Chasmataspida
Encyclopedia
Chasmataspidida are a rare, extinct group of chelicerate arthropods. They are probably related to either 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.
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.
A revision by Markus Poschmann and co-workers 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.
, 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 Scotland. Described by Erik Tetlie and Simon Braddy, it was placed in Diploaspididae, but interpreted as being somewhat more intermediate in form between the Chasmataspis and Diploaspis body plans.
Xiphosura
Xiphosura is an order of marine chelicerates which includes a large number of extinct lineages and only four recent species in the family Limulidae, which include the horseshoe crabs...
) 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
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
and the group is known sporadically in the fossil record through to the mid Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
. 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.
Chasmataspis
The first chasmataspid to be discovered was Chasmataspis laurencii, described by the American palaeontologists Kenneth E. Caster and H. K. Brooks. These Ordovician fossils come from the site of the Douglas DamDouglas Dam
Douglas Dam is a hydroelectric dam on the French Broad River in Sevier County, Tennessee, in the southeastern United States. The dam is operated by the Tennessee Valley Authority , which built the dam in record time in the early 1940s to meet emergency energy demands at the height of World War II...
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.
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.A revision by Markus Poschmann and co-workers 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; 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 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 IslandOctober Revolution Island
October Revolution Island is the largest island of the Severnaya Zemlya group in the Russian Arctic....
, 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.
Loganamaraspis
The latest chasmataspid to be recognised is Loganamaraspis dunlopi from a famous Silurian fossil locality near LesmahagowLesmahagow
Lesmahagow is a small town on the edge of moorland, near Lanark in the central belt of Scotland. It is also known as Abbey Green.-Etymology:The name is possibly a corruption of "Church of St Machutus"...
in Scotland. Described by Erik Tetlie and Simon Braddy, 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
- †Chasmataspis Caster & Brooks, 1956
- †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
- †‘Eurypterus’