Chasmataspidida

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Chasmataspidida
Temporal range: Ordovician–Mid Devonian
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Possible Cambrian record [1]
Hoplitaspis hiawathai 5.png
Fossils of Hoplitaspis hiawathai .
20200607 Chasmataspidida Octoberaspis Hoplitaspis Chasmataspis Diploaspis.png
Reconstruction of Dvulikiaspis menneri (middle top), Octoberaspis ushakovi (top left), Hoplitaspis hiawathai (top right), Chasmataspis laurencii (bottom left) and Diploaspis casteri (bottom right).
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Chelicerata
Clade: Dekatriata
Order:† Chasmataspidida
Caster & Brooks, 1956
Clades
Synonyms
  • Diploaspidida Simonetta & Delle Cave, 1978

Chasmataspidids, sometime referred to as chasmataspids, [1] [2] [3] are a group of extinct chelicerate arthropods that form the order Chasmataspidida. Chasmataspidids are probably related to horseshoe crabs (Xiphosura) and/or sea scorpions (Eurypterida), [4] [1] with more recent studies suggest that they form a clade (Dekatriata) with Eurypterida and Arachnida. [5] [6] [7] [8] Chasmataspidids are known sporadically in the fossil record through to the mid-Devonian, [9] with possible evidence suggesting that they were also present during the late Cambrian. [1] Chasmataspidids are most easily recognised by having an opisthosoma divided into a wide forepart (preabdomen) and a narrow hind part (postabdomen) each comprising 4 and 9 segments respectively. [1] [10] There is some debate about whether they form a natural (i.e. monophyletic) group. [3] [1] [4]

Contents

Distribution

Geographic distribution of chasmataspidids. Geographical distribution of chasmataspidid fossils.png
Geographic distribution of chasmataspidids.

Chasmataspidids survived at least since Ordovician to mid-Devonian in age. As of 2019, most chasmataspidids (with a total of 9 species) are known from the Devonian strata, while the preceding Silurian and Ordovician period each have 3 and 2 species being described. [11] [12] Diploaspis is the only genus of chasmataspidids that unambiguously comprises species from different periods (D. casteri and D. muelleri from Devonian and D. praecursor from silurian). [13] There was also a trace fossil composed of resting imprints with Chasmataspis-like outline discovered from late-Cambrian stratum, which might suggest an earlier occurrence of chasmataspidids. [1]

Morphology

Most chasmataspidids are small arthropods with a body length that did not exceed 3 centimeters, with the ordovician species being exceptionally large, ranging between 10 ( Chasmataspis ) and 29 centimeters ( Hoplitaspis ). [11]

The streamlined body of a chasmataspidid is composed of a rigid prosoma and an externally 13-segmented opisthosoma. As in eurypterids, the dorsal side of the prosoma was covered by a rigid carapace (prosomal dorsal shield) that bore a pair of larger lateral (presumably compound [10] ) eyes and a pair of tiny median ocelli. [10] Chasmataspidids are readily distinguished from other chelicerates by the subdivision of the 13 opisthosomal segments into a widened, 4-segmented preabdomen and a slender, 9-segmented postabdomen. [14] [10] the tergite (dorsal exoskeleton) of the first opisthosomal or preabdominal segment is retained as a narrow element known as 'microtergite', [14] which is not observed in eurypterids. [10] The posterior three preabdominal segments are well developed, forming a rigid box-like section called a 'buckler'. [10] The postabdominal segments are cylindrical, and the last segment terminates with a spine/plate-like telson, which is usually relatively short. [10]

Appendages

Since the appendages of chasmataspidid are rarely preserved in the fossil, most species have only fragile or even no appendicular structures had been described. Based on available materials, the prosoma compose of 6 appendage pairs (appendage I - VI) just like most euchelicerates, which were 1 pair of small chelicerae and 5 pairs of limb-like appendages, although the detail morphology of the former is still unclear. [10] [11] The coxae (basalmost limb segments) of appendage II-VI bore gnathobases. [15] [11] At least the posteriormost appendage pair (appendage VI) of prosoma seems to be differ between families. [11] Appendage of Chasmataspididae known only from 2 disarticulated specimens of appendages which interpreted as appendage VI of Chasmataspis . [11] the appendage bore exopod-like structure on the base and terminated with a chelate (pincer), similar to those of a xiphosuran. [1] On the other hand, Appendage VI modified into a paddle that strikingly resemble to those of a eurypterine (swimming eurypterid) was discovered in some species of Diploaspididae, [14] [11] but the basal diploaspidid Loganamaraspis possibly did not possess this character on Appendage VI. [3] the limb-like appendage II-V of diploaspidids are either featureless [14] or bore rows of spines. [13] [11]

Opisthosomal appendages are even rarely being observed and only known from a few diploaspidid materials. [15] [3] [11] they are at least present on the ventral side of preabdomen, each pair originated from one preabdominal segment. [10] the anteriormost appendicular structure of opisthosoma was metastoma, a plate-like structure interpreted as a fused appendage pair of first opisthosomal segment, [10] situated between the gnathobase of prosomal appendage VI. [11] Beyond the metastoma were 3 pairs of plate-like opercula originated from the 3 buckler segments, with the first operculum pair (genital operculum) bore a medially positioned genital appendage that extend until the posterior region of second operculum pair. [15] [11] Some of the opercula may have book gills just like those of xiphosurans and eurypterids, but the evidence are equivocal. [16] Previous reports of a large operculum cover the whole ventral surface of buckler are most likely a misinterpretation of the ventral buckler wall (sternites or dorsal surface of gill chamber), which were originally enclosed by the opercula in life. [17] [16] The metastoma, opercula and genital appendage are shared characters between chasmataspidid and eurypterid, but unlike the fused first and second operculum pair of eurypterid, the two operculum pairs seems to be unfused in chasmataspidid. [10] Possible chasmataspidid trace fossil from cambrian have imprints resembling 6 pairs of opercula. [1] If the interpretation is true, chasmataspidid may had extra 3 pairs of opercula on the first 3 postabdominal segment as well. [10]

Representative genera

Chasmataspis

Reconstruction of Chasmataspis laurencii. 20200606 Chasmataspis laurencii.png
Reconstruction of Chasmataspis laurencii.

The first chasmataspidid to be discovered was Chasmataspis laurencii, described by the American palaeontologists Kenneth E. Caster and H. K. Brooks in 1956. [18] These Ordovician fossils come from the site of the Douglas Dam in Tennessee, USA. They are the most xiphosuran-like of the known chasmataspidid species, with a horseshoe-shaped carapace. Caster & Brooks raised a new family, Chasmataspididae, to accommodate these specimens. The species was redescribed by Jason Dunlop and colleagues in 2004. [1]

Diploaspis

Reconstruction of Diploaspis casteri. 20200731 Diploaspis casteri.png
Reconstruction of Diploaspis casteri.

The next species to be discovered were Diploaspis casteri and Heteroaspis novojilovi; both described by the Norwegian palaeontologist Leif Størmer from the early Devonian of Alken an der Mosel in Germany in 1972. [19]

A revision by Markus Poschmann and co-workers in 2005 recognised H. novojilovi as a synonym of D. casteri. The two species appear to actually be preservational variants of the same species. Poschmann et al. also described a second species as Diploaspis muelleri. [16]

A third species, Diploaspis praecursor (Late Silurian, Bertie Group, New York State), was described by Lamsdell and Briggs in 2017. [13]


Forfarella

Reconstruction of Forfarella mitchelli. Forfarella mitchelli restoration.png
Reconstruction of Forfarella mitchelli.

Forfarella mitchelli from the early Devonian of the Forfar region in the Midland Valley of Scotland was described by Jason Dunlop and colleagues in 1999; although the fossil had actually been recognised as a chasmataspidid and provisionally labelled as such some years previously by Charles Waterston. Forfarella mitchelli is not very well preserved, but does show the characteristic chasmataspidid body plan. [2]

Achanarraspis

The stratigraphically youngest chasmataspidid is Achanarraspis reedi, described by Lyall Anderson and colleagues in 2000, from the mid-Devonian Achanarras quarry in Caithness, Scotland, a site rich in fish fossils. [17]

Octoberaspis

Reconstruction of Octoberaspis ushakovi 20200605 Octoberaspis ushakovi.png
Reconstruction of Octoberaspis ushakovi

Well preserved chasmataspidids were recovered from the early Devonian of October Revolution Island, part of the Severnaya Zemlya group in the Russian Arctic. Originally briefly described as eurypterids, they were formally described as Octoberaspis ushakovi by Jason Dunlop in 2002. Octoberaspis is one of the few chasmataspidids with well-documented opisthosomal appendages, reveal some characters previously though to be eurypterid-exclusive were also shared by chasmataspidid as well. [15]

Loganamaraspis

Loganamaraspis dunlopi discovered from a famous Silurian fossil locality near Lesmahagow in Scotland. Described by Erik Tetlie and Simon Braddy in 2003, it was placed in Diploaspididae, but interpreted as being somewhat more intermediate in form between the Chasmataspis and Diploaspidid body plans. [3]

Dvulikiaspis

Reconstruction of Dvulikiaspis menneri 20200610 Dvulikiaspis menneri.png
Reconstruction of Dvulikiaspis menneri

Fossils of Dvulikiaspis menneri discovered from the Imangda River of Taymyr Peninsula were originally interpreted as a species of eurypterid genus Stylonurus , and formally described as a new genus of chasmataspidid by David J. Marshall and co-authors in 2014. Dvulikiaspis menneri is one of the few well-preserved chasmataspidid, with distal morphology of appendage II-VI had been revealed. [14]

Hoplitaspis

Reconstruction of Hoplitaspis hiawathai 20200605 Hoplitaspis hiawathai.png
Reconstruction of Hoplitaspis hiawathai

Hoplitaspis hiawathai is the second known species of Ordovician chasmataspidid, discovered from the Big Hill Lagerstätte of Michigan in United States, described by James C. Lamsdell and co-authors in 2019. With nearly complete set of appendages being observable, Hoplitaspis hiawathai is the most complete chasmataspidid known at that time. Each of the paddle of Hoplitaspis hiawathai has a claw instead of an intersegmental element like those of other diploaspidids, providing clues on the relationship between the appendage VI of Chasmataspis and diploaspidids. [11]

Classification

Phylogenetic position

Chelicerata
Summarized phylogenetic position of Chasmataspidida as of the 2010s. [5] [6] [7] [8]

Chasmataspidids have a controversial phylogenetic position within Chelicerata. The first species to be discovered were thought to be unusual fossil xiphosuran, [5] while later species were often based on specimens initially misidentified as eurypterids. [14] Chasmataspidids had been interpreted as relatives/members of either xiphosurans or eurypterids, [20] [4] or forming a clade (Dekatriata) with eurypterids and arachnids. [5] [6] [7] [8] Some studies even suggest that chasmataspidids may not represent a monophyletic taxon - for example as a paraphyletic grade where the eurypterids arose; [3] [4] or a polyphyletic group with Chasmataspis and diploaspidids more closely related to xiphosurans and eurypterids, respectively. [1] The polyphyletic hypothesis was based on the xiphosuran-like characters of Chasmataspis (e.g. genal spines, chelate limbs, fused opisthosomal segments) and eurypterid-like characters found on diploaspidid genera (e.g. paddles on appendage VI). [1] However this interpretation could be unreliable, as the characters are either partially shared by both xiphosurans and eurypterids [1] (e.g. genal spines were found in eurypterid juveniles; [21] some xiphosurans have non-chelate limbs and unfused opisthosoma [22] ) or more likely represent a result of parallel evolution (e.g. the paddles of diploaspidids and swimming eurypterids have different component [11] ). Additionally, the monophyly of chasmataspidids could be supported by the unique component of 4-segmented preabdomen and 9-segmented postabdomen as well. [1] [10] As of the 2010s, many studies supports the monophyly of Chasmataspidida and Dekatriata (Chasmataspidida+Eurypterida+Arachnida). [5] [23] [6] [7] [8] [24] [25] [11]

Interrelationships

Chasmataspidida
Chasmataspididae

Chasmataspis laurencii 20200606 Chasmataspis laurencii.png

Diploaspididae

Loganamaraspis dunlopi

Dvulikiaspis menneri 20200610 Dvulikiaspis menneri.png

Achanarraspis reedi

Heteroaspis stoermeri

Octoberaspis ushakovi 20200605 Octoberaspis ushakovi.png

Diploaspis praecursor 20200607 Diploaspis praecursor.png

Diploaspis casteri 20200731 Diploaspis casteri.png

Diploaspis muelleri

Internal phylogeny of Chasmataspidida based on Selden, Lamsdell & Liu (2015), [6] with addition of Diploaspis praecursor based on Lamsdell & Briggs (2017). [13]

As of 2019, up to 12 genera had been associated within Chasmataspidida. With the exception of Diploaspis which compose of 3 species since 2017, [13] all chasmataspidid genera are monotypic. [9] The order Chasmataspidida subdivided into two families: Chasmataspididae and Diploaspididae. the former consists of Chasmataspis (and possibly also Kiaeria [12] ) while the latter include the remaining genera. [9] Chasmataspididae is defined by a horseshoe-shaped carapace with distinct genal spines and a completely fused preabdomen; [1] while Diploaspididae is defined by a semicircular to subquadrate carapace and a preabdomen with curved, non-trilobate segments. [14]
†ChasmataspididaCaster & Brooks, 1956

Related Research Articles

<span class="mw-page-title-main">Chelicerata</span> Subphylum of arthropods

The subphylum Chelicerata constitutes one of the major subdivisions of the phylum Arthropoda. Chelicerates include the sea spiders, horseshoe crabs, and arachnids, as well as a number of extinct lineages, such as the eurypterids and chasmataspidids.

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

Eurypterids, often informally called sea scorpions, are a group of extinct arthropods that form the order Eurypterida. The earliest known eurypterids date to the Darriwilian stage of the Ordovician period 467.3 million years ago. The group is likely to have appeared first either during the Early Ordovician or Late Cambrian period. With approximately 250 species, the Eurypterida is the most diverse Paleozoic chelicerate order. Following their appearance during the Ordovician, eurypterids became major components of marine faunas during the Silurian, from which the majority of eurypterid species have been described. The Silurian genus Eurypterus accounts for more than 90% of all known eurypterid specimens. Though the group continued to diversify during the subsequent Devonian period, the eurypterids were heavily affected by the Late Devonian extinction event. They declined in numbers and diversity until becoming extinct during the Permian–Triassic extinction event 251.9 million years ago.

<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.

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

Onychopterella is a genus of predatory eurypterid, an extinct group of aquatic arthropods. Fossils of Onychopterella have been discovered in deposits from the Late Ordovician to the Late Silurian. The genus contains three species: O. kokomoensis, the type species, from the Early Pridoli epoch of Indiana; O. pumilus, from the Early Llandovery epoch of Illinois, both from the United States; and O. augusti, from the Late Hirnantian to Early Rhuddanian stages of South Africa.

<i>Willwerathia</i> Genus of Devonian arthropod

Willwerathia is a genus of Devonian arthropod. It is sometimes classified as synziphosurine, a paraphyletic group of horseshoe crab-like fossil chelicerate arthropods, while some studies compare its morphology to an artiopod. Willwerathia known only by one species, Willwerathia laticeps, discovered in deposits of the Devonian period from the Klerf Formation, in the Rhenish Slate Mountains of Germany.

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

Parahughmilleria is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of Parahughmilleria have been discovered in deposits of the Devonian and Silurian age in the United States, Canada, Russia, Germany, Luxembourg and Great Britain, and have been referred to several different species. The first fossils of Parahughmilleria, discovered in the Shawangunk Mountains in 1907, were initially assigned to Eurypterus. It would not be until 54 years later when Parahughmilleria would be described.

<i>Bembicosoma</i> Extinct genus of chelicerate

Bembicosoma is a genus of synziphosurine, a paraphyletic group of fossil chelicerate arthropods. Bembicosoma was regarded as part of the clade Planaterga. Fossils of the single and type species, B. pomphicus, have been discovered in deposits of the Silurian period in the Pentland Hills, Scotland. Bembicosoma had been tentatively assigned as an eurypterid before its synziphosurine affinities revealed.

<i>Bunodes</i> Genus of horseshoe crab relatives

Bunodes is a genus of synziphosurine, a paraphyletic group of fossil chelicerate arthropods. Bunodes was regarded as part of the clade Planaterga. Fossils of the single and type species, B. lunula, have been discovered in deposits of the Silurian period in Ludlow, England. Bunodes is the type genus of the family Bunodidae, the other genera of the same family being Limuloides. There are 64 direct children of Bunodes.

<i>Weinbergina</i> Extinct genus of chelicerate

Weinbergina is a genus of synziphosurine, a paraphyletic group of fossil chelicerate arthropods. Fossils of the single and type species, W. opitzi, have been discovered in deposits of the Devonian period in the Hunsrück Slate, Germany.

<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 from 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">Adelophthalmidae</span> Family of eurypterids

Adelophthalmidae is a family of eurypterids, an extinct group of aquatic arthropods. Adelophthalmidae is the only family classified as part of the superfamily Adelophthalmoidea, which in turn is classified within the infraorder Diploperculata in the suborder Eurypterina.

<span class="mw-page-title-main">Eurypterina</span> Suborder of eurypterid

Eurypterina is one of two suborders of eurypterids, an extinct group of chelicerate arthropods commonly known as "sea scorpions". Eurypterine eurypterids are sometimes informally known as "swimming eurypterids". They are known from fossil deposits worldwide, though primarily in North America and Europe.

<i>Wiedopterus</i> Extinct genus of Devonian eurypterids

Wiedopterus is a genus of eurypterid, an extinct group of aquatic arthropods. The type and only species of Wiedopterus, W. noctua, is known from deposits of Early Devonian age in Germany. The generic name derives from the Wied river, which runs near the site of the initial discovery, and the species name, noctua, derives from Latin noctua (owl) which refers to the superficial resemblance of the carapace to an owl.

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

Borchgrevinkium is an extinct genus of chelicerate arthropod. A fossil of the single and type species, B. taimyrensis, has been discovered in deposits of the Early Devonian period in the Krasnoyarsk Krai, Siberia, Russia. The name of the genus honors Carsten Borchgrevink, an Anglo-Norwegian explorer who participated in many expeditions to Antarctica. Borchgrevinkium represents a poorly known genus whose affinities are uncertain.

<span class="mw-page-title-main">Metastoma</span> Abdominal plate present in some anthropods

The metastoma is a ventral single plate located in the opisthosoma of non-arachnid dekatriatan chelicerates such as eurypterids, chasmataspidids and the genus Houia. The metastoma located between the base of 6th prosomal appendage pair and may had functioned as part of the animal's feeding structures. It most likely represented a fused appendage pair originated from somite 7, thus homologous to the chilaria of horseshoe crab and 4th walking leg pair of sea spider. In eurypterids, the plate was typically cordate (heart-shaped) in shape, though differed in shape in some genera, such as Megalograptus.

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

Dvulikiaspis is a genus of chasmataspidid, a group of extinct aquatic arthropods. Fossils of the single and type species, D. menneri, have been discovered in deposits of the Early Devonian period in the Krasnoyarsk Krai, Siberia, Russia. The name of the genus is composed by the Russian word двуликий (dvulikij), meaning "two-faced", and the Ancient Greek word ἀσπίς (aspis), meaning "shield". The species name honors the discoverer of the holotype of Dvulikiaspis, Vladimir Vasilyevich Menner.

<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.

<i>Camanchia</i> Extinct genus of chelicerate

Camanchia is a genus of synziphosurine, a paraphyletic group of fossil chelicerate arthropods. Camanchia was regarded as part of the clade Prosomapoda. Fossils of the single and type species, C. grovensis, have been discovered in deposits of the Silurian period in Iowa, in the United States. Alongside Venustulus, Camanchia is one of the only Silurian synziphosurine with fossil showing evidence of appendages.

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

Forfarella is a genus of chasmataspidid, a group of extinct aquatic arthropods. Fossils have been discovered in deposits of the Early Devonian period. The single and type species, F. mitchelli, is known from one only specimen found in Scotland, in the United Kingdom. Known as BMNH In 60023, it is poorly preserved and its type locality is uncertain, although it might be the Kelly Den stream section near the village of Arbirlot.

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

Chasmataspis is a genus of chasmataspidid, a group of extinct aquatic chelicerate arthropods. It was found in the Early Ordovician deposits of Tennessee, United States.

References

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  13. 1 2 3 4 5 6 James C. Lamsdell; Derek E. G. Briggs (2017). "The first diploaspidid (Chelicerata: Chasmataspidida) from North America (Silurian, Bertie Group, New York State) is the oldest species of Diploaspis" (PDF). Geological Magazine. 154 (1): 175–180. Bibcode:2017GeoM..154..175L. doi:10.1017/S0016756816000662. S2CID   85560431. Archived from the original (PDF) on 2020-02-27.
  14. 1 2 3 4 5 6 7 8 9 10 Marshall, David J.; Lamsdell, James C.; Shpinev, Evgeniy; Braddy, Simon J. (2014). "A diverse chasmataspidid (Arthropoda: Chelicerata) fauna from the Early Devonian (Lochkovian) of Siberia". Palaeontology. 57 (3): 631–655. doi: 10.1111/pala.12080 . ISSN   1475-4983. S2CID   84434367.
  15. 1 2 3 4 5 6 Jason A. Dunlop (2002). "Arthropods from the Lower Devonian Severnaya Zemlya Formation of October Revolution Island, Russia" (PDF). Geodiversitas . 24 (2): 349–379.
  16. 1 2 3 4 Markus Poschmann, Lyall I. Anderson & Jason A. Dunlop (2005). "Chelicerate arthropods, including the oldest phalangiotarbid arachnid, from the Early Devonian (Siegenian) of the Rhenish Massif, Germany" (PDF). Journal of Paleontology . 79 (1): 110–124. doi:10.1666/0022-3360(2005)079<0110:CAITOP>2.0.CO;2. S2CID   129082668.
  17. 1 2 3 Lyall I. Anderson, Jason A. Dunlop & Nigel H. Trewin (2000). "A Middle Devonian chasmataspid arthropod from Achanarras Quarry, Caithness, Scotland" (PDF). Scottish Journal of Geology . 36 (2): 151–158. doi:10.1144/sjg36020151. S2CID   140167776.
  18. ↑ Kenneth E. Caster and H. K. Brooks (1956). "New fossils from the Canadian–Chazan (Ordovician) hiatus in Tennessee". Bulletins of American Paleontology . 36: 157–199.
  19. 1 2 3 4 Leif Størmer (1972). "Arthropods from the Lower Devonian (Lower Emsian) of Alken an der Mosel, Germany. Part 2: Xiphosura". Senckenbergiana Lethaea . 53: 1–29.
  20. ↑ Shultz, Jeffrey W. (2007-06-01). "A phylogenetic analysis of the arachnid orders based on morphological characters". Zoological Journal of the Linnean Society. 150 (2): 221–265. doi: 10.1111/j.1096-3642.2007.00284.x . ISSN   0024-4082.
  21. ↑ Lamsdell, James C.; Selden, Paul (2013). "Babes in the wood – a unique window into sea scorpion ontogeny". BMC Evolutionary Biology . 13 (98): 1–46. doi: 10.1186/1471-2148-13-98 . PMC   3679797 . PMID   23663507.
  22. ↑ Moore, Rachel A.; Briggs, Derek E. G.; Bartels, Christoph (2005). "A new specimen ofWeinbergina opitzi (Chelicerata: Xiphosura) from the Lower Devonian Hunsriick Slate, Germany". Paläontologische Zeitschrift. 79 (3): 399–408. doi:10.1007/BF02991931. ISSN   0031-0220. S2CID   84994966.
  23. ↑ Legg, David A. (2014). "Sanctacaris uncata: the oldest chelicerate (Arthropoda)". Naturwissenschaften. 101 (12): 1065–1073. doi:10.1007/s00114-014-1245-4. ISSN   0028-1042. PMID   25296691. S2CID   15290784.
  24. ↑ Aria, Cédric; Caron, Jean-Bernard (2017-12-21). "Mandibulate convergence in an armoured Cambrian stem chelicerate". BMC Evolutionary Biology. 17 (1): 261. doi: 10.1186/s12862-017-1088-7 . ISSN   1471-2148. PMC   5738823 . PMID   29262772.
  25. ↑ Aria, Cédric; Caron, Jean-Bernard (2019). "A middle Cambrian arthropod with chelicerae and proto-book gills". Nature. 573 (7775): 586–589. doi:10.1038/s41586-019-1525-4. ISSN   1476-4687. PMID   31511691. S2CID   202550431.
  26. ↑ Dunlop, Jason A.; Poschmann, Markus; Anderson, Lyall I. (December 2001). "On the Emsian (Early Devonian) arthropods of the Rhenish Slate Mountains: 3. The chasmataspididDiploaspis". PalZ. 75 (2): 253–269. doi:10.1007/BF02988018. ISSN   0031-0220. S2CID   128479601.
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