Page 365 - Introduction to Paleobiology and The Fossil Record

352 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Crioceras sp. 1 Crioceras sp. 2
Dactylioceras Normannites
?
Turrilites

Promicroceras
Macroscaphites
Caloceras
Crioceras sp. 3

Scaphites
Ludwigia Sigaloceras Oecoptychius Lytocrioceras
(a)

100 m 100 m 100 m

oxygenated

anoxic anoxic

(b)
Figure 13.21 Life attitudes and buoyancy of the ammonites. (a) Supposed life orientations of a
selection of ammonite genera, with the center of gravity marked ×; the center of buoyancy is marked
with a dot and the extent of the body chamber is indicated with subparallel lines. (b) Relationship of
some ammonite morphotypes to water depth and the development of anoxia. (a, from Trueman, A.E.
1940. Q. J. Geol. Soc. Lond. 96; b, from Batt 1993.)

sacs and body outline is well known from ing with the exoskeletons of the shelled ceph-
several localities in the Jurassic. In contrast, alopods such as the nautiloids and ammonoids.
the skeletons of extinct belemnites are locally The belemnite skeleton is relatively simple,
abundant in Jurassic and Cretaceous rocks. consisting of three main parts: the bullet-
Belemnites had an internal skeleton, contrast- shaped guard is solid and composed of radi-