Zoology 250 Lecture 31
Phylum CHORDATA, Vertebrate Origins
- 1) Chordates are a morphologically diverse group of three subphyla: Urochordata (sea squirts), Cephalochordata (lancelets), Vertebrata
- a) they are united by five key traits: pharyngeal gill slits, endostyle, notochord, dorsal hollow nerve cord, and post-anal tail
- 2) Adult urochordates bear little resemblance to other chordates
- a) most are sessile, are covered by a complex tunic, have a pharynx with many gill slits, and lack a notochord & nerve cord in adults
- b) the 'tadpole' larva possesses the distinctive chordate characters
- c) Cl. Ascidiacea(sea squirts): sessile solitary or colonial filter feeders
- d) Cl. Larvacea (appendicularians): pelagic makers of mucous houses; the likely sister group of all remaining urochordates
- 3) Adult cephalochordates resemble the larvae of primitive vertebrates and possess all the chordate characters; their large pharynx with many U-shaped gill slits also closely resembles that of hemichordates
CONCLUSIONS: The Tree of Life
- 1) The Tree of Life reveals that life has a history: a grand vision
- 2) The Tree of Life can be read partly from traits of living organisms:
- a) Multicellular 'animals' likely arose from colonial protists
- b) Early branches yielded cellular, then tissue (diploblastic, radial), then organ-system (triploblastic, bilateral) grades of organization
- c) Bilaterians split into two major branches (Protostomia & Deuterostomia) within which coeloms evolved independently
- 3) The Tree of Life can also be read partly from fossils and molecules:
- a) A literal reading of the fossil record suggests animal phyla arose in an 'explosion' near the beginning of the Cambrian (~580 MYA)
- b) But recent molecular evidence (Oct. 1996; 8 molecules) suggests protostomes diverged from deuterostomes nearly 1,200 MYA!
- 4) The study of comparative anatomy and development continues to yield essential insights into the evolution of animal form & diversity
- 5) Major questions about the Tree of Life remain (e.g., placement of ctenophores, 'pseudocoelomates', lophophorates, origin of chordates)
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Copyright © 1999 by A. Richard Palmer. All rights reserved.
(revised Jan. 21, 1999)