Chasmataspidida

Chasmataspidida (often referred to informally as chasmataspids) are a rare, extinct group of chelicerate arthropods. They are probably related to horseshoe crabs (Xiphosura) and/or sea scorpions (Eurypterida). Indeed 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 chasmataspids were present during the late Cambrian and the group is known sporadically in the fossil record through to the mid Devonian. Chasmataspids 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 chasmataspid genera

Chasmataspis

The first chasmataspid to be discovered was Chasmataspis laurencii, described by the American palaeontologists Kenneth E. Caster and H. K. Brooks.^[1] These Ordovician fossils come from the site of the Douglas Dam in Tennessee, USA. They are the most xiphosuran-like of the known chasmataspid 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.^[2]

Diploaspis

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.^[3]

A revision by Markus Poschmann and co-workers^[4] 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.

Forfarella

Forfarella mitchelli from the early Devonian of the Forfar region in the Midland Valley of Scotland was described by Jason Dunlop and colleagues;^[5] although the fossil had actually been recognised as a chasmataspid and provisionally labelled as such some years previously by Charles Waterston. Forfarella mitchelli is not very well preserved, but does show the characteristic chasmataspid body plan.

Acahanarraspis

The stratigraphically youngest chasmataspid is Achanarraspis reedi, described by Lyall Anderson and colleagues^[6] from the mid Devonian Achanarras quarry in Caithness, Scotland; a famous fossil fish locality.

Octoberaspis

Well preserved chasmataspids 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.^[7]

Loganamaraspis

The latest chasmataspid to be recognised is Loganamaraspis dunlopi from a famous Silurian fossil locality near Lesmahagow in Scotland. Described by Erik Tetlie and Simon Braddy,^[8] it was placed in Diploaspididae, but interpreted as being somewhat more intermediate in form between the Chasmataspis and Diploaspis body plans.

Classification

• †Chasmataspidida Caster & Brooks, 1956
  • = †Diploaspidida Simonetta & Delle Cave, 1978

• †Chasmataspididae Caster & Brooks, 1956
  • †Chasmataspis Caster & Brooks, 1956
    • †Chasmataspis laurencii Caster & Brooks, 1956 – Ordovician of Tennessee, USA

• †Diploaspididae Størmer, 1972
  • = †Heteroaspididae Størmer, 1972
  • †Achanarraspis Anderson, Dunlop & Trewin, 2000
    • †Achanarraspis reedi Anderson, Dunlop & Trewin, 2000 – Devonian of Scotland
  • †Diploaspis Størmer, 1972
    • = †Heteroaspis Størmer, 1972
    • †Diploaspis casteri Størmer, 1972 – Devonian of Germany
      • = †Heteroaspis novojilovi Størmer, 1972 – Devonian of Germany
    • †Diploaspis muelleri Poschmann, Anderson & Dunlop, 2005 – Devonian of Germany
  • †Forfarella Dunlop, Anderson & Braddy, 1999
    • †Forfarella mitchelli Dunlop, Anderson & Braddy, 1999 – Devonian of Scotland
  • †Loganamaraspis Tetlie & Braddy, 2004
    • †Loganamaraspis dunlopi Tetlie & Braddy, 2004 – Silurian of Scotland
  • †Octoberaspis Dunlop, 2002
    • †Octoberaspis ushakovi Dunlop, 2002 – Devonian of Severnya Zemlya, Russia

• Diploaspidae incertae sedis
  • †‘Eurypterus’
    • †‘Eurypterus’ stoermeri Novojilov, 1959 – Devonian of Siberia

References