Ornitholestes
Name:
Ornitholestes
(Bird robber).
Phonetic: Or-nif-oh-less-tees.
Named By: Henry Fairfield Osborn - 1903.
Classification: Chordata, Reptilia, Dinosauria,
Saurischia, Theropoda, Maniraptora, Ornitholestinae.
Species: O. hermanni (type).
Diet: Carnivore.
Size: Estimated at 2 meters long, possibly
slightly larger.
Known locations: USA - Morrison Formation.
Time period: Kimmeridgian of the Jurassic.
Fossil representation: Skull and partial post
cranial skeleton.
An interesting fact about Ornitholestes is that since it was first discovered in July of 1900, it was the first theropod dinosaur to be discovered in the twentieth century. Since this time, Ornitholestes has been regularly depicted in books, art and even dinosaur documentaries such as the BBC series Walking With Dinosaurs. However, today many of these reconstructions are actually considered to be inaccurate due to original misinterpretation of the fossils as well as assumed ideas about this dinosaur’s behaviour.
Physical appearance
In
general appearance Ornitholestes was a small
theropod dinosaur that was
built for speed and agility rather than strength and brute force.
However the lower leg bones (missing in the type specimen) are
thought to have been slightly shorter than the femur, something that
is contrary to other fast running dinosaurs. This might indicate that
while Ornitholestes was a lightweight reasonably
fast theropod, it
was not suited to chasing other small bipedal dinosaurs. Up to half
of the total body length was made up of the tail which would have
served as a balancing aid.
In
terms of scale the skull of Ornitholestes was not
as large in
proportion to the body as the skulls of other known theropods.
Despite this the skull and lower jaw were robust in their overall
construction. The teeth at the front of the mouth were conical with
slight serrations, which would have made them useful for tearing
pieces of flesh from a carcass. The teeth at the back however were
recurved (pointed to the back of the mouth) with enhanced
serrations which would have allowed them to more effectively slice
through meat.
Ornitholestes
may not have relied upon its teeth and mouth for prey capture however
for two reasons. One is that the relatively small size of the skull
and hence mouth meant that catching prey would be more difficult with
the mouth. Two is the fact that the arms of Ornitholestes
were
comparatively long, two thirds the total length of the legs. These
arms also had hands with long fingers and had joints which allowed them
to move in much wider arcs than larger and more primitive theropods
which relied upon their mouths for prey capture. As such since the
arms were the best adapted body part for capturing prey, it’s logical
to reason that Ornitholestes used them to catch and
hold onto prey
while the mouth was possibly used to deliver a killing bite before
feeding.
The
feature that Ornitholestes is most commonly
depicted with is a small
crest on the tip of its snout. This crest was once thought to have
been a display feature and perhaps a sign of sexual dimorphism between
males and female, however this crest is now thought to have not even
existed at all. Because it had been subjected to intense subterranean
pressures when it went through the fossilisation process, the skull
was badly crushed. Subsequent reviews of Ornitholestes
(Rauhut,
2003 and Carpenter et al. 2005) have now concluded that this
crest was actually a piece of the skull that had been split from the
main bone and pushed upwards, giving the false impression that a
crest was present in the living dinosaur. Subsequently, more modern
depictions of Ornitholestes now show it without the
nasal crest.
There
is long running debate as to whether Ornitholestes
did or did not have
feathers, or at least primitive versions of them. Initially
proposed by Percy Lowe in 1944, the idea was largely dismissed as
fancy by other palaeontologists, but much later discoveries of
fossils from China of the similarly sized Sinocalliopteryx
and
Sinosauropteryx
show that at least some small dinosaurs had a downy
covering of hair-like feathers. Further study and research into wider
dinosaur groups has yielded that many more of the small theropods than
previously thought may have had downy feather covering; we just
didn’t know to look before because those parts had not been preserved
in other fossils. It remains to be seen if Ornitholestes
had a
covering of primitive feathers, but most palaeontologists today are
not so quick to ignore the idea as there predecessors were. If true
then these primitive feathers would have provided Ornitholestes
with
insulation that may also suggested a reasonably fast metabolism and by
extension a dinosaur with an active lifestyle.
Behaviour and ecological niche
Many
people will now about the famous depiction of an Ornitholestes
catching
an Archaeopteryx that was created by the great
paleo artist Charles
R. Knight. This image was based upon the literal translation of the
name Ornitholestes which means ‘bird robber’
and helped fix the
idea that Ornitholestes was a hunter of primitive
birds into the public
imagination. The problem with this picture is that Ornitholestes
is
so far only known from the western United States, while Archaeopteryx
is only known from central Europe. While these two continents were
closer together during the Jurassic, current fossil evidence simply
does not support this exact idea of predator/prey interaction.
Despite this however the concept could yet prove to be quite accurate.
When
looking at the body of Ornitholestes you have three
clues to its
predatory behaviour. One is the lightweight body and quite long legs
which meant that Ornitholestes would have been
fast, but probably not
as fast as other similarly sized dinosaurs that had longer legs. Two
is the small skull size, not really suitable for attacking equally
sized or larger dinosaurs. Third are the grasping arms that could
easily arc to clasp around objects. When you put all of these things
together you end up with a predator that is supremely adapted to
chasing and catching smaller vertebrate animals like lizards and
possibly even primitive mammals that were no bigger than most of
today’s small rodents. Proto-birds would have also been potential
prey had they also been active in the same habitats as Ornitholestes.
Small prey animals like these would also not have required much power
to kill, and the small yet robust skull and jaw may have been
connected by reasonably strong biting muscles capable of closing the
jaws in a killing bite. The slicing teeth at the back of the mouth
would have also made short work of the bodies of small prey.
It
is also not completely out of the question that Ornitholestes
was a
predator of other dinosaurs, particularly juveniles that were not
fully developed enough to escape. Additionally as a carnivore
Ornitholestes probably would also scavenged carrion
whenever the
opportunity presented itself as scavenging requires less energy
expenditure that actively hunting prey. On the predatory food chain
of the Jurassic North America however Ornitholestes
would still have
been reasonably low down. Larger theropods such as Ceratosaurus
and
Allosaurus,
both of them known from the same formation as
Ornitholestes, grew much bigger and even juveniles
of these genera
would have been capable of killing and eating a careless Ornitholestes.
Classification - the
relationship of Ornitholestes to other dinosaurs
Ornitholestes
was named in 1903 by Henry Fairfield Osborne, the man who named
many of some of the most famous dinosaur genera including Tyrannosaurus
in 1905 and Velociraptor
and Oviraptor,
both in 1924. As a
small theropod Ornitholestes ended up being placed
within the
Coelurosauria, the group of theropods that at the time became home
for most small theropod dinosaurs. In 1920 Ornitholestes
effectively ceased to exist when Charles Whitney Gilmore declared it to
be the same dinosaur as Coelurus, another small theropod that is
known from the Morrison Formation, but only by partial remains.
This theory was reinforced in 1934 when Oliver Perry Hay created
the species Coelurus hermanni based from the original description of
Ornitholestes hermanni.
Almost
half of the century passed until Ornitholestes was
resurrected as a
genus by John Oestrum in 1980. The classification of theropods had
moved on a long way since the 1930s, but the exact position of
Ornitholestes still and continues to spark debate
amongst
palaeontologists. In 1988 Gregory S. Paul proposed that
Ornitholestes was similar to Proceratosaurus
on the grounds that they
had a similar skull structure. Indeed Proceratosaurus
does have a
small nasal crest on the tip of its snout, a feature that at the time
was also thought to be present in Ornitholestes.
Paul also detailed
that these two genera were possibly related to Allosaurus,
the top
predator during the late Jurassic.
Although
a good theory at the time, is was eventually realised by
researchers, including Paul, that there is no direct familial link
amongst these three dinosaurs. In fact later study upon
Proceratosaurus has now seen it treated as possibly
the earliest known
tyrannosauroid. Ornitholestes is now back within
the Coelurosauria
which today is the group of theropods that are thought to be closer to
birds than more primitive theropods like carnosaurs such as
Allosaurus. Some palaeontologists have gone even
further by placing
Ornitholestes within the Maniraptora (‘hand
snatchers’) which also
includes dromaeosaurs and troodonts as well as oviraptorids and
therizinosaurs amongst others. Not all palaoentologists are in
agreement as to how Ornitholestes fits into this
sub group, but
Ornitholestes is often considered to be a more
primitive member.
Partial
hand remains that were once attributed to Ornitholestes
are now widely
thought to belong to another dinosaur called Tanycolagreus,
yet
another small theropod from the Morrison Formation that was found a few
hundred meters away from the hand.
Further reading
- Ornitholestes hermanni, a new compsognathoid
dinosaur from the Upper
Jurassic. - Bulletin of the American Museum of Natural History
19(12):459-464. - H. F. Osborn - 1903.
- Skeletal adaptations of Ornitholestes, Struthiomimus,
Tyrannosaurus.
- Bulletin of the American Museum of Natural History. 35 (43): 733–771
- H. F. Osborn - 1917.
- Coelurus and Ornitholestes: Are they the same? - John H. Ostrom. -
In, Aspects of Vertebrate History: Essays in Honor of Edwin Harris
Colbert. Museum of Northern Arizona Press. - Louis L. Jacobs (ed.) -
1980.
- Redescription of the Small Maniraptoran Theropods Ornitholestes
and
Coelurus, by Kenneth Carpenter, Clifford Miles, John
H. Ostrom
& Karen Cloward. - In, The Carnivorous Dinosaurs. Life of the
Past. Indiana University Press. pp. 49–71. - Kenneth Carpenter (ed.) -
2005a.
- New Small Theropod from the Upper Jurassic Morrison Formation of
Wyoming, by Kenneth Carpenter, Clifford Miles & Karen Cloward.
- In, The Carnivorous Dinosaurs. Life of the Past. Indiana University
Press. pp. 49–71. - Kenneth Carpenter (ed.) - 2005b.
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