''Phthipodochiton'' is an extinct genus of

mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...

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 in ...

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. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start ...

fauna of the Lady Burn Starfish beds of

Girvan Girvan ( gd, Inbhir Gharbhain, "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, ...

Scotland Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to the ...

. It shows a mixture of

aplacophora Aplacophora is a presumably paraphyletic taxon. This 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 ...

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 is a proposed clade of molluscs incorporating those groups that have no conch or shell, that is, the Polyplacophora, Caudofoveata (= Chaetodermomorpha) and Solenogastres (= Neomeniomorpha). For a summary, see It is sister to the Con ...

n mollusks. It was previously classified under the

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

Helminthochiton ''Helminthochiton'' is an extinct of polyplacophoran mollusc. Helminthochiton became extinct during the Permian period. ''Helminthochiton grayiae'' was recognised as the type species for ''Septemchiton''. This was named for Elizabeth Gray who ...

'', 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 a separate organ at the terminal part of the leg made ...

. 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 (, ; plural radulae or radulas) is an anatomical structure used by molluscs 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 ...

has been preserved.

Life habits

''Phthipodochiton'' was carnivorous, feeding on

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

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

chitons Chitons () are marine molluscs of varying size in the class Polyplacophora (), formerly known as Amphineura. About 940 extant and 430 fossil species are recognized. They are also sometimes known as gumboots or sea cradles or coat-of-mail shel ...

, ''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 the solenogasters, are one class of small, worm-like, shell-less molluscs (Aplacophora), the other class being the Caudofoveata ( Chaetodermomorpha). MolluscaBase eds. (202 ...

Taxonomy

''Phthipodochiton'' shares similarities with genera as '' Alastega'', '' Robustum'' and ''

Septemchiton ''Septemchiton'' is an extinct genus of polyplacophoran molluscs. ''Septemchiton'' became extinct during the Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geolog ...

''. 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'', For a summary, see and it has also been placed close to '' Matthevia'' and the shelled aplacophoran '' Kulindroplax'' in phylogenetic analyses.

References

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