The Rostroconchia is a

class Class, Classes, or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used d ...

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

mollusc Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...

s dating from the early

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

to the Late Permian. They were initially thought to be

bivalve Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of aquatic animal, aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed b ...

s, but were later given their own class. They have a single shell in their larval stage, and the adult typically has a single, pseudo-bivalved

shell Shell may refer to: Architecture and design * Shell (structure), a thin structure ** Concrete shell, a thin shell of concrete, usually with no interior columns or exterior buttresses Science Biology * Seashell, a hard outer layer of a marine ani ...

enclosing the mantle and muscular foot. The

anterior Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...

part of the shell probably pointed downward and had a gap from which the foot could probably emerge. Rostroconchs probably lived a sedentary semi- infaunal lifestyle. There were probably more than 1,000 species of members of this class. Approximately 3 dozen genera and an even greater number of species have been described. Generally, rostroconchs are small, less than two centimeters in length, but larger forms, found in United States Devonian limestones, can grow to a length of 15 cm.

Morphology and lifestyle

Externally, rostroconchs look much like bivalves and rostroconchs probably had an extendable muscular foot, indicated by a prominent anterior gape in the rostroconch's shell. It seems, however, that the internal anatomy and morphology of the foot were closer to that of the scaphopods. Rostroconchs began their life as a small, bilaterally symmetrical, univalved protoconch

planktonic Plankton are the diverse collection of organisms that drift in water (or air) but are unable to actively propel themselves against currents (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they pro ...

larva. The bilateral shell grew into two valves as the rostroconch entered adulthood. Adult rostroconchs differ from bivalves because they have no functional hinge. Unlike the shell of a bivalve, which was able to move or articulate, the shell layers of a rostroconch — the layers of rigid calcite— continue across the whole dorsal area of the rostroconch. The two valves would have been rigidly fixed in place, and would have to have been broken periodically to allow the rostroconch shell to grow. The posterior of the shell contains a flattened tube that is called the rostrum. The rostroconch probably burrowed itself into sediment, anterior first, leaving the rostrum above the sediment to be used as a water filtration system.

Evolutionary history

'' Heraultipegma'' is the earliest, very primitive, rostroconch genus dating from the Late

Terreneuvian The Terreneuvian is the lowermost and oldest Series (stratigraphy), series of the Cambrian geological System (stratigraphy), system. Its base is defined by the first appearance datum of the trace fossil ''Treptichnus pedum'' around million years ...

. True Rostroconchs appeared during the Ordovician, heavily competing with the bivalves until their decline in the end-early Ordovician turnover. Early, primitive rostroconchs such as '' Ribeiroia'' had a hinge in which all shell layers covered the dorsal region resulting in a very rigid shell. In '' Conocardium'', a more advanced rostroconch, the outer shell layers do not cross the entire margin, suggesting independent steps towards the bivalve flexible hinge. Some evidence suggests that the conocardoid rostroconchs were the predecessors to the Scaphopoda.

References

* PALAEOS, 2002, Rostroconchia. URL: https://web.archive.org/web/20090302181354/http://www.palaeos.com/Invertebrates/Molluscs/BasalMollusca/Rostroconchia/Rostroconchia.html. Accessed November 16, 2008. {{Authority control Prehistoric mollusc taxonomy Cambrian first appearances Permian extinctions Mollusc classes Prehistoric protostome classes Conchifera