Anaspida ("shieldless ones") is an extinct group of Jawless Fish that existed from the early Silurian period to the late Devonian period.
Anatomy
Compared to many other
, such as the
Heterostraci and
Osteostraci, anaspids did not possess a bony shield or armor, hence their name. The anaspid head and body are instead covered in an array of small, weakly mineralized scales, with a row of massive scutes running down the back, and, at least confirmed among the
birkeniids, the body was covered in rows of tile-like scales made of aspidine, an acellular bony tissue.
Anaspids all had prominent, laterally placed eyes with no
sclerotic ring, with the
opened as a row of holes along either side of the animal, typically numbering anywhere from 6-15 pairs. The major
synapomorphy for the anaspids is the large, tri-radiate spine behind the series of the gill openings.
Taxonomy
Now that
Jamoytius and its close cohorts, i.e.,
Euphanerops, have been moved to
Jamoytiiformes,
[Sansom, Robert S., et al. "Taphonomy and affinity of an enigmatic Silurian vertebrate, Jamoytius kerwoodi White." Palaeontology 53.6 (2010): 1393-1409.] Class Anaspida now consists of two orders, the
monotypic taxon Lasaniida, which contains the genus
Lasanius and represents a basal anaspid group,
[Blom, Henning. "New birkeniid anaspid from the Lower Devonian of Scotland and its phylogenetic implications." Palaeontology 55.3 (2012): 641-652.] and
Birkeniida, which contains all other recognized anaspid taxa.
[Blom, Henning, T. Märss, and C. G. Miller. "Silurian and earliest Devonian birkeniid anaspids from the Northern Hemisphere." Earth and Environmental Science Transactions of the Royal Society of Edinburgh 92.03 (2001): 263-323.] Birkeniida is further divided into several families, including Birkeniidae, Pterygolepididae, Rhyncholepididae and Pharyngolepididae, which contain those taxa known from whole body fossils (in addition to several taxa known only from scales) and the family Septentrioniidae, whose subtaxa are known exclusively from scales.
Two recently described genera,
Kerreralepis and
Cowielepis,
[Blom, Henning. "A new anaspid fish from the Middle Silurian Cowie Harbour fish bed of Stonehaven, Scotland." Journal of Vertebrate Paleontology 28.3 (2008): 594-600.] are considered to be Birkeniida
incertae sedis.
Some recent studies have suggested that anaspids are stem-Cyclostomi, more closely related to hagfish and lampreys than to jawed fish.
A newer taxonomy based on the work of Mikko's Phylogeny Archive,
-
Class † Anaspida Janvier 1996 non Williston 1917
Notes
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