is named
in honour of the Reverend William Fox who had discovered the 1868
specimen.
Like
with so many other early genera of dinosaur including the
aforementioned Iguanodon, Hypsilophodon
seems to have been treated as
something of a ‘wastebasket taxon’ where the fossils of
superficially similar dinosaurs have been added to the genus. Later
studies in the late twentieth and early twenty-first centuries have now
raised the consensus that only the Hypsilophodon
fossils from the Isle
of Wight, particularly the Hypsilophodon Bed of
the Wessex
Formation, are actually valid for the Genus. This leads to the
further conclusion that the other fossils once attributed to the genus
from sites in Europe, mainland England to Even South Dakota in the
United States, are actually those of similar but different dinosaur
genera. This also means that many facts presented about Hypsilophodon
when these fossils were still attributed to the genus are also wrong,
which means that today there are a wide range of details regarding
things such as size and geographical and temporal distribution for
Hypsilophodon, depending upon the source.
Discoveries and
evaluation for the genus in the future may mean further revisions
again, but for now it is safer to restrict study of the genus to the
Wessex Formation fossils.
As
a dinosaur Hypsilophodon is something of a paradox
in that as an Early
Cretaceous dinosaur genus it displayed a mix of evolutionary traits.
The fact that it had five fingers on the hands and four toes on the
feet is taken to represent primitive characteristics, while the
observation that the fifth finger was opposable to the other four is
actually quite advanced. It could be however that the five fingered,
four toed form ‘just worked’ for its lifestyle, and so there
were no need for any radical evolutionary changes. Evolution after
all is driven by necessity, and things are not necessary, then
there is no drive for further changes to take place.
Overall
Hypsilophodon seems to have been a runner. Its
long hind legs and
lightweight build have led to popular suggestion that in terms of
form, Hypsilophodon might have been one of the
fastest types of
dinosaur. This in turn could indicate that Hypsilophodon
relied upon
speed and agility to stay out of the mouths of predators. In terms of
posture Hypsilophodon would have almost certainly
kept its spine
horizontally level to the ground while running, something that would
have been helped by the long tail which would have been kept rigid by a
network of tendons and interlocking vertebrae so that it acted as a
counterbalance to the head and body that was carried forward of the
hips. Hypsilophodon could have arched their necks
and may even have
been able to raise their upper bodies a little to look for danger.
When feeding, Hypsilophodon may have also become
semi-quadrupedal,
balancing themselves on their forelimbs as they pitched down to feed
upon low growing plants.
The
overall small size and low height of Hypsilophodon
meant that it was
almost certainly restricted to feeding upon low growing vegetation.
The small area of the beak like anterior (front) portion of the
mouth however meant that Hypsilophodon may have
been more of a
selective browser rather than a generalist of all plant parts/types.
Another primitive trait of Hypsilophodon is the
fact that although it
had a beak like mouth, it still retained teeth in the premaxilla.
This is considered primitive because most dinosaurs that developed
beak like mouths would go on to lose all of the teeth in the anterior
portion of the mouth. It is uncertain as to exactly why Hypsilophodon
retained these teeth, but they may have still served a purpose in
feeding.
The
teeth at the posterior (back) end of the mouth were broad and
developed for shearing. The teeth in the lower jaw erupted at a
slight angle away from the mouth, while those of the upper jaw were
angled slightly inwards. This provided Hypsilophodon
with a broad
shearing edge that would have effectively processed tougher food such
as newly erupted shoots which when newly grown are tightly and firmly
packed together, to possibly even exposed roots. To help with this
food processing, Hypsilophodon is also thought to
have had cheeks so
that food could be held in place as it was sheared between the rear
teeth, as well as being prevented from falling out of the sides of
the mouth as it did so.
Hypsilophodon
was at one time thought to have had armoured plates that ran down it
back, something that once accredited the genus with the title of the
only armoured ornithopod (not to be confused with
‘ornithischian’) dinosaur. This idea stems all the way back to
1874 when a bony plate was identified as being located on the neck
by J. W. Hulke. This led to speculation that such plates ran all
the way down the back in a kind of mail that prevented the teeth of
predators from puncturing through. However the idea was discredited
in 2008 in a study by Butler and Galton who interpreted the dermal
armour as actually being an intercostal plate from the rib cage that
had originally been made of cartilage. Plates like these have also
been recorded as being preserved with other dinosaur genera, and with
this in mind, Hypsilophodon likely still relied
upon evasion tactics
such as speed to avoid predators.
Another
famous dinosaur from the Wessex Formation which Hypsilophodon
could
have possibly lived alongside is the armoured ornithischian
Polacanthus.
Possible predators of Hypsilophodon
include the
tyrannosauroid Eotyrannus,
neovenatorid Neovenator
as well as the
spinosaurid Baryonyx.
Baryonyx for its part is so
far only
represented by teeth in this formation (which may or may not belong
to a tooth taxon called Suchosaurus), but it’s
known fossil
distribution stretches all the way from Southern England to Portugal,
so it’s likely that at some point it crossed over what is now the Isle
of Wight. Also although perceived as a specialist piscivore (fish
hunter), Baryonyx is also a known dinosaur
eater, and a small
ornithopod like Hypsilophodon could have been
easily killed and
consumed by a Baryonyx if it were able to catch one.
Further reading
- On Hypsilophodon foxii, a new dinosaurian
from the Wealden of the
Isle of Wight. - Thomas Henry Huxley - 1870.
- Supplemental note on the anatomy of Hypsilophodon foxii,
J. W.
Hulke - 1874
- The 'dermal armour' of the ornithopod dinosaur Hypsilophodon
from
the Wealden (Early Cretaceous: Barremian) of the Isle of Wight: a
reappraisal. - Richard J. Butler & Peter M. Galton - 2008.
- Notes on Neocomian (Lower Cretaceous) ornithopod dinosaurs from
England - Hypsilophodon, Valdosaurus,
"Camptosaurus",
"Iguanodon" - and referred specimens from Romania and elsewhere.
-
P. M. Galton - 2009.
- A Cretaceous calamity? The Hypsilophodon Bed of the Isle of Wight,
southern England. - Geology Today. 33 (2): 66–70. - Robert A. Coram,
Jonathan D. Radley & David M. Martill - 2017.
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