Mysterious markings in dinosaur bones

by Christina Shears-Ozeki, University of Portsmouth

What do fossil insects and dinosaur bone have in common? The answer is trace fossils.

For more than 20 years palaeontologists have noticed unusual markings on dinosaur bones. The earliest of these occur in fossils from the mid Triassic (228-235 million years ago); others have even been found in bone fragments inside coprolites (fossilised poop).

Extensively bored dinosaur bone showing chambers and channels.

Trace fossils are tracks, trails and other evidence left by ancient organisms. They help palaeontologists interpret ancient environments, the organisms that inhabited them, and their behaviour. So in fact, trace fossils are fossilised “behaviour” represented by footprints, tracks, burrows, borings, coprolites, bite marks, and even constructions such as birds’ nests, bee-hives, and termite mounds.

Recently, dinosaur bones from mid-Cretaceous (125 million year-old) Moroccan rocks were discovered with oval shaped chambers (approx. 5mm diameter), and channels lined with scratch-marks (approx. 1-2mm long). The chambers and channels were filled with the same sediment surrounding the bones; and hairline desiccation cracks on the bones indicated prolonged exposure to ancient air.

Scanning Electron Microscope (SEM) image of channels in the bone with scratch-marks

Thus the marks must have been produced before fossilisation and most likely soon after the dinosaur had died.

But who or what produced these enigmatic, tiny borings; and why were they present? It must have been something small – perhaps an insect? Comparing these marks to those made by living insects on modern bones, indicates that the likely ‘culprits’ are an ancient type of dermestid beetle (“skin beetle”). Modern dermestid beetle larvae prefer warm, dry environments and are frequently used in museums to remove dried flesh from skeletons (hence the name). But if left with a skeleton long enough, they will ‘eat’ into bone to find the juicy marrow inside.

Dermestid beetle a) adult, b) larvae, and c) pupa; images from BugGuide.net

So the marks on the Moroccan dinosaur bones were probably made by ancient beetle larvae ‘teeth’ when they were searching for bone marrow and making pupation chambers. My research involved the identification of the borers, which also allowed me to determine that the climate during and after the time the dinosaurs died was characterised by dry periods that could sometimes last for a long time.

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