Early tetrapod relationships revisited

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 aïstopods, the three selected species of which form a trichotomy; (2) the internal relationships of embolomeres, with Archeria crassidisca and Pholiderpeton scut 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 aïstopods. 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....

Salamander limb development: Integrating genes, morphology, and fossils

“…This is of particular interest when considered in the framework of the controversial origin(s) and relationships of modern amphibians for which three scenarios are being discussed in the literature: the polyphyly hypothesis proposes a sister taxon relationship between salamanders and branchiosaurids, and frogs and amphibamids, respectively, while caecilians are considered most closely related to microsaurs within lepospondyls (e.g., Carroll, 2007;Anderson et al, 2008;Anderson, 2008;). In the most widely accepted temnospondyl hypothesis, modern amphibians form a monophyletic clade (''Lissamphibia'') nested within temnospondyls and most closely related to amphibamids or branchiosaurids (e.g., Bolt, 1991;Milner, 1993;Ruta et al, 2003;Schoch and Milner, 2004;Ruta and Coates, 2007;Sigurdsen and Bolt, 2009). In the lepospondyl hypothesis, frogs, salamanders, and caecilians also form a monophyletic Lissamphibia with lysorophian lepospondyls as their closest relatives (Laurin and Reisz, 1997;Vallin and Laurin, 2004;Laurin, 2007, 2008).…”

Nuclear gene phylogeny of narrow-mouthed toads (Family: Microhylidae) and a discussion of competing hypotheses concerning their biogeographical origins

“…The following calibration points were used: (1) minimum age of the frog-salamander split at 230 Mya (fossil record of frog ancestor Triadobatrachus; (Sanchiz, 1998)); (2) minimum age of the split between Agalychnis and Litoria at 42 Mya (last connection between Australia and South America; (Seddon et al, 1998)); (3) maximum age of the split between Mantidactylus wittei and Mantidactylus sp. from the Comoro islands at 15 Mya (volcanic origin of the oldest Comoro island Mayotte; (Vences et al, 2003b)); (4) minimum age of the Alytes muletensis-Alytes dickhilleni split at 5 Mya (Mediterranean salinity crisis: Fromhage et al, 2004); (5) age interval of the split between diapsids and synapsids at 338-288 Mya (Graur and Martin, 2004); (6) a minimum age of 338 Mya for the divergence between Lissamphibia and Amniota based on the aïstopod fossil, Lethiscus stocki (Ruta et al, 2003); (7) minimum age for the divergence of the South American Pipa and the African Xenopus of 110 Mya, corresponding to the Wnal separation of South America from Africa (Sanmartin and Ronquist, 2004).…”

Ventastega curonica and the origin of tetrapod morphology

“…This contrasts with marked pattern changes in the dermal bones of the cheek, skull roof and palate, and with a restructuring of braincase that resulted in the loss of the intracranial joint, basicranial fenestra and lateral commissure as well as a host of other smaller changes. With a few modifications such as the gradual withdrawal of the notochord and the rearward extension of the parasphenoid across the basicranial fissure, this new braincase morphology remained essentially constant up into the base of the tetrapod crown group 42 . Even the highly specialized braincase of Ichthyostega is recognizably derived from this pattern 4 .…”