Page 368 - Introduction to Paleobiology and The Fossil Record
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SPIRALIANS 2: MOLLUSKS 355
post-spawning mortality model: Antarctic example
shelf basin
spawning ground
migration to spawning grounds
post-spawning mortality
limited mortality
turbidity-current triggered
accumulation and concentration
by shallow-marine processes
belemnite-rich turbidite
sparse belemnites
(a)
predation concentration model
predation
regurgitates
foundered vertebrate with intact gastric mass
(b)
Figure 13.23 Belemnite battlefields and their possible origin: (a) post-spawning mortality model and
(b) predation concentration model. (From Doyle & MacDonald 1993.)
Watsonella dominated rostroconch faunas of probably derived independently from a sepa-
the Tommotian. The rostroconchs diversifi ed rate monoplacophoran ancestor. In some
during the Ordovician to reach an acme in the respects, the rostroconchs may represent a
Katian when all seven families were repre- missing link between the univalved and
sented. They probably occupied similar eco- bivalved molluskan lineages, while their
logical niches to those of the bivalves. unlikely morphology may have contributed
However, there followed a decline in abun- towards their late discovery.
dance and diversity until fi nal extinction at
the end of the Permian when only conocar-
diodes such as Arceodomus were still extant. EVOLUTIONARY TRENDS WITHIN
The rostroconchs occupy a pivotal position THE MOLLUSCA
in molluskan evolution (Runnegar & Pojeta A spectacular variety of mollusk morphotypes
1974). The group developed from within the and life modes evolved during the Phanero-
monoplacophoran plexus with a loss of seg- zoic, from the simple body plan of the arche-
mentation; the rostroconchs themselves gen- mollusk. Despite the diversity of early mollusks
erated both the bivalves and the scaphopods in the Cambrian, the phylum was not notably
whereas the gastropods and cephalopods were conspicuous in the tiered suspension-feeding