Temporal range: 444–419Ma Early Silurian - Late Silurian
Anaspida ("without shield") is an extinct group of primitive jawless vertebrates that lived during the Silurian period. They were once classically regarded as the ancestors of lampreys. 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 Silurian, where they then disappear from the fossil record.
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. 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.
Phylogeny and taxonomy
- Janvier, Philippe (1997) Anaspida The Tree of Life Web Project.
- 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.
- Patterson, Colin (1987). Molecules and morphology in evolution: conflict or compromise?. Cambridge, UK: Cambridge University Press. p. 142. ISBN 0-521-32271-5.
- Janvier, Philippe (2003). Early Vertebrates. Oxford University Press. ISBN 978-0-19-852646-9.