Early tetrapod relationships revisited

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. Wehave incorporated, where possible, original observations of numerous taxa spread throughout the major tetrapodclades. 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. Differencesbetween these trees concern: (1) the internal relationships of a?¨stopods, the three selected species of whichform a trichotomy; (2) the internal relationships of embolomeres, with Archeria crassidisca and Pholiderpeton scutigerumcollapsed in a trichotomy with a clade formed by Anthracosaurus russelli and Pholiderpeton attheyi ; (3) the internalrelationships of derived dissorophoids, with four amphibamid species forming an unresolved node with a cladeconsisting of micromelerpetontids and branchiosaurids and a clade consisting of albanerpetontids plus basalcrown-group lissamphibians ; (4) the position of albenerpetontids and Eocaecilia micropoda, which form an unresolvednode with a trichotomy subtending Karaurus sharovi, Valdotriton gracilis and Triadobatrachus massinoti ; (5) thebranching pattern of derived diplocaulid nectrideans, with Batrachiderpeton reticulatum and Diceratosaurus brevirostriscollapsed in a trichotomy with a clade formed by Diplocaulus magnicornis and Diploceraspis burkei. The results of theoriginal parsimony run – as well as those retrieved from several other treatments of the data set ( e.g. exclusionof postcranial and lower jaw data ; character reweighting ; reverse weighting ) – indicate a deep split of earlytetrapods between lissamphibian- and amniote-related taxa. Colosteids, Crassigyrinus, Whatcheeria and baphetidsare progressively more crownward stem-tetrapods. Caerorhachis, embolomeres, gephyrostegids, Solenodonsaurusand 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 forthe monophyletic status of the latter is weak. Westlothiana and Lepospondyli form the sister group to diadectomorphsand crown-group amniotes. Tuditanomorph and microbrachomorph microsaurs are successivelymore closely related to a clade including proximodistally : (1) lysorophids; (2) Acherontiscus as sister taxon toadelospondyls; (3) scincosaurids plus diplocaulids ; (4) urocordylids plus a?¨stopods. A data set employing cranialcharacters only places microsaurs on the amniote stem, but forces remaining lepospondyls to appear as sistergroup to colosteids on the tetrapod stem in several trees. This arrangement is not significantly worse than the treetopology obtained from the analysis of the complete data set. The pattern of sister group relationships in thecrownward part of the temnospondyl-lissamphibian tree re-emphasizes the important role of dissorophoids inthe lissamphibian origin debate. However, no specific dissorophoid can be identified as the immediate sister taxonto crown-group lissamphibians. The branching sequence of various stem-group amniotes reveals a coherent setof internested character-state changes related to the acquisition of progressively more terrestrial habits in severalPermo-Carboniferous forms.