R. Eric Lombard

REINTERPRETATION OF THE TEMPORAL AND OCCIPITAL REGIONS IN DIADECTES AND THE RELATIONSHIPS OF DIADECTOMORPHS

New materials from the Permo-Pennsylvanian of north-central New Mexico permit a new description of the temporal and occipital regions of the diadectomorph Diadectes. The important issue of the fate of the intertemporal bone is resolved by demonstrating its absence and apparent incorporation into the parietal as a lateral lappet. Four cranial autapomorphies of Diadectes are recognized: 1) loss of contact between postparietal and tabular; 2) supratemporal greatly enlarged with well-developed occipital process; 3) tabular no longer exposed on skull roof, but greatly reduced and incorporated into occipital plate, with a coarse, posteromedially facing surface; and 4) skull roofing bones thick and porous, with a consistent network of U-shaped grooves. The temporal-occipital region of Diadectes is compared with those of holotypic and recently collected specimens of Limnoscelis and Tseajaia, the type genera of the other two recognized diadectomorph families, Limnoscelidae and Tseajaiidae. On the basis of the literature the comparisons are extended to include certain late Paleozoic amniotes: synapsid Pelycosauria, Captorhinomorpha, and the primitive diapsid Petrolacosaurus. The results are subjected to a cladistic analysis, which supports the following hypotheses of relationships: 1) Diadectidae, Tseajaiidae, and Limnoscelidae form a natural group, the Diadectomorpha; 2) Diadectes and Tseajaia share a more recent common ancestor than either does with Limnoscelis; 3) Diadectomorpha, Pelycosauria, and their descendants form an unnamed, primitive sister clade to that consisting of Captorhinomorpha, Petrolacosaurus, and their descendants; and 4) the taxon Cotylosauria (sensu Heaton, 1980), consisting of Diadectomorpha and Seymouriamorpha, is paraphyletic and invalid. The third hypothesis dictates the assignment of Diadectomorpha to Amniota.

CHAPTER 4 – BIOGEOGRAPHY OF PRIMITIVE AMNIOTES

The origin of amniotes must have begun by the middle of Pennsylvanian or earlier. The past three decades have seen a revolution in the way terrestrial vertebrate fossils are studied. Studies by Carroll have proposed a transitional sequence of taxa that included gephyrostegids and soleonodonsaurids, which led to a primitive amniote bauplan exemplified by the small, presumably insectivorous members of the Protothyrididae family. Most of the authors consider Diadectomorpha and Seymouriamorpha to be successively more distant outgroups to the Amniota. In most recent works, the Seymouriamorpha has always been considered to be a stem Amniote group and no evidence exists for their inclusion within the Amnoita. Despite minor differences in recent phylogenetic analyses, the phylogenetic relationships and biogeographic distributions of the following taxa are considered by all to be critical to an understanding of the origin of Amniotes. In several papers describing the vertebrates and sediments of the commonly referred early Permian red-bed deposits of New Mexico, particularly the lowermost levels of the Cutler Formation, the somewhat ambiguous age of Permo-Carboniferous has been applied. It is noteworthy that only one theraspid has been reported from the early Permian called the enigmatic Tetraceratops of North-Central Texas.

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.

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 Structure of the Amphibian Auditory Periphery: A Unique Experiment in Terrestrial Hearing

The amphibian auditory periphery is traditionally viewed from one of two perspectives: (1) that it is degenerate and by implication derived from the amniote condition or (2) that it represents, at least in most frogs, a simple intermediate condition between the ears of rhipidistian fishes and reptiles. These two perspectives, or various hybrids, guide modern workers in the design of their investigations and the interpretation of the data resulting from these studies. The development and diversity of the amphibian auditory periphery is reviewed and compared to that of amniotes. The otic region in key fossil tetrapods is reviewed. Our current concepts of otic homology are reviewed and some alternative homologies are suggested. This review indicates that the most parsimonius perspective on the amphibian auditory periphery is that it represents a unique experiment in aerial hearing unrelated to that found in modern amniotes. The implications of this perspective for modern morphological and experimental work a...

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.

Structural correlates of function in the “opercularis” muscle of amphibians

SummaryThis study characterizes the fine structure of the “opercularis” muscles of selected frogs and salamanders (Genera: Hyla; Desmognathus; Ambystoma). The “opercularis” muscle originates on the shoulder girdle and inserts on the opercular plate in the fenestra ovalis of the otic capsule. Each of the three genera used exhibits one of the major gross dispositions of this muscle found in amphibians. In each case the “opercularis” muscle contains large numbers of tonic fibers: 80% in Hyla; 90% in Desmognathus; 45% in Ambystoma. These fibers correspond to the class-5 tonic fibers of Smith and Ovalle (1973). The remainder of the fibers in the “opercularis” correspond to those in the class-3 “phasic” of Smith and Ovalle. The muscle from which the “opercularis” is derived (levator scapulae in Hyla, cucullaris in Desmognathus) is comprised of fibers which correspond to the class-2 phasic fibers of Smith and Ovalle.The fiber composition of the “opercularis” indicates that it is constructed to sustain contraction over long periods of time. This composition is supportive of the functional role in audition proposed for the muscle by Lombard and Straughan (1974). Evidence is presented that indicates that fiber size may be body size dependent and thus is an inappropriate criterion of fiber type identification.

Mechanisms of underwater hearing in larval and adult tiger salamanders ambystoma tigrinum

1. A standing wave tube apparatus was used to determine the biophysical basis of underwater hearing in Ambystoma tigrinum. 2. A. tigrinum responds to the pressure component of underwater sound, and the mouth cavity appears responsible for transduction of sound pressure. 3. Near-field displacements produced by pulsations of the air-filled mouth cavity apparently stimulate the inner ear. 4. Salamander head preparations with no air-filled mouth cavity respond to the particle motion component of underwater sound, but only at sound pressure levels 40 dB or more above levels producing clear pressure sensitivity in intact salamanders or head preparations including an air-filled mouth cavity.

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.

Hearing in the Neotropical Frog, Atelopus chiriquiensis

Few studies have measured the hearing of frequencies above 1 kHz in anurans lacking middle ear structures. Auditory frequency threshold curves were determined under similar laboratory conditions for Atelopus chiriquiensis, which lacks a tympanum, middle ear cavity, and stapes, and Hyla regilla, a similarly sized species which has these middle ear structures characteristic of most frogs. Both species were found to have similar hearing capabilities, with bimodal threshold curves and peak sensitivities at about 0.2 kHz and 2-3 kHz. This record of hearing sensitivity in a frog lacking a tympanic ear corroborates a similar observation made by Loftus-Hills in 1973 for Pseudophryne semimarmorata, which also lacks most middle ear structures and is in a different family.