''Jamoytius kerwoodi'' was a species of primitive,
eel-like jawless
fish that lived in the
Llandovery epoch of the Early
Silurian
The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...
period.
Long thought of as a "basal anaspid," ''J. kerwoodi'' is now recognized as the best-known member of the
Hyperoartian
order
Order, ORDER or Orders may refer to:
* Categorization, the process in which ideas and objects are recognized, differentiated, and understood
* Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of d ...
Jamoytiiformes
Jamoytiiformes is an extinct order of prehistoric jawless fish in the superclass Anaspidomorphi.
Taxonomy
Taxonomy is based on Mikko's Phylogeny Archive
* Family † Achanarellidae Newman 2002
** Genus †'' Achanarella'' Newman 2002 'Achanare ...
. It had an elongated body, and is thought to have had, in comparison with relatives known from intact bodies like ''Euphanerops'', a dorsal fin and an anal fin near the rearmost third of its body. Earlier reconstructions depict the creature as having side-fins running the length of its body, starting from behind the branchial openings to the tip of its tail: new research demonstrates that such "fins" are actually deformations of the bodywall as the corpse was being squished post-burial.
In life, ''J. kerwoodi'' resembled a lamprey with a very small mouth. Because the fossil had no teeth, teeth-like structures, nor suggestions of either in its mouth, it was not carnivorous like many modern
lampreys. It was more likely to have been a filter-feeder or a detritus-feeder, possibly in the manner of larval lampreys.
The fish had a cartilaginous skeleton, and a branchial basket resembling the
cyclostomes - features that suggest that it was a basal member of that clade. It is also the earliest known vertebrate with
camera-type eyes.
It also possessed weakly mineralised scales.
History of research
''Jamoytius'' was originally named by Errol White on the basis of two specimens (the generic name is a reference to
J. A. Moy-Thomas[Dawkins, Richard The Ancestor's Tale]) and, at the time, it was considered to be the most
basal
Basal or basilar is a term meaning ''base'', ''bottom'', or ''minimum''.
Science
* Basal (anatomy), an anatomical term of location for features associated with the base of an organism or structure
* Basal (medicine), a minimal level that is nec ...
vertebrate known. Since then, it has been reclassified by many workers as having many different affinities, such as an "unspecialized anaspid",
or as a sister taxon to the lampreys,
its difficulty in classification due to difficulties in reconstructing the anatomy;
it does not possess any usual
chordate synapomorphies
In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to have ...
. Currently, ''J. kerwoodi'' is now placed in its own order Jamoytiiformes, together with ''
Euphanerops'' and similar agnathans.
Further reading
* Long, John A. ''The Rise of Fishes: 500 Million Years of Evolution'' Baltimore: The Johns Hopkins University Press, 1996.
Links to images
References
Jamoytiiformes
Silurian jawless fish
Silurian animals of Europe
Prehistoric Hyperoartia genera
Fossil taxa described in 1946
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