Acaenoplax

{{Short description|Extinct genus of molluscs}}

{{Speciesbox

| fossil_range = {{Fossil range|Wenlock}}

| genus = Acaenoplax

| species = hayae

| authority = 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'.{{cite journal|title=An exceptionally preserved vermiform mollusc from the Silurian of England|journal=Nature |volume=410| issue = 6827|pages= 461–463 | date=Mar 2001 | issn = 0028-0836| pmid = 11260711| doi=10.1038/35068549|last1=Sutton | first1=M. D.|last2=Briggs | first2=D. E. G.| authorlink2 = Derek Briggs |last3=Siveter | first3=D. J.|last4=Siveter | first4=D. J.}}{{Cite journal| first= M. D.| first2 = D. E. G.| first3 = D. J.| first4 = D. J.| last = Sutton| last2 = Briggs| last3 = Siveter| last4 = Siveter| title = Computer reconstruction and analysis of the vermiform mollusc Acaenoplax hayae from the Herefordshire Lagerstatte (Silurian, England), and implications for molluscan phylogeny| journal = Palaeontology| volume = 47| issue = 2| pages = 293–318| year = 2004| doi = 10.1111/j.0031-0239.2004.00374.x| authorlink2 = Derek Briggs}}

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,{{Cite journal| last1= Steiner | first1 = G.| last2 = Salvini-Plawen | first2 = L.| title =Acaenoplax—polychaete or mollusc?| journal = Nature| volume = 414| issue = 6864| pages = 601–602; discussion 602| date=Dec 2001 | issn = 0028-0836| pmid = 11740549| doi = 10.1038/414601a}} but although it was originally interpreted as a polychaete, this position is untenable for a number of reasons.{{Cite journal| last2 = Briggs| authorlink2 = Derek Briggs|first1 = M.| first2 = D. E. G.| last4 = Siveter| last3 = Siveter| last1 = Sutton | first3 = D. J.| first4 = D. J.| title =Acaenoplax—polychaete or mollusc?| journal = Nature| volume = 414| pages = 602| year = 2001| doi = 10.1038/414602a | issue=6864| doi-access = free}}