Kulindroplax

Kulindroplax perissokomos is a Silurian mollusk, known from a single fossil from the Coalbrookdale Formation fauna of England. It lived during the Homerian Age (about 425 million years ago).^[1] It is considered a basal aplacophoran. Unlike all modern aplacophorans, which are shell-less, Kulindroplax has a chiton-like shell, and it is considered a transitional fossil in the evolution of molluscs.^[2][3]

Kulindroplax Temporal range: Wenlock PreꞒ Ꞓ O S D C P T J K Pg N
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Aplacophora
Genus:	†Kulindroplax Sutton, Briggs, Siveter, Siveter and Sigwart, 2012[1]
Species:	†K. perissokomos
Binomial name
†Kulindroplax perissokomos Sutton, Briggs, Siveter, Siveter and Sigwart, 2012

The only known specimen, described in 2012, was conserved at the Oxford University Museum of Natural History.^[1]

Morphology

Kulindroplax is about 2 cm (0.79 in) wide and 4 cm (1.6 in) long.^[2] It is the first known mollusk showing an unambiguous combination of valves, or exterior shells, and a worm-like body.^[2] It bears seven similar, unarticulated valves, with a shorter head valve and a taller caudal one, lacking ornaments^[1] and also several densely packed, 1–2 mm long spicules. It has no discernible foot,^[1] and the radula is not preserved.^[1] A gill array is present, along with a respiratory cavity opening posteriorly. These features make it more reminiscent of caudofoveate aplacophorans.^[1]

While aculiferan mollusks usually bear eight dorsal valves (except for multiplacophorans and Acaenoplax), Kulindroplax bears only seven in a single chiton-like row.^[4]

Life habits

Kulindroplax seems adapted to moving through a substrate, with the spicules acting as "sediment ratchets".^[1] In contrast with some modern aplacophorans like the caudofoveates, which live within the sea bottom sediments, Kulindroplax probably crawled on the bottom, requiring a dorsal armour.^[1]

Evolutionary significance

Kulindroplax settles a 20-year-long dispute about the phylogeny of mollusks, namely the relationship of the worm-like, carnivorous Aplacophora within the group, in particular their relationship to the Polyplacophora.^[3] Aplacophorans have been historically variously treated as either a mollusk base-group, distant relatives of Cephalopoda or as a sister group to Polyplacophora in the clade Aculifera.^[1] Both molecular and fossil evidence appeared to support the last hypothesis.^{[5][6][7][8][9]} Including Kulindroplax in a phylogenetic matrix with other mollusks and mollusk-like fossil taxa allows to consistently resolve Aculifera as a sister group to Conchifera, in all variants of the analysis, thus bringing the fossil record in line with the recent molecular evidence.^[1]

While other fossil taxa like Acaenoplax and Phthipodochiton showed intermediate features between aplacophorans and polyplacophorans, no unambiguous fossil with an aplacophoran-like body and a polyplacophoran-like shell has been found before Kulindroplax.

Below is a cladogram of mollusk phylogeny according to Sutton et al., 2012.^[1] Taxa marked with † are extinct.

← Lophotrochozoa	Brachiopoda Odontogriphus† Halwaxiids† Mollusca Conchifera Aculifera Polyplacophora Echinochiton† Neoloricata Aplacophora Epimenia Septemchiton† Chaetoderma Phthipodochiton† Kulindroplax† Acaenoplax† Matthevia†