John R. Bolt

Amphibamus grandiceps AS A JUVENILE DISSOROPHID: EVIDENCE AND IMPLICATIONS

ABSTRACT Amphibamus grandiceps , a small labyrinthodont known only from Mazon Creek, has been suggested as a possible frog ancestor. Much of the argument favoring this has been refuted, but interesting similarities remain between A. grandiceps and frogs. A re-interpretation of A. grandiceps , based on high-fidelity latex casts studied under scanning electron microscope, is presented. This is used to discuss the relationships of A. grandiceps , the evolution of life history strategies in amphibians in general and the implications of life history strategy for functional morphology. Major conclusions are: 1. A. grandiceps is a member of the Dissorophidae, as suggested by other workers. 2. The dentition of A. grandiceps is similar to that of Lissamphibia (a collective term for the three living amphibian orders). A. grandiceps may be the earliest “protolissamphibian” known. 3. The dentition of A. grandiceps is probably characteristic of juvenile dissorophids in general, and possibly of other labyrinthodont groups. 4. The lissamphibian order(s) derived from dissorophids probably arose via progenesis, a form of developmental heterochrony. Progenesis may have been due to selective pressure for small size, acting on adults which in ecological terms were coming under increasingly intense r -selection. In contrast, many large aquatic labyrinthonts were neotenic and may have been K -selected at all growth stages. 5. Some of the most strikingly dissorophid-like characters of lissamphibians may be largely the result of progenesis, rather than any selective advantage of the characters per se . 6. Developmental heterochrony may have played a larger role in amphibian evolution than is usually supposed; in particular, evolution of both dentition and vertebral centra was probably influenced. If so, studies of “functional morphology” must pay at least as much attention to ontogeny as to mechanical efficiency.

Jaw growth and tooth replacement in Captorhinus aguti (Reptilia: Captorhinomorpha): a morphological and histological analysis

ABSTRACT We here report the results of a primarily histological study of jaw growth and tooth replacement in the Early Permian captorhinomorph reptile Captorhinus aguti. Preliminary histological examination shows that “drift” of teeth from the lingual to the labial side of the jaw apparently occurred through a combination of bone growth + remodelling. Simple graphical analysis of proportional changes in the C. aguti skull and jaws over a threefold span of size increase demonstrates that growth was essentially isometric. This fact and our preliminary histological observations are combined into a model of jaw growth plus tooth replacement, in which the positions and relative ages of teeth are described in terms of the Zahnreihen of Edmund (1960). Detailed histological observations are then used to test the model, which is confirmed in all essentials but shown to be incomplete as first formulated. With incorporation of the fact that during growth the older, smaller parts of the dentary and maxilla were progr...

The Lower Permian amphibamid Doleserpeton (Temnospondyli: Dissorophoidea), the interrelationships of amphibamids, and the origin of modern amphibians

ABSTRACT The Lower Permian amphibamid Doleserpeton annectens (Temnospondyli) is of great importance to our understanding of the origin of modern amphibians. Here, we describe this taxon in detail. The pedicellate bicuspid dentition of Doleserpeton is more similar to that of modern amphibians than any other Palaeozoic form, including Gerobatrachus. The dermal bones of the skull are almost completely smooth, unlike other dissorophoids. The braincase and parts of the postcranial skeleton are more highly ossified than in other amphibamids, and Doleserpeton is unlikely to represent a juvenile stage. The ribs are relatively short and posteriorly curved, as in many modern amphibians, but unlike most lepospondyls. The palate shares numerous features with modern amphibians, including the configuration of the interpterygoid vacuities and the structure of the vomers. We propose that the closest sister taxon to Amphibamidae is Dissorophidae (armored dissorophoids) and that the trematopids are more basally situated than has been suggested by some studies. Micropholis is probably not basal to the other amphibamids, and the clade support for a monophyletic Amphibamidae is surprisingly weak unless Dissorophidae and branchiosaurs are included in this taxon. Doleserpeton adds to the strong evidence for a temnospondyl origin of batrachians, particularly for anurans, and the hypothesis proposing a lepospondyl origin of modern amphibians is therefore questioned. Doleserpeton also shares numerous features with Eocaecilia, pointing to a monophyletic origin of lissamphibians from amphibamid temnospondyls.

THE MANDIBLE OF THE PRIMITIVE TETRAPOD GREERERPETON, AND THE EARLY EVOLUTION OF THE TETRAPOD LOWER JAW

Abstract Exceptionally well-preserved Late Mississippian colosteid amphibian specimens occur in southern Illinois; the mandible is described here. Unexpectedly primitive features include toothed adsymphysial and intercoronoid fossa with fenestrate floor. The large adsymphysial bears teeth, forms 50 percent of the symphysis, and meets its antimere in a very coarsely rugose suture. These and other characters are shown to occur also in Greererpeton burkemorani, to which we refer the Illinois specimens. Colosteid mandibles from a Late Mississippian locality in southern Iowa resemble G. burkemorani closely, although they are not conspecific. Our findings are summarized in a PRESERVE-format data table containing 226 characters. G. burkemoranis adsymphysial suture morphology is shared with the baphetid Megalocephalus pachycephalus. However, the relationship of colosteids to other Paleozoic amphibian groups remains unclear, beyond their position as stem tetrapods. The single elongate Meckelian fenestra of colosteids is likely primitive for tetrapods. A three-stage model is proposed for the evolution of Meckelian fenestrae in tetrapods. Based on sutural morphology, G. burkemorani is considered to have a kinetic joint between skull table and cheek. A functional hypothesis is outlined in which movements at this joint are accommodated at the symphysis. A phylogenetically based test of this hypothesis is proposed.

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.

Nature and Quality of the Fossil Evidence for Otic Evolution in Early Tetrapods

A frequent, and frequently appropriate, role of paleontologists at a conference on the evolutionary biology of anything, is to provide temporal perspective via an overview of morphology and function based on fossils. The recipients of such paleontological benediction can then incorporate this testimony of the rocks into their own morphological and functional investigations. We have presented such papers on the otic region (e.g., Lombard and Bolt 1988), and remain convinced of their value, in fact of their necessity in evolutionary biology. However, the usefulness of a scientific paper is heavily dependent on the reader’s ability to assess the quality of both the data and interpretations presented. In the present context, factors affecting “quality” include accuracy of observation, state of preservation of fossils, techniques used to estimate relationships among species and higher taxa, and evidence and assumptions employed in inferences of function. Paleontologists often, and nonpaleontologists nearly inevitably, overlook these points when considering otic evolution.

The lissamphibian humerus and elbow joint, and the origins of modern amphibians

The origins and evolution of the three major clades of modern amphibians are still a source of controversy, and no general consensus exists as to their relationship to the various known Paleozoic taxa. This may indicate that additional character complexes should be studied to resolve their phylogenetic relationship. The salamander elbow joint has been fundamentally misinterpreted in previous morphological descriptions. In caudates and anurans, both the radius and ulna (fused in anurans) articulate with the characteristically large capitulum (radial condyle), although part of the ulnar articulating surface fits into to the smooth trochlear region. The salamander “ulnar condyle” of previous descriptions is in fact the entepicondyle. The condition seen in batrachians (i.e., salamanders and frogs) may be a lissamphibian synapomorphy because the elbow region of the primitive fossil caecilian Eocaecilia resembles those of frogs and salamanders. In addition to the large and bulbous capitulum, all lissamphibian humeri lack an entepicondylar foramen, and possess a distally pointing entepicondyle, a low and rounded ectepicondyle, and an elongated shaft. These characters are identified in key fossil forms to assess the support for the different hypotheses proposed for the evolutionary origins of lissamphibians. Temnospondyli is the only group of early tetrapods that shows a progressive evolution of lissamphibian traits in the humerus and elbow joint. Furthermore, among Paleozoic taxa, the dissorophoid temnospondyl Doleserpeton annectens is the only taxon that has the full set of humeral features shared by all lissamphibians. These results add support for the theory of a monophyletic origin of lissamphibians from dissorophoidtemnospondyls. J. Morphol., 2009.

SIGOURNEA MULTIDENTATA , A NEW STEM TETRAPOD FROM THE UPPER MISSISSIPPIAN OF IOWA, USA

Abstract Sigournea multidentata n. gen. and sp., an early tetrapod, is described from the Late Mississippian Delta locality of southern Iowa, USA. The holotype and only known specimen, a right mandible, is unique in the structure of the symphysial region, and in addition has a unique combination of characters that are shared with other tetrapods. The free ventral border of the single exomeckelian fenestra is formed by infradentary bones. Its visible portion shows several arch bases, separated by notched or straight intervals. This morphology is interpreted as indicating that the arch bases and intervals between them reflected a series of Meckelian fenestrae that were partly exo- and partly endoskeletal, due to the fact that the exoskeletal arch bases continued dorsally in Meckelian cartilage. We suggest that this may exemplify a stage in the evolution of exomeckelian fenestrae, in at least some lineages of early tetrapods. The relations of Sigournea are indeterminate. That it is a tetrapod is indicated by the presence of pit and ridge ornamentation, an open lateral line sulcus, a dorsally directed glenoid, a single elongate exomeckelian fenestra, absence of intercoronoid fossae, absence of coronoid fangs, a single row of marginal teeth on the dentary, and an absence of dentition on the prearticular. It is at present impossible to determine its relationship with other early tetrapods, and we conclude that Sigournea is best considered as an early tetrapod incertae sedis.

A NEW TEMNOSPONDYL AMPHIBIAN FROM THE LATE TRIASSIC OF TEXAS

Abstract A skull representing a new genus and species of Late Triassic temnospondyl, Rileymillerus cosgriffi, is described from the Cooper Canyon Formation, upper Dockum Group of Garza County, Texas. Rileymillerus resembles Latiscopus disjunctus in size and proportions, but the very poorly preserved unique type specimen of L. disjunctus indicates that the taxon should be considered a nomen dubium. Characters of R. cosgriffi include its small size combined with relatively small laterally-facing orbits, relatively high skull, lack of lateral line canals, lateral exposure of the palatine on the skull surface, and lack of otic notch/quadrate angle. No postcranial material can be definitely associated, although we describe a partial vertebral column that might pertain to R. cosgriffi. Relationships of R. cosgriffi are uncertain. The possibilities of a close relationship to Almsauridae, Tupilakosauridae or (especially) Brachyopoidea are explicitly examined, but for the present we consider R. cosgriffi as Temnospondyl incertae sedis. Characters described in the text have been converted to the tripartite (part, feature, state) standardized format developed for the PRESERVE web site, and are presented as a 125-character data matrix in the Appendix.

A microsaur from the Mississippian of Illinois and a standard format for morphological characters

The oldest known microsaur is preserved in a nodule from the Kinkaid Formation (Mississippian; Elvirian) collected near Goreville, Illinois. At least eight individuals are represented: three by partial skulls plus vertebral column segments with associated limb elements, and five by postcrania only. Skulls are crushed, incomplete, and exposed mainly in palatal view. Palatal bones are denticulate and the palatine has in addition a single large tooth. The basipterygoid process is laterally directed and the basipterygoid joint is open. The atlas carries large articulating facets for proatlantes, a pair of which are identifiable in one specimen. These features have not been found previously in a microsaur. All vertebral segments are dominated by a biconcave pleurocentrum; sutures between the pleurocentrum and neural arch are visible in presacral vertebrae. Distinctive microsaurian intercentra occur between all presacral pleurocentra. Their presence reinforces the hypothesis that microsaur intercentra are homologous with those of other early tetrapods. Caudal vertebrae retain separate haemal arches and some have ribs. Observed microsaur synapomorphies include: atlas with large median odontoid; atlas with concave lateral facets for occipital condyle; paired occipital condyles that are broad and concave; and thin, straplike intercentra. No observed features support a sister-group relationship with any other microsaur species, or placement within any higher level microsaur group. Because significant portions of the skeleton are missing or inaccessible, the Goreville microsaur is not formally named. A standardized, hierarchical format for skeletal characters is introduced that facilitates data sharing and comparison and fosters rapid archiving and retrieval.