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
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.
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.
largest of the
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
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.
(with new discoveries this list is likely to change)
Archaeornithomimus (Ancient bird mimic)
Beishanlong (Beishan/White mountains dragon)
Deinocheirus (Terrible hand)
Gallimimus (Chicken mimic)
Garudimimus (Garuda mimic)
Harpymimus (Harpy mimic)
Kinnareemimus (Kinnaree mimic)
Ornithomimus (Bird mimic)
Pelecanimimus (Pelican mimic)
Qiupalong (Qiupa dragon)
Shenzhousaurus (Shenzhous lizard)
Sinornithomimus (Chinese bird mimic)
Struthiomimus (Ostrich mimic)