Anaspida

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An extinct order of fresh- or brackish-water Agnatha, known from the Upper Silurian of Europe and Canada and from the Upper Devonian of Canada. The Middle Silurian Scottish Jamoytius is thought by some to be an anaspid.

Members of this group are small, not exceeding 10 in. (25 cm) in length, and typically have a slender, fusiform body covered with small scales and a rounded, jawless, terminal or sub-terminal mouth (see illustration). Long paired fins are present, at least in some genera, and the tail is unusual in having the muscular lobe turned downward. They were probably active, nectonic swimmers, adapted for feeding on minute particles.

Pharyngolepsis oblongus of the Anaspida, reconstruction. (After A. Ritchie)

A relationship to Osteostraci and Petromyzonida is indicated by the single dorsal nostril lying in front of the pineal eye and between the large paired eyes and by the paired rows of circular gill openings; all three orders are grouped in the superorder Hyperoartii. Anaspida are possibly ancestral to living lampreys. See also Osteostraci; Petromyzontida.

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Anaspida Temporal range: 444–359 Ma PreЄ Є O S D C P T J K Pg N Early Silurian - Late Devonian
Jamoytius kerwoodi
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Subphylum:	Vertebrata
Class:	Anaspida

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] where most species, save for lampreys, went extinct due to the environmental upheaval during that time.

Contents 1 Anatomy 2 Phylogeny and Taxonomy 3 Notes 4 External links

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].

Phylogeny and Taxonomy

Anaspida	Pharyngolepsis Rhyncolepsis Birkeniidae Birkenia Lasanius

Notes

1. ^ 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. 
2. ^ 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. 
3. ^ 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. 
4. ^ Janvier, Philippe (2003-01-01). Early Vertebrates. Oxford University Press. ISBN 978-0-19-852646-9. 
5. ^ 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/. 

External links

• Anaspids at tolweb.org

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