More than two dozen types of ancient animals — including at least two predatory dinosaurs, eight mammals, and a host of amphibians, sharks and other fish — have been unearthed in a vast fossil cache that spans the Wyoming-Montana border, a new study reports.
The animals are so numerous and so diverse — several of them likely new to science — that the site is like an entire ecosystem encased in stone, paleontologists say.
“This documents a nice ‘snapshot’ … of an ancient type of ecosystem, perhaps not so very different from the kinds of ecosystems that exist today in and around small bodies of water,” said Dr. Matthew Carrano, paleontologist at the Smithsonian Institution and lead author of a paper describing the discovery in The Journal of Vertebrate Paleontology.
The ecosystem in this case was a small lake that sat some 111 million years ago on what’s now the high, semi-arid plateau of northern Wyoming. [Learn more about the mountain West’s watery past: “Extinct Tropical Bird Discovered in Wyoming“]
While some excavations within the giant deposit, known as the Cloverly Formation, have yielded large fossils with animals’ bones preserved in situ, Carrano and his team looked for different signs — sites where animals’ bones got washed away and settled together.
These rich bone deposits, called vertebrate microfossil bonebeds, or VMBs, have never been explored before in the area, Carrano said.
“The VMBs are a special mode of preservation,” he said in an interview. “Animals are living and dying in and near these bodies of water, and their remains are falling to the bottom, bit by bit, and getting buried.
“Animals in the sediments churn things up and mix them, so most of the bones don’t survive. But one or a few do from each animal, and this happens for thousands of years. Once the pond silts up, it gets preserved as a rock layer filled with tiny bones.”
The result, he says, is fossil evidence of at least 29 kinds of ancient animals that have never before been recorded in the Cloverly Formation.
Of the total finds, the team reports, 40 percent are of fish, with crocodile-like crocodylians coming next in the ranks, then dinosaurs, turtles, and amphibians.
The challenge, of course, is piecing together the thousands of bones, teeth, and other fossil fragments to determine exactly how many individual animals, representing how many species, can be accounted for at the bottom of this ancient lake.
Yet, in a process somewhat like working a giant phylogenetic jigsaw puzzle, paleontologists can identify these ancient animals over time, and even figure out which ones may have never been named before.
They do it, Carrano explains, by comparing the small fossils to similar bones of known species, “and by the fact that we can tell enough to know at least what type of animal each specimen represents.
“If that type of animal is unknown from this region and time, it’s almost certainly a new species, even if we don’t have enough to name it.
“So, for example, we have teeth from a theropod like Troodon, but it’s not Troodon — much too old in time. Yet we don’t yet know of any Troodon-like animals from the Cloverly, so it’s surely new.”
It will take years for the experts to parse all of the details and pin down exactly how many individuals of how many species this new discovery has turned up.
But, Carrano says, in the end the new fossil find will provide a much fuller picture of life in the ancient West than could ever be apprehended with just a few large fossils.
“Most importantly, it allows us a real window into an ancient ecological community, which we almost never get for dinosaur communities,” he said, “for the simple reason that we usually just collect individual dinosaurs and not all the animals that lived with them too.”
Matthew P.J. Oreska, Matthew T. Carrano, & et al. (2013). Vertebrate paleontology of the Cloverly Formation (Lower Cretaceous), I: faunal composition,biogeographic relationships, and sampling Journal of Vertebrate Paleontology DOI: 10.1080/02724634.2012.717567