Marcello Ruta

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. 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 aistopods, 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 stemtetrapods. 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. 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 identiffed 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.

Dates, nodes and character conflict: addressing the Lissamphibian origin problem

Synopsis Extant amphibians consist of Salientia (frogs), Caudata (salamanders), and Gymnophiona (caecilians). The mutual relationships of these groups are controversial, with either Batrachia (Salientia + Caudata) or Procera (Gymnophiona + Caudata) as emerging clades in recent molecular and morphological analyses. The monophyly of amphibians as a whole is supported by independent data, but their origins and affinities with early tetrapods are debated. A new cladistic analysis of early tetrapods retrieves Temnospondyli (the most species‐rich group of early tetrapods) as the closest relatives of crown group amphibians. One temnospondyl group, the Dissorophoidea, forms a series of consecutive outgroups to crown amphibians. In particular, the Lower Permian amphibamid Doleserpeton is the most derived plesion on the amphibian stem. The Albanerpetontidae, a group of salamander‐like tetrapods ranging from the Jurassic to the Pliocene, are placed as stem Gymnophiona. The shortest trees support the Batrachia hypothesis. However, the Procera hypothesis is not a significantly worse fit for the whole character set. Exhaustive treatment of characters and taxa is the most appropriate way to disentangle contrasting phylogenetic signals in large matrices. Tests of different crown topologies show that placement of amphibians within lepospondyls (e.g. as sister taxon to Lysorophia) is not a significantly worse fit for the whole character set than a close temnospondyl‐lissamphibian relationship. However, the latter phylogenetic hypothesis best captures the most coherent assembly of derived lissamphibian apomorphies.

Evolutionary patterns in early tetrapods. I. Rapid initial diversification followed by decrease in rates of character change

Although numerous studies have examined morphological diversification during major radiations of marine taxa, much less attention has been paid to terrestrial radiations. Here, we examine rates of character change over phylogeny and over time for Palaeozoic limbed tetrapods. Palaeozoic tetrapods show significant decreases in rates of character change whether the rate is measured per sampled cladistic branch or per million years along phylogeny. Given changes per branch, rates decrease significantly from the Devonian through the Pennsylvanian, but not from the Pennsylvanian through the Permian. Given changes per million years, rates decrease significantly over each boundary, although the decrease is least significant over the Pennsylvanian–Permian boundary. Decreasing rates per million years through the Permian might be an artefact of the method being able to ascribe longer durations to Permian branches than to Carboniferous ones; however, it is difficult to ascribe the general pattern of decreasing rates of change over time to sampling biases or methodological biases. Thus, the results implicate biological explanations for this pattern.

Fins to limbs: what the fossils say1

SUMMARY A broad phylogenetic review of fins, limbs, and girdles throughout the stem and base of the crown group is needed to get a comprehensive idea of transformations unique to the assembly of the tetrapod limb ground plan. In the lower part of the tetrapod stem, character state changes at the pectoral level dominate; comparable pelvic level data are limited. In more crownward taxa, pelvic level changes dominate and repeatedly precede similar changes at pectoral level. Concerted change at both levels appears to be the exception rather than the rule. These patterns of change are explored by using alternative treatments of data in phylogenetic analyses. Results highlight a large data gap in the stem group preceding the first appearance of limbs with digits. It is also noted that the record of morphological diversity among stem tetrapods is somewhat worse than that of basal crown group tetrapods. The pre‐limbed evolution of stem tetrapod paired fins is marked by a gradual reduction in axial segment numbers (mesomeres); pectoral fins of the sister group to limbed tetrapods include only three. This reduction in segment number is accompanied by increased regional specialization, and these changes are discussed with reference to the phylogenetic distribution of characteristics of the stylopod, zeugopod, and autopod.

Early tetrapod evolution

Tetrapods include the only fully terrestrial vertebrates, but they also include many amphibious, aquatic and flying groups. They occupy the highest levels of the food chain on land and in aquatic environments. Tetrapod evolution has generated great interest, but the earliest phases of their history are poorly understood. Recent studies have questioned long-accepted hypotheses about the origin of the pentadactyl limb, the phylogeny of tetrapods and the environment in which the first tetrapods lived.

Nice snake, shame about the legs

Snakes are one of the most extraordinary groups of terrestrial vertebrates, with numerous specializations distinguishing them from other squamates (lizards and their allies). Their musculoskeletal system allows creeping, burrowing, swimming and even gliding, and their predatory habits are aided by chemo- and thermoreceptors, an extraordinary degree of cranial kinesis and, sometimes, powerful venoms. Recent discoveries of indisputable early fossil snakes with posterior legs are generating intense debate about the evolutionary origin of these reptiles. New cladistic analyses dispute the precise significance and phylogenetic placement of these fossils. These conflicting hypotheses imply radically different scenarios of snake origins and relationships with wide biological implications.

Brief review of the stylophoran debate

The “calcichordate” theory interprets an extinct group of calcite‐plated invertebrates, the stylophorans, as chordates. In this theory, cornute stylophorans are interpreted as stem chordates, whereas mitrate stylophorans are primitive members of the acraniates, tunicates, and craniates. However, this theory discounts major synapomorphies between cornutes and mitrates. These groups constitute a natural, monophyletic group which is here argued to lie within the echinoderm radiation. The “calcichordate” theory is, therefore, rejected because it relies on assumption‐driven hypotheses of character transformation which are supported by ambiguous, poor, or missing fossil evidence. Stylophorans may lie at the base of the echinoderm clade and primitively lack pentameral symmetry, therefore casting light on the near‐ancestral body organization of the phylum.

Evolutionary patterns in early tetrapods. II. Differing constraints on available character space among clades

Radiations of large clades often accompany rapid morphological diversification. Evolutionary biologists debate the impact of external restrictions imposed by ecology, and intrinsic constraints imposed by development and genetics, on the rate at which morphological innovations are gained. These issues are particularly interesting for groups such as tetrapods, which evolved novel body plans relative to their piscine ancestors and which also invaded new ecosystems following terrestrialization. Prior studies have addressed these issues by looking at either ranges of morphological variation or rates of character change. Here, we address a related but distinct issue: the numbers of characters that freely vary within a clade. We modify techniques similar to those used by ecologists to infer species richnesses to estimate the number of potentially varying characters given the distributions of changes implied by a model phylogeny. Our results suggest both increasing constraints/restrictions and episodes of ‘character release’ (i.e. increasing the number of potentially varying characters). In particular, we show that stem lissamphibians had a restricted character space relative to that of stem amniotes, and that stem amniotes both had restrictions on some parts of character space but also invaded new character space that had been unavailable to stem tetrapods.

A reassessment of the temnospondyl amphibian Perryella olsoni from the Lower Permian of Oklahoma

Additional mechanical preparation of the type material of the temnospondyl amphibian Perryella olsoni (Lower Permian, Wellington Formation, Oklahoma) highlights new cranial and postcranial features and provides additional data on previously known structures. Important new information is available for the quadrate, palatal bones and their associated dentition, parasphenoid, and appendicular skeleton. The revised diagnosis and redescription of Perryella provide the basis for a re-evaluation of its systematic affinities. A cladistic analysis of (mostly) Carboniferous and Permian temnospondyls, together with several Devonian and Carboniferous stem tetrapod outgroups, supports a single origin for temnospondyls. The sequence of branching events within temnospondyls consists of: (1) a paraphyletic Edopoidea; (2) a clade of Zatracheidae, Eryopidae, and basal Archegosauriformes; (3) a monophyletic Dvinosauria; and (4) a monophyletic Dissorophoidea. Perryella is nested within Dvinosauria in an intermediate position between Trimerorhachidae and Dvinosauroidea.

The seymouriamorph tetrapod Ariekanerpeton sigalovi from the Lower Permian of Tadzhikistan. Part I: Cranial anatomy and ontogeny

Redescription of the cranial anatomy of Ariekanerpeton sigalovi (Seymouriamorpha: Discosauriscidae; Lower Permian, Tadzhikistan) highlights new diagnostic features which permit a new skull reconstruction and revised diagnosis. The largest Ariekanerpeton specimens differ from like-sized Discosauriscus , a second member of Discosauriscidae, in showing: oblique anterior margin of tabular ornamented surface; acute, elongate posterolateral corner of supratemporal; postorbital ventrolateral ramus with small anteroventral process and forming oblique suture with jugal dorsomedial ramus; jugal posterior ramus tapering rearward, forming subhorizontal suture with quadratojugal; weak ventral sculpture lateral to pterygoid articular recess; parasphenoid subrhomboidal, with lightly sculptured anterior wedge-like process; more densely arranged denticle rows on top of low radiating ridges on pterygoid palatal ramus. Gill filaments occur in specimens with skull lengths of up to 30 mm. Detailed examination of ontogenetic changes in Ariekanerpeton and comparisons with other seymouriamorphs are presented.