Early tetrapod relationships revisited

Ruta, Marcello and Coates, Michael I. and Quicke, Donald L. J. (2003) Early tetrapod relationships revisited. Biological Reviews, 78 (2). pp. 251-345. ISSN 1464-7931

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Abstract

In an attempt to investigate differences between the most widely discussed hypotheses of early tetrapod relationships,
we assembled a new data matrix including 90 taxa coded for 319 cranial and postcranial characters. We
have incorporated, where possible, original observations of numerous taxa spread throughout the major tetrapod
clades. A stem-based ( total-group) definition of Tetrapoda is preferred over apomorphy- and node-based
(crown-group) definitions. This definition is operational, since it is based on a formal character analysis. A PAUP*
search using a recently implemented version of the parsimony ratchet method yields 64 shortest trees. Differences
between these trees concern: (1) the internal relationships of aı¨stopods, the three selected species of which
form a trichotomy; (2) the internal relationships of embolomeres, with Archeria crassidisca and Pholiderpeton scutigerum
collapsed in a trichotomy with a clade formed by Anthracosaurus russelli and Pholiderpeton attheyi ; (3) the internal
relationships of derived dissorophoids, with four amphibamid species forming an unresolved node with a clade
consisting of micromelerpetontids and branchiosaurids and a clade consisting of albanerpetontids plus basal
crown-group lissamphibians ; (4) the position of albenerpetontids and Eocaecilia micropoda, which form an unresolved
node with a trichotomy subtending Karaurus sharovi, Valdotriton gracilis and Triadobatrachus massinoti ; (5) the
branching pattern of derived diplocaulid nectrideans, with Batrachiderpeton reticulatum and Diceratosaurus brevirostris
collapsed in a trichotomy with a clade formed by Diplocaulus magnicornis and Diploceraspis burkei. The results of the
original parsimony run – as well as those retrieved from several other treatments of the data set ( e.g. exclusion
of postcranial and lower jaw data ; character reweighting ; reverse weighting ) – indicate a deep split of early
tetrapods between lissamphibian- and amniote-related taxa. Colosteids, Crassigyrinus, Whatcheeria and baphetids
are progressively more crownward stem-tetrapods. Caerorhachis, embolomeres, gephyrostegids, Solenodonsaurus
and seymouriamorphs are progressively more crownward stem-amniotes. Eucritta is basal to temnospondyls,
with crown-lissamphibians nested within dissorophoids. Westlothiana is basal to Lepospondyli, but evidence for
the monophyletic status of the latter is weak. Westlothiana and Lepospondyli form the sister group to diadectomorphs
and crown-group amniotes. Tuditanomorph and microbrachomorph microsaurs are successively
more closely related to a clade including proximodistally : (1) lysorophids; (2) Acherontiscus as sister taxon to
adelospondyls; (3) scincosaurids plus diplocaulids ; (4) urocordylids plus aı¨stopods. A data set employing cranial
characters only places microsaurs on the amniote stem, but forces remaining lepospondyls to appear as sister
group to colosteids on the tetrapod stem in several trees. This arrangement is not significantly worse than the tree
topology obtained from the analysis of the complete data set. The pattern of sister group relationships in the
crownward part of the temnospondyl-lissamphibian tree re-emphasizes the important role of dissorophoids in
the lissamphibian origin debate. However, no specific dissorophoid can be identified as the immediate sister taxon
to crown-group lissamphibians. The branching sequence of various stem-group amniotes reveals a coherent set
of internested character-state changes related to the acquisition of progressively more terrestrial habits in several
Permo-Carboniferous forms.

Item Type:Article
Additional Information:In an attempt to investigate differences between the most widely discussed hypotheses of early tetrapod relationships, we assembled a new data matrix including 90 taxa coded for 319 cranial and postcranial characters. We have incorporated, where possible, original observations of numerous taxa spread throughout the major tetrapod clades. A stem-based ( total-group) definition of Tetrapoda is preferred over apomorphy- and node-based (crown-group) definitions. This definition is operational, since it is based on a formal character analysis. A PAUP* search using a recently implemented version of the parsimony ratchet method yields 64 shortest trees. Differences between these trees concern: (1) the internal relationships of aı¨stopods, the three selected species of which form a trichotomy; (2) the internal relationships of embolomeres, with Archeria crassidisca and Pholiderpeton scutigerum collapsed in a trichotomy with a clade formed by Anthracosaurus russelli and Pholiderpeton attheyi ; (3) the internal relationships of derived dissorophoids, with four amphibamid species forming an unresolved node with a clade consisting of micromelerpetontids and branchiosaurids and a clade consisting of albanerpetontids plus basal crown-group lissamphibians ; (4) the position of albenerpetontids and Eocaecilia micropoda, which form an unresolved node with a trichotomy subtending Karaurus sharovi, Valdotriton gracilis and Triadobatrachus massinoti ; (5) the branching pattern of derived diplocaulid nectrideans, with Batrachiderpeton reticulatum and Diceratosaurus brevirostris collapsed in a trichotomy with a clade formed by Diplocaulus magnicornis and Diploceraspis burkei. The results of the original parsimony run – as well as those retrieved from several other treatments of the data set ( e.g. exclusion of postcranial and lower jaw data ; character reweighting ; reverse weighting ) – indicate a deep split of early tetrapods between lissamphibian- and amniote-related taxa. Colosteids, Crassigyrinus, Whatcheeria and baphetids are progressively more crownward stem-tetrapods. Caerorhachis, embolomeres, gephyrostegids, Solenodonsaurus and seymouriamorphs are progressively more crownward stem-amniotes. Eucritta is basal to temnospondyls, with crown-lissamphibians nested within dissorophoids. Westlothiana is basal to Lepospondyli, but evidence for the monophyletic status of the latter is weak. Westlothiana and Lepospondyli form the sister group to diadectomorphs and crown-group amniotes. Tuditanomorph and microbrachomorph microsaurs are successively more closely related to a clade including proximodistally : (1) lysorophids; (2) Acherontiscus as sister taxon to adelospondyls; (3) scincosaurids plus diplocaulids ; (4) urocordylids plus aı¨stopods. A data set employing cranial characters only places microsaurs on the amniote stem, but forces remaining lepospondyls to appear as sister group to colosteids on the tetrapod stem in several trees. This arrangement is not significantly worse than the tree topology obtained from the analysis of the complete data set. The pattern of sister group relationships in the crownward part of the temnospondyl-lissamphibian tree re-emphasizes the important role of dissorophoids in the lissamphibian origin debate. However, no specific dissorophoid can be identified as the immediate sister taxon to crown-group lissamphibians. The branching sequence of various stem-group amniotes reveals a coherent set of internested character-state changes related to the acquisition of progressively more terrestrial habits in several Permo-Carboniferous forms.
Keywords:amniotes, characters, congruence, lissamphibians, parsimony ratchet, taxon exemplar, tetrapods, total-group
Subjects:C Biological Sciences > C300 Zoology
C Biological Sciences > C182 Evolution
F Physical Sciences > F641 Palaeontology
C Biological Sciences > C181 Biodiversity
Divisions:College of Science > School of Life Sciences
ID Code:6453
Deposited By: Marcello Ruta
Deposited On:07 Oct 2012 09:50
Last Modified:13 Mar 2013 09:16

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