by the Tertiary.They were related to the modern squid and octopus and pos-
sessed a long, bulletlike shell.The shell was straight in most species and loosely
coiled in others. The chambered part of the shell was smaller than the
ammonoid, and the outer walls thickened into a fat cigar shape.
The ammonoids were the most significant cephalopods. They had a large
variety of coiled shell forms (Fig. 98), which made them ideal for dating Paleo-
zoic and Mesozoic rocks. Shell designs steadily improved, making ammonoids
the swiftest creatures of the deep.They successfully competed with fish for food
and avoided predators.Ammonoids lived mainly at middle depths and might have
shared many features with living squids and cuttlefish. Some ammonoids grew to
tremendous size with shells up to 7 feet wide.The nautilus, which is commonly
referred to as a living fossil because it is the only living relative of the ammonoids,
lives in the depths of the South Pacific and Indian oceans down to 2,000 feet.
During a major extinction event near the end of the Devonian about
365 million years ago, many tropical marine groups disappeared, possibly due
to climatic cooling. Several extinctions correlate with glaciations.Yet no major
die out occurred during the widespread Carboniferous glaciation, which
enveloped the southern continents around 330 million years ago. The rela-
tively low extinction rates were credited to the limited number of extinction-
prone species following the late Devonian extinction.
The die out of species at the end of the Devonian apparently occurred
over a period of 7 million years. It eliminated species of corals and many other
bottom-dwelling marine organisms. Primitive corals and sponges, which were
prolific limestone reef builders early in the period, suffered heavily during the
extinction and never fully recovered. While these groups vanished, the glass
sponges, which better tolerated cold conditions, rapidly diversified, only to
Figure 97 The
ammonoids were among
the most spectacular
creatures of the Paleozoic
and Mesozoic seas, with
some growing up to 7 feet
across.DEVONIAN FISH