Siphonotretida

Spihonotretids were most abundant in the late Cambrian and Early Ordovician (Furongian to Floian), and were traditionally considered to have gone extinct in the Upper Ordovician ("Ashgill"). and as late as the Ludlow Epoch of the Silurian (Orbaspina). Isolated fragments are even known from the Emsian stage of the Lower Devonian. == Anatomy ==

Siphonotretids had simple, rounded shells, with an ornamentation of hollow spines Similar to acrotretides, the pedicle foramen was set at the apex of the ventral valve, though it is often elongated into a tubular groove opening forwards. This groove lies on a triangular extension of the ventral valve, known as a pseudointerarea, which overhangs the dorsal valve. Unlike acrotretides, the adult shell is spinose while the larval shell lacks a pitted texture. ==List of genera==

• Acanthambonia Cooper, 1956 (Ordovician, "Arenig" to "Ashgill") • Acanthotretella? Holmer & Caron, 2006 (Upper Cambrian to Tremadocian) • Orbaspina Valentine & Brock, 2003 (Silurian, late Llandovery to Ludlow) • Quasithambonia? Bednarczyk & Biernat, 1978 (Ordovician, "Arenig") • Schizambon Walcott, 1884 (Middle Cambrian to Ordovician, "Arenig") • Siphonobolus Havlíček, 1982 (Ordovician, "Tremadoc" to "Arenig") • Siphonotreta de Verneuil, 1845 (Ordovician, "Arenig" to "Caradoc") • Siphonotretella Popov & Holmer, 1994 (Lower Ordovician, "Tremadoc" to "Arenig") ==References==