Phthipodochiton

Phthipodochiton is an extinct genus of molluscs, known from several fossils from the upper Ordovician fauna of the Lady Burn Starfish beds of Girvan, Scotland.^[1] It shows a mixture of aplacophoran body plan and polyplacophoran-like valves, and it is an informative fossil in the evolution of aculiferan mollusks.^[1]

Phthipodochiton Temporal range: Upper Ordovician PreꞒ Ꞓ O S D C P T J K Pg N
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Polyplacophora
Order:	†Paleoloricata
Genus:	†Phthipodochiton Sutton and Sigwart, 2012[1]
Species:	†P. thraivensis
Binomial name
†Phthipodochiton thraivensis Sutton and Sigwart, 2012

It was previously classified under the genus Helminthochiton,^[2] but it has been reassigned to its own genus in 2012.^[1]

Morphology

Phthipodochiton body is worm-like, with eight polyplacophoran-like valves but no true foot.^[3] Head and tail valves are slightly smaller than the intermediate ones.^[1] The only ornaments on the valves appear to be growth lines.^[1] The body is also covered by a sheet of spicules ; no radula has been preserved.^[1]

Life habits

Phthipodochiton was carnivorous, feeding on crinoids,^[3] as shown by a fossil preserved with gut contents.^[4] In contrast with modern chitons, Phthipodochiton probably did not creep on its foot but had a locomotion style similar to that of solenogastres.^[1]

Taxonomy

Phthipodochiton shares similarities with genera as Alastega, Robustum and Septemchiton.^[1] but it is sufficiently distinct from all of them to be considered a separate species. It is considered to belong to the aplacophoran stem lineage, along with Acaenoplax,^[5] and it has also been placed close to Matthevia and the shelled aplacophoran Kulindroplax in phylogenetic analyses.^[6]