Bone Collectors

An unusual find has excited Indian 
palaeontologists, reports Biplab Das

It was a winter afternoon. Two young geologists were combing the sandstone beds of Kutch, Gujarat, in the hope of finding fossils. As the day drew to a close, they found something astonishing - a dinosaur bone. Dinosaurs were exclusively terrestrial animals, so finding a dinosaur bone in shallow marine deposits was intriguing. 

"Closer inspection in the laboratory revealed that the fossil bone belonged to a sauropod dinosaur," says Shiladri Sekhar Das, from the geological studies unit of the Indian Statistical Institute (ISI). "The bone was the proximal part of the left tibia, part of the hind limb." Das was supported in his research by Sudipta Kumar Jana from the department of geological sciences, Jadavpur University. 

Though dinosaur bones are rarely encountered in marine deposits, sporadic finds have been reported throughout the world. A skeleton of a dinosaur called stegosaur was found in the marine sediments of Lias, UK. "The limestone sequence of Haute-Marne of eastern France has also yielded several lower vertebrae of a sauropod dinosaur," says Das. "But the mystery behind all these fossil finds including the present one, is how the dinosaur bones got sequestered in marine deposits." 

To solve it, the first task before the research team was to precisely determine the age of the fossil bone. Indications of this were given by ammonite fossils found in the same sandstone bed as the dinosaur bones. Ammonites were mollusks. They evolved nearly 400 million years ago and became extinct 65 million years ago. Ammonites were sensitive to the slightest change in their marine habitat, and their fossils record minute details of the geological upheavals that they went through. Studying ammonite fossils that preserve the characteristic geological features of an age, geologists can pin down the age of other fossils found alongside them. 

In the present study, the research team found that the fossil bed at Jumara, Kutch, was rich in the Reineckeia anceps species of ammonite. "The fossil bed harbouring the R. anceps of ammonites is called Anceps zone, and is 157.8 million years old - the time when the owner of the saurapod bone, lived and died," write Das and Jana in a recent issue of Current Science. 

Geologically speaking, it was midway through the Jurassic period, when all kinds of dinosaurs roamed the earth. At that time, the sea covered much of the present mainland, extending as much as 200 km eastward from the present shoreline. Much of present-day Rajasthan and Gujarat was swampy forest in which sauropod dinosaurs lived. 

They were hervivores, and even had a rudimentary form of social life. Recent studies by Indian palaeontologists show that they lived at the edge of lakes and used the shore for nesting sites. "The sauropod dinosaur whose hind limb bone was found might have died near one such nesting site," says Das. "During a high tide flooding may have extended the shoreline inward and washed the carcass out to sea." 

While afloat, the dinosaur carcass would have undergone a process called sequential disarticulation of bones. Bacterial decomposition coupled with the action of scavengers would have loosened the grip of muscles on bones, dropping them one by one from the carcass. Gases from the microbial action would have kept the carcass afloat as it drifted further out to sea. 

It has been found that a drifting carcass sheds bones in a specific order. Parts of the body with more flesh and internal fluids shed their bones first. Thus the lower jaw is the first to be released. Then go the cranium and small bones of the forelimb. "Having less flesh and fluid and greater amount of tendon, the bones of the pelvis and hind limb are the last to be dropped off by a drifting carcass," says P. Dodson, who studies marine mammals at the University of Wyoming. 

"A similar chain of events might have happened to the floating carcass of the sauropod dinosaur, which travelled 100 km across the Jurassic sea of Kutch before finally sinking to the bottom at Jumara," Das explains. Another research team from India also proved that the pelvic and tail bones of a sauropod dinosaur were the last to separate and fall from the "drifting sack" of a carcass. "Thus, it supports our claim that the left tibia, a hind limb bone of the sauropod dinosaur, was one of the last few bones that were dropped off the carcass." 

Teeth, rib bones, vertebrae and broken egg shells which have suffered little posthumous drift have been found closer to the shoreline. "From the fossil bone, it can be inferred that from head to tail, the dinosaur's length was around 40 feet," Das says, "and its height was around 10 feet." There is reason to hope that the smaller bones may be parts of the same dinosaur, which were shed closer to the shore. 

On a note of optimism, Das concludes that "Finding its remaining bones will probably provide clues to the dinosaur's death and many other facets of its life."