The evolution of larvae in temnospondyls and the stepwise origin of amphibian metamorphosis

The question of what the ancient life cycle of tetrapods was like forms a key component in understanding the origin of land vertebrates. The existence of distinct larval forms, as exemplified by many lissamphibians, and their transformation into adults is an important aspect in this field. The temnospondyls, the largest clade of Palaeozoic-Mesozoic non-amniote tetrapods, covered a wide ecomorphological range from fully aquatic to terrestrial taxa. In various species, rich ontogenetic data have accumulated over the past 130 years, permitting the study of early phases of temnospondyl development. In temnospondyls, eight ontogenetic phases have been identified in which the skeleton formed. In branchiosaurids and the eryopiform Sclerocephalus, large parts of the ossification sequence are now known. Most taxa in which small specimens are preserved had aquatic larvae with external gills that superficially resemble larval salamanders. In the edopoids, dvinosaurs, and eryopiforms, the larvae developed slowly, with incompletely ossified axial and appendicular skeletons, but possessed a fast-developing dermal skull with strong teeth. Irrespective of adult terrestriality or a fully aquatic life, there was no drastic transformation during later ontogeny, but a slow and steady acquisition of adult features. In dissorophoids, the limbs developed at a much faster pace, whereas skull formation was slowed down, especially in the amphibamiforms, and culminating in the neotenic Branchiosauridae. In the zatracheid Acanthostomatops, slow but profound transformation led to a fully terrestrial adult. The basal dissorophoid Stegops retained rapid development of dermal skull bones and established a fully dentigerous, strongly ossified palate early. In Micromelerpeton, formation of the last skull bones was slightly delayed and metamorphosis remained a long and steady phase of morphological transformations. In amphibamiforms, metamorphosis became more drastic, with an increasing number of events packed into a short phase of ontogeny. This is exemplified by Apateon, Platyrhinops, and Amphibamus in which this condensation was maximised. We distinguish three different types of metamorphosis (morphological, ecological and drastic) that evolved cumulatively in early tetrapods and within temnospondyls.

The oldest known rhynchocephalian reptile from the Middle Triassic (Ladinian) of Germany and its phylogenetic position among Lepidosauromorpha

Skeletal remains of a small lepidosaurian reptile from the Middle Triassic (Ladinian: Longobardian) Erfurt Formation, exposed in a commercial limestone quarry near Vellberg (Germany), represent the oldest rhynchocephalian known to date. The new taxon, Wirtembergia hauboldae, is diagnosed by the following combination of features: Premaxilla with four teeth, first being largest and decreasing in size from first to fourth. Jugal with tiny, spur-like posterior process. Lateral surface of dentary strongly convex dorsoventrally for much of length of bone, bearing distinct longitudinal ridge and sculpturing in large specimens. Coronoid eminence of dentary low, subrectangular, and with dorsoventrally concave lateral surface in larger specimens. Dentition with pleurodont anterior and acrodont posterior teeth. Posterior (=additional) teeth with (in side view) triangular, at mid-crown level labiolingually somewhat flattened crowns, and with oval bases. Phylogenetic analysis recovered the new rhynchocephalian as the earliest-diverging member of its clade known to date.

Using salamanders as model taxa to understand vertebrate feeding constraints during the late Devonian water-to-land transition

The vertebrate water-to-land transition and the rise of tetrapods brought about fundamental changes for the groups undergoing these evolutionary changes (i.e. stem and early tetrapods). These groups were forced to adapt to new conditions, including the distinct physical properties of water and air, requiring fundamental changes in anatomy. Nutrition (or feeding) was one of the prime physiological processes these vertebrates had to successfully adjust to change from aquatic to terrestrial life. The basal gnathostome feeding mode involves either jaw prehension or using water flows to aid in ingestion, transportation and food orientation. Meanwhile, processing was limited primarily to simple chewing bites. However, given their comparatively massive and relatively inflexible hyobranchial system (compared to the more muscular tongue of many tetrapods), it remains fraught with speculation how stem and early tetrapods managed to feed in both media. Here, we explore ontogenetic water-to-land transitions of salamanders as functional analogues to model potential changes in the feeding behaviour of stem and early tetrapods. Our data suggest two scenarios for terrestrial feeding in stem and early tetrapods as well as the presence of complex chewing behaviours, including excursions of the jaw in more than one dimension during early developmental stages. Our results demonstrate that terrestrial feeding may have been possible before flexible tongues evolved. This article is part of the theme issue 'Food processing and nutritional assimilation in animals'.

A new Middle Triassic (Ladinian) trilophosaurid stem-archosaur from Germany increases diversity and temporal range of this clade

We report the first trilophosaurid stem-archosaur from Central Europe, Rutiotomodon tytthos gen. et sp. nov., from the Middle Triassic (Ladinian) Erfurt Formation of Baden-Württemberg (Germany). It is currently known from two jaw fragments with distinctive teeth. The labiolingually wide but mesiodistally narrow maxillary and dentary teeth each have a large labial cusp from which an occlusal ridge extends lingually to a small lingual cusp. A mesial and a distal cingulum extend between the labial and lingual cusps. The mesial and distal faces of the labial cusp each bear three prominent, lingually curved apicobasal ridges (arrises). A referred partial dentary has an edentulous, expanded symphysis similar to the mandibular 'beak' in Trilophosaurus buettneri. A review of Coelodontognathus ricovi, from the Lower Triassic (Olenekian) of southwestern Russia, supports its referral to Trilophosauridae rather than Procolophonidae. Based on this reassessment and the new material from the Middle Triassic, the temporal range of trilophosaurids now spans nearly the entire Triassic Period, from the Olenekian to the Rhaetian. Trilophosaurids present craniodental features that indicate omnivory or herbivory with limited oral food processing. They were more diverse in terms of dental structure (and presumably diet) than previously assumed.

Cranial shape evolution of extant and fossil crocodile newts and its relation to reproduction and ecology

The diversity of the vertebrate cranial shape of phylogenetically related taxa allows conclusions on ecology and life history. As pleurodeline newts (the genera Echinotriton, Pleurodeles and Tylototriton) have polymorphic reproductive modes, they are highly suitable for following cranial shape evolution in relation to reproduction and environment. We investigated interspecific differences externally and differences in the cranial shape of pleurodeline newts via two‐dimensional geometric morphometrics. Our analyses also included the closely related but extinct genus Chelotriton to better follow the evolutionary history of cranial shape. Pleurodeles was morphologically distinct in relation to other phylogenetically basal salamanders. The subgenera within Tylototriton (Tylototriton and Yaotriton) were well separated in morphospace, whereas Echinotriton resembled the subgenus Yaotriton more than Tylototriton. Oviposition site choice correlated with phylogeny and morphology. Only the mating mode, with a random distribution along the phylogenetic tree, separated crocodile newts into two morphologically distinct groups. Extinct Chelotriton likely represented several species and were morphologically and ecologically more similar to Echinotriton and Yaotriton than to Tylototriton subgenera. Our data also provide the first comprehensive morphological support for the molecular phylogeny of pleurodeline newts.

A tiny new Middle Triassic stem-lepidosauromorph from Germany: implications for the early evolution of lepidosauromorphs and the Vellberg fauna

The Middle Triassic was a time of major changes in tetrapod faunas worldwide, but the fossil record for this interval is largely obscure for terrestrial faunas. This poses a severe limitation to our understanding on the earliest stages of diversification of lineages representing some of the most diverse faunas in the world today, such as lepidosauromorphs (e.g., lizards and tuataras). Here, we report a tiny new lepidosauromorph from the Middle Triassic from Vellberg (Germany), which combines a mosaic of features from both early evolving squamates and rhynchocephalians, such as the simultaneous occurrence of a splenial bone and partial development of acrodonty. Phylogenetic analyses applying different optimality criteria, and combined morphological and molecular data, consistently recover the new taxon as a stem-lepidosauromorph, implying stem-lepidosauromorph species coinhabited areas comprising today's central Europe at the same time as the earliest known rhynchocephalians and squamates. It further demonstrates a more complex evolutionary scenario for dental evolution in early lepidosauromorphs, with independent acquisitions of acrodonty early in their evolutionary history. The small size of most terrestrial vertebrates from Vellberg is conspicuous, contrasting to younger Triassic deposits worldwide, but comparable to Early Triassic faunas, suggesting a potential long-lasting Lilliput effect in this fauna.

Morphological evolution of the skull roof in temnospondyl amphibians mirrors conservative ontogenetic patterns

Addressing the patterns of ontogenetic allometry is relevant to understand morphological diversification because allometry might constrain evolution to specific directions of change in shape but also facilitate phenotypic differentiation along lines of least evolutionary resistance. Temnospondyl amphibians are a suitable group to address these issues from a deep-time perspective because different growth stages are known for numerous Palaeozoic and Mesozoic species. Herein we examine the patterns of ontogenetic allometry in the skull roof of 15 temponspondyl species and their relationship with adult morphological evolution. Using geometric morphometrics, we assessed ontogenetic and evolutionary allometries of this cranial part and the distribution of adult shapes in the morphospace to investigate whether these patterns relate to each other and/or to lifestyle and phylogeny. We found conspicuous stereotyped ontogenetic changes of the skull roof which are mirrored at the evolutionary level and consistency of the adult shape with phylogeny rather than lifestyle. These results suggest that the evolution of adult cranial shape was significantly biased by development towards pathways patterned by ontogenetic change in shape. The retrieved conserved patterns agree with a widespread evolutionary craniofacial trend found in amniotes, suggesting that they might have originated early in tetrapod evolutionary history or even earlier.

Microanatomy of the stem-turtle Pappochelys rosinae indicates a predominantly fossorial mode of life and clarifies early steps in the evolution of the shell

Unlike any other tetrapod, turtles form their dorsal bony shell (carapace) not from osteoderms, but by contribution of the ribs and vertebrae that expand into the dermis to form plate-like shell components. Although this was known from embryological studies in extant turtles, important steps in this evolutionary sequence have recently been highlighted by the Triassic taxa Pappochelys, Eorhynchochelys and Odontochelys, and the Permian Eunotosaurus. The discovery of Pappochelys shed light on the origin of the ventral bony shell (plastron), which formed from enlarged gastralia. A major question is whether the turtle shell evolved in the context of a terrestrial or aquatic environment. Whereas Odontochelys was controversially interpreted as aquatic, a terrestrial origin of turtles was proposed based on evidence of fossorial adaptations in Eunotosaurus. We report palaeohistological data for Pappochelys, a taxon that exemplifies earlier evolutionary stages in the formation of the bony shell than Odontochelys. Bone histological evidence reveals (1) evolutionary changes in bone microstructure in ribs and gastralia approaching the turtle condition and (2) evidence for a predominantly amphibious or fossorial mode of life in Pappochelys, which support the hypothesis that crucial steps in the evolution of the shell occurred in a terrestrial rather than fully aquatic environment.

Amphibian skull evolution: the developmental and functional context of simplification, bone loss and heterotopy

Despite their divergent morphology, extant and extinct amphibians share numerous features in the timing and spatial patterning of dermal skull elements. Here, I show how the study of these features leads to a deeper understanding of morphological evolution. Batrachians (salamanders and frogs) have simplified skulls, with dermal bones appearing rudimentary compared with fossil tetrapods, and open cheeks resulting from the absence of other bones. The batrachian skull bones may be derived from those of temnospondyls by truncation of the developmental trajectory. The squamosal, quadratojugal, parietal, prefrontal, parasphenoid, palatine, and pterygoid form rudimentary versions of their homologs in temnospondyls. In addition, failure to ossify and early fusion of bone primordia both result in the absence of further bones that were consistently present in Paleozoic tetrapods. Here, I propose a new hypothesis explaining the observed patterns of bone loss and emargination in a functional context. The starting observation is that jaw-closing muscles are arranged in a different way than in ancestors from the earliest ontogenetic stage onwards, with muscles attaching to the dorsal side of the frontal, parietal, and squamosal. The postparietal and supratemporal start to ossify in a similar way as in branchiosaurids, but are fused to neighboring elements to form continuous attachment areas for the internal adductor. The postfrontal, postorbital, and jugal fail to ossify, as their position is inconsistent with the novel arrangement of adductor muscles. Thus, rearrangement of adductors forms the common theme behind cranial simplification, driven by an evolutionary flattening of the skull in the batrachian stem.

Amphibian skull evolution: The developmental and functional context of simplification, bone loss and heterotopy

Despite their divergent morphology, extant and extinct amphibians share numerous features in the timing and spatial patterning of dermal skull elements. Here, I show how the study of these features leads to a deeper understanding of morphological evolution. Batrachians (salamanders and frogs) have simplified skulls, with dermal bones appearing rudimentary compared with fossil tetrapods, and open cheeks resulting from the absence of other bones. The batrachian skull bones may be derived from those of temnospondyls by truncation of the developmental trajectory. The squamosal, quadratojugal, parietal, prefrontal, parasphenoid, palatine, and pterygoid form rudimentary versions of their homologs in temnospondyls. In addition, failure to ossify and early fusion of bone primordia both result in the absence of further bones that were consistently present in Paleozoic tetrapods. Here, I propose a new hypothesis explaining the observed patterns of bone loss and emargination in a functional context. The starting observation is that jaw-closing muscles are arranged in a different way than in ancestors from the earliest ontogenetic stage onwards, with muscles attaching to the dorsal side of the frontal, parietal, and squamosal. The postparietal and supratemporal start to ossify in a similar way as in branchiosaurids, but are fused to neighboring elements to form continuous attachment areas for the internal adductor. The postfrontal, postorbital, and jugal fail to ossify, as their position is inconsistent with the novel arrangement of adductor muscles. Thus, rearrangement of adductors forms the common theme behind cranial simplification, driven by an evolutionary flattening of the skull in the batrachian stem. J. Exp. Zool. (Mol. Dev. Evol.) 9999B: XX-XX, 2014. © 2014 Wiley Periodicals, Inc.

The girdles of the oldest fossil turtle, Proterochersis robusta, and the age of the turtle crown

Background

Proterochersis robusta from the Late Triassic (Middle Norian) of Germany is the oldest known fossil turtle (i.e. amniote with a fully formed turtle shell), but little is known about its anatomy. A newly prepared, historic specimen provides novel insights into the morphology of the girdles and vertebral column of this taxon and the opportunity to reassess its phylogenetic position.

Results

The anatomy of the pectoral girdle of P. robusta is similar to that of other primitive turtles, including the Late Triassic (Carnian) Proganochelys quenstedti, in having a vertically oriented scapula, a large coracoid foramen, a short acromion process, and bony ridges that connect the acromion process with the dorsal process, glenoid, and coracoid, and by being able to rotate along a vertical axis. The pelvic elements are expanded distally and suturally attached to the shell, but in contrast to modern pleurodiran turtles the pelvis is associated with the sacral ribs.

Conclusions

The primary homology of the character “sutured pelvis” is unproblematic between P. robusta and extant pleurodires. However, integration of all new observations into the most complete phylogenetic analysis that support the pleurodiran nature of P. robusta reveals that this taxon is more parsimoniously placed along the phylogenetic stem of crown Testudines. All current phylogenetic hypotheses therefore support the basal placement of this taxon, imply that the sutured pelvis of this taxon developed independently from that of pleurodires, and conclude that the age of the turtle crown is Middle Jurassic.

Integration of molecules and new fossils supports a Triassic origin for Lepidosauria (lizards, snakes, and tuatara)

BACKGROUND

Lepidosauria (lizards, snakes, tuatara) is a globally distributed and ecologically important group of over 9,000 reptile species. The earliest fossil records are currently restricted to the Late Triassic and often dated to 227 million years ago (Mya). As these early records include taxa that are relatively derived in their morphology (e.g. Brachyrhinodon), an earlier unknown history of Lepidosauria is implied. However, molecular age estimates for Lepidosauria have been problematic; dates for the most recent common ancestor of all lepidosaurs range between approximately 226 and 289 Mya whereas estimates for crown-group Squamata (lizards and snakes) vary more dramatically: 179 to 294 Mya. This uncertainty restricts inferences regarding the patterns of diversification and evolution of Lepidosauria as a whole.

RESULTS

Here we report on a rhynchocephalian fossil from the Middle Triassic of Germany (Vellberg) that represents the oldest known record of a lepidosaur from anywhere in the world. Reliably dated to 238-240 Mya, this material is about 12 million years older than previously known lepidosaur records and is older than some but not all molecular clock estimates for the origin of lepidosaurs. Using RAG1 sequence data from 76 extant taxa and the new fossil specimens two of several calibrations, we estimate that the most recent common ancestor of Lepidosauria lived at least 242 Mya (238-249.5), and crown-group Squamata originated around 193 Mya (176-213).

CONCLUSION

A Early/Middle Triassic date for the origin of Lepidosauria disagrees with previous estimates deep within the Permian and suggests the group evolved as part of the faunal recovery after the end-Permain mass extinction as the climate became more humid. Our origin time for crown-group Squamata coincides with shifts towards warmer climates and dramatic changes in fauna and flora. Most major subclades within Squamata originated in the Cretaceous postdating major continental fragmentation. The Vellberg fossil locality is expected to become an important resource for providing a more balanced picture of the Triassic and for bridging gaps in the fossil record of several other major vertebrate groups.

Integration of molecules and new fossils supports a Triassic origin for Lepidosauria (lizards, snakes, and tuatara)

BACKGROUND

Lepidosauria (lizards, snakes, tuatara) is a globally distributed and ecologically important group of over 9,000 reptile species. The earliest fossil records are currently restricted to the Late Triassic and often dated to 227 million years ago (Mya). As these early records include taxa that are relatively derived in their morphology (e.g. Brachyrhinodon), an earlier unknown history of Lepidosauria is implied. However, molecular age estimates for Lepidosauria have been problematic; dates for the most recent common ancestor of all lepidosaurs range between approximately 226 and 289 Mya whereas estimates for crown-group Squamata (lizards and snakes) vary more dramatically: 179 to 294 Mya. This uncertainty restricts inferences regarding the patterns of diversification and evolution of Lepidosauria as a whole.

RESULTS

Here we report on a rhynchocephalian fossil from the Middle Triassic of Germany (Vellberg) that represents the oldest known record of a lepidosaur from anywhere in the world. Reliably dated to 238-240 Mya, this material is about 12 million years older than previously known lepidosaur records and is older than some but not all molecular clock estimates for the origin of lepidosaurs. Using RAG1 sequence data from 76 extant taxa and the new fossil specimens two of several calibrations, we estimate that the most recent common ancestor of Lepidosauria lived at least 242 Mya (238-249.5), and crown-group Squamata originated around 193 Mya (176-213).

CONCLUSION

A Early/Middle Triassic date for the origin of Lepidosauria disagrees with previous estimates deep within the Permian and suggests the group evolved as part of the faunal recovery after the end-Permain mass extinction as the climate became more humid. Our origin time for crown-group Squamata coincides with shifts towards warmer climates and dramatic changes in fauna and flora. Most major subclades within Squamata originated in the Cretaceous postdating major continental fragmentation. The Vellberg fossil locality is expected to become an important resource for providing a more balanced picture of the Triassic and for bridging gaps in the fossil record of several other major vertebrate groups.

The cranial osteology and feeding ecology of the metriorhynchid crocodylomorph genera Dakosaurus and Plesiosuchus from the late Jurassic of Europe

Background

Dakosaurus and Plesiosuchus are characteristic genera of aquatic, large-bodied, macrophagous metriorhynchid crocodylomorphs. Recent studies show that these genera were apex predators in marine ecosystems during the latter part of the Late Jurassic, with robust skulls and strong bite forces optimized for feeding on large prey.

Methodology/Principal Findings

Here we present comprehensive osteological descriptions and systematic revisions of the type species of both genera, and in doing so we resurrect the genus Plesiosuchus for the species Dakosaurus manselii. Both species are diagnosed with numerous autapomorphies. Dakosaurus maximus has premaxillary ‘lateral plates’; strongly ornamented maxillae; macroziphodont dentition; tightly fitting tooth-to-tooth occlusion; and extensive macrowear on the mesial and distal margins. Plesiosuchus manselii is distinct in having: non-amblygnathous rostrum; long mandibular symphysis; microziphodont teeth; tooth-crown apices that lack spalled surfaces or breaks; and no evidence for occlusal wear facets. Our phylogenetic analysis finds Dakosaurus maximus to be the sister taxon of the South American Dakosaurus andiniensis, and Plesiosuchus manselii in a polytomy at the base of Geosaurini (the subclade of macrophagous metriorhynchids that includes Dakosaurus, Geosaurus and Torvoneustes).

Conclusions/Significance

The sympatry of Dakosaurus and Plesiosuchus is curiously similar to North Atlantic killer whales, which have one larger ‘type’ that lacks tooth-crown breakage being sympatric with a smaller ‘type’ that has extensive crown breakage. Assuming this morphofunctional complex is indicative of diet, then Plesiosuchus would be a specialist feeding on other marine reptiles while Dakosaurus would be a generalist and possible suction-feeder. This hypothesis is supported by Plesiosuchus manselii having a very large optimum gape (gape at which multiple teeth come into contact with a prey-item), while Dakosaurus maximus possesses craniomandibular characteristics observed in extant suction-feeding odontocetes: shortened tooth-row, amblygnathous rostrum and a very short mandibular symphysis. We hypothesise that trophic specialisation enabled these two large-bodied species to coexist in the same ecosystem.

First diagnostic marine reptile remains from the Aalenian (Middle Jurassic): a new ichthyosaur from southwestern Germany

BACKGROUND

The Middle Jurassic was a critical time in the evolutionary history of ichthyosaurs. During this time interval, the diverse, well-studied faunas of the Lower Jurassic were entirely replaced by ophthalmosaurids, a new group that arose sometime prior to the Aalenian-Bajocian boundary and by the latest middle Jurassic comprised the only surviving group of ichthyosaurs. Thus, the Middle Jurassic Aalenian-Bathonian interval (176-165 million years ago) comprises the time frame during which ophthalmosaurids not only originated but also achieved taxonomic dominance. However, diagnostic ichthyosaur remains have been described previously from only a single locality from this interval, from the Bajocian of Argentina.

METHODOLOGY/PRINCIPAL FINDINGS

In this paper, we describe a new species of ichthyosaur based on a partial articulated specimen from the Middle Jurassic of southwestern Germany. This specimen was recovered from the Opalinuston Formation (early Aalenian) and is referable to Stenopterygius aaleniensis sp. nov. reflecting features of the skull and forefin. The genus Stenopterygius is diverse and abundant in the Lower Jurassic of Europe, but its presence has not previously been confirmed in younger (Middle Jurassic) rocks from the northern hemisphere.

CONCLUSIONS/SIGNIFICANCE

This specimen represents the only diagnostic ichthyosaur remains reported from the Aalenian. It bears numerous similarities in size and in morphology to the Lower Jurassic species of the genus Stenopterygius and provides additional evidence that the major ecological changes hypothesized to have occurred at the end of the Toarcian took place sometime after this point and most likely did not occur suddenly. There is currently no evidence for the presence of ophthalmosaurids in the northern hemisphere during the Aalenian-Bathonian interval.

The sail-backed reptile Ctenosauriscus from the latest Early Triassic of Germany and the timing and biogeography of the early archosaur radiation

BACKGROUND

Archosaurs (birds, crocodilians and their extinct relatives including dinosaurs) dominated Mesozoic continental ecosystems from the Late Triassic onwards, and still form a major component of modern ecosystems (>10,000 species). The earliest diverse archosaur faunal assemblages are known from the Middle Triassic (c. 244 Ma), implying that the archosaur radiation began in the Early Triassic (252.3-247.2 Ma). Understanding of this radiation is currently limited by the poor early fossil record of the group in terms of skeletal remains.

METHODOLOGY/PRINCIPAL FINDINGS

We redescribe the anatomy and stratigraphic position of the type specimen of Ctenosauriscus koeneni (Huene), a sail-backed reptile from the Early Triassic (late Olenekian) Solling Formation of northern Germany that potentially represents the oldest known archosaur. We critically discuss previous biomechanical work on the 'sail' of Ctenosauriscus, which is formed by a series of elongated neural spines. In addition, we describe Ctenosauriscus-like postcranial material from the earliest Middle Triassic (early Anisian) Röt Formation of Waldhaus, southwestern Germany. Finally, we review the spatial and temporal distribution of the earliest archosaur fossils and their implications for understanding the dynamics of the archosaur radiation.

CONCLUSIONS/SIGNIFICANCE

Comprehensive numerical phylogenetic analyses demonstrate that both Ctenosauriscus and the Waldhaus taxon are members of a monophyletic grouping of poposauroid archosaurs, Ctenosauriscidae, characterised by greatly elongated neural spines in the posterior cervical to anterior caudal vertebrae. The earliest archosaurs, including Ctenosauriscus, appear in the body fossil record just prior to the Olenekian/Anisian boundary (c. 248 Ma), less than 5 million years after the Permian-Triassic mass extinction. These earliest archosaur assemblages are dominated by ctenosauriscids, which were broadly distributed across northern Pangea and which appear to have been the first global radiation of archosaurs.

Palaeoecological and palaeoenvironmental influences revealed by long-bone palaeohistology: the example of the Permian branchiosaurid Apateon

Apateon, a small temnospondyl from the Permian freshwater-lake deposits of the Saar-Nahe Basin (SW Germany), is known by exceptionally well-preserved material. Here we report the first palaeohistological analysis of Apateon that focuses on its life-history traits and palaeoenvironments. Different samples (different localities and horizons) of Apateon caducus and Apateon pedestris have been analysed. Their stylopod histology shows different growth rhythms that might be correlated to changes in palaeoecosystems: food availability and/or presence of predators. Palaeoenvironmental influences are also recognized during the limb-bone osteogenesis by the expression of simple and/or double patterns of Lines of Arrested Growth (LAG). A double-LAG pattern expresses hibernating and aestivating arrests of growth in extant newts. The fossil samples from the two stratigraphically oldest horizons preserved a similar double-LAG pattern, suggesting that they may have hibernated and aestivated every year because of harsh climatic conditions. The Saar-Nahe lake system probably passed from a higher altitude zone into a lower one, possibly because of subsidence and/or erosion. It could also be correlated to the size of the lakes that differs from one locality to another, inducing different responses of the organisms to the climatic variations.

Riedl's burden and the body plan: selection, constraint, and deep time

Rupert Riedl's concept of burden forms a causal hypothesis on organismic integration and evolutionary constraints. Defined as the hierarchically nested interdependence of characters within the organism, burden was seen as (1) defining and conserving body plans and (2) constraining and directing evolutionary trajectories. A review of the components of the burden concept reveals important consistencies with the modern tenets of evo-devo. This concept differs from the current consensus of evolutionary theory in that it (1) grants evolution less options for changing tightly integrated, "locked-in" characters and (2) in deducing from this an ever decreasing freedom for evolution, with cyclism and typostrophism as resulting macroevolutionary phenomena. Despite these differences, I show that the burden concept was consistent with most major tenets of the Modern Synthesis, and Riedl attempted to explain patterns of large-scale evolutionary trends exclusively by microevolutionary (gradualistic) processes. The burden concept is fruitful and unique in its focus on hierarchically nested constraints and resembles the hierarchical architecture of gene regulatory networks. However, such networks are more high-dimensional and most of their components appear to be easier to evolve than Riedl's burden. Yet in combination with evolvability, a modified concept of burden might contribute substantially to the understanding of organismic integration and the long-term evolution of body plans.

Amphibian development in the fossil record

Ontogenetic series of extinct taxa are extremely rare and when preserved often incomplete and difficult to interpret. However, the fossil record of amphibians includes a number of well-preserved ontogenetic sequences for temnospondyl and lepospondyl taxa, which have provided valuable information about the development of these extinct groups. Here we summarize the current knowledge on fossil ontogenies of amphibians, their potential and limitations for relationship assessments, and discuss the insights they have provided for our understanding of the anatomy, life history, and ecology of extinct amphibians.

Life-cycle evolution as response to diverse lake habitats in Paleozoic amphibians

The evolution of life cycles forms the subject of numerous studies on extant organisms, but is rarely documented in the fossil record. Here, I analyze patterns of development in time-averaged samples of late Carboniferous and early Permian amphibians, and compare them to paleoecological patterns derived from the same deposits located within a large sedimentary basin (Saar-Nahe, Germany). In 300-297 million years (myr) old Sclerocephalus haeuseri (1-1.7 m), adult size, morphology, and the course of ontogeny varied with respect to the habitats in which the species existed. These differences are best exemplified by ontogenetic trajectories, which reveal a full range of modifications correlating with environmental parameters (lake properties, food resources, competitors). In a 2- to 3-myr-long interval, six different lake habitats were inhabited by this species, which responded to changes by modification of growth rate, adult size, developmental sequence, skeletal features, prey preference, and relative degree of terrestriality.

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.

A relict basal tetrapod from Germany: first evidence of a Triassic chroniosuchian outside Russia

Chroniosuchians are basal tetrapods nesting within the paraphyletic anthracosaurs and were so far only well known from the Permian and Triassic of Russia. In this study, we present evidence for their existence in the upper Middle Triassic of Germany, based on diagnostic osteoderms and vertebrae from the Kupferzell and Vellberg localities in southern Germany. The finds are most similar to Synesuchus, a Middle Triassic bystrowianid chroniosuchian from the Northern Ural Pechora region. They demonstrate that by Middle Triassic time, chroniosuchians were much more widespread than previously thought.

Limb ossification in the Paleozoic branchiosaurid Apateon (Temnospondyli) and the early evolution of preaxial dominance in tetrapod limb development

Despite the wide range of shapes and sizes that accompany a vast variety of functions, the development of tetrapod limbs follows a conservative pattern of de novo condensation, branching, and segmentation. Development of the zeugopodium and digital arch typically occurs in a posterior to anterior sequence, referred to as postaxial dominance, with a digital sequence of 4-3-5-2-1. The only exception to this pattern in all of living Tetrapoda can be found in salamanders, which display a preaxial dominance in limb development, a de novo condensation of a basale commune (distal carpal/tarsal 1+2) and a precoccial development of digits I and II. These divergent patterns have puzzled researchers for over a century leading to various explanatory hypotheses. Despite many advances in research on tetrapod limb development, the divergent evolution of these two pathways and its causes are still not understood. Based on an extensive ontogenetic series we investigated the pattern of limb development of the 300 Ma old branchiosaurid amphibian Apateon. This revealed a preaxial dominance in limb development that was previously believed to be unique and derived for modern salamanders. The Branchiosauridae are favored as close relatives of extant salamanders in most phylogenetic hypotheses of the highly controversial origins and relationships of extant amphibians. The findings provide new insights into the evolution of developmental pathways in tetrapod limb development, the relationships of modern amphibians with possible Paleozoic antecedents, and their initial timing of divergence.

Skull ontogeny: developmental patterns of fishes conserved across major tetrapod clades

In vertebrates, the ontogeny of the bony skull forms a particularly complex part of embryonic development. Although this area used to be restricted to neontology, recent discoveries of fossil ontogenies provide an additional source of data. One of the most detailed ossification sequences is known from Permo-Carboniferous amphibians, the branchiosaurids. These temnospondyls form a near-perfect link between the piscine osteichthyans and the various clades of extant tetrapods, retaining a full complement of dermal bones in the skull. For the first time, the broader evolutionary significance of these event sequences is analyzed, focusing on the identification of sequence heterochronies. A set of 120 event pairs was analyzed by event pair cracking, which helped identify active movers. A cladistic analysis of the event pair data was also carried out, highlighting some shared patterns between widely divergent clades of tetrapods. The analyses revealed an unexpected degree of similarity between the widely divergent taxa. Most interesting is the apparently modular composition of the cranial sequence: five clusters of bones were discovered in each of which the elements form in the same time window: (1) jaw bones, (2) marginal palatal elements, (3) circumorbital bones, (4) skull roof elements, and (5) neurocranial ossifications. In the studied taxa, these "modules" have in most cases been shifted fore and back on the trajectory relative to the Amia sequence, but did not disintegrate. Such "modules" might indicate a high degree of evolutionary limitation (constraint).

Metamorphosis and neoteny: alternative pathways in an extinct amphibian clade

The Branchiosauridae was a clade of small amphibians from the Permo-Carboniferous with an overall salamander-like appearance. The clade is distinguished by an extraordinary fossil record that comprises hundreds of well-preserved specimens, representing a wide range of ontogenetic stages. Branchiosaurids had external gills and weakly ossified skeletons, and due to this larval appearance their status as neotenic (perennibranchiate) forms has long been accepted. Despite their extensive fossil record large specimens with an adult morphology appeared to be lacking altogether, but recently two adult specimens were identified in a rich sample of Apateon gracilis collected in the 19th century from a locality near Dresden, Saxony. These specimens are unique among branchiosaurids in showing a high level of ossification, including bones that have never been reported in a branchiosaur. These highlight the successive formation of features believed to indicate terrestrial locomotion, as well as feeding on larger prey items. Moreover, these transformations occurred in a small time window (whereas the degree of size increase is used as a proxy of time) and the degree of concentration of developmental events in branchiosaurids is unique among tetrapods outside the lissamphibians. These specimens are compared with large adults of the neotenic branchiosaurid Apateon caducus from the Saar-Nahe Basin, which despite their larger body size lack the features found in the adult A. gracilis specimens. These specimens give new insight into patterns of metamorphosis (morphological transformation) in branchiosaurids that are believed to be correlated to a change of habitat, and clearly show that different life-history pathways comparable to those of modern salamanders were already established in this Paleozoic clade.

Ontogenetic evidence for the Paleozoic ancestry of salamanders

The phylogenetic positions of frogs, salamanders, and caecilians have been difficult to establish. Data matrices based primarily on Paleozoic taxa support a monophyletic origin of all Lissamphibia but have resulted in widely divergent hypotheses of the nature of their common ancestor. Analysis that concentrates on the character states of the stem taxa of the extant orders, in contrast, suggests a polyphyletic origin from divergent Paleozoic clades. Comparison of patterns of larval development in Paleozoic and modern amphibians provides a means to test previous phylogenies based primarily on adult characteristics. This proves to be highly informative in the case of the origin of salamanders. Putative ancestors of salamanders are recognized from the Permo-Carboniferous boundary of Germany on the basis of ontogenetic changes observed in fossil remains of larval growth series. The entire developmental sequence from hatching to metamorphosis is revealed in an assemblage of over 600 specimens from a single locality, all belonging to the genus Apateon. Apateon forms the most speciose genus of the neotenic temnospondyl family Branchiosauridae. The sequence of ossification of individual bones and the changing configuration of the skull closely parallel those observed in the development of primitive living salamanders. These fossils provide a model of how derived features of the salamander skull may have evolved in the context of feeding specializations that appeared in early larval stages of members of the Branchiosauridae. Larvae of Apateon share many unique derived characters with salamanders of the families Hynobiidae, Salamandridae, and Ambystomatidae, which have not been recognized in any other group of Paleozoic amphibians.