Acaenoplax

Acaenoplax Temporal range: Wenlock PreꞒ Ꞓ O S D C P T J K Pg N
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Mollusca
Class:	incertae sedis
Family:	†Heloplacidae
Genus:	†Acaenoplax
Species:	†A. hayae
Binomial name
†Acaenoplax hayae Sutton et al, 2001

Acaenoplax is an extinct worm-shaped mollusc known from the Coalbrookdale Formation of Herefordshire, England. It lived in the Silurian 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'.[1][2]

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,[3] but although it was originally interpreted as a polychaete,[1] this position is untenable for a number of reasons.[4]

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.[1] There are eighteen rows of spines projecting from ridges in the body surface, which encircle the body except for its bottom surface,[1] which presumably bore a molluscan foot. Its straight gut was preserved in phosphate.[2]

Affinity

Heloplax, Enetoplax and Arctoplax are genera of shell that are closely related to Acaenoplax, but whose soft tissue is not preserved.[1]

See also

• Kulindroplax, a possibly related seven-shelled mollusk from the same Lagerstätte.

References

1. Sutton, M. D.; Briggs, D. E. G.; Siveter, D. J.; Siveter, D. J. (Mar 2001). "An exceptionally preserved vermiform mollusc from the Silurian of England". Nature. 410 (6827): 461–463. doi:10.1038/35068549. ISSN 0028-0836. PMID 11260711.
2. Sutton, M. D.; Briggs, D. E. G.; Siveter, D. J.; Siveter, D. J. (2004). "Computer reconstruction and analysis of the vermiform mollusc Acaenoplax hayae from the Herefordshire Lagerstatte (Silurian, England), and implications for molluscan phylogeny". Palaeontology. 47 (2): 293–318. doi:10.1111/j.0031-0239.2004.00374.x.
3. Steiner, G.; Salvini-Plawen, L. (Dec 2001). "Acaenoplax—polychaete or mollusc?". Nature. 414 (6864): 601–602, discussion 602. doi:10.1038/414601a. ISSN 0028-0836. PMID 11740549.
4. Sutton, M.; Briggs, D. E. G.; Siveter, D. J.; Siveter, D. J. (2001). "Acaenoplax—polychaete or mollusc?". Nature. 414 (6864): 602. doi:10.1038/414602a.