- For the reptiles, see Anapsida; for the crustaceans, see Anaspididae
The Anaspida ("without shield") are stem gnathostomes,[1] and are classically regarded as the ancestors of lampreys.[2] Anaspids were small marine agnathans that lacked scales and paired fins, but have a striking highly hypocercal tail. They first appeared in the early Silurian, and flourished until the Late Devonian extinction,[3] during the late Devonian, where most species, save for lampreys, went extinct due to the environmental upheaval during that time.
[edit] Anatomy
Unusually for an agnathan, anaspids did not possess a bony shield or armor. The head is instead covered in an array of smaller, weakly mineralized scales[4]. They have large laterally placed eyes with no sclerotic ring, and 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[5].
[edit] Phylogeny and Taxonomy
- ^ 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. http://books.google.com/books?id=zeyRZNZl-74C&pg=PA188&dq=Anaspida+%22stem+gnathostomes%22&as_brr=3&ei=2KN-SNeSF6HOjgGr-NDHCA&sig=ACfU3U2_2PVl8IEP8mBW5PCch2Wr3OzTQw.
- ^ Patterson, Colin (1987). Molecules and morphology in evolution: conflict or compromise?. Cambridge, UK: Cambridge University Press. p. 142. ISBN 0-521-32271-5. http://books.google.com/books?id=DL_KQPX3AmIC&pg=PA142&dq=Anaspida+gnathostomes&ei=W6F-SO-2E5XCigHs-L2_CA&sig=ACfU3U203AXE0jY6Q46goxR6Jeiswg7P_A.
- ^ Hall, Brian Keith; Hanken, James (1993). The Skull. Chicago: University of Chicago Press. p. 131. ISBN 0-226-31568-1. http://books.google.com/books?id=fB-hO7i50esC&pg=PA131&dq=Anaspida+gnathostomes&ei=W6F-SO-2E5XCigHs-L2_CA&sig=ACfU3U2Ztylrkp1nwc-wUCb4UF0rLqUF5w.
- ^ Janvier, Philippe (2003-01-01). Early Vertebrates. Oxford University Press. ISBN 9780198526469.
- ^ Janvier, Philippe (1997). "Anaspida. Version 01 January 1997 (under construction)". http://tolweb.org/Anaspida/14837/1997.01.01 in The Tree of Life Web Project, http://tolweb.org/.
[edit] External links
