The ornithomimosaurs are an often underappreciated group of dinosaurs that are frequently interpreted as being very similar to ratite birds (ostriches and emus) in their ecological niche. This is why even though the ornithomimosaurs are often referred to as ‘bird mimic dinosaurs’, they are also known as ‘ostrich mimic dinosaurs’, or just ‘ostrich dinosaurs’.
Early discovery, classification
and distribution
The
scientific history of the ornithomimosaurs stems all the way back
to 1890 when one of the most famous palaeontologists of all time,
Othniel Charles Marsh, described a partial hand and foot. Marsh
noted the similarity in the structure of the foot to that of a bird
which led to the creation of the first ornithomimosaur genus
Ornithomimus.
Since the naming much more fossil material has been
attributed to Ornithomimus, to the point where it
has suffered from
the ‘wastebasket taxon’ effect where superficially similar fossils
were attributed to the genus with hardly any in depth study or
comparison to existing material. Inevitably however other
palaeontologists picked where others left off, with the discovery of
the second ornithomimosaur taking place in 1917 when Henry
Fairfield Osborn named Struthiomimus
from some fossils that had
originally been assigned to Ornithomimus. The
1970’s and
1980’s sparked a small renaissance for the ornithomimosaurs with the
discovery and naming of a few new genera from Asian (principally
Mongolian) fossils. More discoveries took place in the 1990’s
and 2000’s and together these have allowed palaeontologists to
begin piecing together both primitive and advanced forms.
The
term ornithomimosaur is used to describe all members of the
Ornithomimosauria, but the dinosaurs of this group are still more
commonly referred to as ornithomimids. In all actuality however
ornithomimids are but one group of ornithomimosaurs that belong to the
Ornithomimidae family that sits within the Ornithomimosauria.
Ornithomimosaurs are regarded as members of the Coelurosauria, a
group of theropods that includes other types of theropods such as
tyrannosaurs. In fact the similarity in form of the foot bones
between ornithomimosaurs and tyrannosaurs once led to speculation that
they were closely related which in part led to the creation of
Arctometatarsalia by Thomas R. Holtz in 1994 as a new definition
for all of these theropods. Further study into theropods has since
concluded that there is no direct relation between these theropod
groups other than their being coelurosaurians. The features seen as
linking them (refer to body form section below) are now seen as a
case on convergent evolution.
Ornithomimosaurs
are today classed within the Maniraptoriformes, also created by Holtz
but in 1995. Maniraptoriform dinosaurs are loosely called ‘hand
graspers’ and is a reference to the greater dexterity and movement of
the forelimbs that while basic, is still superior to the more
primitive theropod groups. Another group of maniraptoriforme
theropods called the Alvarezsauridae are seen by many to be very
closely related to the ornithomimosaurs. In 2005 Paul Sereno
established the Ornithomimiformes to include the Alvarezsauridae and
Ornithomimosauria into a related group, but this term is not in
widespread use by other palaeontologists.
Most
ornithomimosaur remains are known from North America and Asia, but
the discovery of Pelecanimimus
in Spain suggests that they were at one
time spread across Eurasia. This genus is dated to the Barremian and
with the earliest remains which come from Thailand (Kinnareemimus)
dated to around the Valanginian to Hauterivian periods; this
distribution would have taken place during the early Cretaceous. How
long this distribution lasted is difficult to say but so far most late
Cretaceous genera are only known from Asia and North America. While
this might suggest an eastwards drift across the globe for this
type of dinosaur, it may well be that there are more ornithomimosaur
genera waiting to be discovered.
The ornithomimosaur body form
The
common ornithomimosaur traits are a very lightweight body, long neck
with a small skull (especially when compared to other theropod
types), long thin tail to act as a counter balance, relatively
long fore limbs capable of grasping when they came together, and long
legs well adapted for running. The legs are of particular interest as
the lower leg and foot bones are very long when compared to the upper
leg (thigh) bone. The feet are also digitigrade which means that
ornithomimosaurs walked upon their toes rather than the flats of their
feet. This extended the length of the ‘leg’ even more so that
with every step forward an ornithomimosaur could proportionately cover
a much greater length of ground than other theropods which had shorter
lower leg proportions. For this reason the ornithomimosaurs,
especially the advanced forms, are seen as possibly being the
fastest dinosaurs to ever live.
The
key adaptation to fast running in advanced ornithomimosaur forms is not
just the leg proportions but the structure of the feet.
Ornithomimosaur feet are tridactyl which means they have three toes in
contact with the ground. The middle toe is developed in a way that it
can better withstand the shock of the impact of the foot coming down
against the ground. This feature would not be required to support the
weight of the body when the dinosaur was standing or casually walking
around otherwise it would be seen in most if not all theropod forms.
But an animal running at say thirty kilometres an hour would be
impacting the ground a lot harder with its feet than a similarly sized
and weighted animal that was only running at ten kilometres an hour due
to the greater velocity involved. This specially adapted feature
which has been named the arctometatarsus by Thomas R. Holtz in
1992 is also seen in tyrannosaurs, but the appearance here is
today treated as a case on convergent evolution rather than these two
groups of theropods being directly related because it is only known in
the advanced later forms of both groups.
Although
ornithomimosaurs are thought to have adapted for fast running it should
be noted that many earlier forms such as Sinornithomimus
and
Pelecanimimus seem to have lived in areas where
there was shallow water
due to known features and repeated occurrences of there remains. In
such environments the long legs would actually serve very well as
stilts to keep the body out of the water as the ornithomimosaurs looked
for things to eat. So the questions are, ‘did ornithomimosaurs
learn to run and then start wading as well?’ or ‘did
ornithomimosaurs learn to wade in water and then adapt to running on
the plains?’ Due to the current rarity of the more basal
ornithomimosaurs it’s hard to answer both questions. But one thing to
consider is that the advanced ornithomimosaurs best adapted to
running, the ornithomimids, are mostly only known from the end of
the Late Cretaceous. This might suggest that the fast running
adaptations were built upon the foundation of an earlier physical
adaptation, although the driving force for this is still uncertain.
Ornithomimosaurs
are also known for their toothless beaks but the primitive forms that
were around in the early Cretaceous such as Harpymimus,
Shenzhousaurus
and Pelecanimimus all had teeth. In fact with
some
two hundred and twenty teeth, Pelecanimimus has
the most teeth of any
other known theropod. The teeth of Pelecanimimus
however were very
small; something to be expected when you consider that it was a small
dinosaur. Harpymimus and Shenzhousaurus
also had very small teeth,
but the numbers were greatly reduced and only in the lower jaw,
perhaps serving a function to aid with a specialised form of feeding,
or merely being vestigial. Later ornithomimosaurs lost their teeth
completely and instead made do with a keratinous beak similar to that
of a bird that could bite and tear things along its edge. The change
to a toothless beak has led to lot of debate and speculation which is
covered in more detail in the next section.
Ornithomimosaurs
have reasonably well developed forelimbs for a theropod and it seems
that they had an active purpose in feeding. Proportionately the
forelimbs are quite large in relation to the body, but there is
considerable difference in overall size and proportion of the forearms
between different genera. More primitive forms seem to have had claws
that were relatively straight and only slightly curved, but later on
the claws of ornithomimosaurs became more curved. This change could
have come about through changes between the lifestyles of primitive and
advanced forms.
In
terms of size ornithomimosaurs tend to range between two and four
meters in length, but some easily exceeded this. Gallimimus
is
often credited at being six meters long, although other fossils
suggest it may have approached eight meters. Additionally Beishanlong
is also estimated to have approached a size rivalling this. Perhaps
largest of the ornithomimosaurs was Deinocheirus
which so far is only
known from its near two and half meter long arms. By filling the gaps
with other ornithomimosaurs parts and scaling them up to fit the size
of the arms, it’s probable that Deinocheirus is
the largest known
ornithomimosaur and one of the largest theropod dinosaurs discovered.
More detail about this is on the main Deinocheirus
page and the
additional section about it further down this article.
Most
ornithomimosaurs seem to follow the same principal shape but the
primitive genus of Pelecanimimus is confirmed to
have had a gular pouch
connecting to its lower jaw like a pelican, hence the inspiration for
its name which means ‘pelican mimic’. Pelecanimimus
is also the
first and so far only known ornithomimosaur to have had a small head
crest that rose up from the back of the skull. Another
ornithomimosaur called Garudimimus
was once thought to have had a small
crest in front of its eyes, but this has now been found to actually
be a loose fragment of the skull that was incorrectly assembled as a
crest. The eyes of ornithomimosaurs are usually orientated to face to
the sides so that they have a very wide field of view, but not much
in the way of depth perception.
Because
ornithomimosaurs are treated as being members of the Maniraptoriformes
they are expected by many to have had feathers like many other
dinosaurs that belong to this group have been confirmed to have had.
Unfortunately there is currently no firm evidence to confirm this for
ornithomimosaurs, and the impression that confirms a gular
(throat) pouch in Pelecanimimus does not show
the presence of
feathers. However this does not mean that Pelecanimimus
did not have
feathers at all, just not on this part of the body. If feathers are
to be expected anywhere it would be upon the main body itself where
they could provide the most benefit in terms of insulation.
Additionally the outer extremities such as the lower legs and hands
and even parts of the head and neck probably would not have them anyway
due to their smaller surface area which limits the rate of temperature
exchange. Palaeontologists are looking at areas where feathered
dinosaurs are expected such as the Yixian Formation of Liaoning
Province in China which has held the remains of everything from
primitive birds such as Confuciusornis to
feathered tyrannosaurs like
Dilong
and Yutyrannus.
What did ornithomimosaurs eat?
This
is the question that has sparked the most debate about the
ornithomimosaurs. It’s a safe bet that they were descended from meat
eating dinosaurs, but no one knows if they continued to include meat
in their diets or if they switched to a different method of survival.
Primitive forms like Pelecanimimus were most
probably hunters that
focused upon fish and other aquatic vertebrates since the presence of
the throat pouch indicates that they would have been well suited to
this kind of diet, but this pouch was practically useless for
feeding from plants. The greatly reduced but pointed teeth in the
lower jaws of Harpymimus and Shenzhousaurus
may have also helped hold
onto slippery prey such as fish. Unfortunately it’s not possible to
complete the picture yet because the skull of the oldest known
ornithomimosaur, Kinnareemimus has not been found.
The
toothless beaks of later ornithomimosaurs are so adaptable that they
could have been used for anything from picking select parts of plants
to snatching small vertebrates like lizards, snakes and mammals from
the undergrowth. If indeed ornithomimosaur are like ostriches in
their behaviour then they would have done both, living the life of
omnivores that could change and adapt their diets to take best
advantage of the ecosystems that they lived in. This adaptability
would also mean that they did not have to directly compete against
dedicated plant eaters or meat eaters.
Ornithomimosaurs
might have also been opportunists that fed from carrion, picking
scraps of meat out from between bones of the kills of dedicated
predators. Egg thievery is another option especially when you
consider that their arms were reasonably well adapted for grasping,
and the curved claws of later forms would be better at reaching around
the curvature of an egg. The toothless beak would also have been
better for breaking an eggshell so that the dinosaur could get at the
yolk within. However this method requires the presence of brooding
dinosaurs tending nests, and this may have only occurred during
certain time of the year. Also while the curved claws of later
ornithomimosaurs might have been good for reaching around eggs, they
would have also been good for gripping branches and pulling them down.
This would pull down more of a plant so that an ornithomimosaur could
feed upon a greater area if it ate plants.
Some
of the most exciting discoveries concerning possible diet are
gastroliths that have been recovered from remains of Shenzhousaurus
and
Sinornithomimus. These are regular stones that
have been swallowed by
an animal (in these cases dinosaurs) that cannot process food
completely by chewing in the mouth. These stones are stored in the
stomach where the grinding action of the stones rubbing against each
other break down the food so that it can be more easily digested.
Most of the time gastroliths are associated with herbivorous animals,
especially those that eat tough plants. However there are cases of
animals that eat fish and invertebrates that also have gastroliths in
order to break down things like, scales, bones and shells. This
is especially so in predatory animals that can only swallow captured
prey due to their dental specialisations. If ornithomimosaurs ate
small prey animals, they would have realistically had to swallow them
whole and so the idea that the presence of gastroliths means that they
must have been herbivores is not an absolute one. Further
complications come from the inclusion of Lourinhanosaurus,
a
reasonably large meat eating dinosaur from the Jurassic of Portugal
that also bizarrely seems to have had gastroliths as well.
The
debate about what ornithomimosaurs continues to go on with most
researchers preferring to label ornithomimosaurs, and particularly
ornithomimids as omnivores. Aside from being difficult to disprove it
does seem to be the answer that best fits them. However primitive
forms also seem to be predisposed to being predators of fish and
possibly other small vertebrates. Finally however it may be that
different genera adapted to fill different ecological niches, some
being strict herbivores and feeding from plants, some being
piscivores and stalking shallow waters for fish, predators that
walked through high growing vegetation looking for snakes and lizards,
to omnivore that ate whatever they could find.
Ornithomimosaur behaviour
One
thing that can established for two ornithomimosaur genera,
Ornithomimus and Garudimimus,
is how they lived during the day.
Both of these dinosaurs have been persevered with their scleral rings
(ringed growths of bone that supported the shape of the eyes) and
analysis of these indicates that they were cathemeral. This means
that they are active for short periods at a time during both the day
and the night. Not all ornithomimosaur genera are complete enough to
have their scleral rings preserved so it is hard to ascertain if they
too followed this pattern and while it is possible that they may have
been cathemeral, they may have also been diurnal (active during the
day only) or nocturnal (active at night only).
At
least two ornithomimosaur genera, Gallimimus and Sinornithomimus
are
known to have formed groups. In fact the first two discoveries
concerning Sinornithomimus were of group
concentrations of many
individuals that seem to have died from some kind of natural event.
The interesting thing here is that many of the individuals involved in
these concentrations were juveniles. The first collection of
Sinornithomimus for example was composed of eleven
juvenile individuals
and three others that are thought to have been nearly fully grown
subadults. The second concentration was of thirteen individuals,
all juveniles.
Previously
ornithomimosaurs were thought to have formed family groups since
juvenile Gallimimus were found near adult remains
suggesting that the
parents looked after the young. But in Sinornithomimus
at least it
seems that they mimicked a different bird strategy where juvenile
individuals leave the care of their parents and form groups composed of
other similarly aged immature individuals. The purpose of this is to
free up the parents from care of the offspring so that they can raise a
new brood of young rather than continue to look after the previous
year’s young. This also suggests an r-strategy survival mode where
large numbers of young were raised because most of them would not
survive to adult hood, falling victim to predators or natural events
such as floods.
It
is still unknown if other ornithomimosaurs formed juvenile groups or
continued to move around in herds while adult. Although they were
fast, if an ornithomimosaur was taken by surprise by a predator such
as a juvenile tyrannosaur or dromaeosaur that speed would count for
nothing. By travelling in groups there are at least several pairs of
eyes watching out for danger and if one individual spooks at the sight
of what it thought was a predator than that could trigger an automatic
response from the rest of the group. This does not mean to suggest
that ornithomimosaurs formed huge herds or complicated social
hierarchies, any groups formed by them may have just been loose
gatherings in a similar manner to how some birds today will flock when
travelling or gather together at a site abundant in food and resources
but otherwise live independent lives.
Ornithomimosaurs and their place
in the Cretaceous ecosystems
The
exact placement of ornithomimosaurs is troublesome to establish in most
cases since we are still not a hundred per cent certain as to what they
all ate. The easiest is probably Pelecanimimus
since it is known to
have feature that would have helped it to hunt things like fish.
However from the same formation that the Pelecanimimus
holotype is
known from comes Baryonyx,
a much larger spinosaur that although
probably was a specialised fish hunter, would have been capable of
making a meal of a Pelecanimimus if it got too
close.
There
is also a fossil vertebra with scrapes caused by the teeth of a
Saurornitholestes. Other dromaeosaurs and also
similarly sized
troodonts would have been potential predators in both North America and
Asia. Although an ornithomimosaur was likely the faster runner once
it got going, these dinosaurs would have been small enough to get
close and have the speed and weapons to make a strike. It also needs
to be remembered that tyrannosaurs, not so much the adults but the
smaller juveniles could have been a particular threat. Juvenile
tyrannosaurs have different body proportions to adults, particularly
the lower leg which in some juveniles and very similar to the leg
proportions of some ornithomimosaurs. This brings the frightening
prospect that when young, tyrannosaurs were potentially just as fast
as some ornithomimids, especially when you factor in the same middle
toe development that helped with faster running. In the grander
ecological picture this specialisation in juvenile tyrannosaurs hunting
ornithomimosaurs is actually quite sound as this would stop them
competing with adult tyrannosaurs that were not as well adapted to
catch ornithomimosaurs, but better able to catch and kill the
dinosaurs that the juveniles could not such as ceratopsian dinosaurs
and large hadrosaurs.
If
ornithomimosaurs were omnivores they would have been what are termed
generalists which were better able to adapt to new conditions and
environments. They could also avoid directly competing with one group
of plant or meat eating dinosaurs and have a better chance of
coexisting with other forms. With this in mind ornithomimosaurs would
not particularly dominate the landscape but rather live on the
ecological fringes that could not support a group of dinosaurs that had
a more specialised preference. Even if some genera were more
predisposed to hunting than others, their small skulls meant that
they could only hunt for small vertebrate prey or scavenge carrion.
While they would be up in the predatory food chain above these smaller
animals, they would not have been much of a threat to other dinosaurs
save for egg stealing of taking hatchling dinosaurs small enough to be
swallowed. They would themselves have been possible prey for any
predator of roughly equal size or bigger to themselves.
Deinocheirus,
largest of the
ornithomimosaurs?
In
1970 the discovery of a two hundred and forty centimetre long pair
of forelimb raised a lot of eyebrows although no one was sure what it
was, something that became a statement of many children’s dinosaur
books which saw the arms included to impress readers more than
anything else. The long claws on the end of the arm were usually
portrayed as being killing devices for rending the flesh of prey
leading to suggestions that Deinocheirus
‘must’ have been a
fearsome predator. Most palaeontologists however are not keen to
assume things and are forever asking questions and comparing
fossils to other extinct and extant (living) animal species.
Comparison to all other currently known dinosaur types has yielded one
conclusion: the arms of Deinocheirus most
closely match those of
the ornithomimosaurs.
If
this conclusion is correct, and current fossil evidence suggests that
it is, Deinocheirus would have at least been
within the region of ten
meters long, and possibly several meters longer, depending upon the
size and proportion of the arm to the body which is seen to show
considerable variation between different ornithomimosaurs. In terms
of length and possibly even height this would also see Deinocheirus
rival other large theropods such as Tyrannosaurus,
Giganotosaurus
and
even the lower estimates of Spinosaurus
which today is regarded as
being the largest (at least in terms of length) theropod currently
known.
Unfortunately
just like most other ornithomimosaurs, it remains impossible to
establish what Deinocheirus ate, a problem
confounded by the fact
that the skull is unknown. A large ornithomimosaur might have had a
greater tendency to hunt other dinosaurs, but assuming the missing
skull was small in proportion to the body like in other
ornithomimosaurs, only small dinosaurs would have been viable prey.
Two problems here are they would have been harder to catch, and
unless eaten in large quantities, not return the energy invested in
catching them.
For
Deinocheirus at least the notion of it being a plant
eater is actually
more plausible. Plants do not offer the same quick return of energy
as eating meat does, but plants don’t try to run away when you eat
them either. Deinocheirus could have taken its
time eating, and by
being bigger than other most other herbivores, reached areas that
were beyond the browsing limit of smaller plant eating dinosaurs.
While regarded as an ornithomimosaur this would see Deinocheirus
living more like a large therizinosaur. Additionally the large size
and bulky claws that were probably used to hook and pull down branches
could have been turned against predators that tried to attack. Given
its large size, this would make Deinocheirus a
difficult target for
even the largest of its known contemporary predators such as
Tarbosaurus.
Ornithomimosaurs in popular
culture
It’s
a shame that as far as the early twenty-first century, one of the
most enigmatic groups of dinosaurs has been poorly represented outside
of palaeontology, although this could be partly down to the fact that
there is so much uncertainty regarding them. Perhaps the best known
early depiction of an ornithomimid was in the 1969 film The Valley
of Gwangi where a stop motion Ornithomimus appeared
briefly before
being killed by an Allosaurus.
This scene was mirrored in the 1993
blockbuster Jurassic Park where a herd of Gallimimus
was attacked by
the Tyrannosaurus which succeeded in killing one.
In the 1985
television documentary Dinosaur! another stop motion ornithomimid,
this time Struthiomimus, is depicted stealing
and eating an egg from
a nest of hadrosaurs. The Struthiomimus was then
attacked and killed
by a pair of Deinonychus,
although this is inaccurate because
Deinonychus are only known from much earlier in
the Cretaceous
whereas Struthiomimus is strictly late Cretaceous.
So far though
ornithomimosaurs seem to serve more as ‘filler material’ to give
the predators something to hunt.
Early
forms such as Ornithomimus and Struthiomimus
have also regularly
appeared in dinosaur books, partly due to the facts that they are
known from more remains and have been known for longer. However
Gallimimus is now enjoying increased popularity
thanks to its
appearance in Jurassic Park, and Deinocheirus has
been regularly
portrayed, albeit as an ‘unknown dinosaur’. Ornithomimosaurs
might always be in the shadow of other kinds of dinosaurs such as
tyrannosaurs, sauropods and ceratopsian, perhaps not unlike they
were in life, but as more discoveries are made and new genera named,
there popularity as a group can only rise.
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