The Kirengellids are a group of problematic Cambrian

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

shells of marine organisms. The shells bear a number of paired

muscle scar Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of m ...

s on the inner surface of the

valve A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fitting ...

. These fossils have conventionally been regarded as

monoplacophora Monoplacophora , meaning "bearing one plate", is a polyphyletic superclass of molluscs with a cap-like shell inhabiting deep sea environments . Extant representatives were not recognized as such until 1952; previously they were known only from ...

n molluscs, and possibly ancestral to gastropods or

cephalopod A cephalopod is any member of the molluscan class Cephalopoda ( Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, ...

s. They were presumed to be exogastric on the presumption that their larger muscle scars were anterior, but it may be dangerous to compare these scars with molluscan musculature. In any case, they coiled in the opposite direction to '' Romaniella''. However, their calcitic shells, the position of the muscle scars, and putative association with secondary shell elements, make a

brachiopod Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, ...

affinity possible, by analogy with the mobergellans: a group of phosphatic shells from the same time period, with a similar set of muscle scars. 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.

Included taxa

After * ''Kirengella'' Rozov, 1968 (Upper Cambrian) ** † ''Kirengella alta'' Whitfield 1889 ** ''Kirengella ayaktchica'' - type species ** ''Kirengella expansus'' ** † ''Kirengella kultavasaensis'' Doguzhaeva 1972 ** ''Kirengella oregonensis'' ** ''Kirengella pyramidalis'' ** † ''Kirengella rectilateralis'' Berkey 1898 ** † ''Kirengella stabilis'' Berkey 1898 ** ''Kirengella washingtonense'' * ''Hypseloconus'' (Upper Cambrian) * ''Lenaella'' (Tremadoc / Lower Ordovician) * ''Nyuella'' (Tremadoc / Lower Ordovician) * ''Romaniella'' (Arenig / late Lower Ordovician) * ''Moyerokania'' (Arenig / late Lower Ordovician) * ''Angarella'' (Arenig / late Lower Ordovician) * ''Pygmaeoconus'' (Llanvirn / early Middle Ordovician)

References

* S. N. Rozov. 1975. ''A new order of the Monoplacophora''. Paleontological Journal 15(1):39-43 * G.P. Wahlman. 1992. Middle and Upper Ordovician symmetrical univalved mollusks (Monoplacophora and Bellerophontina) of the Cincinnati Arch region. United States Geological Survey Professional Paper 1066(O):1-123 Cambrian animals {{paleo-mollusc-stub