Sinornithomimus
Name:
Sinornithomimus
(Chinese bird mimic).
Phonetic: Sy-nor-nif-h-mime-us.
Named By: Yoshitsugu Kobayashi and L� Junchang -
2003.
Classification: Chordata, Reptilia, Dinosauria,
Saurischia, Theropoda, Ornithomimidae.
Species: S. dongi (type).
Diet: Uncertain, but thought to be more
herbivorous rather than omnivorous like similar dinosaurs.
Size: 2 meters long.
Known locations: Mongolia - Ulansuhai Formation.
Time period: Due to the uncertain nature of the
Ulansuhai Formation at the time of writing, Sinornithomimus
lived either during the Aptian to Albian or the Turonian stages of the
Cretaceous, but the latter is more commonly accepted.
Fossil representation: Many individuals including
juveniles.
Surprisingly
what is perhaps the most important ornithomimosaur
so far discovered by
the early twentyfirst century is also one of the least known outside of
palaeontology. Initially thought to be a primitive ornithomimosaur,
Sinornithomimus is now thought to be quite a bit
more advanced than
early ornithomimosaurs like Harpymimus,
but not quite as advanced as
those that lived towards the end of Cretaceous such as Ornithomimus.
This intermediate development might also support an intermediate
placement during the Turonian period of the Cretaceous which is roughly
the half way mark between primitive and advanced ornithomimosaur
forms. A feature that makes Sinornithomimus
immediately stand out
from other forms is the neck that is much shorter in proportion to the
body than in other forms.
The
first thing that made palaeontologists sit up and take note about
Sinornithomimus is how individuals of this genus
were found. The
first discovery by Dong Zhiming (after whom the type species is
named) was not of one but fourteen individuals, eleven of which
were juveniles while three were subadults, possible adults.
Amazingly in 2001 another expedition made a near identical
discovery when a group of thirteen juveniles were discovered.
Additionally neither of these groups has been interpreted as being a
slow and steady build-up of individuals, but instead of groups that
were subjected to sudden or fast acting phenomena that killed all the
group members within a similar time period. Current thinking has the
Sinornithomimus groups stuck in mud in an aquatic
environment where
they eventually died from starvation or possible predation because they
could not escape. Some remains seem to have been pulled about and
scattered, especially in the upper body areas which is seen as a sign
of scavenging.
As
you have probably already noticed the key thing here is that most of
the group members were juveniles with only a few near adult
individuals. Before there was speculation that ornithomimosaurs went
around in family groups with support for the idea coming from some
genera such as Gallimimus,
but two collections of juvenile groups
suggests that as soon as juveniles were independent enough to move
around and feed by themselves they left their parents to join up with
other juveniles where they would live together until adulthood. This
behaviour is not that farfetched when you consider that many types
of birds do exactly this today, the benefits being that the parents
are then free to raise another brood rather than continue to look after
the previous year’s young. Once they had turned adult,
Sinornithomimus would have left these juvenile
groups, but may still
have formed loose groups with other adults before pairing up with
another and raising their own brood so that the cycle continues to
repeat itself.
The
above would suggest that Sinornithomimus and
possibly other
ornithomimosaurs relied upon what is called an r-strategy survival
mode. In the simplest terms this is where parent animals raise a
relatively large number of offspring each year with the expectation
that many to most of them won’t survive to adulthood. The fact that
two groups of juveniles were killed together by what seems to have been
a naturally occurring event would suggest that the environment did
contain hazardous areas that could easily catch out and cause the
deaths of the animals living there. Clustering in groups meant that
Sinornithomimus would have had better protection
from predatory
dinosaurs of the time such as Shaochilong
and Chilantaisaurus.
Many
individuals looking for danger are more likely to spot it than just one
and predators always have a harder time keeping their focus upon a
targeted individual when it is moving around within a group. This is
not always successful however and predators are sometimes successful in
striking a group, especially when an individual gets separated. The
first groups remains where only three subadults were present may be
indicative that the others had already left to start their adult
lives, or that they simply didn’t make it that far.
Another
thing area that Sinornithomimus has helped to fill
in the gaps is
gastroliths. For those who are not familiar, gastroliths are stones
which are swallowed and stored in the stomachs of some animals that
usually cannot process food in their mouths by chewing, so they rely
upon the grinding action of the stones moving around in their stomachs
to do the job for them. Most, particularly the more advanced form
ornithomimosaurs did not have teeth, just a keratinous beak that
allowed them to grip and snip things. Beaks however are good for a
variety of dietary options from plants to small animals and carrion,
so exactly what ornithomimosaurs ate has always remained a mystery.
Because gastroliths are most commonly found in herbivorous (plant
eating) animals this has led to speculation that Sinornithomimus
at
least may have been more herbivorous than the omnivore that eats both
plants and animals that other ornithomimosaurs are depicted as being.
However,
while it’s very possible that Sinornithomimus
was a plant eater,
it is not unknown for predatory animals to also have them. Animals
that eat fish and invertebrates for example often swallow stones to rub
off scales and wear down and crack shells and the fossils remains of
aquatic crustaceans were found along the Sinornithomimus
revealing that
they were at least present. Very primitive ornithomimosaurs like
Pelecanimimus
are also thought to have been fish eaters which further
establish a link to a predatory past of the ornithomimosaur group. It
would also make more sense for juvenile animals to feed upon animal
protein as this would have a much higher nutritional gain for them than
eating plants. Again looking at birds, many species will only feed
their young invertebrates when they are in the nest, but will also
incorporate seeds and green shoots into their diets when adult. Going
a little bit further away from Sinornithomimus,
there is evidence of
a large predatory theropod from Portugal called Lourinhanosaurus
which
also seems to have had gastroliths within its stomach.
While
the gastroliths within Sinornithomimus can be
interpreted as being for
herbivory, they can also be interpreted in other ways including
carnivory or omnivory like with other ornithomimosaurs. Further
remains may yet one day reveal a more definitive answer. Another
ornithomimosaur that has been found with gastroliths is Shenzhousaurus.
Further reading
- A new ornithomimid dinosaur with gregarious habits from the Late
Cretaceous of China. - Acta Palaeontologica Polonica 48(2):235-259. -
Y. Kobayashi & J. Lu - 2003.
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