Page 363 - Introduction to Paleobiology and The Fossil Record

350 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Era Period
0 Quaternary
Cenozoic Tertiary Nautiloidea Teuthoidea Sepioidea Octopoda Ammonoidea
1.5
60
Cretaceous Belemnitida Lytoceratida Ammonitida Phylloceratida
Mesozoic Jurassic
130 Ancyloceratida

190
Triassic
220 Aulacoceratida Ceratitida
Time (Ma) 280 Carboniferous Prolecanitida Goniatitida
Permian

340 Actinoceratoidea Bactritida Clymeniida Anarcestida
Paleozoic Silurian
400 Devonian
440 Endoceratoidea
Ordovician
500 Plectronoceras
Cambrian
570
Figure 13.19 Stratigraphic 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 broad cadicones. Evolute planulates and ser-
penticones, together with small planulates,
The pioneer work by Arthur Trueman (Univer- were probably pelagic in the upper parts of the
sity of Glasgow) on the buoyancy and orienta- water column. However, most oxycones were
tion of the ammonite shell established the restricted to shallow-shelf depths. Some het-
probable life attitudes for even the most bizarre eromorphs were nektobenthonic, whereas a
heteromorph forms (Fig. 13.21a). Theoreti- few floated in the surface waters.

cally, at least, virtually all ammonoids could In many ammonite faunas the consistent
favorably adjust their attitude and buoyancy co-occurrence of large and small similarly
in the water column. Most ammonoids were ornamented mature shells at specifi c horizons
probably part of the mobile benthos, although suggests that the macroconch and microconch
after death their gas-filled shells could be may be related sexual dimorphs (the male and

widely distributed by oceanic currents. Many female of the species). The macroconch was
groups of ammonoids were endemic, and the probably the female, though this may not
shovel-like jaws of some groups were most always have been the case. The ammonoids

efficient at the sediment–water interface. probably originated from the bactritid ortho-
Richard Batt’s studies (1993) on Cretaceous cone nautiloids, with protoconchs and large
ammonite morphotypes from the United States body chambers, during the earliest Devonian.
have established a series of shell types related The anarcestide goniatites, with simple sutural
to life modes and environments (Fig. 13.21b). patterns, were relatively scarce during the
For example, evolute heavily ornamented Mid Devonian. However, by the Famennian,
forms were probably nektobenthonic, as were other groups such as the clymeniids, with a
spiny cadicones and spherocones, nodose dorsally situated siphuncle, were common.
spherocones and platycones, together with The goniatitides expanded during the