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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 x; 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.)

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 x; 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 several localities in the Jurassic. In contrast, the skeletons of extinct belemnites are locally abundant in Jurassic and Cretaceous rocks. Belemnites had an internal skeleton, contrast ing with the exoskeletons of the shelled ceph-alopods such as the nautiloids and ammonoids. The belemnite skeleton is relatively simple, consisting of three main parts: the bullet-shaped guard is solid and composed of radi ally arranged needles of calcite with, at its anterior end, a conical depression or alveolus that houses the apical portion of the conical phragmocone, consisting of concave septa and a ventral siphuncle, and the spatulate pro-ostracum (corresponding to the dorsal wall of the body chamber of ectocochliate forms) that extends anteriorly (Fig. 13.22). This assemblage, situated on the dorsal side of the animal, is analogous to the chambered shells of nautiloids and ammonoids. Soft parts of belemnites, including the contents of ink sacs and tentacle hooks, are also occasionally preserved.

ventral head (anterior)

anterior (dorsal)

ventral head (anterior)

anterior (dorsal)

opening o^^ posterior (ventral) mantle cavity dorsal (posterior)

opening o^^ posterior (ventral) mantle cavity internal digestive beak radula remnant diverticulum cecum

internal digestive beak radula remnant diverticulum cecum

mouth brain tentacle funnel mantle

(b) caVity heart anus kidney

Hibolites

Middle Jurassic - Late Cretaceous

Hibolites

Middle Jurassic - Late Cretaceous

Hastites

Late - Middle Jurassic

Duvalia

Middle Jurassic -

Late Cretaceous

Duvalia

Middle Jurassic -

Late Cretaceous

Hastites

Late - Middle Jurassic

Actinocamax Belemnitella

Upper Cretaceous Upper Cretaceous

Figure 13.22 Coleoid morphology: (a) reconstruction of a living belemnite, (b) soft-part morphology of the belemnites, (c) internal skeleton of the belemnites, and (d) some belemnite genera. (From Peel et al. 1985.)

Actinocamax Belemnitella

Upper Cretaceous Upper Cretaceous

Figure 13.22 Coleoid morphology: (a) reconstruction of a living belemnite, (b) soft-part morphology of the belemnites, (c) internal skeleton of the belemnites, and (d) some belemnite genera. (From Peel et al. 1985.)

By analogy with modern squids, the belem-nites were probably rapidly-moving predators living in shoals with their body level regulated by the guard. The animal thus probably maintained a horizontal attitude within the water column, preferring the open ocean. Data from the stomach contents of ichthyosaurs confirm that these mollusks formed part of their diet.

Some of the oldest records of belemnites, for example Jeletzkya from the mid-Carboniferous of Illinois, are tentative. The first unequivocal belemnites are from the Middle Triassic rocks of Sichuan Province, China where several species of Sinobelemnites occur. Belemnites became extinct at the end of the Cretaceous; later records are reworked or based on misinterpretations.

Some of the first supposed belemnites, like the Carboniferous Paleoconus, were relatively short stubby forms. In the Early Jurassic, Megateuthis was a long, slender form, whereas Dactyloteuthis was laterally flattened; the later Jurassic Hibolites is spear-shaped. The Cretaceous Belemnitella has a large bullet-shaped guard, whereas that of Duvalia has a flattened spatulate shape (Fig. 13.22d). However, despite differences in the detailed morphology of the endoskeltons across genera, many authorities consider that most of the Mesozoic belemnites probably looked very similar, but there are still enough features to measure on their skeletons and discriminate taxa (Box 13.8).

The compact calcareous guards of the bel-emnites have proved ideal for the analysis of oxygen isotope ratios (O16 : O18) relating to paleotemperature conditions in the Jurassic and Cretaceous seas. These data have indicated warm peaks during the Albian and the Coniacian-Santonian (mid-Cretaceous) with a gradual cooling from the Campanian (Late Cretaceous) onwards. And as with many other Mesozoic groups, belemnite distributions show separate low-latitude Tethyan and high-latitude Boreal assemblages.

Spectacular mass accumulations of belem-nite rostra are relatively common in Mesozoic sediments and, although some authors have used these assemblages in paleocurrent studies, few have addressed their mode of accumulation. Dense accumulations of bullet-shaped belemnite rostra have promoted the term "belemnite battlefields" for such distinctive shell beds (Fig. 13.23). These accumulations conform to five genetic types (Doyle & MacDonald 1993): (1) post-spawning mortalities (Fig. 13.23a); (2) catastrophic mass mortalities; (3) predation concentrates, either in situ or regurgitated (Fig. 13.23b); (4) condensation deposits perhaps aided by winnowing and sediment by-pass; and (5) resedimented deposits derived from usually condensed accumulations. Many of these so-called bel-emnite battlefields are then partly natural occurrences, reflecting the biology of the animals (numbers 1-3), but it is important to distinguish these from sedimentary accumulations (numbers 4 and 5) that say nothing about belemnite behavior.

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