Anaspida

(paleontology) An order of extinct fresh- or brackish-water vertebrates in the class Agnatha.

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.

The noun has one meaning:

Meaning #1: extinct order of jawless vertebrates
  Synonym: order Anaspida

Anaspida Fossil 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.

The Anaspida 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. They first appeared sometime during the early Silurian, and flourished until the Late Devonian Extinction Event,^[3] during the late Devonian, where most species, save for lampreys, went extinct due to the environmental upheaval during that time. The gills opened as a row of holes along either side of the animal that typically numbered anywhere from from 6-15 pairs.

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. 

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