Scientists have reported the discovery of a hadrosaur pedal ungual (the bone on the end of a toe that supported the keratin claw or hoof), that shows a series of small bite marks made by a theropod dinosaur. The toe claw seems to have been bitten repeatedly and although scrapes and scratches on fossil bones that are incidental feeding traces left by meat-eating dinosaurs have been well documented, these bite marks might represent something very different.

Did a baby tyrannosaur or possibly a dromaeosaurid gnaw on the toe bone of a dead duck-billed dinosaur?

A young tyrannosaur gnaws on a hadrosaur toe bone
A life reconstruction of a juvenile tyrannosaur biting down on the hadrosaur pedal ungual causing the bite marks that have been preserved in the fossil (TMP 2018.012.0123). Picture credit: Joshua Doyon.

Gnawing Behaviour

Gnawing behaviour is synonymous with many types of mammals, specifically members of the Carnivora and rodents (Rodentia), but it is not commonly associated with the Dinosauria. Coprolites thought to have come from tyrannosaurs contain a lot of bone fragments, tests demonstrate that large tyrannosaurids were capable of crushing bone and it had been thought that coprolite bone content came about as bones were ingested through general consumption.

However, a trio of scientists – Caleb Brown and Darren Tanke from the Royal Tyrrell Museum of Palaeontology (Alberta) in collaboration with Dr David Hone, Senior Lecturer in Zoology at the University of London, have recently published a paper in PeerJ, that suggests that the unusual bite marks on the hadrosaur pedal ungual might represent dinosaur gnawing behaviour.

A hadrosaur pedal ungual with gnaw marks
Ammonium chloride powder coated photographs of the hadrosaurid pedal showing bite marks (viewed from the bottom – ventral/plantar view). (A) View of TMP 2018.012.0123 (A), with marks highlighted in blue (A’). Close-up of the bitten region (B), with marks highlighted in blue and numbered in Arabic numerals (B’). Note scale bars = 1 cm.

Documenting Unusual Dinosaur Behaviour

The fossil toe claw bone (specimen number TMP 2018.012.0123), comes from a bonebed (bonebed 50) that contains the disarticulated remains of several different types of duck-billed dinosaur including Corythosaurus. Although the bone came from an adult, it is not possible to confirm the dinosaur species. Thirteen, distinct and highly localised tooth marks have been identified. Their pattern suggests that a small, meat-eating dinosaur delivered up to six repeated, powerful bites to the claw bone. There would have been very little meat on this part of the hadrosaur’s body, gnawing on the pedal ungual represents an unusual and rare form of behaviour.

The researchers reviewed pedal unguals of duck-billed dinosaurs from the Dinosaur Park Formation. They identified tooth marks and feeding traces on four other toe claw bones, but this represents less than 1% of all the hadrosaur toe bones found and feeding traces were much more common on other bones.

Evidence of late stage carcase consumption by a dromaeosaur or young tyrannosaur on a pedal ungual of a hadrosaur.
Right articulated hadrosaurid pes in dorsal view (A), with ungual of digit three highlighted (white) and the position of the tooth marks (ventral side) indicated in black. Shaded line drawing of the ventral view of the ungual (B), showing the position of the bite marks (black). Close-up view of bite mark size (C) and (D) Close-up view of bite marks showing potential alignment of tooth row parallel with the long axes of the tooth marks. Hollow fills in C indicate potential bite marks missing from rows/columns. Note scale bars = 1 cm.

Dromaeosaur or Tyrannosaur?

The tracemaker cannot be definitively identified but the researchers rule out crocodilians, small mammal feeding traces and snake bites, leaving a theropod dinosaur as the likely tracemaker whose unusual behaviour has been recorded in the fossil. The number of theropods capable of causing such marks and known from the Dinosaur Park Formation is relatively small. The scientists considered dromaeosaurids and their close relatives the Troodontidae, as the tooth marks could have been made by a large troodontid such as Latenivenatrix. The team also considered whether the tracemaker was a young tyrannosaurid.

Given the lack of evidence of denticle spacing present on the bite marks, and that both Tyrannosauridae and Dromaeosauridae were capable of delivering bites resulting in deep furrows and pits to the bone surface, the team speculated that either a dromaeosaur (such as Dromaeosaurus or Saurornitholestes), caused the damage or perhaps the marks were made by a very young tyrannosaurid. Two genera of tyrannosaur are known from the Dinosaur Park Formation, namely Gorgosaurus and Daspletosaurus.

Perhaps, a very young Gorgosaurus, the lowest ranked animal in the pack was left to pull at and gnaw on the toe of the hadrosaur, whilst the rest pack gorged themselves on the more attractive, nutrient rich parts of the carcase.

Can Dogs Provide an Answer?

Anyone who has kept horses and dogs will tell you that when the horse’s hooves are trimmed dogs love to eat the trimmings. The hooves are made from keratin, the same protein responsible for the toe claw on the hadrosaur. Dogs can get very excited when the farrier starts to tidy up the hooves, they seem to crave the soft, recently trimmed parts of the hoof.

Many dog treats are made from horse’s hooves. Could your pet dog provide an insight into dinosaur feeding behaviour?

A trio of Japanese hunting dogs.
Most dogs like to consume hoof trimmings. The hooves of horses and duck-billed dinosaurs are made of the same material (keratin), could a young tyrannosaur have craved the taste of the toe claws of hadrosaurs?

Could a tyrannosaur similarly have craved the taste of the toe claw of a duck-billed dinosaur?

The scientific paper: “Rare evidence for ‘gnawing-like’ behavior in a small-bodied theropod dinosaur” by Caleb M. Brown, Darren H. Tanke and David W. E. Hone published in PeerJ.

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