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

2 Comments

1. 1. Cymbaline 04:57 PM 1/7/11

   Thank you for acknowledging that MOST dinosaurs perished at K-T, not ALL. The ones who did not perish: BIRDS. Thanks, carry on, and stop calling dinosaurs reptiles, because they aren't reptiles any more than birds are reptiles. Dinosaurs (including birds) and mammals are warm-blooded. Thanks.

   Reply | Report Abuse | Link to this

2. 2. Garland 08:59 PM 2/6/11

   The "warm-blooded" as in mammals isn't actually proven any more than as birds aren't descendants of the huge dinosaurs, they are cousins, the feathered dinosaurs that lived back in the day, but the lack of bird fossils probably tells something of the time when avian and the dino-hipped birds differentiated. Funnily enough, the so called "bird-hipped" dinosaurs aren't related to birds. The predatory dinos like allosaurs and raptors most likely all had feathers. Allosaurs were a hundred million years before even the first smallest raptor dino ever walked this earth, feathers and all, though, and birds, like mammals, were very small back then.
   
   Big bones tend to be preserved, so allosaurs, whose later big descendant was the Tyrannosaurus genus, were only a for a few million years the top predators. I wonder why people always are interested in the top predators, and not the top grazers.
   
   Well, they did grow to huge sizes, but the reason that mammals don't grow so large anymore is probably in the environment and the mammal dependency on their mothers for sustenance, rather than getting out there and surviving on their own from the start.

   Reply | Report Abuse | Link to this