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.     

Salamander-like development in a seymouriamorph revealed by palaeohistology

The amniotes generally lay eggs on land and are thereby differentiated from lissamphibians (salamanders, frogs and caecilians) by their developmental pattern. Although a number of 330-300-Myr old fossils are regarded as early tetrapods placed close to amniotes on the basis of anatomical data, we still do not know whether their developmental pattern was more similar to those of lissamphibians or amniotes. Here we report palaeohistological and skeletochronological evidence supporting a salamander-like development in the seymouriamorph Discosauriscus. Its long-bone growth pattern, slow diaphyseal growth rate and delayed sexual maturity (at more than 10 years old) are more comparable with growth features of extant salamanders rather than extant amniotes, even though they are mostly hypothesized to be phylogenetically closer to living amniotes than salamanders.     

Mitogenomic perspectives on the origin and phylogeny of living amphibians

Establishing the relationships among modern amphibians (lissamphibians) and their ancient relatives is necessary for our understanding of early tetrapod evolution. However, the phylogeny is still intractable because of the highly specialized anatomy and poor fossil record of lissamphibians. Paleobiologists are still not sure whether lissamphibians are monophyletic or polyphyletic, and which ancient group (temnospondyls or lepospondyls) is most closely related to them. In an attempt to address these problems, eight mitochondrial genomes of living amphibians were determined and compared with previously published amphibian sequences. A comprehensive molecular phylogenetic analysis of nucleotide sequences yields a highly resolved tree congruent with the traditional hypotheses (Batrachia). By using a molecular clock-independent approach for inferring dating information from molecular phylogenies, we present here the first molecular timescale for lissamphibian evolution, which suggests that lissamphibians first emerged about 330 million years ago. By observing the fit between molecular and fossil times, we suggest that the temnospondyl-origin hypothesis for lissamphibians is more credible than other hypotheses. Moreover, under this timescale, the potential geographic origins of the main living amphibian groups are discussed: (i) advanced frogs (neobatrachians) may possess an Africa-India origin; (ii) salamanders may have originated in east Asia; (iii) the tropic forest of the Triassic Pangaea may be the place of origin for the ancient caecilians. An accurate phylogeny with divergence times can be also helpful to direct the search for "missing" fossils, and can benefit comparative studies of amphibian evolution.     

Molecular Evidence for the Early History of Living Amphibians

The evolutionary relationships of the three orders of living amphibians (lissamphibians) has been difficult to resolve, partly because of their specialized morphologies. Traditionally, frogs and salamanders are considered to be closest relatives, and all three orders are thought to have arisen in the Paleozoic (>250 myr). Here, we present evidence from the DNA sequences of four mitochondrial genes (2.7 kilobases) that challenges the conventional hypothesis and supports a salamander–caecilian relationship. This, in light of the fossil record and distribution of the families, suggests a more recent (Mesozoic) origin for salamanders and caecilians directly linked to the initial breakup of the supercontinent Pangaea. We propose that this single geologic event isolated salamanders and archaeobatrachian frogs on the northern continents (Laurasia) and the caecilians and neobatrachian frogs on the southern continents (Gondwana). Among the neobatrachian frog families, molecular evidence supports a South American clade and an African clade, inferred here to be the result of mid-Cretaceous vicariance.     

Metamorphosis of the feeding mechanism in tiger salamanders (Ambystoma tigrinum): the ontogeny of cranial muscle mass

Most previous research on metamorphosis of the musculoskeletal system in vertebrates has focused on the transformation of the skeleton. In this paper we focus on the transformation of the muscles of the head during metamorphosis in tiger salamanders ( Ambystoma tigrinum ) in order (1) to provide new data on changes in myology during ontogeny, and (2) to aid in interpreting previous data on the metamorphosis of function in the head of salamanders.
The physiological cross-sectional area of nine head muscles was calculated by measuring fibre angles, fibre lengths, and muscle mass in two samples of tiger salamanders obtained just before and just after metamorphosis. The major mouth-opening muscles (rectus cervicis and depressor mandibulae) exhibit a significant decrease in estimated maximum tetanic tension (MTT) across metamorphosis of about 36%. The jaw-closing muscles (adductor mandibulae internus and externus) and the head-lifting muscles (epaxials) also decrease in MTT but not significantly. The muscles associated with tongue projection during feeding on land (the subarcualis rectus I, geniohyoideus, interhyoideus and intermandibularis) all show a slight increase in MTT at metamorphosis.
Metamorphic transformation of feeding behaviour in Ambystoma tigrinum involves changes in performance, the design of skeletal elements, changes in muscle force-generating capability, and changes in hydrodynamic design from unidirectional flow in larvae to bidirectional flow during aquatic feeding after metamorphosis. Although muscle activity patterns during aquatic feeding do not change across metamorphosis, tongue-based terrestrial feeding involves a suite of novel muscle activity patterns, morphological characters acquired at metamorphosis, and a metamorphic increase in the masses of muscles important in tongue projection.     

Thyroid hormone responsive QTL and the evolution of paedomorphic salamanders

The transformation of ancestral phenotypes into novel traits is poorly understood for many examples of evolutionary novelty. Ancestrally, salamanders have a biphasic life cycle with an aquatic larval stage, a brief and pronounced metamorphosis, followed by a terrestrial adult stage. Repeatedly during evolution, metamorphic timing has been delayed to exploit growth-permissive environments, resulting in paedomorphic salamanders that retain larval traits as adults. We used thyroid hormone (TH) to rescue metamorphic phenotypes in paedomorphic salamanders and then identified quantitative trait loci (QTL) for life history traits that are associated with amphibian life cycle evolution: metamorphic timing and adult body size. We demonstrate that paedomorphic tiger salamanders (Ambystoma tigrinum complex) carry alleles at three moderate effect QTL (met1–3) that vary in responsiveness to TH and additively affect metamorphic timing. Salamanders that delay metamorphosis attain significantly larger body sizes as adults and met2 explains a significant portion of this variation. Thus, substitution of alleles at TH-responsive loci suggests an adaptive pleiotropic basis for two key life-history traits in amphibians: body size and metamorphic timing. Our study demonstrates a likely pathway for the evolution of novel paedomorphic species from metamorphic ancestors via selection of TH-response alleles that delay metamorphic timing and increase adult body size.     

The importance of global parsimony and historical bias in understanding tetrapod evolution. Part I. Systematics,middle ear evolution and jaw suspension

A phylogenetic analysis based on a data matrix of 43 taxa and 155 osteological characters has produced a new hypothesis of tetrapod phylogeny that is drastically different from the established consensus. Among Paleozoic taxa, only diadectomorphs appear to be related to amniotes. In contrast to previous hypotheses, lissamphibians appear to have been derived from lepospondyls. Seymouriamorphs, gephyrostegids, embolomeres, temnospondyls, and loxommatids are stem-tetrapods. The new phylogeny suggests that the absence of a tympanic middle ear in salamanders and gymnophiones is a primitive character.     

Generating,growing and transforming skeletal shape: insights from amphibian pharyngeal arch cartilages

Amphibians that undergo a metamorphosis provide an unparalleled opportunity to investigate how skeletal shape is generated, preserved, and transformed in development. Their pharyngeal arch (PA) cartilages, which support breathing and feeding behaviors, form embryonically from cranial neural crest cells, grow isometrically at larval stages, and abruptly change shape during metamorphosis. Further, the shape changes occur in three different ways: some adult cartilages form de novo, others emerge from within resorbing larval cartilages and some larval cartilages reshape themselves at the cellular level. Isometric growth followed by abrupt shape change is unique to amphibian PA cartilages, which suggests that the origin and evolution of amphibian metamorphosis has been influenced by the tissue properties of cartilage. This essay reviews the functional role of the PA skeleton in frogs and salamanders and presents a mechanistic framework for understanding how its shape is generated, preserved, and transformed at the levels of cell behavior and specification mechanisms.     

Development and evolution of the tetrapod skull–neck boundary

The origin and evolution of the vertebrate skull have been topics of intense study for more than two centuries. Whereas early theories of skull origin, such as the influential vertebral theory, have been largely refuted with respect to the anterior (pre‐otic) region of the skull, the posterior (post‐otic) region is known to be derived from the anteriormost paraxial segments, i.e. the somites. Here we review the morphology and development of the occiput in both living and extinct tetrapods, taking into account revised knowledge of skull development by augmenting historical accounts with recent data. When occipital composition is evaluated relative to its position along the neural axis, and specifically to the hypoglossal nerve complex, much of the apparent interspecific variation in the location of the skull–neck boundary stabilizes in a phylogenetically informative way. Based on this criterion, three distinct conditions are identified in (i) frogs, (ii) salamanders and caecilians, and (iii) amniotes. The position of the posteriormost occipital segment relative to the hypoglossal nerve is key to understanding the evolution of the posterior limit of the skull. By using cranial foramina as osteological proxies of the hypoglossal nerve, a survey of fossil taxa reveals the amniote condition to be present at the base of Tetrapoda. This result challenges traditional theories of cranial evolution, which posit translocation of the occiput to a more posterior location in amniotes relative to lissamphibians (frogs, salamanders, caecilians), and instead supports the largely overlooked hypothesis that the reduced occiput in lissamphibians is secondarily derived. Recent advances in our understanding of the genetic basis of axial patterning and its regulation in amniotes support the hypothesis that the lissamphibian occipital form may have arisen as the product of a homeotic shift in segment fate from an amniote‐like condition.     

Monophyly and affinities of albanerpetontid amphibians (Temnospondyli; Lissamphibia)

The Albanerpetontidae are Middle Jurassic-Miocene amphibians that have variously been regarded as caudates (salamanders), a clade distinct from caudates, or incertae sedis lissamphibians. Here I test for monophyly of the Albanerpetontidae and examine the affinities of the group, within the framework of a more inclusive Temnospondyli, by performing a cladistic analysis using 59 informative characters scored for four non-lissamphibian temnospondyl genera, stem- and crown-clade caudates, salientians (frogs), gymnophionans (caecilians), and the two recognized albanerpetontid genera Albanerpeton and Celtedens . Monophyly of the Albanerpetontidae is corroborated. I interpret synapomorphies of the marginal teeth (non-pedicellate; crowns chisel like, labiolingually compressed, with three mesiodistally aligned cuspules) in albanerpetontids as being associated with a shearing bite. Other synapomorphies evidently strengthened and increased the mobility of the skull, mandible, and cervical region for burrowing, feeding, or both. Nested sets of synapomorphies place the Albanerpetontidae within the Lissamphibia, as the sistertaxon of Caudata plus Salientia. None of the 17 characters previously advanced as albanerpetontid-caudate synapomorphies convincingly places the Albanerpetontidae within the Caudata or allies the two groups as sistertaxa. Albanerpetontids are better interpreted not as aberrant caudates, but as a distinct clade of lissamphibians in which numerous apomorphies are superimposed upon an otherwise primitive lissamphibian body plan.     

Evolution of salamander life cycles: a major-effect quantitative trait locus contributes to discrete and continuous variation for metamorphic timing

The evolution of alternate modes of development may occur through genetic changes in metamorphic timing. This hypothesis was examined by crossing salamanders that express alternate developmental modes: metamorphosis vs. paedomorphosis. Three strains were used in the crossing design: Ambystoma tigrinum tigrinum (Att; metamorph), wild-caught A. mexicanum (Am; paedomorph), and laboratory Am (paedomorph). Att/Am hybrids were created for each Am strain and then backcrossed to their respective Am line. Previous studies have shown that a dominant allele from Att (met(Att)) and a recessive allele from lab Am (met(lab)) results in metamorphosis in Att/Am hybrids, and met(Att)/met(lab) and met(lab)/met(lab) backcross genotypes are strongly associated with metamorphosis and paedomorphosis, respectively. We typed a molecular marker (contig325) linked to met and found that met(Att)/met(lab) and met(Att)/met(wild) were associated with metamorphosis in 99% of the cases examined. However, the frequency of paedomorphosis was 4.5 times higher for met(lab)/met(lab) than for met(wild)/met(wild). We also found that met(Att)/met(wild) and met(wild)/met(wild) genotypes discriminated distributions of early and late metamorphosing individuals. Two forms of phenotypic variation are contributed by met: continuous variation of metamorphic age and expression of discrete, alternate morphs. We suggest that the evolution of paedomorphosis is associated with genetic changes that delay metamorphic timing in biphasic life cycles.     

The skull and jaw musculature as guides to the ancestry of salamanders

The fossil record provides no evidence supporting a unique common ancestry for frogs, salamanders and apodans. The ancestors of the modern orders may have diverged from one another as recently as 250 million years ago, or as long ago as 400 million years according to current theories of various authors. In order to evaluate the evolutionary patterns of the modern orders it is necessary to determine whether their last common ancestor was a rhipidistian fish, a very primitive amphibian, a labyrimhodom or a ‘lissamphibian’. The broad cranial similarities of frogs and salamanders, especially the dominance of the braincase as a supporting element, can be associated with the small size of the skull in their immediate ancestors. Hynobiids show the most primitive cranial pattern known among the living salamander families and “provide a model for determining the nature of the ancestors of the entire order. Features expected in ancestral salamanders include: (1) Emargination of the cheek; (2) Movable suspensorium formed by the quadrate, squamosal and pterygoid; (3) Occipital condyle posterior to jaw articulation; (4) Distinct prootic and opisthotic; (5) Absence ol otic notch; (6) Stapes forming a structural link between braincase and cheek. In the otic region, cheek and jaw suspension, the primitive salamander pattern (resembles most closely the microsaurs among known Paleozoic amphibians, and shows no significant features in common with either ancestral frogs or the majority of labyrinth odonts. The basic pattern of the adductor jaw musculature is consistent within both frogs and salamanders, but major differences are evident between the two groups. The dominance of the adductor mandibulae externus in salamanders can be associated with the open cheek in all members of that order, and the small size of this muscle in frogs can be associated with the large otic notch. The spread of different muscles over the otic capsule, the longus head ol the adductor mandibulae posterior in frogs and the superficial head of the adductor mandibulae internus in salamanders, indicates that fenestration of the skull posterodorsal to the orbit occurred separately in the ancestors of the two groups. Reconstruction of the probable pattern of the jaw musculature in Paleozoic amphibians indicates that frogs and salamanders might have evolved from a condition hypothesized for primitive labyrinthodonts, but the presence of a large otic notch in dissorophids suggests specialization toward the anuran, not the urodele condition. The presence of either an einarginated cheek or an embayment of the lateral surface of the dentary and the absence of an otic notch in microsaurs indicate a salamander-like distribution of die adductor jaw muscles. The ancestors of frogs and salamanders probably diverged from one another in the early Carboniferous, Frogs later evolved from small labyrinthodonts and salamanders from microsaurs. Features considered typical of lissamphibians evolved separately in the two groups in the late Permian andTriassic.     

The Evolution of the Motor Control of Feeding in Amphibians

Based on studies of a few model taxa, amphibians have been consideredstereotyped in their feeding movements relative to other vertebrates.However, recent studies on a wide variety of amphibian specieshave revealed great diversity in feeding mechanics and kinematics,and illustrate that stereotypy is the exception rather thanthe rule in amphibian feeding. Apparent stereotypy in some taxamay be an artifact of unnatural laboratory conditions. The commonancestor of lissamphibians was probably capable of some modulationof feeding movements, and descendants have evolved along twotrajectories with regard to motor control: (1) an increase inmodulation via feedback or feed-forward mechanisms, as exemplifiedby ballistic-tongued plethodontid salamanders and hydrostatic-tonguedfrogs, and (2) a decrease in variation dictated by biomechanicsthat require tight coordination between different body parts,such as the tongue and jaws in toads and other frogs with ballistictongue projection. Multi-joint coordination of rapid movementsmay hamper accurate tongue placement in ballistic-tongued frogsas compared to both short-tongued frogs and ballistic tongued-salamandersthat face simpler motor control tasks. Decoupling of tongueand jaw movements is associated with increased accuracy in bothhydrostatic-tongued frogs and ballistic-tongued salamanders.     

Metamorphosis in the Urodelan amphibians: patterns, regulation mechanisms, and evolution

Patterns of metamorphosis and mechanisms of its regulation in primitive and advanced salamanders are compared. It is found that urodelan evolution was characterised by the following trends: 1) increase in the number of metamorphosing systems; 2) increase in the amplitude of metamorphic transformations of each particular system due to the progressive divergence of the larval and the adult morphology; 3) synchronization of metamorphic transformations and their concentration within a relatively short period of ontogeny; 4) increase in the role of the thyroid hormones (TH) in the regulation of metamorphosis. Structures that are induced by factors other than TH and develop independently of TH in primitive Urodela species acquire TH-dependence in phyletically more advanced salamanders. For instance, morphogenetic induction as a mechanism of ontogeny regulation is substituted by endocrine induction with TH as the inducing factor. The switch from morphogenetic to endocrine induction stimulates the following events: 1) optimization of ontogeny; 2) reduction of the metamorphosis duration; 3) formation of the dissociability of larval and post-metamorphic stages of ontogeny, which, in its turn, is a precondition for the swith to necrobiotic metamorphosis and to the direct development.     

Vertebral development and amphibian evolution

Amphibians provide an unparalleled opportunity to integrate studies of development and evolution through the investigation of the fossil record of larval stages. The pattern of vertebral development in modern frogs strongly resembles that of Paleozoic labyrinthodonts in the great delay in the ossification of the vertebrae, with the centra forming much later than the neural arches. Slow ossification of the trunk vertebrae in frogs and the absence of ossification in the tail facilitate the rapid loss of the tail during metamorphosis, and may reflect retention of the pattern in their specific Paleozoic ancestors. Salamanders and caecilians ossify their centra at a much earlier stage than frogs, which resembles the condition in Paleozoic lepospondyls. The clearly distinct patterns and rates of vertebral development may indicate phylogenetic separation between the ultimate ancestors of frogs and those of salamanders and caecilians within the early radiation of ancestral tetrapods. This divergence may date from the Lower Carboniferous. Comparison with the molecular regulation of vertebral development described in modern mammals and birds suggests that the rapid chondrification of the centra in salamanders relative to that of frogs may result from the earlier migration of sclerotomal cells expressing Pax1 to the area surrounding the notochord.     

Osseous pathologies in the lungless salamander Desmognathus fuscus (Plethodontidae)

The majority of reported pathologies in lissamphibians (salamanders, caecilians and frogs) include limb deformities such as missing limbs, multiple extra limbs and digits, or incomplete limb formation. However, comparatively little is known about congenital vertebral malformations or posttraumatic pathologies (e.g. injuries, infections) in the vertebral column of salamanders. In the present study, we describe eight vertebral deformities in three cleared and stained specimens of Desmognathus fuscus. Two specimens display developmental deformities which range from a potential non-segmented wedge vertebra to fully segmented hemivertebrae. The vertebral pathology in the third specimens possibly results from a parasitic infection. Apparently, these osseous deformities were not severe enough to prohibit survival of the specimens.     

Fetal adaptations for viviparity in amphibians

Live‐bearing has evolved in all three orders of amphibians—frogs, salamanders, and caecilians. Developing young may be either yolk dependent, or maternal nutrients may be supplied after yolk is resorbed, depending on the species. Among frogs, embryos in two distantly related lineages develop in the skin of the maternal parents' backs; they are born either as advanced larvae or fully metamorphosed froglets, depending on the species. In other frogs, and in salamanders and caecilians, viviparity is intraoviductal; one lineage of salamanders includes species that are yolk dependent and born either as larvae or metamorphs, or that practice cannibalism and are born as metamorphs. Live‐bearing caecilians all, so far as is known, exhaust yolk before hatching and mothers provide nutrients during the rest of the relatively long gestation period. The developing young that have maternal nutrition have a number of heterochronic changes, such as precocious development of the feeding apparatus and the gut. Furthermore, several of the fetal adaptations, such as a specialized dentition and a prolonged metamorphosis, are homoplasious and present in members of two or all three of the amphibian orders. At the same time, we know little about the developmental and functional bases for fetal adaptations, and less about the factors that drive their evolution and facilitate their maintenance. J. Morphol. 276:941–960, 2015. © 2014 Wiley Periodicals, Inc.     

Metamorphosis and fish vision

Many species of fish exhibit metamorphosis in which dramatic external transformations occur as a consequence of coordinated changes in gene expression within an organism. Because postembryonic development and change appears to be the rule rather than the exception in teleost fish species, we view metamorphosis as one of many developmental strategies in fish which have continued plasticity as a common theme. Metamorphic changes are manifested in the visual system by modification of photoreceptor peak sensitivity rod photoreceptor cell addition, and retinal reorganization. These changes correspond to significant changes in the natural habitat of the animal and in its visual capabilities as demonstrated behaviorally. Thyroxine is the main metamorphic hormone as has also been found in amphibia. The sequence of metamorphic events occur in all teleosts, but they are compressed in time in direct developing animals suggesting that such animals might prove useful for understanding the evolution of metamorphosis in fish. It seems likely that rod photoreceptors may have evolved in conjunction with the change from larval to juvenile stage through metamorphosis in indirect developing fishes. During evolution, the contraction and/or loss of the larval stage has resulted in earlier appearance of rod photoreceptors during development although they always arise later than cone photoreceptors. This ontogenetic developmental sequence supports Walls's (1942) proposal that cones are phylogenetically older than rods and suggests that rods may have evolved several times.     

Metamorphosis and fish vision

Many species of fish exhibit metamorphosis in which dramatic external transformations occur as a consequence of coordinated changes in gene expression within an organism. Because postembryonic development and change appears to be the rule rather than the exception in teleost fish species, we view metamorphosis as one of many developmental strategies in fish which have continued plasticity as a common theme. Metamorphic changes are manifested in the visual system by modification of photoreceptor peak sensitivity, rod photoreceptor cell addition, and retinal reorganization. These changes correspond to significant changes in the natural habitat of the animal and in its visual capabilities as demonstrated behaviorally. Thyroxine is the main metamorphic hormone as has also been found in amphibia. The sequence of metamorphic events occur in all teleosts, but they are compressed in time in direct developing animals suggesting that such animals might prove useful for understanding the evolution of metamorphosis in fish. It seems likely that rod photoreceptors may have evolved in conjunction with the change from larval to juvenile stage through metamorphosis in indirect developing fishes. During evolution, the contraction and/or loss of the larval stage has resulted in earlier appearance of rod photoreceptors during development although they always arise later than cone photoreceptors. This ontogenetic developmental sequence supports Walls's (1942) proposal that cones are phylogenetically older than rods and suggests that rods may have evolved several times.