The Rostroconchia is a

class Class 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 differently ...

of extinct molluscs dating from the early

Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ag ...

to the

Late Permian Late may refer to: * LATE, an acronym which could stand for: ** Limbic-predominant age-related TDP-43 encephalopathy, a proposed form of dementia ** Local-authority trading enterprise, a New Zealand business law ** Local average treatment effect, ...

. They were initially thought to be

bivalve Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, biv ...

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 ** Thin-shell structure Science Biology * Seashell, a hard o ...

enclosing the mantle and muscular foot. The

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

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 Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is ''flora'', and for fungi, it is ''funga''. Flora, fauna, funga and other forms of life are collectively referred to as ''biota''. Zoolo ...

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 Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, biv ...

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 found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cruc ...

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. 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 ''Ribeiroia'' () is a genus of trematode parasites that sequentially infect freshwater snails in the family Planorbidae (ramshorn snails) as first intermediate hosts, fish and larval amphibians as second intermediate hosts, and birds and mamma ...

'' 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. {{Taxonbar, from=Q1422647 Prehistoric mollusc taxonomy Cambrian first appearances Permian extinctions Mollusc classes Prehistoric protostome classes