New Study Suggests that Therizinosaurids Had Highly Developed Senses

They may look like a dinosaur that has been designed by a committee, but a new study by a team of international researchers into the sensory abilities of therizinosaurs suggests that these feathered theropods had highly developed senses.  Using sophisticated computerised tomography (CT scans) to analyse a therizinosaur skull discovered in Mongolia, the palaeontologists found that these bizarre-looking members of the Dinosauria had a well developed sense of smell, most likely excellent eyesight and a strong sense of balance.

Sensory Abilities of Therizinosaurs

The therizinosaurids are a rare clade of the Dinosauria whose fossils have been found in North America and Asia.  Almost exclusively known from Cretaceous material, these dinosaurs are members of the Theropoda but they seem to have adapted to a more sedentary, herbivorous lifestyle, despite being distantly related to fearsome dinosaurs such as Tyrannosaurus rex and Giganotosaurus carolini.

The therizinosauroids are such unusual dinosaurs that until sufficient fossil material was found to prove their affinities with the Dinosauria they were thought by many palaeontologists not to be dinosaurs at all.  The first member of the therizinosaur family to be formally named and described was Therizinosaurus (Therizinosaurus cheloniformis).  It was named and described in 1954 by the eminent Soviet scientist Evgeny Aleksandrovich Maleev, following an intensive study of some limb bones, strange claws and flattened ribs found by a joint Soviet/Mongolian expedition to a remote area of Mongolia in 1948.

This genus is also the largest known for the therizinosauroids, with some scientists estimating that this creature was more than ten metres in length and weighed as much as five metric tonnes.

A Typical Therizinosaur – Beipiaosaurus

A dinosaur designed by a committee – the term we use to describe therizinosaurs.

Picture credit: Everything Dinosaur

The picture (above) shows a drawing of a typical therizinosaur, it is based on a model of a Beipiaosaurus produced by Safari Ltd.

To view this model range: Therizinosaurs and Prehistoric Animal Figures (Safari Ltd).

Taxonomic Relationships

The exact taxonomic relationships between different genera of therizinosaur is debated by palaeontologists; as is the groups affinities with the rest of the Theropoda.  Therizinosaurs seemed to share some general characteristics, they had small heads, perched on long necks, round, tubby bodies that gave them an almost pot-bellied look, short stocky legs and relatively long arms.  The hands ended in three-fingered claws, the most famous of the therizinosaur autapomorphies (distinctive anatomical features), the claws of therizinosaurs were exceptionally long.  The claws of  T. cheloniformis,  for example could have been up to a metre in length.   These claws were not strongly curved but rather flattened.  When first studied these claws reminded the scientists of scythes, so with more fossil finds, the clade of “scythe” lizards or the therizinosaurs was established.

A number of therizinosaurs such as Beipiaosaurus (B. inexpectus), are believed to have been covered in coats of simple, downy feathers.  These feathers were not like the flight feathers of modern birds but helped to insulate these animals and keep them warm.  This is evidence to suggest that these dinosaurs were endothermic (warm-blooded).

What these animals ate is also debated by palaeontologists, some believe that these dinosaurs were omnivores, feeding on vegetation but also catching small lizards, birds and mammals.  Others think that the long, flattened blade like claws and their strong arms were used to break open termite nests and feed on the insects inside.  Many palaeontologists have concluded that with their extended bellies, blunt beaks and small, leaf-shaped, non-serrated teeth coupled with the presence of prominent cheeks, these dinosaurs were largely herbivorous if not entirely vegetarian.

Finding an Ecological Niche

It has been speculated that these animals lived mainly in arboreal environments and filled a ecological niche similar to that filled by the later chalicotheres and the giant sloths.  Well developed senses are not usually associated with animals that occupy these niches in an ecosystem, so a team of international researchers from the Earth Sciences Department of the University of Bristol (England), two American Universities assisted by an associate at the University of Ulaanbaatar (Mongolia) set about exploring the brains of therizinosaurs.

It is not possible to study the brain tissue of dinosaurs, such soft tissue is very rarely fossilised and quickly rots when an organism dies, however, by assembling a skull and examining the spaces that would have been occupied by soft tissues such as brains and nerves, palaeontologists can build up a detailed picture of the shape and function of extinct animal’s brains.  By studying the morphology and size of nerve openings into the skull from the orbit and olfactory areas, scientists can deduce how well developed certain senses were.  If the inner ear is available for study then an understanding of the animal’s sense of balance can be achieved.

Elikosaurus andrewsi

Using a superbly preserved and virtually complete skull of the Late Cretaceous therizinosaur known as Elikosaurus andrewsi, a typical Mongolian therizinosaur that lived around 90 million years ago (Turonian faunal stage of the Cretaceous), the team were able to build up a picture of the senses of this dinosaur.  Sophisticated computer programmes interpreted the data from numerous CT scans of the fossil skull and this enabled the team to build up a three-dimensional picture of the internal structure of the dinosaur’s head.

The Skull of the Therizinosaur used in the Study (Elikosaurus andrewsi)

The large orbit (eye socket) of a therizinosaur. Study suggests these creatures had highly developed senses.

Picture credit: Emily Rayfield, University of Bristol

The study, the results of which have been published in the online scientific journal “The Public Library of Science” reveal that therizinosaurs had a remarkably well developed sense of smell, excellent eyesight and a strong sense of balance. When the skull of a therizinosaur is examined the large orbit (eye socket) can be clearly seen, and the long muzzle provides an extended naris which may have helped improve the animal’s ability to smell.  All this data was interpreted and managed by what turns out to be an enlarged forebrain. It seems that these slow-moving, herbivores with their over-sized bellies and strange claws may have had keen senses and a very well developed sense of balance.  It is likely that these attributes are a carry over from when the ancestors of the therizinosaurs were active predators.

The work of this international team of researchers has important implications for our understanding of how sensory functions evolved and changed in the Dinosauria.  It also sheds a little light on how the sensory organs of birds (closely related to theropod dinosaurs), evolved.

It is difficult to think of these dinosaurs as “smart”, their brains were small in proportion to their body size, but this latest sophisticated study further assuages the old fashioned theory that these reptiles were extremely slow witted, with dull reactions and in comparison to modern mammals, the dinosaurs were very poorly adapted to their environments being doomed to ultimate extinction.  In reality, the Dinosauria, were extremely well adapted to their environments and they evolved into a myriad of forms so that they could exploit a whole range of opportunities in the Mesozoic world.

Everything Dinosaur acknowledges the help of the University of  Bristol with this article.  Inside the head of a dinosaur: Research reveals new information on the evolution of dinosaur senses.