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 probably 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 Cambrian when, for example, Heraultipegma and post-spawning mortality model: Antarctic example shelf -
spawning ground post-spawning mortality accumulation and concentration by shallow-marine processes
basin migration to spawning grounds
^ limited mortality turbidity-current triggered w /
^ limited mortality turbidity-current triggered belemnite-rich turbidite accumulation and concentration by shallow-marine processes
sparse belemnites belemnite-rich turbidite sparse belemnites predation concentration model predation concentration model
regurgitates foundered vertebrate with intact gastric mass regurgitates foundered vertebrate with intact gastric mass
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 the Tommotian. The rostroconchs diversified during the Ordovician to reach an acme in the Katian when all seven families were represented. They probably occupied similar ecological niches to those of the bivalves. However, there followed a decline in abundance and diversity until final extinction at the end of the Permian when only conocar-diodes such as Arceodomus were still extant.
The rostroconchs occupy a pivotal position in molluskan evolution (Runnegar & Pojeta 1974). The group developed from within the monoplacophoran plexus with a loss of segmentation; the rostroconchs themselves generated both the bivalves and the scaphopods whereas the gastropods and cephalopods were probably derived independently from a separate monoplacophoran ancestor. In some respects, the rostroconchs may represent a missing link between the univalved and bivalved molluskan lineages, while their unlikely morphology may have contributed towards their late discovery.
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