Canada’s Smallest Dinosaur Hints at Reptilian not Mammalian Predatory Niche
A research paper has just been published announcing the discovery of the smallest dinosaur known to date from North America. This tiny theropod, identified from fossilised pelvic bones is believed to have been no bigger than a crow and probably lived in the undergrowth or up trees, filling a niche held by small mammals in today’s ecosystems.
This new dinosaur species, remains of which come from the fossil rich sediments of Alberta, Canada, has been named Hesperonychus is believed to a close relative of the Chinese dinosaur Microraptor gui which some scientists claim is the smallest dinosaur discovered to date.
The tiny fossilised hip bones had been collected many years before, but they had been misidentified as being those from an extinct genus of lizard, it was only when the material was re-examined that the mistake was noticed. The research paper on this tiny carnivore has just been published in the scientific journal “The Proceedings of National Academy of Sciences”.
This little dinosaur identified from a fused pelvic girdle (indicating an adult specimen, not a juvenile animal), is believed to be a member of the dromaeosaur family. These bipedal dinosaurs were relatively common in the Late Cretaceous, but this particular member of the “swift lizards” is the smallest found in North America to date. An examination of other fossil material has revealed some fossil finger and toe bones, including the retractable pedal ungual (toe claw), characteristic of the dromaeosaur group.
Pictures show the tiny Hesperonychus toe claw balanced on a Canadian coin for scale. It is not known whether this small dinosaur would have been covered in feathers, but scientists have speculated that it probably was. The feathers would have helped insulate this small meat-eater and help prevent it losing too much heat, regulating its body temperature. They may also have helped it glide from tree to tree as it searched for food.
The specimen helps to confirm that reptiles, such as this tiny dinosaur and not mammals, filled the role of small predators during the Mesozoic. There were certainly small mammals around at the time but they did not have a monopoly on the small, insect eating niche, perhaps dinosaurs like Hesperonychus hunted during the day and the mammals came out to hunt a night.
Dr Phillip Currie, palaeontologist from the University of Alberta and co-author of the research paper commented:
“Despite the discovery of exquisitely preserved skeletons of small bird-like dinosaurs in Asia, they are exceedingly rare in North America.”
Despite many major dinosaur finds in formations such as Judith River and the Dinosaur Provincial Park Formation, Dr Currie and his colleagues had been pondering why so few small fossils have been unearthed in Alberta, Canada – one of the world’s richest sites for large-dinosaur bones of the Cretaceous. The team suspect that there were many small dinosaurs around during the age of reptiles, however, their remains did not preserve well so they are relatively scarce in the fossil record.
Dr Currie went onto state:
“There were many large dinosaurs running around eating them, and small bones are easily washed away by rivers [common in this region during the Cretaceous period]“.
The new discovery casts more doubt on whether mammals would have acted as small predators in Cretaceous-era North America. The fossilised pelvis came from an animal that weighed no more than 1.9kg (4.2lb) and appears distinctively reptilian. Team members at Everything Dinosaur have had the opportunity to visit some of the fossil storage sites at Canadian museums such as the Royal Tyrrell Museum and we once calculated that there were so many fossils awaiting cleaning and study that it would take all the staff over 100 years to clear the back log. Such is the rich diversity of fossils found in formations such as the Dinosaur Provincial Park, an ecosystem closely studied by Dr Currie and his fellow palaeontologists.
Commenting on the find, identified as an insect eating, bipedal, dinosaur, Dr Currie added:
“This tells us that [as in Asia], North American dinosaurs likely out-competed mammals for both large and small predator niches”.
The authors of the research also suggest that this little dinosaur may provide a clue to help resolve the debate as to whether flight originated in arboreal dinosaurs “tree down” or whether it evolved from dinosaurs that had a cursorial existence “ground up”. Based on the size of the hip bones and the position of the pubis a bone that makes up the pelvic structure, normally this bone is to be found between the hind legs and with theropods it has in many cases a characteristic “boot” shape to the tip. However, with Hesperonychus the pubis is bent, this indicates that it may have had an arboreal existence.
“We know this dinosaur was a tree-climber”, Dr Currie explained.
“It likely used the long feathers on its limbs to glide or parachute from tree to tree”.
Certainly, it would make sense if this little dinosaur was able to climb trees, it would have required an excellent sense of balance, strong limbs and stereoscopic sight, all qualities believed to have been possessed by Dromaeosaurs. By climbing trees, it would have been able to catch insects and other invertebrates as well as avoiding many of the larger predators that shared its environment.
The scale of this little biped, is difficult to imagine, so the University of Calgary provided a diagram, illustrating the size of Hesperonychus in comparison with other contemporary theropods that also roamed Alberta at the time this little dinosaur existed.
Scale Drawing of Late Cretaceous Theropods from Alberta
Picture credit: University of Calgary/PNAS
The specimen, Hesperonychus elizabethae – named after its collector Dr Elizabeth Nicholls – was reclassified by palaeontologist Dr Nicholas Longrich, a co-author of the paper, from the University of Calgary.
It has long been suspected that there were large numbers of small dinosaurs living in forest and woodland habitats, but forest environments have very poor fossil preservation potential and any small dinosaur remains on the forest floor would soon have been scavenged or broken up. Hence, the fact that very small dinosaurs are probably under-represented in the existing fossil record.