Figure 13.19 Stratigraphie ranges of the main ammonite taxa together with the other main cephalopod groups. (Based on Ward, P. 1987. Natural History of Nautilus. Allen & Unwin, Boston.)

Ammonoid ecology and evolution

The pioneer work by Arthur Trueman (University of Glasgow) on the buoyancy and orientation of the ammonite shell established the probable life attitudes for even the most bizarre heteromorph forms (Fig. 13.21a). Theoretically, at least, virtually all ammonoids could favorably adjust their attitude and buoyancy in the water column. Most ammonoids were probably part of the mobile benthos, although after death their gas-filled shells could be widely distributed by oceanic currents. Many groups of ammonoids were endemic, and the shovel-like jaws of some groups were most efficient at the sediment-water interface. Richard Batt's studies (1993) on Cretaceous ammonite morphotypes from the United States have established a series of shell types related to life modes and environments (Fig. 13.21b). For example, evolute heavily ornamented forms were probably nektobenthonic, as were spiny cadicones and spherocones, nodose spherocones and platycones, together with broad cadicones. Evolute planulates and ser-penticones, together with small planulates, were probably pelagic in the upper parts of the water column. However, most oxycones were restricted to shallow-shelf depths. Some het-eromorphs were nektobenthonic, whereas a few floated in the surface waters.

In many ammonite faunas the consistent co-occurrence of large and small similarly ornamented mature shells at specific horizons suggests that the macroconch and microconch may be related sexual dimorphs (the male and female of the species). The macroconch was probably the female, though this may not always have been the case. The ammonoids probably originated from the bactritid ortho-cone nautiloids, with protoconchs and large body chambers, during the earliest Devonian. The anarcestide goniatites, with simple sutural patterns, were relatively scarce during the Mid Devonian. However, by the Famennian, other groups such as the clymeniids, with a dorsally situated siphuncle, were common. The goniatitides expanded during the

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