Ammonoids

There's no hint of a shell; previous theories had postulated a shelled common ancestor to squid, nautiloids, and ammonoids, but either this was a specialized branch that lost the shell, or modern cephalopod groups independently re-evolved the structure. ❋ Unknown (2010)

Goniatite ammonoids are common, and coleoids appear. ❋ Unknown (2005)

Productid and spriferid brachiopods, bivalves, foraminifera, and ammonoids all abundant. ❋ Unknown (2005)

Given that ammonoids added to their spiral shells as they grew, moving their soft bodies into the forwardmost chambers as they did so, new shell material eventually overgrew the source of the irritation and encapsulated it. ❋ Brian Switek (2011)

The ammonoids were making blister pearls - a particular type of pearl that is created on the interior of the shell when something irritates the mollusk's soft mantle. ❋ Brian Switek (2011)

A restoration of the ammonite Asteroceras, resembling what the ammonoids in the study might have looked like in life. ❋ Brian Switek (2011)

Pits were not found in all ammonoids, but in some genera - such as ❋ Brian Switek (2011)

As it turns out, ammonoids - the extinct, coil-shelled cousins of modern squid and nautilus - made pearls, too, but they did so to stave off the onslaught of prehistoric parasites. ❋ Brian Switek (2011)

The precise type of parasite which infested the ammonoids is unknown, but Klug and colleagues have an inkling. ❋ Brian Switek (2011)

Reached by e-mail, Klug noted that "The ammonoids' reactions to the parasites changed through ammonoid evolution," but cautions that "we cannot study the detailed reactions" of these interactions since both the hosts and parasites are long gone. ❋ Brian Switek (2011)

The fact that this evidence of mollusk parasitism coincides with an evolutionary radiation of early, jawed fish may indicate that the trematodes were using the ammonoids as a pathway to their vertebrate hosts, although this hypothesis has yet to be confirmed with hard fossil evidence. ❋ Brian Switek (2011)

"So far, no ammonite has been found with really well-preserved soft-tissues," Klug said, "so up to now, it is impossible to tell what these parasites did to their host." bivalve mollusk specimens of about the same geological age as the ammonoids also have Housian pits, and the patterns are similar to those in modern-day shells which have been infested with trematodes. ❋ Brian Switek (2011)

The parasites were changing to keep up with the rapid adaptation of the ammonoids. ❋ Brian Switek (2011)

I see the gap theory as the only way to account for Genesis's failure to specifically mention crinoids, trilobites, or ammonoids. ❋ Unknown (2009)

There’s no hint of a shell; previous theories had postulated a shelled common ancestor to squid, nautiloids, and ammonoids, but either this was a specialized branch that lost the shell, or modern cephalopod groups independently re-evolved the structure. ❋ Unknown (2010)