''Kulindroplax perissokomos'' is a

Silurian The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...

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

, known from a single

fossil 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 preserve ...

from the

Coalbrookdale Formation Coalbrookdale Formation, earlier known as Wenlock Shale or Wenlock Shale Formation and also referred to as Herefordshire Lagerstätte in palaeontology, is a fossil-rich deposit ('' Konservat-Lagerstätte'') in Powys and Herefordshire at the Engl ...

fauna of

England England is a Countries of the United Kingdom, country that is part of the United Kingdom. It is located on the island of Great Britain, of which it covers about 62%, and List of islands of England, more than 100 smaller adjacent islands. It ...

. It lived during the

Homerian In the geologic timescale, the Homerian is an age of the Wenlock Epoch of the Silurian Period of the Paleozoic Era of the Phanerozoic Eon that is comprehended between 430.5 ± 0.7 Ma and 427.4 ± 0.5 Ma (million years ago), approximately. ...

Age Age or AGE may refer to: Time and its effects * Age, the amount of time someone has been alive or something has existed ** East Asian age reckoning, an Asian system of marking age starting at 1 * Ageing or aging, the process of becoming older ...

(about 425

million years ago Million years ago, abbreviated as Mya, Myr (megayear) or Ma (megaannum), is a unit of time equal to (i.e. years), or approximately 31.6 teraseconds. Usage Myr is in common use in fields such as Earth science and cosmology. Myr is also used w ...

). It is considered a basal

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. Unlike all modern aplacophorans, which are shell-less, ''Kulindroplax'' has a

chiton 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 sea cradles or coat-of-mail shells or suck ...

-like shell, and it is considered a

transitional fossil A transitional fossil is any fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group. This is especially important where the descendant group is sharply differentiated by gross ...

in the

evolution Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...

of molluscs. The only known specimen, described in 2012, was conserved at the

Oxford University Museum of Natural History The Oxford University Museum of Natural History (OUMNH) is a museum displaying many of the University of Oxford's natural history specimens, located on Parks Road in Oxford, England. It also contains a lecture theatre which is used by the univers ...

Morphology

''Kulindroplax'' is about wide and long. It is the first known mollusk showing an unambiguous combination of

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, ...

, or exterior shells, and a worm-like body. It bears seven similar, unarticulated valves, with a shorter head valve and a taller caudal one, lacking ornaments and also several densely packed, long

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 ( ...

. It has no discernible

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 ...

, and the

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 ...

is not preserved. A

gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...

array is present, along with a respiratory cavity opening posteriorly. These features make it more reminiscent of caudofoveate aplacophorans. While

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 usually bear eight dorsal valves (except for

multiplacophora Multiplacophora is a stem-group of chitons with a number of plates arranged in 7 rows along the body. They date to at least the Upper Cambrian, but two lower Cambrian fossil A fossil (from Classical Latin , ) is any preserved remains, ...

ns and ''

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 ...

''), ''Kulindroplax'' bears only seven in a single

-like row.

Life habits

''Kulindroplax'' seems adapted to moving through a substrate, with the spicules acting as "sediment ratchets". 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.

Evolutionary significance

''Kulindroplax'' settles a 20-year-long dispute about the

phylogeny A phylogenetic tree or phylogeny is a graphical representation which shows the evolutionary history between a set of species or Taxon, taxa during a specific time.Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, M ...

of mollusks, namely the relationship of the worm-like, carnivorous

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 ...

within the group, in particular their relationship to the Polyplacophora. Aplacophorans have been historically variously treated as either a mollusk base-group, distant relatives of

Cephalopoda A cephalopod is any member of the molluscan class Cephalopoda ( Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, ...

or as a sister group to Polyplacophora in the

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

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 ...

. Both molecular and fossil evidence appeared to support the last hypothesis. For a summary, see 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 Conchifera is a subphylum of the phylum Mollusca, containing five extant classes: Monoplacophora, Cephalopoda, Gastropoda, Bivalvia, and Scaphopoda. Conchiferans can bear a single shell as in snails and ammonites, a single pair of shell ...

, in all variants of the analysis, thus bringing the fossil record in line with the recent molecular evidence. While other fossil taxa like ''

'' 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 A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

of mollusk phylogeny according to Sutton ''et al.'', 2012.

Taxa In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...

marked with † are

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

References

{{Taxonbar, from=Q6443023 Monotypic prehistoric mollusc genera Aplacophorans Silurian molluscs Transitional fossils Wenlock series fossils Silurian animals of Europe