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Early tetrapod relationships revisited.

Marcello Ruta¹, Michael I Coates, Donald L J Quicke

¹Department of Organismal Biology and Anatomy, The University of Chicago, 1027 East 57th Street, Chicago, IL 60637-1508, USA. mruta@midway.uchicago.edu

Biological Reviews of the Cambridge Philosophical Society

|June 14, 2003

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This study reveals a deep split in early tetrapod evolution between amphibian- and amniote-related lineages. It clarifies relationships within major groups, suggesting dissorophoids are key to amphibian origins.

Area of Science:

Paleontology
Evolutionary Biology
Comparative Anatomy

Background:

Understanding early tetrapod relationships is crucial for tracing vertebrate evolution.
Previous hypotheses on early tetrapod phylogeny lack consensus.
A comprehensive dataset is needed to resolve phylogenetic uncertainties.

Purpose of the Study:

To investigate and resolve conflicting hypotheses of early tetrapod relationships.
To establish a robust phylogeny using a new, extensive data matrix.
To clarify the evolutionary position of key fossil groups like lepospondyls and dissorophoids.

Main Methods:

Assembled a new data matrix of 90 taxa and 319 cranial and postcranial characters.
Utilized a stem-based (total-group) definition of Tetrapoda.

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Employed a parsimony ratchet method (PAUP*) to analyze the data, yielding 64 shortest trees.

Main Results:

A deep split was identified between lissamphibian- and amniote-related early tetrapods.
Clarified internal relationships within aïstopods, embolomeres, dissorophoids, and nectrideans.
Dissorophoids play a significant role in lissamphibian origins, though no single immediate sister taxon was identified.

Conclusions:

The study provides a refined phylogeny for early tetrapods, supporting a major divergence early in their evolutionary history.
The findings highlight the importance of dissorophoids in understanding the origin of modern amphibians (lissamphibians).
The analysis offers insights into the sequential acquisition of terrestrial adaptations in stem-group amniotes.