ammonoids ideal for dating Paleozoic and Mesozoic rocks. Throughout the
Mesozoic, ammonite shell designs steadily improved.The cephalopod became
one of the swiftest creatures of the deep sea, competing successfully with fish.
For 350 million years, 10,000 ammonite species roamed the seas. Of the
25 families of widely ranging ammonoids living in the late Triassic, however,
all but one or two became extinct at the end of the period, when half of all
species died out. The ammonites that managed to escape extinction eventu-
ally evolved into dozens of ammonite families during the Jurassic and Creta-
ceous. The ammonites lived mainly at the middle depths and might have
shared many features with living squid and cuttlefish.The nautilus, commonly
referred to as a living fossil because it is the ammonite’s only living relative,
lives at extreme depths of 2,000 feet.
The belemnoids, with long, bulletlike shells, originated from more prim-
itive nautiloids and were related to the modern squid and octopuses. They
were abundant during the Jurassic and Cretaceous and became extinct by the
Tertiary.The shell was straight in most species and loosely coiled in others.The
chambered part of the shell was smaller than that of the ammonoid, and the
outer walls thickened into a fat cigar shape.
Unfortunately, after surviving the critical transition from the Permian to
the Triassic and recovering from serious setbacks during the Mesozoic, the
ammonites suffered final extinction at the end of the Cretaceous.At that time,
the recession of the seas reduced their shallow-water habitats worldwide.The
ammonites declined over a period of about 2 million years, possibly becom-
ing extinct 100,000 years before the end of the Cretaceous.
A fast-swimming, shell-crushing marine predator called ichthyosaur
apparently preyed on ammonites by first puncturing the shell from behind,
causing it to fill with water and sink to the bottom.Then, the attack could be
made on the more vulnerable front side of the ammonite.These highly aggres-
Figure 162Squid were
among the most successful
cephalopods.CRETACEOUS CORALS