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Siphonotretida
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Siphonotretida is an extinct order of in the class . The order is equivalent to the sole superfamily Siphonotretoidea, itself containing the sole family Siphonotretidae. Siphonotretoids were originally named as a superfamily of , before being raised to their own order.

(2025). 9780813731087, Geological Society of America; University of Kansas. .


Evolution
Spihonotretids were most abundant in the and ( to ), and were traditionally considered to have gone extinct in the ("Ashgill"). More recently, new siphonotretids have been described as early as Cambrian Stage 4 ( )
(2025). 9780813731360, Geological Society of America; University of Kansas. .
and as late as the Epoch of the ( ). Isolated fragments are even known from the stage of the . Archaic Cambrian-style siphonotretids such as Schizambon and , with basic forms of ornamentation, populated the shores of , , and by the start of the Ordovician. In the late , advanced Ordovician-style spiny siphonotretids spread out from temperate waters around Gondwana and mostly replaced their older relatives.


Anatomy
Siphonotretids had simple, rounded shells, with an ornamentation of hollow spines or rarely pointed tubercles. The shell is usually ventribiconvex (both valves convex, the ventral valve moreso) and composed of microscopic granules of . The inner surface of the shell tends to be weakly mineralized, so many aspects of the musculature and other soft anatomy are difficult to estimate in most species. Available data supports comparison to the internal structures of lingulids. Siphonotretids may be related to the linguloid families Lingulellotretidae or .

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.

The possible siphonotretid is known from several exceptionally-preserved specimens which reveal lingulid-like traits such as , a spirolophous and U-shaped gut. However, the shell was poorly mineralized and sends out a very long, stalk-like pedicle, which in one specimen was attached to a fragment of algae. Combined with a lightweight shell, the pedicle likely helped to suspend the body above the seabed, an lifestyle dissimilar to the (burrowing) lingulids.


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