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

In an ammonite ocean
As with other extinct animals, "it's very hard to piece together their mode of life," Landman says of ammonites.

And that question had been circulating in the field for some time, says University of Glasgow cephalopod expert Alistair McGowan, who was not involved in the new research. "The issue of how ammonoids feed and what they ate has been a difficult one to comes to grips with," McGowan notes. "Until now there has been much speculation about what ammonoids ate, but the evidence has been, at best, circumstantial."

In addition to digitally reconstructing the teeth and jaws of these extinct animals, Landman and his colleagues also found bits of the food itself, including a tiny fragment of a sea snail that might have been caught in the water column.

Because ammonites were such prevalent denizens of the ancient seas, the fact that they are not going after fish or larger prey, but rather "just plucking off plankton from the water," as Landman describes, has substantial implications for piecing together the Mesozoic era's marine food web. Not only does it mean that "there must have been a lot of plankton in the sea," but also that "these ammonites are ingesting all of the plankton and secreting it as fecal pellets" that then fall to the seafloor, the realm of bivalves, he explains.

A deadly diet
Knowing what these extinct mollusks ate also "is quite significant for theories about why ammonoids became extinct at the end of the Cretaceous [period]," McGowan explains. Researchers have theorized that plankton populations took a quick and substantial hit at the K–T boundary—likely due to lack of sunlight following a massive asteroid impact and its dust fallout. And living so close to the bottom of this food web, ammonites would have seen their main sustenance vanish rapidly.

The new analysis is by no means a comprehensive survey of the whole subclass' diet. Being only a peek into the mouths of three individual Baculites, it is but "a snapshot of the final meals of these specimens," McGowan says. But it is an important step toward productive future research, he notes.

Landman and his colleagues are now headed back out into the field to find fossils closer to the K–T boundary of 65.5 million years ago to try to get a better picture of these organisms right before they went extinct.

Learning more about these extinct ammonite diets "really adds to the sense of diversity of the cephalopod group," Landman says, noting as a contrasting example voracious Humboldt squid. With the new support for plankton-eating ammonites, "you had this whole niche, which is not represented at all in modern-day cephalopods" that, he adds, seem to have been "channeled into this carnivorous niche."

The technique has also cleared a new channel, says McGowan, who is studying "micro-wear" patterns on cephalopod jaws using scanning electron microscopes. "I think it is a stunning piece of technical work," he notes. Unlike earlier computed tomography (CT) machines, these next-generation x-ray versions use even more energy to image structures down to the micron level, Landman explains. The data then get fed into sophisticated software that reconstructs and analyzes the information.

Although the process can be pricey, McGowan says, "this paper justifies the expense of examining further ammonoid specimens." The findings also underscore the importance of revisiting older pieces from collections as new methodologies become available, he notes. "The advent of new technologies and methods for studying museum collections, whether paintings, plots or placoderms, often produces significant and unforeseen new results and data."