May 6, 2009
One of the first things I learned about dinosaur fossils was that soft tissues are never preserved. Impressions of skin, hair, and even internal organs can leave their mark in the fossil record, but no one is ever going to find an intact, non-fossilized Tyrannosaurus heart. Like many of the things that “everyone knows,” though, it now seems that this view is not exactly right. In very exceptional circumstances , remnants of dinosaur soft tissue can be preserved, and a recently published paper in the journal Science throws new support to this controversial hypothesis.
For several years now paleontologists have been debating whether structures found inside a Tyrannosaurus femur were preserved soft tissue structures or something else, like bacteria, that took the shape of things like blood vessels. The pioneering scientist behind this research has been Mary Schweitzer. The new report by her and her colleagues focuses on a new case of soft tissue preservation, but it is not about Tyrannosaurus. Instead it features preserved soft tissue structures from the hadrosaur Brachylophosaurus, a dinosaur from the other great branch of the dinosaur family tree, the Ornithischia.
The researchers who found the Brachylophosaurus leg in which the soft tissue structures were found were careful right from the start. They did not expose the bones in the field but kept it in a plaster jacket until they got it into a lab. Only then did they expose it and quickly take their samples to prevent possible contamination or degradation of what might be inside the leg. What Schweitzer and her colleagues found were bone cells, blood vessels, and what appeared to be degraded blood products, real remnants of dinosaur soft tissue and not bacterial biofilm. They tested the material, re-tested it, and even sent it to other labs, and the overwhelming consensus was that the material truly was the ancient leftovers of dinosaur soft tissue.
The team was even able to recover some protein sequences from this material. It came from collagen protein, one of the materials in bone, and the scientists were able to construct an evolutionary tree by comparing the Brachylophosaurus sequences to ones from Tyrannosaurus and living animals. What they found was that Brachylophosaurus grouped most closely with Tyrannosaurus, with birds being the next closest group to both. While in the right ballpark, this does not quite fit the fossil evidence. Tyrannosaurus and Brachylophosaurus shared an ancient common ancestor over 230 million years ago, but birds are more closely related to Tyrannosaurus than Tyrannosaurus is to Brachylophosaurus. The reason why this is not shown in the evolutionary tree is that the protein sequences recovered for both dinosaurs are very incomplete, but the fact that the two dinosaurs grouped close together throws some support to the idea that ancient proteins may be used to inform evolutionary trees.
It is still unknown how soft tissue structures and bits of protein have come to be preserved for over 80 million years, but finds like this suggest there is a lot of fossilization (and dinosaurs) that we are only now just learning about. As outlined in Jack Horner’s recent book How to Build a Dinosaur, a new area of paleontology is opening up in which knowledge of microbiology and genetics is just as important as knowing skeletal anatomy. This is only the beginning, and if students follow Schweitzer’s lead into paleomicrobiology who knows what amazing finds might be made?
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