Transitions between Major Classes: Vertebrates


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.

Keywords: evolution; palaeontology; vertebrata; tetrapoda; amniota; mammalia; aves

Figure 1.

Hypothesis of relationships of major vertebrate groups with their most commonly agreed on sister taxa. Not all hierarchical levels of vertebrate taxa are represented; rather, major vertebrate classes are included with representatives of their most commonly accepted sister taxa.

Figure 2.

Summary of fish and tetrapod taxa spanning the initial water‐to‐land transition from the Late Devonian crossopterygian fish Eusthenopteron to the Early Carboniferous tetrapod Pederpes. Schematic body reconstructions in left lateral aspect are accompanied by the corresponding skull reconstructions for each taxon (where known) to demonstrate changes in proportions of dermal roofing bones.

Figure 3.

Representative transitional vertebrate taxa: (a) the ichthyostegid amphibian Acanthostega, (b) the ichthyostegid amphibian Ichthyosetga, (c) and (d) dorsal and occipital views of the high‐fibre herbivore and near amniote Diadectes. The element indicated in red is the parietal bone (P). (e) and (f) Right lateral and dorsal views of the advanced therapsid Morganucodon. (g) Reconstruction of feathered juvenile dromaeosaurid dinosaur Sinornithosaurus. (a) and (b) After Lombard and Sumida ; (e) and (f) adapted from Benton ; (g) used with permission of the artist.



Ahlberg PE , Clack JA , Lukševič E , Blom H and Zupinš I (2008) Ventastega curonica and the origin of tetrapod morphology. Nature 453: 1199–1204.

Benton M (2005) Vertebrate Palaeontology. Oxford: Blackwell Publishing.

Brainerd EL , Ditelberg JS and Bramble DM (1993) Lung ventilation in salamanders and the evolution of vertebrate air‐breathing mechanisms. Biological Journal of the Linnean Society 49: 163–183.

Carrier DR and Farmer CG (2000) The evolution of pelvic aspiration in archosaurs. Paleobiology 26: 271–293.

Clack JA (2002) An early tetrapod from Romer's gap. Nature 418: 72–76.

Coates MI and Clack JA (1995) Romer's gap: tetrapod origins and terrestriality. Bulletin of the Natural History Museum of Paris 17: 359–372.

Dial KP (2003) Wing assisted incline running and the evolution of flight. Science 299: 402–404.

Hopson JA (1994) Synapsid evolution and the radiation of non‐eutherian mammals. In: Prothero DR and Schoch RM (eds) Major Features of Vertebrate Evolution, pp 190–219. Knoxville, TN: Paleontological Society, Short Courses in Paleontology, University of Tennessee.

Lee MYS and Spencer P (1997) Crown clades, key characters and taxonomic stability: when is an amniote not an amniote? In: Sumida SS and Martin KLM (eds) Amniote Origins: Completing the Transition to Land, pp 61–84. San Diego: Academic Press.

Lombard RE and Sumida SS (1992) Recent progress in understanding early tetrapods. American Zoologist 32: 609–622.

Reilly SM and White TD (2003) Hypaxial motor patterns and the function of epipubic bones in primitive mammals. Science 299: 400–402.

Sumida SS and Martin KLM (1997) Amniote Origins: Completing the Transition to Land. San Diego: Academic Press.

Xu X , Wang X , Kuang X , Zhang F and Du X (2003) Four‐winged dinosaurs from China. Nature 421: 335–340.

Further Reading

Anderson JS and Sues H‐D (2007) Major Transitions in Vertebrate Evolution. Bloomington: Indiana University Press.

Clack JA (2002) Gaining Ground, The Origin and Evolution of Tetrapods. Bloomington: Indiana University Press.

Currie PJ , Koppelhus EB , Shugar MA and Wright JL (2004) Feathered Dragons. Bloomington: Indiana University Press.

Schultze H‐P and Trueb L (1991) Origins of the Higher Groups of Tetrapods – Controversy and Consensus. Cornell: Cornell University Press.

Sumida SS and Brochu CA (2000) Phylogenetic context for the origin of feathers. American Zoologist 40: 486–503.

Sumida SS and Martin KLM (1997) Amniote Origins: Completing the Transition to Land. San Diego: Academic Press.

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How to Cite close
Sumida, Stuart S, and Devlin, Kathleen R(Sep 2013) Transitions between Major Classes: Vertebrates. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001662.pub3]