Although it got a brief treatment in the book Horns and Beaks, many people have been waiting for more information on the exceptionally-preserved Brachylophosaurus skeleton named “Leonardo.” Due to be unveiled next week at the Houston Museum of Natural Science (the date was pushed back due to Hurricane Ike; the museum and Leonardo were unharmed), the fossil provides a unique look at the soft tissues of this particular dinosaur.
Dinosaur “mummies” have been found before, dating back to the 19th century, but in many cases little more than skin impressions were preserved. Leonardo, by contrast, is so well preserved that paleontologists are getting a look at the internal anatomy of the dinosaur, and the new paper (out in Palaios) focuses on gut contents.
Within the body cavity of Leonardo there is a large amount of plant material. The question is, however, whether those plants represent gut contents or found their way into the specimen by some other route. If the body cavity was preserved intact (i.e. there were no gaping holes in it) then there could be little doubt that the plant material represents gut contents, but if the cavity was punctured other possibilities open up. The gut contents might have stayed in the body and been reworked, they could have been mixed with outside plant material, or the original contents could have been removed and then replaced. The actual soft tissue of the digestive tract would have decomposed fairly quickly after death and so the contents would not have been in their original position even if they were preserved, but the first task of the researchers was to determine whether they were dealing with gut contents or washed-in plants (or both).
The paleontologists were not looking at whole leaves, stems, or other plant parts, though. The preserved plant material was very fragmentary, best described as “fragile, black crumbs.” These little bits were substantially different from the woody material found in some hardosaur coprolites (or fossil poo), meaning that (if the fragments were true gut contents) shortly before death Leonardo had a lunch of leaves. The fragments were consistent with what would be expected for partially-digested hadrosaur meals, of course, but it did not make identification of the plants especially easy. This became even more vexing when the researchers realized that the high amount of clay inside the body cavity most probably meant that the body cavity was breached. There was a possibility that some of this material came in from the outside.
[As is pointed out in the paper, many animals have clay or soil inside their digestive tract as a result of purposefully eating it or the material coming in with the food. In Leonardo’s case, though, the sheer amount of clay showed that it was probably not intentionally swallowed. If the hadrosaur did so then it would have eaten more clay than plants!]
If there was a mix of gut contents and material from outside, however, it would be expected that there would be different characteristics between the partially digested food and the washed-in plants. Being that the material was generally uniform, the alternatives became narrowed down to either preserved gut contents or outside material. While both options were plausible, the retention of gut contents seems more likely.
Exceptional preservation is typically a result of rapid burial; the longer a carcass is left out to rot the more it will be picked apart or otherwise broken down before preservation. Given that Leonardo is perhaps the most exceptionally-preserved large dinosaur yet found, the skeleton was probably buried very quickly after death. This means that if the plant material in the body cavity came from the outside it would have had to find a way in after the body wall was breached, something that is unlikely. Indeed, the type of plant material inside the body was not found outside the body in the surrounding sediment; it is more probable that the crumbly plant fossils were actual gut contents.
About 75 million years ago, in what is now Montana, a subadult Brachylophosaurus died. While it was not killed or consumed by a predator, the exact cause of death is still a mystery. Either in death or shortly afterward the body of the dinosaur was quickly covered by sediment, perhaps by a flooded river. Piled beneath the mud and clay the dinosaur was beyond the reach of the large predators (except, perhaps, its tail, which may have been tugged on by some scavenger), and its body slowly started to decompose. The internal organs went first, with a breach in the body wall letting enough water and sediment in to wash the gut contents around inside the body cavity. This process preserved some aspects of the anatomy but obscured others, but tens of millions of years later the descendants of the tiny mammals that lived in the corners of the Mesozoic world would unearth Leonardo’s remains. Paleontologists have only just begun to understand what the skeleton can teach us about dinosaurs, and I’m sure we will be talking about Leonardo for many years to come.
J. S. Tweet, K. Chin, D. R. Braman, N. L. Murphy (2008). Probable Gut Contents Within A Specimen Of Brachylophosaurus Canadensis (Dinosauria: Hadrosauridae) From the Upper Cretaceous Judith River Formation Of Montana PALAIOS, 23 (9), 624-635 DOI: 10.2110/palo.2007.p07-044r