ANCIENT ESCARGOT: Under the ocean during the age of the dinosaurs, ammonites fed primarily on plankton--and the occasional snail--new imaging research suggests. The diverse ammonite group includes these classically curved species, as well as the more cylindrical Baculites that were examined in the new study. Image: WIKIMEDIA COMMONS
As tremendous ichthyosaurs and plesiosaurs swam through Mesozoic seas, a smaller, more common creature also was cruising the currents, playing an outsized role in marine ecosystems.
Ammonites, extinct members of the cephalopod group (which includes nautiluses, squids and octopuses), are so diverse and prevalent in the fossil record that they are used by paleontologists as markers to signal different geologic strata. But along with most dinosaurs, these hard-shelled creatures disappeared during the massive Cretaceous–Paleogene (K–T) extinction.
New details of their cloistered anatomy, revealed by next generation three-dimensional imaging, help to explain the reason for their rapid demise—and their important role in the ancient ocean ecosystem: It turns out that these multitudinous mollusks were primarily plankton-eaters.
"We are piecing together how it worked in ways that we couldn't have imagined," Neil Landman, a curator of paleontology at the American Museum of Natural History in New York City and collaborator on the new research, says of the marine food web that existed during the reign of the ammonites: 407 million to 65.5 million years ago. The new findings were published online January 6 in Science, and preliminary results were presented at a conference in September 2010.
Landman and his colleagues used synchrotron x-ray microtomographic imaging to peer through the mineralized mass of these fossilized mollusks' shells and discover subtle aspects of the inner anatomy. Training this relatively new technology on a few Baculites (a Mesozoic ammonite genus) the researchers found that, like the hard chitin beaks of squids and octopuses, the soft-bodied Baculites had hard so-called radulas for mastication.
The teeth and jaws had previously been glimpsed in serendipitously broken or weathered specimens, but these fragile structures had never been studied in great detail. Although researchers in the field were not surprised to see that these invertebrates had jaws, "the real difficulty is ever getting close enough to see what they're like"—and thus, how they might have been used, Landman says.
Video courtesy of Stewart Wills/Sophia Cai/Isabelle Kruta/Science