One of the challenges of paleontology is that we are trying to infer and entire animal just from the hard parts that fossilize, mostly bones and teeth (and sometimes just teeth). But if we look at animals today there are a lot of details we could not guess from their bones alone – the mane of a lion, the humps on camels, or the amazing display of peacocks. Soft tissue is rarely preserved, so in many cases we simply have to guess. If we look at the classic depictions of dinosaurs, we see creatures that are all drab in color and don’t have any significant soft-tissue adaptations. We are getting better and inferring color from fossilized melanosomes. We are also discovering that many dinosaurs had feathers, and those feathers were probably colorful. In fact, that clade (just think of birds) have lots of integumentary adaptations, not just feathers. Turkeys are a good example. It’s just as likely, therefore, that dinosaurs had lots of interesting integumentary structures hanging off of them.

A recent study highlights one method that scientists can use to gain more information about the soft tissue of dinosaurs. They looked at the teeth of T. rex, crocodilians, and lizards. Their question was – did T. rex have a toothy smile like an alligator, or fleshy lips covering their teeth like lizards? They have been imagined both ways, but Jurassic Park probably solidified the image for many people of a toothy T. rex. There is definitely something more menacing about a grill of visible deadly carnivore teeth.

What they did was look at the teeth, which is the hardest part of vertebrates and fossilize very well. Specifically they looked at the wear on the teeth. If fleshy lips covered the teeth, they would have been protected from wear along the covered surface. When looking at the teeth of crocodilians vs lizards we see this difference. So all we have to do is examine T-rex teeth to see if they have wear patterns that look like a crocodiles or a lizards. They also did an analysis of skull size and tooth length to see if there was also a relationship there. They found:

Contrary to depictions that have dominated for more than a century, they found that theropods, including T. rex, had lips that covered their teeth, leaving them looking more like modern Komodo dragons than crocodiles.

So at least the theropod dinosaurs, the group to which T. rex belongs, probably had lips covering their teeth. This probably will not be the last significant update to our reconstruction of what T. rex, or other dinosaurs, looked like. It is interesting to think about – what possibilities are there for inferring soft tissue in extinct animals. Here are the methods that I am aware of or can think of.

The first is clearly from looking at the bones themselves. Bones have insertion points for tendons, and we can infer from their shape what stresses they would have been under. We can then make comparisons to living animals to help us put flesh on the bones. This methods works well for basic musculature and structure, but not so much for outer soft tissue, which can have a dramatic effect of how an animal looks.

Fossilized melanosomes are helping us determine what color animals were, especially if they had feathers. It is possible that we may discover other proteins that fossilize well that might help us determine something about the outer (most integumentary) structures of extinct animals.

Trace fossils are another category of evidence, such as the soft tissue impression that some fossilized animals made in the substrate in which they were fossilized. If, for example, an animals lays down in mud when it dies and is then covered by mud or sediment so it is preserved, and then the mud solidifies, we get a sort of cast of their skin. Another trace fossil is footprints, from which we can see the size of their feet, claws, the length of their strides, and an estimate of their weight.

One interesting line of evidence of cave drawings from people. Some human ancestors drew the animals they hunted on cave walls, and in at least one case this provided evidence that a now extinct animal had a large hump – previously unknown from fossil evidence alone.

There is also DNA evidence. We are increasingly able to isolate and expand DNA remnants from inside bone and other trace fossils. It may be possible learn something about what ancient animals looked like from fragments of their DNA. Dinosaurs are likely outside the window of DNA survival, but recent evidence puts the upper limit at about a million years, and it may be a bit more. But probably not the 65 million year we would need for dino DNA. For this applications, however, we don’t necessarily need a full genome. Just evidence of specific genes might give us information.

Animals being frozen in permafrost is another source of soft tissue. This requires continuous existence of the ice for thousands of years. There is permafrost 650,000 years old. Antarctica has the oldest glacier ice, probably 20 million years old, with the oldest ice core recovered being  2.7 million years. So – still no dinosaurs, but there could be some interesting animals frozen under the Antarctic ice.

If we let ourselves speculate about science fiction we can also always hope that an alien probe has been secretly circling the Earth for hundreds of millions of years, recording the evolution of life on Earth, and waiting for us to discover it so it can give us a massive treasure trove of information. Unlikely, but fun to think about.

Meanwhile, we have the above methods, and there is still a lot of work to be done.