The number of known specimens of Jurassic period turtles has at least doubled, thanks to the discovery of a large bone bed in China.
Found in the northwest province of Xinjiang, the fossil deposit contains the remains of as many as 1,800 turtles.
“Bones upon bones, we couldn’t believe our eyes,” Oliver Wings, a paleontologist and guest researcher at the Museum für Naturkunde in Berlin, said.
The fossils were found in April 2008 in China’s Turpan Basin. The region, which is likely China’s hottest and driest, lies below sea level.
“The area is absolutely desolate with almost no green, but a series of oases are nearby that used to serve as stopping posts along the silk road,” Dr. Walter G. Joyce, a paleontologist affiliated with Germany’s University of Tubingen and Yale University, said in an email response to questions.
The fossils are concentrated within an area of less than a hectare in size. A hectare is equal to 10,000 square meters.
“Some of the shells were stacked up on top of one another in the rock,” Joyce said.
A paleontology graduate student working with Wings on a prospecting expedition unexpectedly found the turtle remains.
“Given that the mesa is rather narrow, a colleague decided to check out the other side and was surprised to see turtles litter the surface,” Joyce said.
Researchers removed a half-meter square block of the rock containing a high concentration of the fossils for study off-site. They photographed the individual fossils contained in it and, from the number of individual remains found, determined that the density of specimens within the bone bed is equal to about 36 per square meter.
Joyce explained that a second area of the bone bed has a lower concentration of fossils and that the team estimated the number of individuals in that section by counting the number of shell pieces found in that area.
“What we got at the end is a conservative estimate,” he said. “It is very likely that many more turtles died at the site!”
Joyce added that the specimens are likely to increase the number of known Jurassic turtle individuals by at least two-fold.
During the Jurassic period, which ended about 145 million years ago, the area of Xingjiang in which the fossils were discovered was verdant, with many lakes and rivers.
Scientists are able to infer the climactic conditions of the area tens of millions of decades ago by closing examining the sediments that surround the fossils. The distinctive red sediments in the particular geologic formation in which the fossils were found indicates that, at the time they were laid down, the climate in the area was seasonally arid.
Joyce clarified that the “characteristic assemblage of plant and animal fossils” and “such rarities as the turtle mass grave we found” also give researchers enough information to support that conclusion.
The rock layer that holds the fossils, called the Qigu Formation, includes the remains of freshwater mollusks, which indicates the area was periodically subjected to river flows or under a lake.
The fossils are not scattered throughout the thickness of the Qigu Formation. According to the paper describing the discovery, the depth of the bone bed does not exceed 20 centimeters.
Wings and Joyce, together with their colleagues, therefore concluded that the turtles likely gathered in a dry lake bed to await rain. When the rain fell, it produced a flood that moved the turtles into one place, where sediment buried them.
“We know from modern semi-tropical areas, regions with dry and wet seasons, that aquatic turtles will accumulate in retreating rivers and ponds as droughts progress and often die when these water dry out completely,” Joyce said. “This is particularly typical for Australia, but also seen in parts of Africa, South America, and even Florida.”
The turtles may belong to a previously unknown species.
“We presume we found a new species, but we are not sure yet,” Joyce said.
The fossils date to the middle Jurassic, about 164 million years ago. The oldest known turtle species dates to about 220 million years ago. Turtles have existed on Earth for a period longer than snakes, lizards, or crocodiles.
Joyce said that the find is likely to help paleontologists gain a greater understanding of Mesozoic turtle diversity.
“We know from living organisms that individuals of some species look very similar, whereas individuals of other species show a lot of variation,” he said. “As a paleontologist, it is therefore always difficult to assess if your fossil looks different from others because it is a new species, or because it is part of an already known, but variable species. With data from the new site we will finally be able to more rigorously assess patterns of diversity of turtles in the Jurassic of Asia.”
Wings and Joyce, together with other scientists, conducted further examination of the specimens and surrounding rock layer on-site in April 2009 and October 2011. Their research was published in the Oct. 21 edition of Naturwissenschaften.
Journal reference: Oliver Wings, Márton Rabi, Jörg W. Schneider, Leonie Schwermann, Ge Sun, Chang-Fu Zhou, Walter G. Joyce. An enormous Jurassic turtle bone bed from the Turpan Basin of Xinjiang, China. Naturwissenschaften, 2012. DOI: 10.1007/s00114-012-0974-5.