''Acaenoplax'' is an extinct worm-shaped
mollusc
Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...
known from the
Coalbrookdale Formation of
Herefordshire
Herefordshire ( ) is a Ceremonial counties of England, ceremonial county in the West Midlands (region), West Midlands of England, bordered by Shropshire to the north, Worcestershire to the east, Gloucestershire to the south-east, and the Welsh ...
, England. It lived in the
Silurian
The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...
period. It was a couple of centimetres long and half a centimetre wide, and comprises serially repeated units with seven or eight shells, and rings of 'spines'.
Some of its characters are reminiscent of the
polychaete worms, and the character combinations do not place it obviously in the stem of any modern mollusc group,
but although it was originally interpreted as a polychaete,
this position is untenable for a number of reasons.
Morphology
The organism resembles a bristled worm, but bears a number of shells on its upper surface. The first shell is cap-like, whereas the others are saddle-shaped. The rearmost shell is almost rectangular, whereas the others are more circular, with spines on the rear surface of the third to sixth shells. The originally-aragonitic shells do not overlap.
There are eighteen rows of spines projecting from ridges in the body surface, which encircle the body except for its bottom surface,
which presumably bore a molluscan foot. Its straight gut was preserved in phosphate.
Affinity
''
Heloplax'', ''
Enetoplax'' and ''
Arctoplax'' are genera of shell that are closely related to ''Acaenoplax'', but whose soft tissue is not preserved.
See also
* ''
Kulindroplax'', a possibly related seven-shelled mollusk from the same ''Lagerstätte''.
References
Silurian molluscs
Wenlock series fossils
Silurian animals of Europe
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