Page 364 - Introduction to Paleobiology and The Fossil Record

SPIRALIANS 2: MOLLUSKS 351

Box 13.7 Ammonite heteromorphs

One of the more spectacular aspects of ammonite evolution was the appearance of bizarre hetero-
morphic (“different shape”) shells in many lineages at a number of different times (Fig. 13.20).
Heteromorphs first appeared during the Devonian, but were particularly significant in Late Triassic

and Late Cretaceous faunas. Some such as Choristoceras, Leptoceras and Spiroceras appeared merely
to uncoil; Hamites, Macroscaphites and Scaphites partly uncoiled and developed U-bends; whereas
Noestlingoceras, Notoceras and Turrilites mimicked gastropods and Nipponites adopted shapes
based on a series of connected U-bends. Initially, the heteromorph was considered as a decadent
degenerate animal anticipating the extinction of a lineage. Nevertheless, some heteromorphs appar-
ently gave rise to more normally coiled descendants and their association with extinction events only
is far from true. Additionally, functional modeling suggests many were perfectly adapted to both
nektobenthonic and pelagic life modes. Moreover Stephane Reboulet and her colleagues (2005) have
shown that among the ammonites in the Albian rocks of the Vocontian Basin, southern France, het-
eromorphs probably were better adapted to compete in meso- and oligostrophic conditions than
many other groups.

Spiroceras
Nipponites (Cretaceous)
Turrilites (Cretaceous) (Jurassic) Macroscaphites
(Cretaceous)

Hamulina (Cretaceous)

Choristoceras Hyphantoceras Ostlingoceras
(Triassic) (Cretaceous) (Cretaceous)
Figure 13.20 Some heteromorph ammonites.

Carboniferous, together with the prolecan- the living animal, perhaps aiding more vigor-
tides, where all the subsequent ammonoids ous movement of the animal and its shell.
probably originated. During the Triassic, the
ceratitides diversified, peaking in the Late

Triassic; but by the Jurassic the smooth in- Coleoidea
volute phylloceratides, the lytoceratides and The subclass Coleoidea contains cuttlefi sh,
the ammonitides were all well established. squids and octopuses, the latter including the
Complex septa and sutures may have increased paper nautilus, Argonauta. Coleoids show the
the strengths of the ammonoid phragmocone, dibranchiate condition, with a single pair of
protecting the shell against possible implosion gills within the mantle cavity. Although argo-
at deeper levels in the water column. More nauts can be traced back to the Mid Tertiary,
intricate septa also provided a larger surface the living coleoid orders generally have a poor
area for the attachment of the soft parts of fossil record, but preservation of arms, ink