Dinosaurian death in decidedly drenched dirt.

Well, make that just wet enough for some less than ideal results for at least one group of dinosaurs!

Just got done reading a paper entitled Mud Trapped Herd Captures Evidence of Distinctive Dinosaur Sociality, and it's a really cool read; especially if you have an interest in both taphonomy (what happens to an animal--in this case fossilized--after it dies) and dinosaurian behavior.

An excavation done in the Ulansuhai formation (late Cretaceous in age) of inner Mongolia turned up a group of at least thirteen separate specimens of the ornithomimid Sinornithomimus who were all found on a single geologic horizon and in very close proximity. Utilizing the sort of detective work essential for a field like paleontology (like a sleuth in some crime thriller, we have to use as many tools and conceptions as we can to get to the bottom of the case!), the paleontologists steadily unearthed a fascinating story.

First off, the geology of the locality held many different hints of information--like the presence of fine-grained layered deposits of rock and dirt that would alternate between very thick and very thin sections laid down in such a way as to suggest a non-fluvial (non-river caused) depositional environment. Various fossil ostracods (a sort of shrimp-like animal) helped support this.

However, the rocks told more of a story--in one layer in particular, this lakeside was drying up and fast--leaving various features of an arid environment (evaporites and the like). It was this horizon where the dinosaurs were found.

Twelve of the thirteen ornithomimids were found in an upright posture, stuck in a highly viscous material created by certain kinds of drying clays. Most (ten out of thirteen) of the skeletons were angled between 87º and 188º, which also lends itself towards a mass death at a single instant in time (a chance alignment for that number of specimens within that range of direction is very, very low).

One of the many really neat things about this find is the fact that the directionality the animals died in greatly influenced the bones and pieces that were missing from the skeletons--animals that died with their belly facing the ground most often had their gastralia (ribs found in front of the gut) and their stomach contents at the time of death preserved, but were missing dorsal (back) vertebrae and their respective ribs. On the other hand, the animal that was found on its left side had right rib elements missing. This is highly indicative of scavenging--after death and before complete burial, the exposed surfaces were eaten by opportunistic feeders.

Interestingly, the ilium (the largest of the hip bones) was missing in most of the specimens, and --though it can't be said for certain--this is also likely due to scavenging given that a. it probably would have been exposed to the surface on most of the specimens and b. it has a massive amount of muscle tissue to attract local carnivores. However, several of the Ilia appear to have weathered off, and very well may have survived to the point of burial.

Bone histology studies (research into the microstructure of the animal's bones) as well as relative size comparisons show that these animals were all juveniles or sub-adults, which is absolutely awesome in many ways--many bone beds containing the skeletons of non-avian juvenile dinosaurs have been found before, but because there was little taphonomic evidence to say for certain that they dead as a group or if it was a chance washing, this aspect of dinosaurian behavior was still slightly up in the air. But because this bone bed also contains similar age ranges, and the detail of taphonomy is so intense, it helps prove that previous similar finds were no accident!

All of this information helps us paint a picture of the scene--a drying lake bed, many millions of years ago attracts a roaming group of juvenile Sinornithomimus (the adults and infants are presumably nesting and traveling elsewhere). They're parched, thirsty beyond belief--and in temperatures well over 100 degrees, water is not common. They reach the watering hole, and though the water is stagnant and awful tasting, it was the most water they've had in days--but something goes wrong: they're stuck. They struggle, letting out distress calls in hopes that others are near--and as they try to pull themselves away from the sticky mud's grasp, they push themselves further in yet. After what could be hours, they fall over dead--struck by overexerting themselves in the scalding desert heat. The lake bed progressively dries further, and scavengers take the opportunity to snag a meal of what's exposed. Eventually the bodies are entirely buried with mud, and become fossilized over the aeons--lost in memory until a chance encounter with an entirely different group of bipedal animals, who map and detail their discovery for the world to see.

Reference--

Varricchio, D.J., Sereno, P. C., Zhao, X., Tan, L., Wilson ,J. A.,and Lyon, G. H.2008.Mud−trapped herd captures evidence

of distinctive dinosaur sociality. Acta Palaeontologica Polonica 53 (4): 567–578.