Page 357 - Introduction to Paleobiology and The Fossil Record
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344 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
described in detail by Stephen Jay Gould, The origin of the cephalopods remains con-
suggest that new subspecies evolving by allo- troversial, although most agree the group was
patric speciation arose suddenly by pedomor- derived from a monoplacophoran-like ances-
phosis (see p. 145). These rapid speciation tor. John Peel (1991) suggested that the group
events, separated by intervals of stasis, are is derived from within the class Helcionel-
strong supportive evidence of the punctuated loida; both groups are characterized by endo-
equilibrium model of microevolutionary gastric coiling and, moreover, the helcionelloids
change. Moreover, in a classic study of Late predate the appearance of the cephalopods by
Tertiary snails from Lake Turkana, Kenya, some 10 million years. Another group of gas-
Peter Williamson (1981) suggested there had tropod-like shells, the tergomyans, with apical
been punctuated changes in 14 separate lin- septa, might also have been ancestral, only
eages (see also p. 123). they lack perforate septa.
CLASS CEPHALOPODA Nautiloidea
The cephalopods are the most highly orga- Most information about nautiloids comes
nized of the mollusks, with the greatest com- from studies of the behavior and morphology
plexity of any of the spiralian groups. The of the living Nautilus that occurs mainly in
close association of a well-defi ned head with the southwest Pacific, normally at depths of
the foot modified into tentacles is the source 5–550 m (Box 13.6). It pursues a nocturnal,
of their name, meaning “head-footed”. High nektobenthonic life mode as both a carnivore
metabolic and mobility rates, a well- and scavenger; however it is prey to animals
developed nervous system, and sharp eyesight with powerful jaws such as the perch, marine
associated with an advanced brain, are ideal turtles and sperm whales.
adaptations for a carnivorous predatory life Living Nautilus has its head, tentacles, foot
mode. The funnel or hyponome is also modi- and hyponome concentrated near the aper-
fied from the foot, and squirts out water from ture of the body chamber; the visceral mass
the mantle cavity providing the animal with containing other vital organs is situated to the
a form of jet propulsion. rear of the body chamber (Fig. 13.14). The
Modern cephalopods belong to two groups. surrounding mantle extends posteriorly as the
Firstly, living Nautilus has an external coiled siphuncular cord connecting all the previous,
shell with a thin internal mantle and nearly now empty, chambers that together constitute
100 tentacles. Only five species of this genus the phragmocone. Each chamber is parti-
are extant although it was once used as an tioned from those adjacent by a sheet of cal-
analog for the behavior of all extinct exter- careous material, the septum; the suture is
nally-shelled cephalopods such as the ammo- formed where each septum is cemented to the
noids. Secondly, the coleoids; these have outer shell. The form of the suture, or the
internal shells and thick external mantles. suture pattern, is used in the classifi cation of
They include the 10-tentacled extinct belem- externally-shelled cephalopods. The conch is
nites, the squids and cuttlefi sh; the octopods usually oriented as follows: anterior at the
have eight tentacles and have lost their skele- aperture, posterior at the point furthest from
ton. These living forms are most common the aperture, the venter on the side with the
in shallow-water belts around the ocean hyponome, usually the outside, and the
margins. dorsum opposite. Despite the simplicity of
A tripartite division of the cephalopods this arrangement, fossil nautiloids developed
into three subclasses includes: (i) Nautiloidea, a wide range of shell morphologies
with straight or coiled external shells with (Fig. 13.15).
simple sutures (Late Cambrian to Recent); (ii)
Ammonoidea, with coiled, commonly ribbed
external shells with complex sutures (Early Ammonoidea
Devonian to latest Cretaceous, possibly earli- The ammonite usually had a planispirally
est Paleogene); and (iii) Coleoidea, with coiled shell comprising the protoconch, phrag-
straight or coiled internal skeletons (Carbon- mocone and body chamber (Fig. 13.16). The
iferous to Recent). protoconch or larval shell records the earliest
