Phthipodochiton
{{Short description|Extinct genus of mollusc}}
{{Italic title}}
{{Speciesbox
|genus=Phthipodochiton
|species = thraivensis
|fossil_range = {{Fossil range|Upper Ordovician}}
|authority = Sutton and Sigwart, 2012
|parent_authority = Sutton and Sigwart, 2012{{Cite journal | last1 = Sutton | first1 = M. D. | last2 = Sigwart | first2 = J. D. | doi = 10.1111/j.1475-4983.2011.01126.x | title = A chiton without a foot | journal = Palaeontology | volume = 55 | issue = 2 | pages = 401 | year = 2012 | bibcode = 2012Palgy..55..401S | s2cid = 86554201 | doi-access = free }}
}}
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. It shows a mixture of aplacophoran body plan and polyplacophoran-like valves, and it is an informative fossil in the evolution of aculiferan mollusks.
It was previously classified under the genus Helminthochiton,{{Fossil chiton catalogue}} but it has been reassigned to its own genus in 2012.
Morphology
Phthipodochiton body is worm-like, with eight polyplacophoran-like valves but no true foot,{{cite web|title=Aculifera|url=http://palaeos.com/metazoa/mollusca/aculifera/aculifera.html|work=Palaeos|access-date=21 November 2012}} though a pedal groove may be present. Head and tail valves are slightly smaller than the intermediate ones. The only ornaments on the valves appear to be growth lines. The body is also covered by a sheet of spicules ; no radula has been preserved.
Life habits
Phthipodochiton was carnivorous, feeding on crinoids, as shown by a fossil preserved with gut contents.{{Cite journal | last1 = Donovan | first1 = S. K. | last2 = Sutton | first2 = M. D. | last3 = Sigwart | first3 = J. D. | title = The last meal of the Late Ordovician mollusc 'Helminthochiton' thraivensis Reed, 1911, from the Lady Burn Starfish Beds, southwest Scotland | doi = 10.1002/gj.1286 | journal = Geological Journal | date = 2011 | volume = 46 | issue = 5 | pages = 451–463 | bibcode = 2011GeolJ..46..451D | s2cid = 86539199 | url = https://pure.qub.ac.uk/ws/files/759573/Donovanetal_2010b.pdf }} In contrast with modern chitons, Phthipodochiton probably did not creep on its foot but had a locomotion style similar to that of solenogastres.
Taxonomy
Phthipodochiton shares similarities with genera as Alastega, Robustum and Septemchiton. 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,{{Cite journal| last1 = Sigwart | first1 = J. D.| last2 = Sutton | first2= M. D.| title = Deep molluscan phylogeny: synthesis of palaeontological and neontological data| journal = Proceedings of the Royal Society B: Biological Sciences| issue = 1624| volume = 274| pages = 2413–2419| date =Oct 2007| pmid = 17652065| pmc = 2274978| doi = 10.1098/rspb.2007.0701}} For a summary, see {{cite web| title=The Mollusca | publisher=University of California Museum of Paleontology| url=http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/mollusca.php| access-date=2 October 2008}} and it has also been placed close to Matthevia and the shelled aplacophoran Kulindroplax in phylogenetic analyses.{{Cite journal | last1 = Sutton | first1 = M. D. | last2 = Briggs | first2 = D. E. G. | last3 = Siveter | first3 = D. J. | last4 = Siveter | first4 = D. J. | last5 = Sigwart | first5 = J. D. | title = A Silurian armoured aplacophoran and implications for molluscan phylogeny | doi = 10.1038/nature11328 | journal = Nature | volume = 490 | issue = 7418 | pages = 94–97 | year = 2012 | pmid = 23038472| bibcode = 2012Natur.490...94S | s2cid = 4422386 }}