Anaspida

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Anaspida
Temporal range: 444–359Ma
Early Silurian - Late Devonian
Cowielepis ritchiei.jpg
Cowielepis
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Clade: Craniata
Subphylum: Vertebrata
Class: Anaspida
Traquair, 1899
For the reptiles, see Anapsida; for the crustaceans, see Anaspididae
Anaspids are characterized by a large, tri-radiate spine (red) posteriorly to the series of branchial openings. Typical anaspids are restricted to the Silurian but some doubtful forms occur in the Late Devonian. 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]

Anaspida ("without shield") is an extinct group of primitive jawless vertebrates that lived during the Silurian and Devonian periods.[2] They are classically regarded as the ancestors of lampreys.[3] Anaspids were small marine agnathans that lacked heavy bony shield and paired fins, but have a striking highly hypocercal tail. They first appeared in the early Silurian, and flourished until the Late Devonian extinction,[4] where most species, save for lampreys, went extinct due to the environmental upheaval during that time.

Anatomy[edit]

Unusually for typical, prehistoric agnathans, anaspids did not possess a bony shield or armor. The head is instead covered in an array of smaller, weakly mineralized scales.[5] They have large, laterally placed eyes with no sclerotic ring, with the gills opened as a row of holes along either side of the animal, typically numbering anything from 6-15 pairs. The major synapomorphy for the anaspids is the large, tri-radiate spine behind the series of the gill openings.[1]

Phylogeny and taxonomy[edit]

Anaspida


Pharyngolepis





Rhyncholepis


Birkeniidae

Birkenia



Lasanius





Notes[edit]

  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. ^ Hall, Brian Keith; Hanken, James (1993). The Skull. Chicago: University of Chicago Press. p. 131. ISBN 0-226-31568-1. 
  5. ^ Janvier, Philippe (2003). Early Vertebrates. Oxford University Press. ISBN 978-0-19-852646-9. 

External links[edit]