Ammonites, extinct members of the cephalopod group (which includes nautiluses, squid and octopuses), are so diverse and prevalent in the fossil record that they are used by paleontologists as markers to signal different geological strata. But along with most dinosaurs, these hard-shelled creatures disappeared during the massive Cretaceous-Paleogene (or 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.
Using 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 squid 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. These invertebrates had jaws.
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 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.
Researchers have theorized that plankton populations took a quick and substantial hit at the Cretaceous-Paleogene boundary—likely due to lack of sunlight following a massive asteroid impact and its dust fall-out. And living so close to the bottom of this food web, ammonites would have seen their main sustenance vanish rapidly.
Landman and his colleagues are now headed back out into the field to find fossils closer to the Cretaceous-Paleogene boundary of 65.5 million years ago to try to get a better picture of these organisms right before they went extinct.
The technique has also cleared a new channel, says McGowan, who is studying “microwear” 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.
– from scientificamerican.com