Page 359 - Introduction to Paleobiology and The Fossil Record
P. 359
346 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
ammonitic patterns have both the lobes and
saddles fluted and frilled. Based on these
sutural patterns, three groups among the
ammonoids can be recognized in a general
way: the goniatites are typical of the Devo-
nian-Permian, the ceratites of the Triassic,
and the ammonites dominated the Jurassic
and Cretaceous. Nevertheless, these sutural
patterns may be cross-stratigraphic, with
Cretaceous taxa having both goniatitic and
ceratitic grades of suture in homeomorphs of
more typical Devonian and Triassic forms.
The siphuncle connects the outer body
chamber with the phragmocone that includes
all the empty, previous chambers. Septal necks
act like washers, guiding the passage of the
siphuncle through each septum. Excepting
the clymeniids, the siphuncle is situated along
the outer ventral margin of the shell. Seawater
may be pumped in or out of the chambers
through the siphuncle in order to alter the
buoyancy of the ammonite, similar to mecha-
nisms in the nautiloids and in submarines.
The body chamber contains the soft parts
of the ammonite. The aperture may be modi-
fied laterally with lappets and ventrally with
the rostrum. In many taxa, aptychi sealed the
aperture externally, although these plates may
also have been part of the jaw apparatus.
Main ammonoid groups
The subclass Ammonoidea is currently split
into nine orders. The first three, the Anarces-
tida, the Clymeniida and the Goniatitida,
have goniatitic sutures and are included in the
order Goniatitida. The anarcestides charac-
terize Early to Mid Devonian faunas when
forms such as Anarcestes and Prolobites dis-
Figure 13.15 Life attitudes and external played tightly coiled shells together with a
morphologies of the nautiloids. (From Peel et al. ventral siphuncle. The clymeniids were the
1985.)
only ammonoids with a dorsal siphuncle; they
radiated in Late Devonian faunas in Europe
and North Africa, where the group is impor-
rounded lobes and saddles, characterizes tant for biostratigraphic correlation. The
mainly nautiloids ranging in age from Late order developed a variety of shell shapes: Pro-
Cambrian to Late Triassic. Anarcestid and gonioclymenia is evolute with simple ribs,
agoniatitic patterns, however, have a narrow Soliclymenia evolved triangular whorls, and
mid-ventral lobe and a broad lateral lobe with Parawocklumeria is a globular involute form
additional lobes and saddles, and range in age with a trilobed appearance. As a whole, the
from the Early to Mid Devonian. Goniatitic goniatitides ranged in age from the Mid Devo-
sutures are characterized by sharp lobes and nian to Late Permian, with typical goniatitic
rounded saddles, and are found in Late Devo- sutures consisting of eight lobes and ventral
nian-Permian ammonoids. Ceratitic sutures siphuncles. Goniatites, for example, was a
show frilled lobes and undivided saddles, and spherical inflated form with spiral striations,
