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Transitions between Major Classes: Vertebrates

  1. Stuart S Sumida,
  2. Kathleen R Devlin

Published Online: 19 SEP 2013

DOI: 10.1002/9780470015902.a0001662.pub3



How to Cite

Sumida, S. S. and Devlin, K. R. 2013. Transitions between Major Classes: Vertebrates. eLS. .

Author Information

  1. California State University, San Bernardino, California, USA

Publication History

  1. Published Online: 19 SEP 2013


The advent of cladistic analyses of relationships between organisms has provided more rigorous means of defining lineages, although fossil records suggest that boundaries between vertebrate clades are not always distinct. At the beginning of the transition from water to land, the earliest tetrapods retain many fish-like features, and at the end of this continuum numerous terrestrial adaptations may be seen in groups not traditionally classified as amniotes. Characters of the jaw and middle ear, classically associated with Mammalia, differentiated as a functional complex that developed across the therapsid–mammal transition. Feathers and features associated with lightening the skeleton are also found in terrestrial dromaeosaurid dinosaurs, and are not exclusive to birds. That each of these transitions took place through a series of graded steps suggests that they should be considered more as a process than as a point in time or geologic history.

Key Concepts:

  • The transitions between what are considered major taxonomic groups of vertebrates are not distinct, but rather are ‘blurry’ as they reflect processes and the gradual accumulation of suites functional anatomical and physiological complexes.

  • The water-to-land transition is not a single event. It is a continuum ranging across nearly 50 My from the first tetrapods to the first amniotes.

  • With new information including the details of transitional forms such as Tiktaalik and Ventastega, understanding of the changes in the limb skeleton from sarcopterygian fishes to the earliest tetrapods has been refined significantly.

  • The Late Carboniferous to Early Permian Diadectomorpha have emerged as the closest relatives of crown-group amniotes, sharing numerous anatomical features with them.

  • Dromaeosaurid dinosaurs demonstrate a variety of experiments in feather distribution and potential flight adaptations as the group from which Aves evolved.

  • The lineage leading to extant mammals began the process of transformation of the feeding apparatus, associated modifications of the middle ear and (probable) soft tissue associations in groups generally considered to be closely related but outside of traditionally defined mammals.


  • evolution;
  • palaeontology;
  • vertebrata;
  • tetrapoda;
  • amniota;
  • mammalia;
  • aves