April 2006
Heteromorph ammonites collected on the Trent and Puntledge Rivers.
Previous to the Cretaceous, ammonites typically were compact, coiled, outward spiralling discs with each spiral built upon and touching the previous spiral (known as planispiral coils). During the Cretaceous, these regularly coiled ammonites declined as the suborder of ammonite known as Ancyloceratina exploded onto the scene with a vast array of new--and often peculiar--uncoiled shell designs. These uncoiled ammonites, which are characteristic of the Cretaceous, are called heteromorph ammonites.
Heteromorph ammonites were around in the Jurassic, but they weren't common, with nature still favoring the compact planispiral design. But during the Cretaceous, long before the extinction of the dinosaurs at the Cretaceous-Tertiary boundary, these regularly coiled ammonites were in decline. The heteromorphs on the other hand were diversifying into every niche with their fantastic, odd-shaped structures. So common and distinctive had they become that Paleontologists now use them as index fossils for calibrating the age of rock formations and other fossils contained within.
One of the most common heteromorphs was the widespread Baculites, an ammonite that was completely straight. Polyptychoceras was another straightened ammonite with three or four straight, parallel sections of shell joined by short curved sections, giving it the name 'paper clip' ammonite, which they resembled.
Some heteromorph ammonites, like Glyptoxoceras with its ribbing, resembled an elephant's trunk carrying something up to its mouth. Other resembled sea horses, hooks, and trombones. Still others were a tangle of irregular whorls. Both Bostrychoceras and Hyphantoceras had complex helically spiralling whorls that came back on themselves.
Why nature went with these irregular designs is uncertain. The awkward design and ribbing would have made them poor swimmers in comparison to the compact planispiral ammonites. And judging from the fossil records, it also made them quite fragile in comparison as well. Heteromorph ammonites are plentiful in the fossil beds on Vancouver Island, but you rarely see complete specimens. Instead, you find small fragments an inch or two in length. The same cannot be said of the planispiral ammonites such as Hauericeras gardeni that are often found as near complete specimens.
So if the advantage wasn't speed or strength, why did nature uncoil the ammonites in the Upper Cretaceous? Whatever the reason, the fact the they were probably poor swimmers leads paleontologists to think that many of the heteromorphs were planktonic feeders drifting in the open ocean, the way jellyfish do today. If this is the case perhaps, the body design helped orientate the ammonites in the water column. Or perhaps, it was related to reproduction. It wouldn't be the first time that evolutionary changes were related to sex. Indeed, many ammonite species are believed to exhibit sexual dimorphism with the females being larger than the males. Or the answer could be none of the above.
Why nature uncoiled the ammonites may remain a mystery. One things is certain: clues are everywhere locked in the shale formations on Vancouver Island waiting to be unearthed. If you search hard enough, you will undoubtedly come across some fragments of these hetermorph ammonites in your travels. Finding complete specimens, however, will require a great deal of effort and luck. Fortunately, the hunt will take you through some spectacular countryside. Continued . . .
Vancouver Island Cretaceous | Evolution with aTwist | The Where and How