Acheronauta

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Acheronauta
Temporal range: Telychian–Sheinwoodian
Acheronauta-B.png
Reconstruction of A. stimulapis (based on Morph B specimens)
Acheronauta-A.png
Reconstruction of Morph A
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Genus:† Acheronauta
Pulsipher, 2022
Species:
†A. stimulapis
Binomial name
†Acheronauta stimulapis
Pulsipher, 2022

Acheronauta is a genus of extinct worm-shaped arthropod that lived in the early Silurian (Telychian-Sheinwoodian stages) Waukesha biota fossil site in southeast Wisconsin. [1] This arthropod was first discovered alongside the biota in 1985, but was not fully described until October 2022. [1] [2] [3] This creature was recognized and described as a possible early mandibulate (the grouping of arthropods including crustaceans and hexapods). [1] This description is very important as much of the fauna of the biota remain undescribed, and its discovery has allowed for paleontologists to get a better grasp of the diversity of the arthropod fauna at the site. [1] Multiple phylogenetic analyses were performed, and it was found that this arthropod forms a previously undiscovered clade with the Devonian stem-arthropod Captopodus , and the somewhat enigmatic group Thylacocephala. [1]

Contents

Acheronauta has been assessed as a possible basal mandibulate, which are distinguished from other arthropods due to the presence of mandibles. [4] 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. [1]

Discovery and naming

Before it was named, Acheronauta was recognized as one of the most abundant arthropod fossils from the biota (only behind a so far undescribed dalmantid trilobite, and several leperditicopid ostracods). Leading up to its description, this arthropod was not well studied, and remained an enigmatic genus. [3] About twenty-three fossils are known of this arthropod that were found in the 1980s and 1990s due to the initial quarrying activity that revealed the lagerstätte. Currently these fossils are housed in the UWGM, along with many of the other fossils from the site. [3] [2] The fossils of this creature were originally assumed to be thylacocephalans without a whole-body carapace. [1]

Acheronauta derives from the Latin word Acheronta, which is the latin version of the Greek word Acheron, which itself is another name for the River of Woe, a river of the Greek underworld. The last part of the genus 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. [1] [3] The species name Stimulapis derives from the latin words Stimulas which means sting, and apis which means bee. The specific epithet translates to ‘sting like a bee’, which according to the authors of the paper is in honor of the American boxer Muhammad Ali. [1]

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. [1]

Description

The twenty-three or so specimens of this arthropod show it had two tagmata, a head covering carapace that bore a number of head appendages, 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 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. On the anterior area of the head was an ocular somite that bore a pair of teardrop-shaped eyes that possessed facets. The presence of a mandible in this creature seems to provide more evidence for a mandibulate affinity. [1] 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. 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 materials are not observed from this species, except in some cases of there being poorly preserved gut tracts. [1] There is also some evidence for muscle fibers 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. [1]

Classification

Arthropoda

Three versions of a Bayesian phylogenetic analysis were cunducted because of the ambiguity of some of the features of A.stimulapis. [1] 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. [5] The studies originally provided evidence for this genus being related to either the thylacocephalans or the remipedes (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. [1] 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, [5] [6] was found to occupy a position between the Artiopoda and mandibulates, making it slightly related to A.stimulapis, and a basal taxon to the total-group mandibulata. [1] This is actually consistent with the recent rejection of this genus being a cheloniellid within the artiopoda. [7]

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 as well as an opportunistic predator. The exopods and endopods on the appendages of this arthropod may have functioned like a masticatory basket similar to that of most modern crustaceans. On the other hand, the large size of the appendages and their lack of setae would have probably limited the arthropod to the niche of ensnaring small prey, and not suspension feeding. [1]

This arthropod lived alongside many other unique taxa that made up the fauna of the biota. [3] Living in benthic parts of intertidal areas were arthropods like Parioscorpio, and the synziphosurine arthropod Venustuluswaukeshaensis. Also living in these areas were a wide variety of worms like annelids, palaeoscolecids, and polychaetes. [1] [3] [8] Possibly hunting in the midwater was Thylacares brandonensis, a thylacocephalan arthropod, and in terms of geographical location, the closest relative to A.stimulapis. [9] Living in the area were a number of bivalved arthropods like phyllocarid and ostracod crustceans. [10] [3] Also found in the area was the enigmatic "Butterfly animal", which is an undescribed arthropod with unknown taxonomic affiliates. [2] Hard shelled organisms like nautiloid cephalopods, brachiopods, clams, corals, crinoids, and sponges are rare in these sediments, and the ones that are found are not well preserved. [2] [3] This is strange because these organisms are normally common in Silurian aged sediments. [2] The reason for this is due to the unique preservation bias this site has, where the preservation of lightly skeletonized or soft bodied fossils was more favored than in other conditions. [2] 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 cnidarians closely related to stalked jellyfish. [3] [11] Predatory chordates, like Panderodus are also known from these deposits. Other chordates, including potential conodonts have also been found. [2] [12]

Related Research Articles

<span class="mw-page-title-main">Cheloniellida</span> Order of arthropods (fossil)

Cheloniellida is a taxon of extinct Paleozoic arthropods. As of 2018, 7 monotypic genera of cheloniellids had been formally described, whose fossils are found in marine strata ranging from Ordovician to Devonian in age. Cheloniellida has a controversial phylogenetic position, with previous studies associated it as either a member or relative of various fossil and extant arthropod taxa. It was later accepted as a member of Vicissicaudata within Artiopoda.

<span class="mw-page-title-main">Hunsrück Slate</span> Geological formation in Germany

The Hunsrück Slate is a Lower Devonian lithostratigraphic unit, a type of rock strata, in the German regions of the Hunsrück and Taunus. It is a lagerstätte famous for exceptional preservation of a highly diverse fossil fauna assemblage.

In the geological timescale, the Llandovery Epoch occurred at the beginning of the Silurian Period. The Llandoverian Epoch follows the massive Ordovician-Silurian extinction events, which led to a large decrease in biodiversity and an opening up of ecosystems.

<span class="mw-page-title-main">Thylacocephala</span> Extinct group of arthropods

The Thylacocephala are group of extinct probable mandibulate arthropods, that have been considered by some researchers as having possible crustacean affinities. As a class they have a short research history, having been erected in the early 1980s.

<i>Waptia</i> Cambrian arthropod

Waptia is an extinct genus of arthropod from the Middle Cambrian of North America. It grew to a length of 6.65 cm (3 in), and had a large bivalved carapace and a segmented body terminating into a pair of tail flaps. It was an active swimmer and likely a predator of soft-bodied prey. It is also one of the oldest animals with direct evidence of brood care. Waptia fieldensis is the only species classified under the genus Waptia, and is known from the Burgess Shale Lagerstätte of British Columbia, Canada. Specimens of Waptia are also known from the Spence Shale of Utah, United States.

<i>Occacaris</i> Extinct genus of arthropods

Occacaris oviformis is an extinct nektonic predatory arthropod from the Lower Cambrian Maotianshan shale Lagerstätte. It bears a superficial resemblance to the Cambrian arthropod, Canadaspis, though, was much smaller, and had a pair of "great appendages", with which it may have grasped prey. It was originally considered to belong to Megacheira, however it is questioned in later study.

<span class="mw-page-title-main">Synziphosurina</span> Group of arthropods

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 form the monophyletic Xiphosura sensu stricto and only regarded as horseshoe crabs under a broader sense. Synziphosurines survived at least since early Ordovician to early Carboniferous in ages, with most species are known from the in-between Silurian strata.

<span class="mw-page-title-main">Paleontology in Wisconsin</span>

Paleontology in Wisconsin refers to paleontological research occurring within or conducted by people from the U.S. state of Wisconsin. The state has fossils from the Precambrian, much of the Paleozoic, some a parts of the Mesozoic and the later part of the Cenozoic. Most of the Paleozoic rocks are marine in origin. Because of the thick blanket of Pleistocene glacial sediment that covers the rock strata in most of the state, Wisconsin’s fossil record is relatively sparse. In spite of this, certain Wisconsin paleontological occurrences provide exceptional insights concerning the history and diversity of life on Earth.

<i>Thylacares</i> Extinct genus of crustaceans from Wisconsin

Thylacares is a genus of Thylacocephalan containing only the single species Thylacares brandonensis. Found in Silurian period strata from the Brandon Bridge Formation in Waukesha, Wisconsin, U.S., the species is distinguishable from other Thylacocephalans by its smaller raptorial appendages and compound eyes. The body is fully encased in a bivalve shell, with only the eyes protruding on stalks. The species' trunk is composed of about 22 segments. It was once considered the oldest know thylacocephalan, before more older specimens were found in Ordovician rocks.

<i>Parioscorpio</i> Extinct genus of enigmatic arthropod

Parioscorpio is an extinct genus of arthropod containing the species P. venator known from the Silurian-aged Waukesha Biota of the Brandon Bridge Formation near Waukesha, Wisconsin. This animal has gone through a confusing taxonomic history, being called an arachnid, crustacean, and an artiopodan arthropod at various points. This animal is one of the more famous fossil finds from Wisconsin, due to the media coverage it received based on its original description in 2020 as a basal scorpion.

<span class="mw-page-title-main">Waukesha Biota</span> Lagerstätte Fossil site in Waukesha County, Wisconsin, U.S.

The Waukesha Biota is an important fossil site located in Waukesha County and Franklin, Milwaukee County within the state of Wisconsin. This biota is preserved in certain strata within the Brandon Bridge Formation, which dates to the early Silurian period. It is known for the exceptional preservation of soft-bodied organisms, including many species found nowhere else in rocks of similar age. The site's discovery was announced in 1985, leading to a plethora of discoveries. This biota is one of the few well studied Lagerstätten from the Silurian, making it important in our understanding of the period's biodiversity. Some of the species are not easily classified into known animal groups, showing that much research remains to be done on this site. Other taxa that are normally common in Silurian deposits are rare here, but trilobites are quite common.

<i>Venustulus</i> Extinct genus of chelicerate from Wisconsin

Venustulus is a genus of synziphosurine, a paraphyletic group of fossil chelicerate arthropods. Venustulus was regarded as part of the clade Prosomapoda. Fossils of the single and type species, V. waukeshaensis, have been discovered in deposits of the Silurian period in Wisconsin, in the United States. Venustulus is one of the few synziphosurine genera with fossil showing evidence of appendages, the other ones being Weinbergina, Anderella and Camanchia. Despite often being aligned close to horseshoe crabs, it has been found that Venustulus and its relatives form a group made up of various basal euchelicerate arthropods more distant to the xiphosurans.

<span class="mw-page-title-main">Hymenocarina</span> Extinct order of arthropods

Hymenocarina is an order of extinct arthropods known from the Cambrian. They possess bivalved carapaces, typically with exposed posteriors. Members of the group are morphologically diverse and had a variety of ecologies, including as filter feeders and as predators. Recent research has generally considered them to be stem or crown group members of Mandibulata, due to the presence of mandibles in some species.

<i>Cheloniellon</i> Extinct genus of arthropods

Cheloniellon is a monotypic genus of cheloniellid arthropod, known only by one species, Cheloniellon calmani, discovered from the Lower Devonian Hunsrück Slate of Germany.

<i>Panderodus</i> A venomous Conodont from the Early Paleozoic

Panderodus Is an extinct genus of jawless fish belonging to the order Conodonta. This genus had a long temporal range, surviving from the middle Ordovician to late Devonian. In 2021, extremely rare body fossils of Panderodus from the Waukesha Biota were described, and it revealed that Panderodus had a more thick body compared to the more slender bodies of more advanced conodonts. It also revealed that this conodont was a macrophagous predator, meaning it went after large prey.

<span class="mw-page-title-main">Coalbrookdale Formation</span> Fossil-rich deposit in the UK

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 England–Wales border in UK. It belongs to the Wenlock Series of the Silurian Period within the Homerian Age. It is known for its well-preserved fossils of various invertebrate animals many of which are in their three-dimensional structures. Some of the fossils are regarded as earliest evidences and evolutionary origin of some of the major groups of modern animals.

<i>Balhuticaris</i> Extinct genus of arthropods

Balhuticaris is a genus of extinct bivalved hymenocarine arthropod that lived in the Cambrian aged Burgess Shale in what is now British Columbia around 506 million years ago. This extremely multisegmented arthropod is the largest member of the group, and it was even one of the largest animals of the Cambrian, with individuals reaching lengths of 245 mm (9 in). Fossils of this animal suggests that gigantism occurred in more groups of Arthropoda than had been previously thought. It also presents the possibility that bivalved arthropods were very diverse, and filled in a lot of ecological niches.

Tanazios is a genus of Silurian stem-mandibulate.

Captopodus is an extinct genus of stem-mandibulate known from the Early Devonian. This creature was described in 2012 from four fossils found in the Hunsrück Slate, an early Devonian lagerstätten in Germany that represents one of the few marine sites from the Devonian with soft tissue preservation.

<i>Cascolus</i> Silurian genus of arthropods

Cascolus is an extinct genus of stem-mandibulate known from the Coalbrookdale Formation.

References

  1. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Pulsipher, M. A.; Anderson, E. P.; Wright, L. S.; Kluessendorf, J.; Mikulic, D. G.; Schiffbauer, J. D. (2022). "Description of Acheronauta gen. nov., a possible mandibulate from the Silurian Waukesha Lagerstätte, Wisconsin, USA". Journal of Systematic Palaeontology. 20 (1). 2109216. doi:10.1080/14772019.2022.2109216. S2CID   252839113.
  2. 1 2 3 4 5 6 7 Mikulic, D.G.; Briggs, D.E.G.; Kluessendorf, J. (1985). "A new exceptionally preserved biota from the Lower Silurian of Wisconsin, U.S.A." Philosophical Transactions of the Royal Society of London B. 311 (1148): 75–85. Bibcode:1985RSPTB.311...75M. doi:10.1098/rstb.1985.0140.
  3. 1 2 3 4 5 6 7 8 9 Wendruff, A.J.; Babcock, L.E.; Kluessendorf, J.; Mikulic, D.G. (2020). "Paleobiology and Taphonomy of exceptionally preserved organisms from the Waukesha Biota (Silurian), Wisconsin, USA". Palaeogeography, Palaeoclimatology, Palaeoecology. 546: 109631. Bibcode:2020PPP...54609631W. doi:10.1016/j.palaeo.2020.109631. S2CID   212824469.
  4. ↑ Latin Dictionary Founded on Andrews' edition of Freund's Latin dictionary revised by Charlton T. Lewis, Ph.D. and. Charles Short, LL.D. Oxford. Clarendon Press. 1879.
  5. 1 2 Anderson, Evan P; Schiffbauer, James D.; Jacquet, Sarah M.; Lamsdell, James C.; Kluessendorf, Joanne; Mikulic, Donald G. (2021). "Stranger than a scorpion: a reassessment of Parioscorpio venator, a problematic arthropod from the Llandoverian Waukesha Lagerstätte". Palaeontology. 64 (3): 429–474. Bibcode:2021Palgy..64..429A. doi:10.1111/pala.12534. ISSN   1475-4983. S2CID   234812878.
  6. ↑ Braddy, S.J.; Dunlop, J.A. (2021). "A sting in the tale of Parioscorpio venator from the Silurian of Wisconsin: is it a cheloniellid arthropod?". Lethaia. 54 (1): 1–7. Bibcode:2021Letha..54..603B. doi:10.1111/let.12457. S2CID   245285654.
  7. ↑ Van Roy, Peter; Rak, Štěpán; Budil, Petr; Fatka, Oldřich (2022-06-13). "Redescription of the cheloniellid euarthropod Triopus draboviensis from the Upper Ordovician of Bohemia, with comments on the affinities of Parioscorpio venator". Geological Magazine. 159 (9): 1471–1489. Bibcode:2022GeoM..159.1471V. doi:10.1017/s0016756822000292. ISSN   0016-7568. S2CID   249652930.
  8. ↑ Moore, Rachel A.; Briggs, Derek E. G.; Braddy, Simon J.; Anderson, Lyall I.; Mikulic, Donald G.; Kluessendorf, Joanne (2005). "A New synziphosurine (Chelicerata : Xiphosura) from the Late Llandovery (Silurian) Waukesha Lagerstatte, Wisconsin, USA". Journal of Paleontology. 79 (2): 242–250. doi:10.1666/0022-3360(2005)079<0242:ANSCXF>2.0.CO;2. ISSN   0022-3360. S2CID   56570105.
  9. ↑ Haug, C.; Briggs, D.E.G.; Mikulic, D.G.; Kluessendorf, J.; Haug, J.T. (2014). "The implications of a Silurian and other thylacocephalan crustaceans for the functional morphology and systematic affinities of the group". BMC Evolutionary Biology. 14 (159): 1–15. Bibcode:2014BMCEE..14..159H. doi: 10.1186/s12862-014-0159-2 . PMC   4448278 . PMID   25927449.
  10. ↑ Jones, W.T.; Feldman, R.M.; Schweitzer, C.E. (2015). "Ceratiocaris from the Silurian Waukesha Biota, Wisconsin". Journal of Paleontology. 89 (6): 1007–1021. Bibcode:2015JPal...89.1007J. doi:10.1017/jpa.2016.22. S2CID   131127241.
  11. ↑ Waggoner, Ben M.; Smith, David (1994). "Study of conulariid and related phosphatic conical exoskeletons from the Prague Basin (Czech Republic)". In G. L. Albanesi; M. S. Beresi; S. H. Peralta (eds.). Ordovician from the Andes. Serie Correlación Geológica Nº 17. INSTITUTO SUPERIOR DE CORRELACIÓN GEOLÓGICA.
  12. ↑ Murdock, Duncan J. E.; Smith, M. Paul (2021). "Panderodus from the Waukesha Lagerstätte of Wisconsin, USA: a primitive macrophagous vertebrate predator". Papers in Palaeontology . 7 (4): 1977–1993. Bibcode:2021PPal....7.1977M. doi:10.1002/spp2.1389. S2CID   237769553.
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