''Pentapantopus'' is a

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 ...

of fossil pycnogonid (

sea spider Sea spiders are marine arthropods of the class (biology), class Pycnogonida, hence they are also called pycnogonids (; named after ''Pycnogonum'', the type genus; with the suffix '). The class includes the only now-living order (biology), order P ...

). The only known

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

is ''Pentapantopus vogteli'' from the

Hunsrück Slate The Hunsrück () is a long, triangular, pronounced upland in Rhineland-Palatinate, Germany. It is bounded by the valleys of the Moselle-Saar (north-to-west), the Nahe (south), and the Rhine (east). It is continued by the Taunus mountains, past ...

Germany Germany, officially the Federal Republic of Germany, is a country in Central Europe. It lies between the Baltic Sea and the North Sea to the north and the Alps to the south. Its sixteen States of Germany, constituent states have a total popu ...

. This sea spider was thought to have had five pairs of legs; however, a 2024 study disproved this. It is recognizable by its flatten, tuberculated legs with the first pair having less segments than other sea spiders.

Description

''Pentapantopus'' is a relatively small pycnogonid with a measured body length of up to 1.2 cm. It somewhat resembles the

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 ...

Haliestes ''Haliestes'' is a genus of sea spider (pycnogonid) from the Silurian aged Coalbrookdale Formation of England. It contains a single species, ''Haliestes dasos''. The species was first described by David Siveter et al. in 2004. Morphology ''H ...

'', which was used to infer various details that are not well-preserved (e.g. cephalon, palps, ovigers, leg annulations, abdomen) on the former's redescription in 2024. The cephalon is poorly preserved. The narrowed trunk has lateral processes measured as long as wide. The segmentation of the reduced abdomen is unclear, but it might have had 3 segments based on ''Haliestes''. The

proboscis A proboscis () is an elongated appendage from the head of an animal, either a vertebrate or an invertebrate. In invertebrates, the term usually refers to tubular arthropod mouthparts, mouthparts used for feeding and sucking. In vertebrates, a pr ...

was folded underneath its body, which might reflect its mobility. The chelifores have at least 3 segments (

podomere The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments (called podomeres) are of Latin origin, and may be confused with terms for bones: ''coxa'' (meaning hip, : ...

): a 2-segmented

pincer Pincer may refer to: *Pincers (tool) *Pincer (biology), part of an animal *Pincer ligand, a terdentate, often planar molecule that tightly binds a variety of metal ions *Pincer (Go), a move in the game of Go *"Pincers!", an episode of the TV series ...

and an unsegmented scape, although the original description identified a 2-segmented scape. The

palps Pedipalps (commonly shortened to palps or palpi) are the secondary pair of forward appendages among chelicerates – a group of arthropods including spiders, scorpions, horseshoe crabs, and sea spiders. The pedipalps are lateral to the chelicera ...

and ovigers have poorly preserved segmentation, although the former may have a terminal claw. When ''Pentapantopus'' was first described in 2013, it was thought to be a polymerous (extra-legged) species that had 5 pairs of legs (hence the name). However, with the description of another specimen in 2024, this has been refuted in favour of the common arrangement of 4 pairs of legs, as the previous "fifth leg" in the incomplete specimen was likely a misinterpretation of the opposite fourth leg. Each leg begins with a possibly annulated coxa and ends with a long, hook-like terminal claw. Each podomere from the fourth segment is wide and flattened, with most of them bearing tubercles and sparse pairs of long setae along the inner side. Within this section, the first pair of walking legs has one less segment than the other three, resulting in a total count of 7 segments for the first leg, which is unusual for a pycnogonid, in contrast to the usual 8 segments of the remaining 3 pairs.

Taxonomy

In the original description, ''Pentapantopus'' was thought to be a

crown-group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...

pycnogonid (Pantopoda), based on the purported polymerous legs, a rare but derived feature only occurring in Pantopoda. This was questioned by the redescription in 2024, as there were considered to be only 4 pairs of legs, and other newly discovered features (alternated leg segments and the possibility of bearing clawed palps, annulated coxae and segmented abdomen) suggest it is unlikely to be a pantopod. Both the original paper and redescription agreed on its similarities to ''Haliestes'', the latter also suggest their legs are the same type as ''

Palaeoisopus ''Palaeoisopus'' is a genus of fossil pycnogonid (sea spider). The only known species is ''Palaeoisopus problematicus'' from the Lower Devonian Hunsrück Slate of Germany. It is characterized by several features unusual for a pycnogonid, such as ...

'', but it is uncertain if this type represents a

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 ...

evolutionary grade A grade is a taxon united by a level of morphological or physiological complexity. The term was coined by British biologist Julian Huxley, to contrast with clade, a strictly phylogenetic unit. Phylogenetics The concept of evolutionary grades ...

stem-group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...

Pycnogonida.

Etymology

The name ''Pentapantopus'' derives from the word "penta" meaning "5” in reference to the belief that it had five pairs of walking legs, alongside "pantopus", a common suffix for sea spiders. The species name ''vogteli'' honours Hans Vogtel, a former slate worker who helped find numerous fossils during the process of roof-slate production.

References

{{Taxonbar, from=Q107252665 Pycnogonid genera Hunsrück Slate fossils Devonian arthropods of Europe Fossil taxa described in 2013 Monotypic chelicerate genera Fossils of Germany