Fossils of Burrowing Dinosaur Found in Montana

Scientists have speculated that with dinosaurs dominating life on Earth during the Mesozoic they will have filled a lot of environmental niches, specialising in types of food and different ways of living.  Now a team of palaeontologists from Montana State University have found evidence that some dinosaurs spent at least part of their lives in burrows.

Working in an area of south-western Montana, the leader of the university team, David Varricchio discovered an unusual deposit of sandstone protruding from the surrounding sediments.  Choosing to excavate this peculiar feature, the team dug through the sandstone and discovered a mass of small dinosaur bones tangled up in a single layer.  The remains represent a new species of Ornithopod, perhaps closely related to Hypsilophodontia.

Three individual skeletons were located, each one virtually complete, there was two smaller animals and a larger one.  Could this be an underground den where an adult looked after the young?

Further excavation of the 95-million-year-old sediment (dated to the middle Cretaceous – Cenomanian faunal stage), revealed that the sandstone mass was S-shaped and about 2.1 metres in length. For most of its length, the sinuous feature had an oval cross section about 30 centimetres in diameter and about 40 cm high.  However, at its lower end—where the dinosaur bones were found—it broadened to a width of 45 cm, could this be a nursery chamber?  The elongated sandstone mass intersected with three distinct layers of rock derived from mud and clay, perhaps a sign that those strata were in place before the material that formed the sandstone was deposited.

Close study of the fossil remains showed that they were unbroken and showed no signs of having been weathered before they were buried. The bones show no signs of predation either, so it is unlikely a meat-eater dragged the bodies into the burrow.  David Varricchio and his university team have proposed that the mass of sandstone represents a sudden influx of material that filled in a burrow, such as that from a sandstorm trapping its occupants and suffocating them.

These findings have been reported at the scientific symposium and meeting of the Society of Vertebrate Palaeontology in Texas.  This unique discovery adds weight to the theory that some dinosaurs cared for their young and showed a strong maternal or paternal nature.

A Diagram of the Fossil Site showing the Composition of the Burrow

Diagram Credit: L. Hall – Montana State University

The red arrows in the diagram indicate the position of fossil bones.

The dinosaur has been named Oryctodromeus cubicularis (means digging runner of the lair).  The size of the juveniles and the complexity of the burrow indicates that this particular species may have spent a long time looking after young.  The adult was approximately 2.1 metres in length with an estimated weight of 25 kilogrammes.  The juveniles were approximately 1.3 metres long.

Reviewing the papers produced by the Montana team, Dr Paul Sereno commented “all the pieces are there to support the burrowing interpretation”.  Dr Sereno has specialised in Ornithischian cladograms so was able to add credence to the hypothesis that this small biped was a Hypsilophodont.

Being bipedal enabled this little animal to evolve modified forelimbs that could have helped with the digging of dens and burrows.  Although study of the adult arm bone (humerus, ulna and radius) may indicate large muscle attachments to assist with digging, the absence of a manus (hand) prevents scientists from speculating on further anatomical modifications to assist with excavation.  Similar forelimb modifications are seen in modern animals such as rabbits that also have a running and burrowing existence.

The dinosaur’s wide, broad hips as well as being a trait of herbivores may also have assisted with burrow digging.  Oryctodromeus could have braced itself in a wide stance while burrowing.  Modifications to the skull and jaw bones may also have helped this little animal manipulate soil.

An Artist’s Impression of Oryctodromeus

Burrowing Dinosaurs

Picture Credit: Lee Hall (Montana State University)

The large orbit in the skull may indicate that this animal had big eyes and this may indicate that Oryctodromeus was nocturnal.  The presence of a burrow provides evidence of social behaviour, with a prolonged period of parental care coupled with aspects of territory management and possibly social interactions if rival pairs of these animals wandered into another Oryctodromeus’s domain.  Some scientists have speculated that some other small Ornithopods may also have lived part of their lives in burrows.  Their forelimbs may also show modifications for digging.

The technical scientific term for burrowing animals is fossorial, this word and the term fossil are derived from the same Latin source – fossillis; which is related to digging.

Certainly, a little pack of Oryctodromeus working alongside palaeontologists would have been very useful when it came to excavating fossils.

Digging for fossils is certainly very hard work, especially when the matrix is unforgiving and hard such as compacted sandstone.  Machinery cannot be used close to any fossil bearing sediment in case of damage so much of the close excavation is down to elbow grease.  However, when it comes to removing the matrix around fossil bone a steady hand and a lot of patience is required.

Young dinosaur fans can experience this for themselves with Everything Dinosaur’s Dig-a-Dino Series which features excavation kits of prehistoric animals such as Tricertops, Velociraptor, Stegosaurus, Tyrannosaurus rex and the Pterosaur Pteranodon:

As an alternative, the smaller Dinosaur Fossil Finds provide a similar experience but inside the gypsum based block there is a mini dinosaur fossil.  There are six models in this series including Pachycephalosaurus and Diplodocus.

Dinosaur Crafts: Dinosaur Crafts for Kids

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