Page 367 - Introduction to Paleobiology and The Fossil Record

354 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
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 main-
tained a horizontal attitude within the water
column, preferring the open ocean. Data from
the stomach contents of ichthyosaurs confi rm
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 fl attened; 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 discrimi-
nate taxa (Box 13.8).
The compact calcareous guards of the bel-
emnites have proved ideal for the analysis of
18
16
oxygen isotope ratios (O : O ) relating to
paleotemperature conditions in the Jurassic
and Cretaceous seas. These data have indi-
cated 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 accumula-
tion. 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 & Mac-
Donald 1993): (1) post-spawning mortalities
(Fig. 13.23a); (2) catastrophic mass mortali-
ties; (3) predation concentrates, either in situ
or regurgitated (Fig. 13.23b); (4) condensa-
tion 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 accumula-
tions (numbers 4 and 5) that say nothing
about belemnite behavior.
CLASS SCAPHOPODA
Scaphopods are generally rare as fossils. The
Scaphopoda, or elephant-tusk shells, have a
single, slightly curved high conical shell, open
at both ends (Fig. 13.25a). They lack gills and
eyes, but have a mouth equipped with a radula
and surrounded by tentacles; they also possess
a foot, similar to that of the bivalves, adapted
for burrowing. Scaphopods are mainly car-
nivorous, feeding on small organisms such as
foraminiferans and spending much of their
life in quasi-infaunal positions within soft
sediment in deeper-water environments. The
first scaphopods appeared during the Devo-
nian and apparently had similar lifestyles to
living forms such as Dentalium.
CLASS ROSTROCONCHA
Relatively recently a small class of mollusks,
superficially resembling bivalves but lacking
a functional hinge, has been documented
from the Paleozoic. Over 35 genera have been
described; most were originally described as
bivalved arthropods. The rostroconchs prob-
ably had a foot that emerged through the
anterior gape between the shells. However,
the two shells are in fact fused along the mid-
dorsal line, and posteriorly the shells are
extended as a platform or rostrum (Fig.
13.25b). Ontogeny occurs from an initial dis-
soconch with the bilobed form developing
from the disproportionate growth of shell
from the lateral lobes of the mantle. The
group appeared first during the Early Cam-
brian when, for example, Heraultipegma and