''Phthipodochiton'' is an extinct genus of

mollusc Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...

s, known from several

fossils A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

from the upper

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

fauna of the Lady Burn Starfish beds of

Girvan Girvan (, "mouth of the River Girvan") is a burgh and harbour town in Carrick, South Ayrshire, Scotland. Girvan is situated on the east coast of the Firth of Clyde, with a population of about 6,450. It lies south of Ayr, and north of St ...

Scotland Scotland is a Countries of the United Kingdom, country that is part of the United Kingdom. It contains nearly one-third of the United Kingdom's land area, consisting of the northern part of the island of Great Britain and more than 790 adjac ...

. It shows a mixture of

aplacophora Aplacophora is a class of small, deep-water, exclusively benthic, marine molluscs found in all oceans of the world. MolluscaBase eds. (2021). MolluscaBase. Aplacophora. Accessed through: World Register of Marine Species at: http://www.marinespec ...

n body plan and polyplacophoran-like

valves A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fittings, ...

, and it is an informative fossil in the evolution of

aculifera Aculifera (older name: Amphineura) is a clade of molluscs incorporating those groups that have no conch or shell, that is, the Polyplacophora, Caudofoveata and Solenogastres. For a summary, see It is a sister group to the Conchifera. Monophyly ...

n mollusks. It was previously classified under the

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

'' Helminthochiton'', 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 The foot (: feet) is an anatomical structure found in many vertebrates. It is the terminal portion of a limb which bears weight and allows locomotion. In many animals with feet, the foot is an organ at the terminal part of the leg made up o ...

, 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 Spicules are any of various small needle-like anatomical structures occurring in organisms Spicule may also refer to: *Spicule (sponge), small skeletal elements of sea sponges *Spicule (nematode), reproductive structures found in male nematodes ( ...

; no

radula The radula (; : radulae or radulas) is an anatomical structure used by mollusks for feeding, sometimes compared to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters ...

has been preserved.

Life habits

''Phthipodochiton'' was carnivorous, feeding on

crinoids Crinoids are marine invertebrates that make up the class Crinoidea. Crinoids that remain attached to the sea floor by a stalk in their adult form are commonly called sea lilies, while the unstalked forms, called feather stars or comatulids, are ...

, as shown by a fossil preserved with gut contents. In contrast with modern

chitons Chitons () are marine (ocean), marine molluscs of varying size in the class (biology), class Polyplacophora ( ), formerly known as Amphineura. About 940 Extant taxon, extant and 430 fossil species are recognized. They are also sometimes known ...

, ''Phthipodochiton'' probably did not creep on its foot but had a locomotion style similar to that of

solenogastres The Solenogastres (less often referred to as Neomeniomorpha), common name, commonly known as solenogasters, are a class of small, worm-like, shell-less Mollusca, molluscs (Aplacophora), the other class being the Caudofoveata (Chaetodermomorpha).M ...

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 ''Acaenoplax'' is an extinct worm-shaped mollusc known from the Coalbrookdale Formation of Herefordshire, England. It lived in the Silurian The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Or ...

'', For a summary, see and it has also been placed close to ''

Matthevia ''Matthevia'' is a genus of Cambrian molluscs, perhaps related to the chiton Chitons () are marine molluscs of varying size in the class Polyplacophora ( ), formerly known as Amphineura. About 940 extant and 430 fossil species are recogniz ...

'' and the shelled aplacophoran '' Kulindroplax'' in phylogenetic analyses.

References

{{Taxonbar, from=Q7188221 Monotypic prehistoric chiton genera Ordovician molluscs Transitional fossils Fossils of Great Britain