''Acheronauta'' is a genus of extinct worm-shaped

arthropod Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...

that lived in the early

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 ...

(

Telychian In the geologic timescale, the Telychian is the third and final age of the Llandovery Epoch of the Silurian Period of the Paleozoic Era of the Phanerozoic Eon. The Telychian Age was between 438.5 ± 1.2 million years ago (Ma) and 433.4 ± 0.8 M ...

Sheinwoodian In the geologic timescale, the Sheinwoodian is the age of the Wenlock Epoch of the Silurian Period of the Paleozoic Era of the Phanerozoic Eon that is comprehended between 433.4 ± 0.8 Ma and 430.5 ± 0.7 Ma (million years ago), approximately ...

stages)

Waukesha biota The Waukesha Biota (also known as Waukesha Lagerstätte, Brandon Bridge Lagerstätte, or Brandon Bridge fauna) is an important fossil site located in Waukesha County and Franklin, Milwaukee County within the state of Wisconsin. This biota is pr ...

fossil site in southeast

Wisconsin Wisconsin ( ) is a U.S. state, state in the Great Lakes region, Great Lakes region of the Upper Midwest of the United States. It borders Minnesota to the west, Iowa to the southwest, Illinois to the south, Lake Michigan to the east, Michig ...

. This arthropod was first discovered alongside the biota in 1985, but was not fully described until October 2022. This creature was recognized and described as a possible early

mandibulate The clade Mandibulata constitutes one of the major subdivisions of the phylum Arthropoda, alongside Chelicerata. Mandibulates include the crustaceans, myriapods (centipedes and millipedes, among others), and all true insects. The name "Mandibulat ...

(the grouping of arthropods including

crustacean Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...

s and hexapods). This description is very important as much of the

fauna Fauna (: faunae or faunas) is all of the animal life present in a particular region or time. The corresponding terms for plants and fungi are ''flora'' and '' funga'', respectively. Flora, fauna, funga and other forms of life are collectively ...

of the biota remain undescribed, and its discovery has allowed for

paleontologists Paleontology, also spelled as palaeontology or palæontology, is the scientific study of the life of the past, mainly but not exclusively through the study of fossils. Paleontologists use fossils as a means to classify organisms, measure geolo ...

to get a better grasp of the diversity of the arthropod fauna at the site. Multiple phylogenetic analyses were performed, and it was found that this arthropod forms a previously undiscovered

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 ...

with the

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

stem-arthropod '' Captopodus'', and the somewhat enigmatic group

Thylacocephala The Thylacocephala (from the Greek or ', meaning " pouch", and or ' meaning "head") are group of extinct probable mandibulate arthropods, that have been considered by some researchers as having possible crustacean affinities. As a class they ...

. ''Acheronauta'' has been assessed as a possible basal mandibulate, which are distinguished from other arthropods due to the presence of

mandibles In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...

. While its specific placement in the mandibulate family tree has not been fully known, it is accepted to occupy a position near the base of the grouping.

Discovery and naming

Before it was named, ''Acheronauta'' was recognized as one of the most abundant arthropod

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 ...

s from the biota (only behind the dalmantid

trilobite Trilobites (; meaning "three-lobed entities") are extinction, extinct marine arthropods that form the class (biology), class Trilobita. One of the earliest groups of arthropods to appear in the fossil record, trilobites were among the most succ ...

Waukeshaaspis ''Waukeshaaspis'' is an extinct genus of trilobite (a diverse group of marine arthropods) known from the lower Silurian aged Waukesha Biota. A single species is currently known, ''Waukeshaaspis eatonae'', which is known from strata belonging to t ...

'', and several leperditicopid

ostracod Ostracods, or ostracodes, are a Class (biology), class of the crustacean, Crustacea (class Ostracoda), sometimes known as seed shrimp. Some 33,000 species (only 13,000 of which are extant taxon, extant) have been identified,Brandão, S.N.; Antoni ...

s). Leading up to its description, this arthropod was not well studied, and remained an enigmatic genus. About twenty-three fossils are known of this arthropod that were found in the 1980s and 1990s due to the initial

quarrying A quarry is a type of open-pit mine in which dimension stone, rock, construction aggregate, riprap, sand, gravel, or slate is excavated from the ground. The operation of quarries is regulated in some jurisdictions to manage their s ...

activity that revealed the

lagerstätte A Fossil-Lagerstätte (, from ''Lager'' 'storage, lair' '' Stätte'' 'place'; plural ''Lagerstätten'') is a sedimentary deposit that preserves an exceptionally high amount of palaeontological information. ''Konzentrat-Lagerstätten'' preserv ...

. Currently these fossils are housed in the UWGM, along with many of the other fossils from the site. The fossils of this creature were originally assumed to be thylacocephalans without a whole-body carapace. ''Acheronauta'' derives from the

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

word ''Acheronta'', which is the latin version of the Greek word ''Acheron'', which itself is another name for the River of Woe, a

river A river is a natural stream of fresh water that flows on land or inside Subterranean river, caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of ...

of the

Greek underworld In Greek mythology, the underworld or Hades () is a distinct realm (one of the three realms that make up the cosmos) where an individual goes after death. The earliest idea of afterlife in Greek myth is that, at the moment of death, an individu ...

. The last part of 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 ...

name ''Nauta'' means "sailor". ''Acheronauta'' thus is translated as ‘sailor on the River of Woe'. This name is a reference to the harsh environments of Silurian Waukesha that allowed for this site to be preserved. The

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), ...

name ''Stimulapis'' derives from the latin words ''Stimulas'' which means sting, and ''apis'' which means

bee Bees are winged insects closely related to wasps and ants, known for their roles in pollination and, in the case of the best-known bee species, the western honey bee, for producing honey. Bees are a monophyletic lineage within the superfamil ...

. The specific

epithet An epithet (, ), also a byname, is a descriptive term (word or phrase) commonly accompanying or occurring in place of the name of a real or fictitious person, place, or thing. It is usually literally descriptive, as in Alfred the Great, Suleima ...

translates to ‘sting like a bee’, which according to the authors of the paper is in honor of the American

boxer Boxer most commonly refers to: *Boxer (boxing), a competitor in the sport of boxing * Boxer (dog), a breed of dog Boxer or boxers may also refer to: Animal kingdom * Boxer crab * Boxer shrimp, a small group of decapod crustaceans * Boxer snipe ee ...

Muhammad Ali Muhammad Ali (; born Cassius Marcellus Clay Jr.; January 17, 1942 – June 3, 2016) was an American professional boxer and social activist. A global cultural icon, widely known by the nickname "The Greatest", he is often regarded as the gr ...

. Many of the specimens of ''A.'' ''stimulapis'' do not preserve the full anatomy of the arthropod, however the large number of specimens helped paleontologists reconstruct it.

Description

The twenty-three or so specimens of this arthropod show it had two tagmata, a

head A head is the part of an organism which usually includes the ears, brain, forehead, cheeks, chin, eyes, nose, and mouth, each of which aid in various sensory functions such as sight, hearing, smell, and taste. Some very simple ani ...

covering

carapace A carapace is a dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tortoises, the unde ...

that bore a number of head

appendage An appendage (or outgrowth) is an external body part or natural prolongation that protrudes from an organism's body such as an arm or a leg. Protrusions from single-celled bacteria and archaea are known as cell-surface appendages or surface app ...

s, and an elongated multisegmented trunk region that was composed of around forty-four pairs of segments that bore small swimming appendages. The carapace of the

organism An organism is any life, living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have be ...

was small, only shielding the head and the first segments of the main body. This feature looks like the bivalved carapaces of other arthropods like the hymenocarines, but due to the lack of dorsally oriented fossils means that there is no confirmation of the presence of a

hinge line A hinge line is an imaginary longitudinal line along the dorsal edge of the shell of a bivalve mollusk where the two valves hinge or articulate. The hinge line can easily be perceived in these images of a mussel shell and an ark shell. The hing ...

. On the anterior area of the head was an ocular

somite The somites (outdated term: primitive segments) are a set of bilaterally paired blocks of paraxial mesoderm that form in the embryogenesis, embryonic stage of somitogenesis, along the head-to-tail axis in segmentation (biology), segmented animals. ...

that bore a pair of teardrop-shaped

eye An eye is a sensory organ that allows an organism to perceive visual information. It detects light and converts it into electro-chemical impulses in neurons (neurones). It is part of an organism's visual system. In higher organisms, the ey ...

s that possessed

facet Facets () are flat faces on geometric shapes. The organization of naturally occurring facets was key to early developments in crystallography, since they reflect the underlying symmetry of the crystal structure. Gemstones commonly have facets cu ...

s. The presence of a

mandible In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...

in this creature seems to provide more evidence for a mandibulate affinity. The mandibles found display rows of simple teeth that sat on what looks like a grinding surface. Behind the somite were five head appendages with the first set being arthrodized antennae, mandibles, and the others being posterior biramous appendages that appear to end in a single

claw A claw is a curved, pointed appendage found at the end of a toe or finger in most amniotes (mammals, reptiles, birds). Some invertebrates such as beetles and spiders have somewhat similar fine, hooked structures at the end of the leg or Arthro ...

. The first three segments of the trunk region are smaller than the others. Each trunk segment bore a pair of small, paddle-shaped swimming appendages.

Organ Organ and organs may refer to: Biology * Organ (biology), a group of tissues organized to serve a common function * Organ system, a collection of organs that function together to carry out specific functions within the body. Musical instruments ...

materials are not observed from this species, except in some cases of there being poorly preserved gut tracts. There is also some evidence for

muscle fibers Skeletal muscle (commonly referred to as muscle) is one of the three types of vertebrate muscle tissue, the others being cardiac muscle and smooth muscle. They are part of the somatic nervous system, voluntary muscular system and typically are a ...

being present within the head of some of the fossils. While there is not enough evidence to make the argument of there being two species, there are differences in some of the fossils of this arthropod that the authors have labeled as "Morph A" and "Morph B". The main distinction between these two morphs is the shape of the trunk terminus. The trunk of "Morph A" fossils is shorter than that of "Morph B" fossils, showing fewer than thirty appendages. The second distinction is the shape of the carapace. "Morph A" fossils show a blunt anterior margin, with the eyes extending beyond said margin. "Morph B" fossils on the other hand show a pronounced anterodorsal keel coming off of this margin, with the carapace being slightly deeper, and the eyes being slightly more posterior in location.

Classification

Three versions of a Bayesian phylogenetic analysis were cunducted because of the ambiguity of some of the features of ''A.'' ''stimulapis''. Coded within these analyses were more than a hundred taxa of arthropods, including ''Captopodus poschmanni'', and the thylacocephalans. Interestingly, the authors of the paper also coded in ''Parioscorpio venator'', an enigmatic arthropod that coexisted with ''A.'' ''stimulapis''. The studies originally provided evidence for this genus being related to either the thylacocephalans or the

remipedes Remipedia is a class (biology), class of blind crustaceans, closely related to Hexapoda, hexapods, found in coastal aquifers which contain saline groundwater, with populations identified in almost every ocean basin so far explored, including in A ...

(a grouping of vermiform crustaceans found in underground bodies of water), however remipedes don't have a carapace and only possess one tagma. The other analyses show definite evidence of it being related to, and forming a clade with ''C. poschmanni'' and the thylacocephalans. The study placed these three taxa as close to the base of the mandibulate family tree, being more basal than the hymenocarines. The primary interpretation showed this genus as a sister group to the thylacocephalans and ''C. poschmanni''. This study found that instead of being members of crustacea or eucrustcea, the thylacocephalans were found to be placed outside of the crown-group crustaceans and myirapods as stem-group mandibulates. Additionally, ''Parioscorpio'', which has remained enigmatic for over four years since its description in 2020, was found to occupy a position between the

Artiopoda Artiopoda is a clade of extinct arthropods that includes trilobites and their close relatives. It was erected by Hou and Bergström in 1997 to encompass a wide diversity of arthropods that would traditionally have been assigned to the Trilobitomor ...

and mandibulates, making it slightly related to ''A.'' ''stimulapis'', and a basal taxon to the total-group mandibulata. This is actually consistent with the recent rejection of this genus being a

cheloniellid Cheloniellida is a taxon (usually referred to as an order) of extinct Paleozoic arthropods. As of 2018,Wendruff, Andrew James, et al. "New cheloniellid arthropod with large raptorial appendages from the Silurian of Wisconsin, USA." BioRxiv (2018) ...

within the artiopoda.

Autecology and paleoecology

Based on certain characteristics of this creature (including the specialization of its appendages, and its large size compared to many of the other taxa from the lagerstätte), this arthropod was found to be a

scavenger Scavengers are animals that consume Corpse decomposition, dead organisms that have died from causes other than predation or have been killed by other predators. While scavenging generally refers to carnivores feeding on carrion, it is also a he ...

as well as an opportunistic predator. The

exopod 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 ...

s and

endopod 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, : ...

s on the appendages of this arthropod may have functioned like a

masticatory Chewing or mastication is the process by which food is crushed and ground by the teeth. It is the first step in the process of digestion, allowing a greater surface area for digestive enzymes to break down the foods. During the mastication proc ...

basket similar to that of most modern crustaceans. On the other hand, the large size of the appendages and their lack of

seta In biology, setae (; seta ; ) are any of a number of different bristle- or hair-like structures on living organisms. Animal setae Protostomes Depending partly on their form and function, protostome setae may be called macrotrichia, chaetae, ...

e would have probably limited the arthropod to the

niche Niche may refer to: Science *Developmental niche, a concept for understanding the cultural context of child development and growth *Ecological niche, a term describing the relational position of an organism's species *Niche differentiation, in ec ...

of ensnaring small prey, and not

suspension feeding Filter feeders are aquatic animals that acquire nutrients by feeding on organic matters, food particles or smaller organisms (bacteria, microalgae and zooplanktons) suspended in water, typically by having the water pass over or through a specia ...

. This arthropod lived alongside many other unique taxa that made up the fauna of the biota. Living in

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...

parts of

intertidal The intertidal zone or foreshore is the area above water level at low tide and underwater at high tide; in other words, it is the part of the littoral zone within the tidal range. This area can include several types of habitats with various sp ...

areas were arthropods like ''Parioscorpio'', and the

synziphosurine Synziphosurina is a paraphyletic group of chelicerate arthropods previously thought to be basal horseshoe crabs (Xiphosura). It was later identified as a grade composed of various basal euchelicerates, eventually excluded from the monophyletic Xi ...

arthropod ''Venustulus'' ''waukeshaensis''. Also living in these areas were a wide variety of

worm Worms are many different distantly related bilateria, bilateral animals that typically have a long cylindrical tube-like body, no limb (anatomy), limbs, and usually no eyes. Worms vary in size from microscopic to over in length for marine ...

s like

annelid The annelids (), also known as the segmented worms, are animals that comprise the phylum Annelida (; ). The phylum contains over 22,000 extant species, including ragworms, earthworms, and leeches. The species exist in and have adapted to vario ...

palaeoscolecid The palaeoscolecids are a group of extinct ecdysozoan worms resembling armoured priapulids. They are known from the Lower Cambrian to the lower Ludfordian (late Silurian); they are mainly found as disarticulated sclerites, but are also preserved ...

s, and

polychaete Polychaeta () is a paraphyletic class of generally marine Annelid, annelid worms, common name, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called c ...

s. Possibly hunting in the midwater was ''Thylacares brandonensis'', a thylacocephalan arthropod, and in terms of

geographical Geography (from Ancient Greek ; combining 'Earth' and 'write', literally 'Earth writing') is the study of the lands, features, inhabitants, and phenomena of Earth. Geography is an all-encompassing discipline that seeks an understanding o ...

location, the closest relative to ''A.'' ''stimulapis''. Living in the area were a number of bivalved arthropods like

phyllocarid Phyllocarida is a subclass of crustaceans, comprising the extant order Leptostraca and the extinct orders Hymenostraca and Archaeostraca. This clade of marine crustaceans diversified extensively during the Ordovician The Ordovician ( ) is a ...

and ostracod crustceans. Also found in the area was the enigmatic "Butterfly animal", which is an undescribed arthropod with unknown taxonomic affiliates. Hard shelled

s like

nautiloid Nautiloids are a group of cephalopods (Mollusca) which originated in the Late Cambrian and are represented today by the living ''Nautilus'' and ''Allonautilus''. Fossil nautiloids are diverse and species rich, with over 2,500 recorded species. Th ...

cephalopods,

brachiopod Brachiopods (), phylum (biology), phylum Brachiopoda, are a phylum of animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear e ...

clam Clam is a common name for several kinds of bivalve mollusc. The word is often applied only to those that are deemed edible and live as infauna, spending most of their lives halfway buried in the sand of the sea floor or riverbeds. Clams h ...

coral Corals are colonial marine invertebrates within the subphylum Anthozoa of the phylum Cnidaria. They typically form compact Colony (biology), colonies of many identical individual polyp (zoology), polyps. Coral species include the important Coral ...

crinoid 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 ...

s, and

sponges Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a basal clade and a sister taxon of the diploblasts. They are sessile filter feeders that are bound to the seabed, and ar ...

are rare in these sediments, and the ones that are found are not well preserved. This is strange because these organisms are normally common in Silurian aged sediments. The reason for this is due to the unique

preservation bias Taphonomy is the study of how organisms decay and become fossilized or preserved in the paleontological record. The term ''taphonomy'' (from Greek , 'burial' and , 'law') was introduced to paleontology in 1940 by Soviet scientist Ivan Efremov ...

this site has, where the preservation of lightly skeletonized or soft bodied fossils was more favored than in other conditions. Aside from trilobites the only other group of hard shelled organisms that were common in these sediments were the conulariids, an enigmatic group of shelled invertebrates that were most likely

cnidaria Cnidaria ( ) is a phylum under kingdom Animalia containing over 11,000 species of aquatic invertebrates found both in fresh water, freshwater and marine environments (predominantly the latter), including jellyfish, hydroid (zoology), hydroids, ...

ns closely related to

stalked jellyfish Stauromedusae are the stalked jellyfishes. They are the sole living members of the class Staurozoa and belong to the medusozoa subphylum of Cnidaria. They are unique among medusa jellyfish in that they do not have an alternation of polyp and med ...

. Predatory

chordate A chordate ( ) is a bilaterian animal belonging to the phylum Chordata ( ). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics ( synapomorphies) that distinguish them from ot ...

s, like '' Panderodus'' are also known from these deposits. Other chordates, including potential conodonts have also been found.

References

{{Taxonbar, from=Q115519031 Fossil taxa described in 2022 Monotypic prehistoric arthropod genera Silurian arthropods of North America