Ornithomimus
Name:
Ornithomimus
(Bird mimic).
Phonetic: Or-nif-oh-mime-us.
Named By: Othniel Charles Marsh - 1890.
Synonyms: Dromiceiomimus.
Classification: Chordata, Reptilia, Dinosauria,
Saurischia, Theropoda, Ornithomimidae, Ornithomiminae.
Species: O. velox (type),
O.
edmontonicus. Many other species have been named in the
past, but most of these are considered dubious and/or have been
assigned to other existing species and genera.
Diet: Possible omnivore.
Size: Exact size depends upon the species in
question. Larger individuals approximately 3.8 meters long.
Known locations: USA and Canada.
Time period: Campanian to Maastrichtian of the
Cretaceous.
Fossil representation: Many individuals, but often
incomplete.
Named
in 1890 from a partial hand and foot, Ornithomimus
became the
first known ornithomimid,
which is why it is considered the type
genus of the Ornithomimidae. The ornithomimds (also sometimes
called 'ostrich dinosaurs’) were small dinosaurs that usually
ranged between three and four meters long, although some were
slightly larger. During the late Cretaceous ornithomimds seem to have
been particularly widespread across North America and Asia where they
relied upon speed for survival in habitats that were dominated by
tyrannosaurs, and shared with horned and armoured dinosaurs like
ceratopsians and ankylosaurs.
Amongst
the dinosaurs Ornithomimus has one of the most
complicated taxonomic
histories that continue to cause much confusion amongst researchers.
In a nut shell, because it was one of the first ornithomimds
discovered, Ornithomimus along with Struthiomimus
were effectively
used as wastebasket taxons where any remains remotely resembling the
earlier material were almost automatically attributed to one of these
genera as a new species. This also happened for the first two
dinosaur genera ever named, Megalosaurus
and Iguanodon,
although
perhaps a better analogy would come from the pterosaurs Pterodactylus
and Rhamphorhynchus.
Both of these genera once had an incredible
number of species attributed to them, just for most of them to be
found to be the same as the type species or actually different genera
completely.
The
classification of Ornithomimus didn’t really get
under way properly
until 1972 when palaeontologist Dale Russel undertook a review of
the ornithomimids. The result of Russel’s study was that Ornithomimus
along with Struthiomimus were each themselves valid
genera and that
Ornithomimus had two distinct species, O.
velox and O.
edmontonicus, even though the line between these two
species was
occasionally blurry. Russel additionally created two new ornithomimid
genera of Archaeornithomimus
and Dromiceiomimus on
the basis that some
Ornithomimus remains could not be placed with
the genus.
Other
palaeontologists have continued to study Ornithomimus
and other closely
related genera, but results and conclusions have been mixed between
different researchers. Further species have been named only to be
contested by others who claim there is not enough difference.
Additionally the earlier creation of Dromiceiomimus
is now seen as
invalid and this genus is now usually attributed as a synonym to
Ornithomimus where the fossil material originally
belonged. It is
hard to say for certain what the future holds for Ornithomimus,
although its place a valid genus is not in dispute. Further remains
and study however may yet bring a change in the number of valid species.
It
is actually a lot easier to talk about Ornithomimus
as a dinosaur
rather than its taxonomic history. Like with all other
ornithomimids, Ornithomimus is noted for having
long legs, the
lower portions of which were considerably longer than the femur. This
is a sure sign that Ornithomimus was a dinosaur
that was built for
speed and with the exception of other ornithomimids, was probably the
fastest dinosaur in its ecosystem. This speed would have probably
been enough to keep it out of the way of big predators like
Tyrannosaurus
and Albertosaurus,
although it would be interesting to
see how Ornithomimus faired against smaller and
faster predatory
dinosaurs, including juveniles of the big tyrannosaurs. To help
reduce weight the bones of Ornithomimus, like
other ornithomimids,
were hollow. Another adaptation to fast running is the tail which in
life would have been carried off the ground and supported with tendons
so that it could be used as a stiff balancing aid.
The
obvious tactic for a predator to employ against a fast animal like
Ornithomimus would be ambush, dashing out from
cover and closing the
gap between themselves and their target before the prey could turn and
use their speed to escape. This was not all that easy to do either
however, as Ornithomimus had a head that was
carried high from the
body by a long neck. This meant that Ornithomimus
could see over
cover and see other dinosaurs approaching from a long way off which
would make sneaking up on one very difficult. The large orbital
fenestrae of the skull also suggest proportionately larger eyes to fit
within them. This indicates that Ornithomimus
would have had keen
vision, possibly even for seeing at night. Analysis of sceleral
rings has indicated a cathemeral lifestyle, which means that
Ornithomimus would have been active for short
periods during both the
day and night time hours.
Another
feature of the skull is the enlarged brain cavity that also suggests
that Ornithomimus had a large and well developed
brain for its size.
Although it is tempting to say that this may have made Ornithomimus
an
intelligent dinosaur, it is not so much the case of how big the brain
is as a whole, but the development of certain parts. The
development of the brain in Ornithomimus is usually
taken as being
towards proprioception, or in simple terms better control over the
body and its movements. This is a very logical conclusion when you
think that a bipedal dinosaur that was capable of breaking the speed
limit on some of our roads would need excellent coordination when
running over rough terrain while being chased by a predator.
Rather
than having more typical jaws with teeth, Ornithomimus
actually had a
mouth that was more like a beak which could have been used for a
variety of feeding styles. As a theropod Ornithomimus
is thought to
have descended from carnivorous ancestors, but it is still uncertain
if it ate meat as well. The beak-like mouth could have been used to
selectively browse certain parts of plants, or alternatively
Ornithomimus could have used it to pick up insects.
Ornithomimus may
have also preyed upon small animals like lizards and primitive mammals
that it may have caught by using its long neck to reach into the
undergrowth. It is also possible that all of the above might be
correct and that Ornithomimus was an omnivore which
browsed the
landscape eating whatever it could find and thereby not competing with
other more specialised groups of dinosaurs. Regardless of what it
ate, the lack of teeth would at least suggest that Ornithomimus
swallowed food whole.
The
arms of Ornithomimus are also quite long with long
fingers to match.
It has been suggested that these long arms were used to reach out and
grab branches in the same way that a sloth feeds, although it’s
uncertain how much benefit if any Ornithomimus
would have gained from
this because it already had a long relatively long neck for this
purpose already. It’s possible that the long arms and hands may have
played a bigger part in prey capture, since the mouth lacked teeth to
grip hold of prey. If Ornithomimus raided nests
for eggs as has been
suggested in the past, then it may have held onto eggs within its
hands while the beak-like mouth broke the shell and dipped into the
yolk within.
One
area where Ornithomimus differs from other
ornithomimids is that it had
a shorter actual body length. This may indicate an adaptation for
greater agility in this genus as a shorter bodied animal would be able
to make tighter turns than a longer bodied animal. Another area of
study now concerning Ornithomimus is if it actually
had hair like
feathers over its body. For a long time there was no evidence to
directly support the idea for feathers on Ornithomimus,
but new
specimens and studies of Ornithomimus are
increasingly suggesting that
Ornithomimus, and perhaps most if not all
ornithomimid dinosaurs
actually had feathers. These feathers seem to have been small and
pernaceous meaning that they were not suited to flight and served more
of an insulatory version. Feather data for Ornithomimus
is still in
its early stages, but Ornithomimus seems to have
mostly feathers upon
the body, tail and neck. No feathers seem to have been present
upon the legs.
Further reading
- Description of new dinosaurian reptiles. - The American Journal of
Science, series 3 39:81-86. - Othniel Charles Marsh - 1890.
- Notice of new reptiles from the Laramie Formation. - American Journal
of Science 43:449-453. - Othniel Charles Marsh - 1892.
- A new Ornithomimus with complete abdominal
cuirass. - The Canadian
Field-Naturalist 47(5): 79-83. - C. M. Sternberg - 1933.
- A specimen of Ornithomimus velox (Theropoda,
Ornithomimidae) from the
terminal Cretaceous Kaiparowits Formation of southern Utah. - Journal
of Paleontology, 59(5): 1091-1099. - DeCourtean & Russel - 1985.
- A juvenile Ornithomimus antiquus (Dinosauria:
Theropoda:
Ornithomimosauria), from the Upper Cretaceous Kirtland Formation
(De-na-zin Member), San Juan Basin, New Mexico. - New Mexico Geological
Society Guidebook, 48th Field Conference, Mesozoic Geology and
Paleontology of the Four Corners Region. 249-254. - Sullivan - 1997.
- A reevaluation of the genus Ornithomimus based on
new preparation of
the holotype of O. velox and new fossil
discoveries. - Journal of
Vertebrate Paleontology. - L. Claessens, M. Loewen & Z.
Lavender - 2011.
- A redescription of Ornithomimus velox Marsh, 1890 (Dinosauria,
Theropoda). - Journal of Vertebrate Paleontology. - Leon P. A. M.
Claessens & Mark A. Loewen - 2015.
- A densely feathered ornithomimid (Dinosauria: Theropoda) from the
Upper Cretaceous Dinosaur Park Formation, Alberta, Canada". Cretaceous
Research. 58: 108–117. - Aaron J.van der Reest, Alexander P.Wolfe
& Philip J.Currie - 2016.
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