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."

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