Anaspida ("shieldless ones") is an extinct group of jawless fish that existed from the early Silurian period to the late Devonian period.[2] They were classically regarded as the ancestors of lampreys,[3] but it is denied in recent phylogenetic analysis,[4] although some analysis show these group would be at least related.[5] Anaspids were small marine fish that lacked a heavy bony shield and paired fins, but were distinctively hypocercal.

Anaspida
Temporal range: 444–358.9 Ma early Silurian - Late Devonian
Cowielepis
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Superclass: Anaspidomorphi
Class: Anaspida
Traquair, 1899
Type species
Birkenia elegans
Traquair 1898
Orders
Synonyms
  • Anaspidi
  • Birkeniae
Anaspids are characterized by a large, tri-radiate spine (red) posteriorly to the series of branchial openings. It is assumed that the most primitive anaspids, such as Pharyngolepis (top), possessed a long, ribbon-shaped, ventrolateral fin-fold (green). More advanced forms, such as Rhyncholepis (bottom), possessed a shorter paired fin-fold (green) and enlarged, spine-shaped, median dorsal scutes. – Philippe Janvier[1]

Anatomy

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Compared to many other ostracoderms, 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.[6] Anaspids all had prominent, laterally placed eyes with no sclerotic ring, with the gills 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.[1]

Taxonomy

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Now that Jamoytius and its close cohorts, i.e., Euphanerops, have been moved to Jamoytiiformes,[7] Class Anaspida now consists of two orders, the monogeneric Lasaniida, which contains the genus Lasanius and represents a basal anaspid group,[8] and Birkeniida, which contains all other recognized anaspid taxa.[9] 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.[9] Two recently described genera, Kerreralepis[8] and Cowielepis,[10] are considered to be Birkeniida incertae sedis.

Some recent studies have suggested that anaspids are stem-cyclostomes, more closely related to hagfish and lampreys than to jawed fish.[5]

A newer taxonomy based on the work of Mikko's Phylogeny Archive,[11] Nelson, Grande and Wilson 2016[12] and van der Laan 2018.[13]

 
Life reconstruction of Lasanius problematicus


Notes

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  1. ^ a b Janvier, Philippe (1997) Anaspida The Tree of Life Web Project.
  2. ^ Ahlberg, Per Erik (2001). Major events in early vertebrate evolution: palaeontology, phylogeny, genetics, and development. Washington, DC: Taylor & Francis. p. 188. ISBN 0-415-23370-4.
  3. ^ Patterson, Colin (1987). Molecules and morphology in evolution: conflict or compromise?. Cambridge, UK: Cambridge University Press. p. 142. ISBN 0-521-32271-5.
  4. ^ Keating, Joseph N.; Donoghue, Philip C. J. (2016-03-16). "Histology and affinity of anaspids, and the early evolution of the vertebrate dermal skeleton". Proceedings of the Royal Society B: Biological Sciences. 283 (1826): 20152917. doi:10.1098/rspb.2015.2917. ISSN 0962-8452. PMC 4810860.
  5. ^ a b Reeves, Jane C.; Wogelius, Roy A.; Keating, Joseph N.; Sansom, Robert S. (March 2023). Cavin, Lionel (ed.). "Lasanius , an exceptionally preserved Silurian jawless fish from Scotland". Palaeontology. 66 (2): e12643. doi:10.1111/pala.12643. ISSN 0031-0239.
  6. ^ Janvier, Philippe (2003). Early Vertebrates. Oxford University Press. ISBN 978-0-19-852646-9.
  7. ^ Sansom, Robert S., et al. "Taphonomy and affinity of an enigmatic Silurian vertebrate, Jamoytius kerwoodi White." Palaeontology 53.6 (2010): 1393-1409.
  8. ^ a b Blom, Henning. "New birkeniid anaspid from the Lower Devonian of Scotland and its phylogenetic implications." Palaeontology 55.3 (2012): 641-652.
  9. ^ a b 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.
  10. ^ 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.
  11. ^ Haaramo, Mikko (2003). "†Anaspida – anaspids". In Mikko's Phylogeny Archive. Retrieved November 25, 2018.
  12. ^ Nelson, Joseph S.; Grande, Terry C.; Wilson, Mark V. H. (2016). Fishes of the World (5th ed.). John Wiley & Sons. ISBN 9781118342336.
  13. ^ van der Laan, Richard (2018). "Family-group names of fossil fishes". European Journal of Taxonomy. doi:10.5852/ejt.2018.466.
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