Kirengellida

The Kirengellids are a group of problematic Cambrian fossil shells of marine organisms. The shells bear a number of paired muscle scars on the inner surface of the valve.

Phylum:Mollusca

Class:Monoplacophora

Subclass:Tergomya

Order:†Kirengellida
Rozov, 1975

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Kirengellida Temporal range: Late Cambrian–Mid Ordovician PreꞒ Ꞓ O S D C P T J K Pg N
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Monoplacophora
Subclass:	Tergomya
Order:	†Kirengellida Rozov, 1975
Superfamilies[1]
Hypseloconoidea Kirengelloidea
Synonyms[1]
Romaniellida

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These fossils have conventionally been regarded as monoplacophoran molluscs, and possibly ancestral to gastropods or cephalopods.^[2] They were presumed to be exogastric on the presumption that their larger muscle scars were anterior,^[3][4] but it may be dangerous to compare these scars with molluscan musculature.^[5] In any case, they coiled in the opposite direction to Romaniella.^[5] However, their calcitic shells, the position of the muscle scars, and putative association with secondary shell elements, make a brachiopod affinity possible, by analogy with the mobergellans: a group of phosphatic shells from the same time period, with a similar set of muscle scars.^[5] There is also strong similarity to the contemporary brachiopod group, the Craniopsids. In the case of this diagnosis, a simple lophophore apparatus is postulated to sit between the muscle scars and the edges of the shell.^[5] On the other hand, Vendrasco (2012) reaffirmed the interpretation of the kirengellids as molluscs, noting that the Kirengella muscle scar pattern is also similar to what occurs in monoplacophorans.^[6][7] Bouchet et al. (2017) classified Kirengellida as an order within the subclass Tergomya.^[8]