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 apparently 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.
Carboniferous, together with the prolecan-tides, where all the subsequent ammonoids probably originated. During the Triassic, the ceratitides diversified, peaking in the Late Triassic; but by the Jurassic the smooth involute phylloceratides, the lytoceratides and the ammonitides were all well established. Complex septa and sutures may have increased the strengths of the ammonoid phragmocone, protecting the shell against possible implosion at deeper levels in the water column. More intricate septa also provided a larger surface area for the attachment of the soft parts of the living animal, perhaps aiding more vigorous movement of the animal and its shell.
The subclass Coleoidea contains cuttlefish, squids and octopuses, the latter including the paper nautilus, Argonauta. Coleoids show the dibranchiate condition, with a single pair of gills within the mantle cavity. Although argonauts can be traced back to the Mid Tertiary, the living coleoid orders generally have a poor fossil record, but preservation of arms, ink
Was this article helpful?