Description
Birkenia elegans birkenia fossil
Physical description
Birkenia elegans is a primitive fish belonging to the group called the Anaspida (informally called anaspids). Unlike its cousin Lasanius problematicus, which occurs at the same formation, Birkenia has intricate scales covering the entire body. Its most characteristic feature can be found on its back; a row of spines which, at the front, is marked by bi-directional spines, also known as ‘the crown’. This adaptation would have made Birkenia a tough swallow for any predator in these Silurian waters.
Age and Distribution
Birkenia is from the Lower Silurian Fish Bed Formation of Scotland, approximately 430 million years old. Other fishes occurring at this horizon are the anaspid Lasanius problematicus, the thelodonts Lanarkia spinulosa, Lanarkia horrida, and Lanarkia lanceolata, the osteostracan Ateleaspis tessellata, and the recently-discovered euphaneropid Ciderius cooperi. A eurypterid called Lanarkopterus dolichoschelus also occurs at this fauna, as well as a scala of enigmatic fossils such as Taitia catena and Dictyocaris sp.
About the Anaspida Birkenia fossil
Evolutionary significance
The Anaspida are known exclusively from Silurian and Devonian formations of the northern hemisphere. Birkenia is the type species after which the birkeniid anaspids are named. This type of anaspid is characterised by a body covered entirely by scales, as opposed to the scaleless anaspid Lasanius problematicus. Lasanius and Birkenia are the first described anaspids known from articulated fossils. Only a handful of localities yield articulated anaspids as most anaspids are known from isolated scales.
Taxonomy
Birkenia is a Birkeniid anaspid, part of the class Anaspida, which has been placed with the informally termed supergroup ‘ostracoderms’. These fishes are regarded as the jawless precursors to the jawed fishes. The relationship between anaspids and other groups of early fishes remains up for debate, as there is a lack of data about much of its anatomy.
