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Animal EvolutionInterrelationships of the Living Phyla$
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Claus Nielsen

Print publication date: 2011

Print ISBN-13: 9780199606023

Published to Oxford Scholarship Online: December 2013

DOI: 10.1093/acprof:oso/9780199606023.001.0001

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(p.348) 62 CHORDATA
Animal Evolution

Claus Nielsen

Oxford University Press

The Chordata is a monophyletic group consisting of three living phyla: Cephalochordata, Urochordata, and Vertebrata. Morphology-based studies on the origin and evolution of the chordates suggested that the segmented and unsegmented urochordates were sister groups. However, most of the newer studies on molecular phylogeny indicate sister-group relationships between the urochordates and vertebrates within the segmented Chordata, implying that the segmentation disappeared in the urochordates. Recent evidence supports the idea that Urochordata and Vertebrata together form the clade Olfactores. The distinguishing feature of the chordates is the notochord, a cylinder of cells surrounded by a strengthened basal membrane.

Keywords:   chordates, Chordata, Cephalochordata, Urochordata, Vertebrata, evolution, urochordates, molecular phylogeny, vertebrates, notochord

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