What Is a Dinosaur Mummy? Specimens That Kept Their Skin
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
A handful of specimens preserve skin, and in a few cases the shape of the body, rather than only bone. They are not mummies in the Egyptian sense, and the name has stuck to the rarest and most informative fossils in the field.
What the word means here
The term is applied to specimens preserving extensive soft tissue impressions or mineralised soft tissue in addition to the skeleton, generally including skin over much of the body and occasionally the outline of muscles, tendons and the beak or claw sheaths. The process has nothing to do with deliberate preservation and begins with a carcass drying out before burial, so the skin becomes tough and leathery and collapses onto the skeleton rather than decaying away. Rapid burial then preserves that arrangement, either as an impression in the surrounding sediment or through mineral replacement of the tissue itself. The result is a fossil that records the external appearance of an animal directly rather than through inference, which is why a single such specimen can be worth more scientifically than many complete skeletons.
What they show
The information such specimens supply is not available any other way:
- •Scale patterns and their variation across the body, including differences between regions that no reconstruction would have predicted
- •Skin thickness and texture, which constrains how an animal looked in life rather than how its skeleton looked
- •Body outline, since the soft tissue shows where muscle and fat sat and how much of it there was
- •Keratinous coverings on beaks, claws and horns, which are longer than the bone beneath and rarely preserved
- •Structures with no bony support at all, including crests, frills and wattles, which are simply invisible otherwise
- •In exceptional cases, pigment structures indicating colour and patterning
The famous specimens
A small number of finds account for most of what is known. Early twentieth-century collecting in North America produced several duck-billed dinosaur specimens with extensive skin impressions, which established the basic scale patterns for that group and which were described at the time in language that gave the category its name. A remarkably complete armoured dinosaur recovered from a marine deposit in Canada preserved the animal in three dimensions with armour in place, skin intact and even the outlines of the keratin sheaths over the spikes, and analysis of it recovered pigment evidence indicating a reddish colouration with countershading. Further specimens of various groups have added skin from different body regions. Each has required years of preparation, since the tissue is fragile and distinguishing it from the surrounding rock is painstaking work.
What counts as skin evidence
Soft tissue evidence arrives in several forms of varying quality and the distinctions matter for what can be concluded. An impression is a mould of the surface left in sediment, recording texture and pattern without preserving any original material, and it is the commonest form. Replacement preserves the shape of the tissue in mineral, which can retain internal structure and is far more informative. Carbonised films leave a dark residue of the original organic material, which is how many feathers are preserved and which permits chemical analysis. Amber encloses tissue directly and preserves fine detail, though the pieces are small and the famous examples are feathers and small animals rather than anything large. Each type is found under different conditions, which means the record of external appearance is not just patchy but patchy in a biased way, favouring particular environments and particular parts of the body.
How rare they are
Preservation of this kind requires a specific and unlikely sequence, which is why the specimens are so few. The carcass must dry before decay and scavenging destroy it, which requires an arid or otherwise dessicating setting. It must then be buried rapidly, which usually means a flood or a sediment flow, and those conditions are largely incompatible with the drying that preceded them, so the animal must have been moved or the conditions must have changed. Chemical conditions at the burial site must then favour mineral replacement rather than dissolution. Each step is improbable and the combination is more so. The consequence is that such fossils are concentrated in a handful of formations where the necessary sequence occurred repeatedly, and that whole groups of dinosaurs are known from no such specimen at all, so the direct evidence about external appearance is patchy in a way that is easy to forget when looking at confident reconstructions.
The takeaway
Skin dries before burial, collapses onto the skeleton and is then preserved as impression or replaced tissue, which records external appearance directly rather than by inference. Such specimens supply scale patterns, body outline, keratin sheaths and structures with no bony support. The sequence required is improbable at every step, so the direct evidence about how dinosaurs looked is concentrated in a few formations.