Jason S. Anderson

A stem batrachian from the Early Permian of Texas and the origin of frogs and salamanders

The origin of extant amphibians (Lissamphibia: frogs, salamanders and caecilians) is one of the most controversial questions in vertebrate evolution, owing to large morphological and temporal gaps in the fossil record. Current discussions focus on three competing hypotheses: a monophyletic origin within either Temnospondyli or Lepospondyli, or a polyphyletic origin with frogs and salamanders arising among temnospondyls and caecilians among the lepospondyls. Recent molecular analyses are also controversial, with estimations for the batrachian (frog–salamander) divergence significantly older than the palaeontological evidence supports. Here we report the discovery of an amphibamid temnospondyl from the Early Permian of Texas that bridges the gap between other Palaeozoic amphibians and the earliest known salientians and caudatans from the Mesozoic. The presence of a mosaic of salientian and caudatan characters in this small fossil makes it a key taxon close to the batrachian (frog and salamander) divergence. Phylogenetic analysis suggests that the batrachian divergence occurred in the Middle Permian, rather than the late Carboniferous as recently estimated using molecular clocks, but the divergence with caecilians corresponds to the deep split between temnospondyls and lepospondyls, which is congruent with the molecular estimates.

The phylogenetic definition of reptilia.

Naming taxa is an important endeavor in the documentation of life by systematists, whether it is conducted in the context of traditional rank-based classification or within a phylogenetic framework. Proponents of the phylogenetic approach distinguish between the diagnosis of a group and its definition (Ghiselin, 1984; Rowe, 1987, 1988), and this distinction forms the basis for a phylogenetically based method of naming taxa formerly referred to as Phylogenetic Taxonomy (de Queiroz and Gauthier, 1990, 1992, 1994) and now called Phylogenetic Nomenclature (PN; Cantino et al., 1999; Gauthier and de Queiroz, 2001; Bryant and Cantino, 2002). Emphasis in naming has been placed on ancestry using phylogenetic definitions, and the widespread adoption of nodeand stem-based definitions (apomorphy-based definitions have yet to receive as widespread acceptance, but see Gauthier and de Queiroz, 2001; Anderson, 2002; Laurin and Anderson, 2004) has led to a proliferation of new names and definitions. This shift in nomenclatural practice has, unfortunately, fostered a growth in redundant names and definitions for well-known taxa (Benton, 2000). The PhyloCode (Cantino and de Queiroz, 2003) has modified the rule of priority as used in other codes (i.e., International Code of Zoological Nomenclature) to determine which of two or more possible names with equivalent definitions is valid (Brochu and Sumrall, 2001), or which of several definitions for a given name is valid (Cantino and de Queiroz, 2003). Unfortunately, it is now apparent that some of the definitions for wellknown taxon names established early in the emergence of PN were not devised following conventions now widely accepted, by either defining groups in an overly restrictive manner, or via selection of reference taxa without due consideration of the ramifications of differing tree topologies (Anderson, 2002; Laurin and Anderson, 2004). It has become evident in broad-scale amniote taxonomy that the first published definition for Reptilia (Gauthier et al., 1988a), which would have priority under a binding PhyloCode, is problematic because of the dramatic controversies over the affinities of the specifier taxon Testudines (see Zardoya and Meyer, 2001 for review of hypotheses for turtle relationships). Recent morphological and molecular studies have challenged conventional hypotheses concerning the affinities of turtles, and this has led to unexpected and undocumented changes in the composition of the well-known taxon Reptilia, with additional ramifications for the nomenclature of some of its included taxa. We examine the consequences of the application of priority to the nomen Reptilia as our understanding of early amniote interrelationships has progressed over the past two decades, and offer a new definition that brings the phylogenetic concept of this taxon name into line with both currently accepted conventions of PN and historical usage. This new definition corrects an error created by the combination of the selection of a higher taxon (rather than a species) as a specifier, and an unexpected topology. We believe that now is an appropriate time to examine the definitions established when PN was in its earliest stages, and hope to correct what we consider to be a poorly formulated definition upon publication of a binding PhyloCode.

Focal Review: The Origin(s) of Modern Amphibians

The recent description of the stem batrachian Gerobatrachus has changed the terms of the ongoing debate on the origin of extant amphibians (Lissamphibia: frogs, salamanders, and the limbless caecilians). This important fossil, through a shared mosaic of unique derived salientian and urodele characters, links frogs and salamanders with an archaic group of fossil amphibians known as amphibamid temnospondyls. The present paper reviews the impact of this fossil on morphological and molecular phylogenies, and divergence timing estimates based on molecular models and the fossil record. In morphology, most recent efforts have focused on better characterizing the anatomy and relationships of amphibamid temnospondyls. Progress has also been made with the complete description of the earliest caecilian Eocaecilia; however, the question of caecilian origins remains unresolved at present. The large scale phylogenetic analyses all agree on the overall tetrapod tree phylogenetic structure, and the largest analyses agree that the origin of at least frogs and salamanders among fossils from family Amphibamidae. Conversely, all molecular based analyses find a monophyletic Lissamphibia, and a Batrachia terminal dichotomy, which raises questions over either the validity of morphological analyses that support lissamphibian polyphyly or about the possibility of long branch attraction given the short internal divergences and long subsequent branches. Paradoxically, the estimated date of the lissamphibian divergence best matches the fossil record if timed to the split between lepospondyls and temnospondyls. Future research should focus on development and fine details of cranial anatomy of fossil and extant amphibians to produce new evidence and clarity into the question of lissamphibian, and especially caecilian, origins.

Georgenthalia clavinasica, A New Genus and Species of Dissorophoid Temnospondyl from the Early Permian of Germany, and the Relationships of the Family Amphibamidae

Abstract Georgenthalia clavinasica, a new genus and species of amphibamid dissorophoid temnospondyl, is represented by a small, complete, postmetamorphic skull from the Lower Permian Bromacker locality, Germany. It is only the third non-amniote of an assemblage of 12 terrestrial tetrapod taxa known from this locality. It is characterized by a broadly rounded skull with large orbits, a short postorbital length, and a unique keyhole-shaped external naris superficially resembling that of trematopid dissorophoids. New features that help to discriminate between amphibamid species are highlighted. Phylogenetically informative characters present in G. clavinasica include: anterolateral flaring of the lateral margin of frontals; narrow interorbital width; ventral orbital process of the prefrontal contacts palatine; palatine exposed on lateral surface of ventral rim of orbit; large otic notches closely approach the orbits; absence of a supratympanic flange of otic notch; long supratympanic process of squamosal with flange-like process that underlaps the midcentral portion of supratemporal. Several features support a highly terrestrial habitus of G. clavinasica, which is consistent with the interpretation of the fossiliferous beds of the Bromacker quarry as representing an upland terrestrial environment in which limnic conditions were characterized by ephemeral lakes and ponds. A new clade, Olsoniformes, is named for Dissorophidae and Trematopidae (dissorophoids exclusive of Amphibamidae), and new phylogenetic definitions for Amphibamidae, Trematopidae, and Dissorophidae are presented. The small neotenic Micromelerpetontidae, and presumably also the neotenic and larval Branchiosauridae, fall within the newly defined clade Amphibamidae in the current study, and future work should focus on clarifying the nature of this relationship.

The armoured dissorophid Cacops from the Early Permian of Oklahoma and the exploitation of the terrestrial realm by amphibians.

Cacops, one of the most distinctive Paleozoic amphibians, is part of a clade of dissorophoid temnospondyls that diversified in the equatorial region of Pangea during the Late Carboniferous and Early Permian, persisting into the Late Permian in Central Russia and China. Dissorophids were a successful group of fully terrestrial, often spectacularly armoured predators, the only amphibians apparently able to coexist with amniotes when the latter started to dominate terrestrial ecosystems. In this paper, we describe excellent new skulls from the Early Permian of Oklahoma attributed to Cacops, Cacops morrisi sp. nov. and provide for the first time detailed information about this iconic dissorophid. These specimens show anatomical and ontogenetic features that will impact on future studies on the evolution of terrestriality in tetrapods. For example, the large, posteriorly closed tympanic embayment has fine striations on an otherwise smooth surface, documenting the oldest known clear evidence for the presence of a tympanic membrane in the fossil record, a structure that is used for hearing airborne sound in extant tetrapods. The skull of C. morrisi also has several features associated with predatory behaviour, indicating that this dissorophid may have been one of the top terrestrial predators of its time.

Variation in the Skull of Anchiceratops (Dinosauria, Ceratopsidae) from the Horseshoe Canyon Formation (Upper Cretaceous) of Alberta

ABSTRACT Anchiceratops is a chasmosaurine ceratopsid from the Upper Cretaceous Horseshoe Canyon Formation (HCF) of Alberta. It is distinguished primarily by its unique parietosquamosal frill ornamentation and possibly by the presence of a ventrally flexed olfactory bulb of the brain. Although Anchiceratops is known from at least ten partial skulls, only two of these have been formally described. These skulls are not stratigraphically segregated, but they differ markedly in their proportions (e.g., supraorbital horncore and frill dimensions), causing previous authors to account for this disparity with reference to either interspecific or sexual differences. Both of these hypotheses assume that variation in Anchiceratops is dimorphic; however, this assumption has never been tested with reference to all available material. The present study describes all material from the HCF that can be positively attributed to Anchiceratops, and tests the assumption of dimorphism by subjecting this material to a series of morphometric analyses. We find no compelling evidence for dimorphism in Anchiceratops, although sample size is still too small for convincing statistical support. We conclude that there is a single, variable species of Anchiceratops, A. ornatus. Average sedimentation rates for the HCF suggest that A. ornatus is a particularly long-lived species compared with other ceratopsids (∼1.5–2.0 Ma), and the paleoecological implications of this are discussed. A cladistic analysis that includes the new data presented here indicates that Anchiceratops is more closely related to Chasmosaurus than to Triceratops, in contrast with previous studies.

Meaning of the Name Tetrapoda in the Scientific Literature: An Exchange

This exchange stems from an on-going debate between advocates of crown clade (e.g., Gauthier et al., 1988; Rowe, 1988; Laurin, 1998) and those of apomorphybased (e.g., Ahlberg and Clack, 1998; Lee, 1999; Padian et al., 1999; Anderson, 2001) definitions of widely used taxon names (see Table 1 for a glossary of terms). Both types of definitions have advantages: apomorphy-based definitions usually retain a composition of taxa similar to that proposed in most paleontological studies (Lee, 1999), whereas crown clade definitions correspond more closely to the usage of these names by comparative biologists working on extant taxa (de Queiroz and Gauthier, 1992; but see Bryant, 1994; Lee, 1996). Here, we concentrate on the name Tetrapoda, which has been the subject of recent discussion (Anderson, 2001, 2002; Laurin, 2002), but many of the arguments presented here could be applied to other widely used names, such as Mammalia, Aves, and Vertebrata. Laurin (1998, 2002) adopted the crown clade definition of the name Tetrapoda (the last common ancestor of amniotes and lissamphibians, and all its descendants) proposed by Gauthier et al. (1988). Anderson (2002) objected to the crown clade definition of Tetrapoda and, following Lee (1999), advocated adoption of an apomorphy-based definition of this name (the first sarcopterygian to have possessed digits homologous with those in Homo sapiens, and all its descendants). The following exchange is aimed at clarifying the respective advantages of both types of definitions. Established usage of taxon names is important because the systematic community will soon define many widely used names, and the PhyloCode (Cantino and de Queiroz, 2000) recommends, when defining a name, to disrupt current and historical usage as little as possible (Recommendations 10A, 11A). Furthermore, as stated by Anderson (2002: 824), “the PhyloCode provides mechanisms for the amendment (Article 13) or suppression (Article 15) of definitions with priority if they should contravene long accepted usage and thus create instability in nomenclature.” These considerations are important because name definitions under the PhyloCode delimit taxa (i.e., determine their inclusiveness when applied in the context of a particular phylogenetic hypothesis), whereas definitions under traditional rank-based codes do not. Within the traditional rankbased nomenclature system, debates about the inclusiveness can go on indefinitely, whereas within the PhyloCode system, such debates will be minimized (given a particular phylogenetic hypothesis) after the International Committee on Phylogenetic Nomenclature (ICPN) determines which of two conflicting phylogenetic definitions, or names with the same definition, should be retained. The ICPN will have ruling authority (although we expect such actions will be rare if the rules of the PhyloCode are followed, similar to current practice under the International Code of Zoological Nomenclature), but it is more productive to discuss various competing definitions prior to the implementation of the PhyloCode. By ensuring that the most appropriate definition becomes established at the initiation of the PhyloCode system, much future controversy and ICPN action can be avoided. The case of the name Tetrapoda is not unique. Similar disagreements between proponents of crown clade and apomorphy-based definitions have involved several other widely used names, such as Aves (reviewed by Gauthier and de Queiroz, 2001) and Mammalia (Rowe, 1988; Desui, 1991; Rowe and Gauthier, 1992; Wible and Hopson, 1995; Ji et al., 1999; Sidor, 2001). Thus, the current debate on the meaning of the word Tetrapoda can serve as a case study that may help with the resolution of similar disputes in the future. In this point–counterpoint, we debate four key issues related to the definition of the name Tetrapoda.

Skull Ecomorphology of Megaherbivorous Dinosaurs from the Dinosaur Park Formation (Upper Campanian) of Alberta, Canada

Megaherbivorous dinosaur coexistence on the Late Cretaceous island continent of Laramidia has long puzzled researchers, owing to the mystery of how so many large herbivores (6–8 sympatric species, in many instances) could coexist on such a small (4–7 million km2) landmass. Various explanations have been put forth, one of which–dietary niche partitioning–forms the focus of this study. Here, we apply traditional morphometric methods to the skulls of megaherbivorous dinosaurs from the Dinosaur Park Formation (upper Campanian) of Alberta to infer the ecomorphology of these animals and to test the niche partitioning hypothesis. We find evidence for niche partitioning not only among contemporaneous ankylosaurs, ceratopsids, and hadrosaurids, but also within these clades at the family and subfamily levels. Consubfamilial ceratopsids and hadrosaurids differ insignificantly in their inferred ecomorphologies, which may explain why they rarely overlap stratigraphically: interspecific competition prevented their coexistence.

The Braincase of Eocaecilia micropodia (Lissamphibia, Gymnophiona) and the Origin of Caecilians

The scant fossil record of caecilians has obscured the origin and evolution of this lissamphibian group. Eocaecilia micropodia from the Lower Jurassic of North America remains the only stem-group caecilian with an almost complete skull preserved. However, this taxon has been controversial, engendering re-evaluation of traits considered to be plesiomorphic for extant caecilians. Both the validity of the placement of E. micropodia as a stem caecilian and estimates of the plesiomorphic condition of extant caecilians have been questioned. In order to address these issues, the braincase of E. micropodia was examined via micro-computed tomography. The braincase is considered to be a more reliable phylogenetic indicator than peripheral regions of the skull. These data reveal significant new information, including the possession of an ossified nasal septum, ossified anterior wall of the sphenethmoid, long anterolateral processes on the sphenethmoid, and paired olfactory nerve foramina, which are known only to occur in extant caecilians; the latter are possibly related to the evolution of the tentacle, a caecilian autapomorphy. A phylogenetic analysis that included 64 non-amniote taxa and 308 characters represents the first extensive test of the phylogenetic affinities of E. micropodia. The results place E. micropodia securely on the stem of extant caecilians, representing a clade within Temnospondyli that is the sister taxon to batrachians plus Gerobatrachus. Ancestral character state reconstruction confirms the braincase of E. micropodia to be largely representative of the plesiomorphic condition of extant caecilians. Additionally, the results refine the context within which the evolution of the caecilian form can be evaluated. The robust construction and pattern of the dermal skull of E. micropodia is interpreted as symplesiomorphic with advanced dissorophoid temnospondyls, rather than being autapomorphic in its robust construction. Together these data increase confidence in incorporating E. micropodia into discussions of caecilian evolution.

REVISION OF THE AÏSTOPOD GENUS PHLEGETHONTIA (TETRAPODA: LEPOSPONDYLI)

Abstract The aïstopod family Phlegethontiidae is restudied based on new specimens from Pit 11 of Mazon Creek, Illinois, and the coal shales of Nýřany, Czech Republic, as well as most available specimens from North America. Phlegethontiids have highly fenestrate skulls, with orbits placed just anterior their skulls mid point. Dermal skull bones are greatly reduced in number and limited in extent, whereas the endochondral braincase is hyperossified. The frontals are fused medially and enclose the parietal foramen and anterior sagittal crest. As in most other aïstopods, the quadrate, pterygoid, and epipterygoid are fused into a composite bone, the palatoquadrate complex. Details of cranial anatomy contradict a previous model of cranial kinesis by severely limiting the skulls potential mobility. Remnants of the pectoral girdle are present, perhaps due to the presence of an operculum–opercularis-like connection to the stapes. No remnants of the pelvis are present. Three species are recognised within the family. Phlegethontia linearis has short anterior vertebrae, high neural spines on at least the anterior four vertebrae, and vertebrae number between 230–250 in total. Phlegethontia longissima has low neural spines throughout the column, anterior vertebrae that are twice as long as P. linearis, and only 200–210 total vertebrae. Sillerpeton permianum, known from a single braincase and an unassociated string of vertebrae, is distinguished from Phlegethontia by the retention of a separate foramen for the passage of the occulomotor nerve. Phlegethontia “phanerhalpa” is a tiny braincase fragment that differs from the other species of Phlegethontia only in the placement of the jugular foramen relative to the centre of the foramen magnum. This is probably a size-related feature, and P. “phanerhalpa” is considered a nomen dubium.