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Temporal range: Cambrian–Devonian
Hoplitaspis hiawathai 7.png
Fossil of Hoplitaspis hiawathai.
Scientific classification e
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Chelicerata
Clade: Dekatriata
Order: Chasmataspidida
Caster & Brooks, 1956
  • Diploaspidida Simonetta & Delle Cave, 1978

Chasmataspidids, often referred to informally as chasmataspids, are a group of extinct arthropods that form the order Chasmataspidida. Chasmataspidids are probably related to horseshoe crabs (Xiphosura) and/or sea scorpions (Eurypterida).[1] The first species to be discovered were thought to be unusual fossil horseshoe crabs, while later species were often based on specimens initially misidentified as eurypterids. There is some evidence that chasmataspidids were present during the late Cambrian and the group is known sporadically in the fossil record through to the mid-Devonian. Chasmataspidids are most easily recognised by having an abdomen divided into a short forepart (or mesosoma) and a longer hindpart (or metastoma) comprising nine segments. There is some debate about whether they form a natural (i.e. monophyletic) group.



The first chasmataspidid to be discovered was Chasmataspis laurencii, described by the American palaeontologists Kenneth E. Caster and H. K. Brooks.[2] 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 headshield. Caster & Brooks raised a new family, Chasmataspididae, to accommodate these specimens. The species was redescribed by Jason Dunlop and colleagues.[3]


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.[4]

A revision by Markus Poschmann and co-workers[5] recognised H. novojilovi as a synonym of D. casteri. The two genera appear to actually be preservational variants of the same species. Poschmann et al.. also described a second species as Diploaspis muelleri.

A third species, Diploaspis praecursor (Late Silurian, Bertie Group, New York State),[6] was described by Lamsdell and Briggs in 2017.


Forfarella mitchelli from the early Devonian of the Forfar region in the Midland Valley of Scotland was described by Jason Dunlop and colleagues;[7] 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.


The stratigraphically youngest chasmataspidid is Achanarraspis reedi, described by Lyall Anderson and colleagues[8] from the mid-Devonian Achanarras quarry in Caithness, Scotland; a famous fossil fish locality.


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.[9]


One of the most recently discovered chasmataspidid to be recognised is Loganamaraspis dunlopi from a famous Silurian fossil locality near Lesmahagow in Scotland. Described by Erik Tetlie and Simon Braddy,[10] it was placed in Diploaspididae, but interpreted as being somewhat more intermediate in form between the Chasmataspis and Diploaspis body plans.


The order contains:[11]
Chasmataspidida Caster & Brooks, 1956


  1. ^ Garwood, Russell J.; Dunlop, Jason A. (2014). "Three-dimensional reconstruction and the phylogeny of extinct chelicerate orders". PeerJ. 2: e641. doi:10.7717/peerj.641. PMC 4232842. PMID 25405073.
  2. ^ 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.
  3. ^ Jason A. Dunlop, Lyall I. Anderson & Simon J. Braddy (2004). "A redescription of Chasmataspis laurencii Caster & Brooks (Chelicerata: Chasmataspidida) from the Middle Ordovician of Tennessee, USA, with remarks on chasmataspid phylogeny" (PDF). Transactions of the Royal Society of Edinburgh: Earth Sciences. 94 (4): 207–225. doi:10.1017/S0263593300000626.
  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.
  5. ^ 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.
  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". Geological Magazine. 154 (1): 175–180. Bibcode:2017GeoM..154..175L. doi:10.1017/S0016756816000662.
  7. ^ Jason A. Dunlop, L. I. Anderson & S. J. Braddy (1999). "A new chasmataspid (Chelicerata: Chasmataspida) from the Lower Devonian of the Midland Valley of Scotland" (PDF). Transactions of the Royal Society of Edinburgh: Earth Sciences. 89 (3): 161–165. doi:10.1017/s0263593300007100.
  8. ^ 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.
  9. ^ Jason A. Dunlop (2002). "Arthropods from the Lower Devonian Severnaya Zemlya Formation of October Revolution Island, Russia" (PDF). Geodiversitas. 24 (2): 349–379.
  10. ^ O. Erik Tetlie & Simon J. Braddy (2003). "The first Silurian chasmataspid, Loganamaraspis dunlopi gen. et sp. nov. (Chelicerata: Chasmataspidida) from Lesmahagow, Scotland, and its implications for eurypterid phylogeny". Transactions of the Royal Society of Edinburgh: Earth Sciences. 94 (3): 227–234. doi:10.1017/S0263593300000638.
  11. ^ Dunlop, J. A., Penney, D. & Jekel, D. 2015. A summary list of fossil spiders and their relatives. In World Spider Catalog. Natural History Museum Bern, online at, version 18.5 (PDF).
  12. ^ Lamsdell, James C. (2019). "A chasmataspidid affinity for the putative xiphosuran Kiaeria Størmer, 1934". Paläontologische Zeitschrift: 1–5. doi:10.1007/s12542-019-00493-8.