Rostroconchia
Encyclopedia
The Rostroconchia is a class
of extinct mollusc
s dating from the early Cambrian
to the late Permian
. They were initially thought to be bivalves, 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 enclosing the mantle
and muscular foot. The anterior 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.
Rostroconchs began their life as a small, bilaterally symmetrical, univalved protoconch planktonic 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. In this way, the rostroconch shell is more like a taco shell than a clam shell. 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 to have burrowed itself into the sediment anterior first and the rostrum was above the sediment used possibly as a water filtration system.
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 where the predecessors to the Scaphopoda.
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
of extinct mollusc
Mollusca
The Mollusca , common name molluscs or mollusksSpelled mollusks in the USA, see reasons given in Rosenberg's ; for the spelling mollusc see the reasons given by , is a large phylum of invertebrate animals. There are around 85,000 recognized extant species of molluscs. Mollusca is the largest...
s dating from the early Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
to the late Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
. They were initially thought to be bivalves, 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 enclosing the mantle
Mantle (mollusc)
The mantle is a significant part of the anatomy of molluscs: it is the dorsal body wall which covers the visceral mass and usually protrudes in the form of flaps well beyond the visceral mass itself.In many, but by no means all, species of molluscs, the epidermis of the mantle secretes...
and muscular foot. The anterior part of the shell probably pointed downward and had a gap from which the foot could probably emerge. Rostroconchs probably lived a sedentary
Sedentary lifestyle
Sedentary lifestyle is a medical term used to denote a type of lifestyle with no or irregular physical activity. A person who lives a sedentary lifestyle may colloquially be known as a couch potato. It is commonly found in both the developed and developing world...
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 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. In this way, the rostroconch shell is more like a taco shell than a clam shell. 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 to have burrowed itself into the sediment anterior first and the rostrum was above the sediment used possibly 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 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 where the predecessors to the Scaphopoda.