The First of the Bone-heads

A team of North American scientists have announced the discovery of a new species of pachycephalosaur.  Pachycephalosaurs are bird-hipped members of the Dinosauria commonly referred to as the bone-headed dinosaurs.  Pachycephalosaurids are known from Upper Cretaceous fossil bearing sediments of the northern latitudes, most notably from North America.

Much of our knowledge about this group comes from a few well-preserved skeletons ascribed to the genus Stegoceras.  Other species have been identified based on the fossilised remains of their thickened skulls.  Indeed, so thick are the skull bones (frontal and parietal), that often these are the only fossils ascribed to a particular genera, the rest of the skeleton material presumably having been lost to erosion or failing to be preserved.

Bone-headed Dinosaur

This new species, named Acrotholus audeti may be one of the oldest types of pachycephalosaur discovered to date, certainly a candidate for the oldest pachycephalosaur known from North America.  The genus name means “high dome” a reference to the thickened skull, whilst the specific name honours Roy Audet, on whose land the best-preserved specimen of skull material was found back in 2008.  The scientists from the Royal Ontario Museum/University of Toronto along with co-researcher Dr Michael Ryan, curator of vertebrate palaeontology at the Cleveland Museum of Natural History have used the example of this Pachycephalosaur to attest that there may have been many more different kinds of smaller dinosaurs than previously thought.

The team state that the fragile, lighter bones of smaller dinosaurs would not have survived to the present day, unlike some of the bones of their giant cousins.  In this way, the fossil record may be biased towards large dinosaurs and the small dinosaurs may be under represented.

The  Partial Skull Fossil of Acrotholus audeti (2008 Specimen)

The very thick skull of this new pachycephalosaur.

Picture credit: Royal Ontario Museum

A Scientific Paper

Writing in the scientific publication, the “Journal of Nature Communications”, the research team describe their new dinosaur based on the remains of two skull domes, both from the Milk River Formation of southern Alberta (Canada).  The first specimen resides in the collection of the Royal Ontario Museum (Toronto), it was collected more than fifty years ago, the second specimen, the better preserved example, was discovered on rancher’s Roy Audet’s land in 2008.

The fossils have been dated to approximately 85 million years ago (Santonian faunal stage of the Late Cretaceous).  This makes the fossils ascribed to A. audeti some of the oldest pachycephalosaur material known.  Palaeontologists remain uncertain as to where and when the first true pachycephalosaurs evolved.  Certainly, sometime in the Cretaceous a group of small, ornithischian dinosaurs began to evolve reinforced skulls, but the paucity of the fossil record prevents scientists from tracing this clade’s particular family tree.

Some pachycephalosaur material has been described from Upper Jurassic aged sediments, but since the fossil material is extremely fragmentary the placement within the Pachycephalosauridae is controversial.

Scientific Illustration of the New Pachycephalosaur Fossil Material

Dorsal and anterior views of the skull material.

Picture credit: Journal of Nature Communications

Living Alongside the Megafauna

When extant ecosystems are studied, it is very evident that alongside the megafauna there is a whole array of smaller animals living in the same habitat.  It is likely that ecosystems in the Mesozoic were very similar with many more types of small dinosaur (under one hundred kilogrammes), living amongst the much larger dinosaurs.  However, small dinosaur fossils are less common than the fossils of larger animals in the fossil record.  Is this preservation bias or a true reflection of the nature of dinosaur dominated communities?

The carcases of smaller animals would have been more easily consumed by scavengers, small bones would have been readily destroyed due to natural decay processes.  Even if they did fossilise and survived to the present day, as an example, a femur (thigh bone) of a fox-sized dinosaur would have been likely to be lost to the forces of erosion, abrasion, attrition etc. than the metatarsals (toe bones) of a dinosaur the size of an African elephant.

For models and replicas of Late Cretaceous North American dinosaurs including pachycephalosaurs: Age of Dinosaurs Prehistoric Animal Figures (PNSO).

Intriguingly, the robust and very thick skull domes of pachycephalosaurs that have survived to the present day may provide scientists with an insight into the diversity of small, ornithischian dinosaurs that were around in North America during the Late Cretaceous.  The team that worked on the Acrotholus specimens, palaeontologists from the Royal Ontario Museum as well as graduates from the University of Toronto working in collaboration with scientists from the Cleveland Museum of Natural History, suggest that there may have been many more types of smaller dinosaurs than previously thought.

An Illustration of Acrotholus audeti

In the shadow of larger dinosaurs, dog-sized Acrotholus walks next to larger dinosaur tracks.

Picture credit: Julius Csotonyi

Dr Michael Ryan (Cleveland Museum of Natural History) stated:

“We can predict that many new small dinosaur species like Acrotholus audeti are waiting to be discovered by researchers willing to sort through the many small bones that they pick up in the field.”

The fossils of this new member of the pachcephalosaurids, will be put on display this week at the Royal Ontario Museum, part of an exhibit that provides more information on the vertebrates found in the strata of the Milk River Formation.  Visitors to the museum will be able to see the skull bones of A. audeti and perhaps try to work out why these dinosaurs had such thickened skulls.  The skull bones of Acrotholus would have been up to ten centimetres thick, were the domed skulls used for display or did these dinosaurs butt heads just like some sheep and goats do today?

New Meat-Eating Dinosaur from China

Fossils uncovered in 2006 by a team of international researchers as they explored a remote part of Xinjiang Province have been identified as a new species of predatory theropod dinosaur.  This new, meat-eating dinosaur discovery has been tentatively ascribed to the Coelurosauria.  Although this is the seventh theropod and the fourth coelurosaur discovered in strata from the Shishugou Formation, it represents the earliest fossil coelurosaur discovered from this Formation to date.

New Dinosaur Species

Fossils of small dinosaurs are exceptionally rare, those specimens known from the Middle Jurassic tend to consist of isolated fragments of bone, or individual teeth.  Although far from complete, the fossil material represents an individual animal and it consists of the skull and mandible, limb bones, vertebrae, digits and elements from the pelvic girdle.  The three previously discovered coelurosaurs from the same Chinese location (Guanlong, Haplocheirus and Zuolong) were found at higher stratigraphical layers, thus indicating that this new discovery is much older than those previously found.

It is thought that this new discovery dates from approximately 161 million years ago, (Callovian faunal stage).  This would suggest that this little carnivore roamed China during the Middle Jurassic, however, this specimen could also be referred to as Late Jurassic as the radiometric dates for the strata from which the fossil was excavated, fall within the margin of error delineating the geological boundary between the Middle and Late Jurassic.

The Skull of Aorun zhaio

The coin provides a scale.

Picture credit: James Clark/George Washington University

The picture above shows the skull with a coin providing scale.

Aorun zhaio

The new species has been named Aorun zhaio (The Dragon King of the West), from “Ao Run” taken from the Mandarin Chinese language and the Chinese epic folklore tale, “Journey to the West”.  The species name honours  Professor Zhao Xi-jin, a renowned vertebrate palaeontologist who has been prominent in helping to open up this remote part of China to scientists from outside of the country.

The field team led by James Clark (Ronald B. Weintraub Associate Professor of Biology) at the George Washington University (United States), suggest that this dinosaur was less than a metre long when it died and it probably weighed little more than fifteen hundred grammes.  A detailed analysis of the fossilised bones suggest that this dinosaur was less than twelve months old when it died.  It may not represent an example of a small theropod, as an adult, this dinosaur could have grown up to be an apex predator, perhaps similar in size to the Chinese allosaurid Sinraptor.

A Juvenile Specimen

Dr Clark and his then, doctoral student, Jonah Choiniere were able to establish that this little dinosaur was less than a year old when it died and got buried in stream sediments.  As the fossils represent a very young animal, an animal whose features would change as it grew, it is very difficult to pin down where in the phylogeny within the Coelurosauria this specimen should be placed.  A scientific paper, published in the “Journal of Systematic Palaeontology” outlines the discovery, describes the fossil material and discusses the importance of this specimen with regards to understanding the evolution and radiation of the smaller theropods.

An Interpretation of the Fossil Material

Small, bipedal and possibly feathered.

Picture credit: Everything Dinosaur

The illustration above shows Aorun zhaio as an agile, fast-running dinosaur with a slightly elongated snout and a long tail.  As other members of the coelurosaur clade are believed to have been feathered, in this illustration A. zhaio is depicted as a feathered dinosaur.

For models and replicas of feathered dinosaurs and Sinraptor (whilst stocks last): PNSO Age of Dinosaurs Models and Replicas.

A spokesperson from Everything Dinosaur commented:

“Teeth from the upper and the lower jaw have been found.  Their shape indicates that this little dinosaur was a meat-eater, hunting smaller reptiles, insects and perhaps even primitive mammals.  Reptiles such as snakes, turtles and crocodiles today lay many eggs, but few of the hatch-lings survive to adult hood.  It is likely that this coelurosaur was part of a large brood, but like many of its siblings it did not live very long”.

Many people consider the meat-eating dinosaurs to be all very large animals, but today the Order of Mammalia known as the Carnivora consists of a wide range of differently sized predators.  There are wolves and tigers within this Order but also weasels and small cats.  It was the same during the Mesozoic, some theropod dinosaurs were very large, but others remained small.  Each type of  theropod dinosaur evolved to fit a particular ecological niche.