Cranial musculature in the larva of the caecilian,Ichthyophis kohtaoensis (Lissamphibia: Gymnophiona)

The anatomy of the head muscles in caecilians (Amphibia: Gymnophiona): Variation in relation to phylogeny and ecology?

In limbless fossorial vertebrates such as caecilians (Gymnophiona), head-first burrowing imposes severe constraints on the morphology and overall size of the head. As such, caecilians developed a unique jaw-closing system involving the large and well-developed m. interhyoideus posterior, which is positioned in such a way that it does not significantly increase head diameter. Caecilians also possess unique muscles among amphibians. Understanding the diversity in the architecture and size of the cranial muscles may provide insights into how a typical amphibian system was adapted for a head-first burrowing lifestyle. In this study, we use dissection and non-destructive contrast-enhanced micro-computed tomography (μCT) scanning to describe and compare the cranial musculature of 13 species of caecilians. Our results show that the general organization of the head musculature is rather constant across extant caecilians. However, the early-diverging Rhinatrema bivittatum mainly relies on the 'ancestral' amphibian jaw-closing mechanism dominated by the m. adductores mandibulae, whereas other caecilians switched to the use of the derived dual jaw-closing mechanism involving the additional recruitment of the m. interhyoideus posterior. Additionally, the aquatic Typhlonectes show a greater investment in hyoid musculature than terrestrial caecilians, which is likely related to greater demands for ventilating their large lungs, and perhaps also an increased use of suction feeding. In addition to three-dimensional interactive models, our study provides the required quantitative data to permit the generation of accurate biomechanical models allowing the testing of further functional hypotheses.

The relationship between head shape, head musculature and bite force in caecilians (Amphibia: Gymnophiona)

Caecilians are enigmatic limbless amphibians that, with a few exceptions all have an at least partly burrowing lifestyle. Although it has been suggested that caecilian evolution resulted in sturdy and compact skulls as an adaptation to their head-first burrowing habits, no relationship between skull shape and burrowing performance has been demonstrated to date. However, the unique dual jaw-closing mechanism and the osteological variability of their temporal region suggest a potential relationship between skull shape and feeding mechanics. Here, we explored the relationships between skull shape, head musculature, and in vivo bite forces. Although there is a correlation between bite force and external head shape, no relationship between bite force and skull shape could be detected. Whereas our data suggest that muscles are the principal drivers of variation in bite force, the shape of the skull is constrained by factors other than demands for bite force generation. However, a strong covariation between the cranium and mandible exists. Moreover, both cranium and mandible shape covary with jaw muscle architecture. Caecilians show a gradient between species with a long retroarticular process associated with a large and pennate-fibered m. interhyoideus posterior and species with a short process but long and parallel-fibered jaw adductors. Our results demonstrate the complexity of the relationship between form and function of this jaw system. Further studies that focus on factors such as gape distance or jaw velocity will be needed in order to fully understand the evolution of feeding mechanics in caecilians.

Evolution of the strikingly diverse submandibular muscles in Anura

The most ventral muscles of the head (the mm. submentalis, intermandibularis, and interhyoideus) provide support to the gular region and lift the buccal floor during ventilation and feeding. These muscles show limited variation in most gnathostomes, but in Anura they exhibit a surprising diversity. The few studies that have explored this character system highlighted its potential as a source of phylogenetic information. In this paper we explored the diversity of this character system studying specimens of 567 anuran species and reviewing published data to cover a total of 1321 species, belonging to 53 of the 54 currently recognized anuran families, as well as caudates and caecilians. We defined 27 discrete characters including the number of muscle bellies, supplementary layers, hypertrophy and diversity of elastic fibres, and pigmentation, among others, and optimized them on a comprehensive phylogenetic hypothesis. We recognized 223 unambiguously optimized synapomorphies for numerous clades on different scales, including three for Anura and many for suprafamiliar clades with poor phenotypic support. Finally, we discussed the evolution of this highly diverse character system, including homology, development, and its functional role in vocalization and feeding. Interestingly, the striking levels of variation in some structures contrast with the amount of phylogenetic inertia, allowing us to recognize several general patterns. Supplementary elements of the m. intermandibularis evolved first as broad layers occuring in more than half of extant anuran species and then concentrated forming discreet bellies in several clades. The anterior portion of the gular region is not sexually dimorphic, and is likely related to ventilation and tongue protraction. Conversely, the diversity of the m. interhyoideus is strongly linked to vocal sacs, which are present only in adult males, suggesting the presence of two independent modules.

Brain Reconstruction Across the Fish-Tetrapod Transition; Insights From Modern Amphibians

The fish-tetrapod transition (which incorporates the related fin-limb and water-land transitions) is celebrated as one of the most important junctions in vertebrate evolution. Sarcopterygian fishes (the “lobe-fins”) are today represented by lungfishes and coelacanths, but during the Paleozoic they were much more diverse. It was some of these sarcopterygians, a lineage of the tetrapodomorph fishes, that gave rise to tetrapods (terrestrial vertebrates with limbs bearing digits). This spectacular leap took place during the Devonian Period. Due to the nature of preservation, it is the hard parts of an animal’s body that are most likely to fossilize, while soft tissues such as muscular and brain tissues, typically fail to do so. Thus, our understanding of the adaptations of the hard skeletal structures of vertebrates is considerably greater than that of the soft tissue systems. Fortunately, the braincases of early vertebrates are often ossified and thereby have the potential to provide detailed morphological information. However, the correspondence between brain and endocast (an internal mold of the cavity) has historically been considered poor in most “lower” vertebrates and consequently neglected in such studies of brain evolution. Despite this, recent work documenting the spatial relationship in extant basal sarcopterygians (coelacanth, lungfish, axolotl, and salamander) has highlighted that this is not uniformly the case. Herein, we quantify and illustrate the brain-endocast relationship in four additional extant basal tetrapod exemplars: neobatrachian anurans (frogs) Breviceps poweri and Ceratophrys ornata; and gymnophionans (caecilians) Gegeneophis ramaswamii and Rhinatrema bivittatum. We show that anurans and caecilians appear to have brains that fill their endocasts to a similar degree to that of lungfishes and salamanders, but not coelacanth. Ceratophrys has considerably lower correspondence between the brain and endocast in the olfactory tract and mesencephalic regions, while Breviceps has low correspondence along its ventral endocranial margin. The brains of caecilians reflect their endocasts most closely (vol. ∼70%). The telencephalon is tightly fitted within the endocast in all four taxa. Our findings highlight the need to adequately assess the brain-endocast relationship in a broad range of vertebrates, in order to inform neural reconstructions of fossil taxa using the Extant Phylogenetic Bracket approach and future studies of brain evolution.

Evolution of the head-trunk interface in tetrapod vertebrates

Vertebrate neck musculature spans the transition zone between head and trunk. The extent to which the cucullaris muscle is a cranial muscle allied with the gill levators of anamniotes or is instead a trunk muscle is an ongoing debate. Novel computed tomography datasets reveal broad conservation of the cucullaris in gnathostomes, including coelacanth and caecilian, two sarcopterygians previously thought to lack it. In chicken, lateral plate mesoderm (LPM) adjacent to occipital somites is a recently identified embryonic source of cervical musculature. We fate-map this mesoderm in the axolotl (Ambystoma mexicanum), which retains external gills, and demonstrate its contribution to posterior gill-levator muscles and the cucullaris. Accordingly, LPM adjacent to the occipital somites should be regarded as posterior cranial mesoderm. The axial position of the head-trunk border in axolotl is congruent between LPM and somitic mesoderm, unlike in chicken and possibly other amniotes.

Anatomy, function, and evolution of jaw and hyobranchial muscles in cryptobranchoid salamander larvae

Larval salamanders (Lissamphibia: Caudata) are known to be effective suction feeders in their aquatic environments, although they will eventually transform into terrestrial tongue feeding adults during metamorphosis. Early tetrapods may have had a similar biphasic life cycle and this makes larval salamanders a particularly interesting model to study the anatomy, function, development, and evolution of the feeding apparatus in terrestrial vertebrates. Here, we provide a description of the muscles that are involved in the feeding strike in salamander larvae of the Hynobiidae and compare them to larvae of the paedomorphic Cryptobranchidae. We provide a functional and evolutionary interpretation for the observed muscle characters. The cranial muscles in larvae from species of the Hynobiidae and Cryptobranchidae are generally very similar. Most notable are the differences in the presence of the m. hyomandibularis, a muscle that connects the hyobranchial apparatus with the lower jaw. We found this muscle only in Onychodactylus japonicus (Hynobiidae) but not in other hynobiid or cryptobranchid salamanders. Interestingly, the m. hyomandibularis in O. japonicus originates from the ceratobranchial I and not the ceratohyal, and thus exhibits what was previously assumed to be the derived condition. Finally, we applied a biomechanical model to simulate suction feeding in larval salamanders. We provide evidence that a flattened shape of the hyobranchial apparatus in its resting position is beneficial for a fast and successful suction feeding strike.

Cranial muscle development in the model organism ambystoma mexicanum: implications for tetrapod and vertebrate comparative and evolutionary morphology and notes on ontogeny and phylogeny

There is still confusion about the homology of several cranial muscles in salamanders with those of other vertebrates. This is true, in part, because of the fact that many muscles present in early ontogeny of amphibians disappear during development and specifically during metamorphosis. Resolving this confusion is important for the understanding of the comparative and evolutionary morphology of vertebrates and tetrapods because amphibians are the phylogenetically most plesiomorphic tetrapods, concerning for example their myology, and include two often used model organisms, Xenopus laevis (anuran) and Ambystoma mexicanum (urodele). Here we provide the first detailed report of the cranial muscle development in axolotl from early ontogenetic stages to the adult stage. We describe different and complementary types of general muscle morphogenetic gradients in the head: from anterior to posterior, from lateral to medial, and from origin to insertion. Furthermore, even during the development of neotenic salamanders such as axolotls, various larval muscles become indistinct, contradicting the commonly accepted view that during ontogeny the tendency is mostly toward the differentiation of muscles. We provide an updated comparison between these muscles and the muscles of other vertebrates, a discussion of the homologies and evolution, and show that the order in which the muscles appear during axolotl ontogeny is in general similar to their appearance in phylogeny (e.g. differentiation of adductor mandibulae muscles from one anlage to four muscles), with only a few remarkable exceptions, as for example the dilatator laryngis that appears evolutionary later but in the development before the intermandibularis.

Cranial muscles in amphibians: development, novelties and the role of cranial neural crest cells

Our research on the evolution of the vertebrate head focuses on understanding the developmental origins of morphological novelties. Using a broad comparative approach in amphibians, and comparisons with the well-studied quail-chicken system, we investigate how evolutionarily conserved or variable different aspects of head development are. Here we review research on the often overlooked development of cranial muscles, and on its dependence on cranial cartilage development. In general, cranial muscle cell migration and the spatiotemporal pattern of cranial muscle formation appears to be very conserved among the few species of vertebrates that have been studied. However, fate-mapping of somites in the Mexican axolotl revealed differences in the specific formation of hypobranchial muscles (tongue muscles) in comparison to the chicken. The proper development of cranial muscles has been shown to be strongly dependent on the mostly neural crest-derived cartilage elements in the larval head of amphibians. For example, a morpholino-based knock-down of the transcription factor FoxN3 in Xenopus laevis has drastic indirect effects on cranial muscle patterning, although the direct function of the gene is mostly connected to neural crest development. Furthermore, extirpation of single migratory streams of cranial neural crest cells in combination with fate-mapping in a frog shows that individual cranial muscles and their neural crest-derived connective tissue attachments originate from the same visceral arch, even when the muscles attach to skeletal components that are derived from a different arch. The same pattern has also been found in the chicken embryo, the only other species that has been thoroughly investigated, and thus might be a conserved pattern in vertebrates that reflects the fundamental nature of a mechanism that keeps the segmental order of the head in place despite drastic changes in adult anatomy. There is a need for detailed comparative fate-mapping of pre-otic paraxial mesoderm in amphibians, to determine developmental causes underlying the complicated changes in cranial muscle development and architecture within amphibians, and in particular how the novel mouth apparatus in frog tadpoles evolved. This will also form a foundation for further research into the molecular mechanisms that regulate rostral head morphogenesis. Our empirical studies are discussed within a theoretical framework concerned with the evolutionary origin and developmental basis of novel anatomical structures in general. We argue that a common developmental origin is not a fool-proof guide to homology, and that a view that sees only structures without homologs as novel is too restricted, because novelties must be produced by changes in the same framework of developmental processes. At the level of developmental processes and mechanisms, novel structures are therefore likely to have homologs, and we need to develop a hierarchical concept of novelty that takes this into account.

Cranial muscles of the anurans Leiopelma hochstetteri and Ascaphus truei and the homologies of the mandibular adductors in Lissamphibia and other gnathostomes

The frogs Ascaphus truei and Leiopelma hochstetteri are members of the most basal lineages of extant anurans. Their cranial muscles have not been previously described in full and are investigated here by dissection. Comparison of these taxa is used to review a controversy regarding the homologies of the jaw adductor muscles in Lissamphibia, to place these homologies in a wider gnathostome context, and to define features that may be useful for cladistic analysis of Anura. A new muscle is defined in Ascaphus and is designated m. levator anguli oris. The differences noted between Ascaphus and Leiopelma are in the penetration of the jaw adductor muscles by the mandibular nerve (V3). In the traditional view of this anatomy, the paths of the trigeminal nerve branches define homologous muscles. This scheme results in major differences among frogs, salamanders, and caecilians. The alternative view is that the topology of origins, insertions, and fiber directions are defining features, and the nerves penetrate the muscle mass in a variable way. The results given here support the latter view. A new model is proposed for Lissamphibia, whereby the adductor posterior (levator articularis) is a separate entity, and the rest of the adductor mass is configured around it as a folded sheet. This hypothesis is examined in other gnathostomes, including coelacanth and lungfish, and a possible sequence for the evolution of the jaw muscles is demonstrated. In this system, the main jaw adductor in teleost fish is not considered homologous with that of tetrapods. This hypothesis is consistent with available data on the domain of expression of the homeobox gene engrailed 2, which has previously not been considered indicative of homology. Terminology is discussed, and "adductor mandibulae" is preferred to "levator mandibulae" to align with usage in other gnathostomes.

Ontogenetic differences in the feeding biomechanics of oviparous and viviparous caecilians (Lissamphibia: Gymnophiona)

Caecilians have a unique dual jaw-closing system in that jaw closure is driven by the ancestral jaw-closing muscles (mm. levatores mandibulae) plus a secondarily recruited hyobranchial muscle (m. interhyoideus posterior). There is a variety of feeding habits (suction feeding, skin feeding, intrauterine scraping, and biting) during ontogeny that relate to reproductive modes in different caecilian species. This study examines the cranial biomechanics of caecilians in the suction-feeding larva of Ichthyophis cf. kohtaoensis, in the embryo and juvenile of the skin-feeding Boulengerula taitana, and in a newborn of the intrauterine feeder Typhlonectes natans. A lever arm model was applied to calculate effective mechanical advantages of jaw-closing muscles over gape angles and to predict total bite force in developing caecilians. In I. cf. kohtaoensis, Notable differences were found in the larval jaw-closing system compared to that of the adult. The suction-feeding larva of I. cf. kohtaoensis has comparatively large mm. levatores mandibulae that insert with an acute muscle fiber angle to the lower jaw and a m. interhyoideus posterior that has its optimal leverage at small gape angles. Conversely, the skin-feeding juvenile of B. taitana and the neonate T. natans are very similar in the feeding parameters considered herein compared to adult caecilians. Some ontogenetic variation in the feeding system of B. taitana before the onset of feeding was present. This study contributes to our understanding of the functional demands that feeding habits put on the development of cranial structures.

Timing of organogenesis support basal position of turtles in the amniote tree of life

Background

The phylogenetic position of turtles is the most disputed aspect in the reconstruction of the land vertebrate tree of life. This controversy has arisen after many different kinds and revisions of investigations of molecular and morphological data. Three main hypotheses of living sister-groups of turtles have resulted from them: all reptiles, crocodiles + birds or squamates + tuatara. Although embryology has played a major role in morphological studies of vertebrate phylogeny, data on developmental timing have never been examined to explore and test the alternative phylogenetic hypotheses. We conducted a comprehensive study of published and new embryological data comprising 15 turtle and eight tetrapod species belonging to other taxa, integrating for the first time data on the side-necked turtle clade.

Results

The timing of events in organogenesis of diverse character complexes in all body regions is not uniform across amniotes and can be analysed using a parsimony-based method. Changes in the relative timing of particular events diagnose many clades of amniotes and include a phylogenetic signal. A basal position of turtles to the living saurian clades is clearly supported by timing of organogenesis data.

Conclusion

The clear signal of a basal position of turtles provided by heterochronic data implies significant convergence in either molecular, adult morphological or developmental timing characters, as only one of the alternative solutions to the phylogenetic conundrum can be right. The development of a standard reference series of embryological events in amniotes as presented here should enable future improvements and expansion of sampling and thus the examination of other hypotheses about phylogeny and patterns of the evolution of land vertebrate development.

Caecilian jaw-closing mechanics: integrating two muscle systems

Caecilians (Lissamphibia: Gymnophiona) are unique among vertebrates in having two sets of jaw-closing muscles, one on either side of the jaw joint. Using data from high-resolution X-ray radiation computed tomography scans, we modelled the effect of these two muscle groups (mm. levatores mandibulae and m. interhyoideus posterior) on bite force over a range of gape angles, employing a simplified lever arm mechanism that takes into account muscle cross-sectional area and fibre angle. Measurements of lever arm lengths, muscle fibre orientations and physiological cross-sectional area of cranial muscles were available from three caecilian species: Ichthyophis cf. kohtaoensis; Siphonops annulatus; and Typhlonectes natans. The maximal gape of caecilians is restricted by a critical gape angle above which the mm. levatores mandibulae will open the jaw and destabilize the mandibular joint. The presence of destabilizing forces in the caecilian jaw mechanism may be compensated for by a mandibular joint in that the fossa is wrapped around the condyle to resist dislocation. The caecilian skull is streptostylic; the quadrate-squamosal complex moves with respect to the rest of the skull. This increases the leverage of the jaw-closing muscles. We also demonstrate that the unusual jaw joint requires streptostyly because there is a dorsolateral movement of the quadrate-squamosal complex when the jaw closes. The combination of the two jaw-closing systems results in high bite forces over a wide range of gape angles, an important advantage for generalist feeders such as caecilians. The relative sizes and leverage mechanics of the two closing systems allow one to exert more force when the other has a poor mechanical advantage. This effect is seen in all three species we examined. In the aquatic T. natans, with its less well-roofed skull, there is a larger contribution of the mm. levatores mandibulae to total bite force than in the terrestrial I. cf. kohtaoensis and S. annulatus.

From fish to modern humans--comparative anatomy, homologies and evolution of the head and neck musculature

In a recent paper Diogo (2008) reported the results of the first part of an investigation of the comparative anatomy, homologies and evolution of the head and neck muscles of osteichthyans (bony fish + tetrapods). That report mainly focused on actinopterygian fish, but also compared these fish with certain non-mammalian sarcopterygians. The present paper focuses mainly on sarcopterygians, and particularly on how the head and neck muscles have evolved during the transitions from sarcopterygian fish and non-mammalian tetrapods to monotreme and therian mammals, including modern humans. The data obtained from our dissections of the head and neck muscles of representative members of sarcopterygian fish, amphibians, reptiles, monotremes and therian mammals, such as rodents, tree-shrews, colugos and primates, including modern humans, are compared with the information available in the literature. Our observations and comparisons indicate that the number of mandibular and true branchial muscles (sensu this work) present in modern humans is smaller than that found in mammals such as tree-shrews, rats and monotremes, as well as in reptiles such as lizards. Regarding the pharyngeal musculature, there is an increase in the number of muscles at the time of the evolutionary transition leading to therian mammals, but there was no significant increase during the transition leading to the emergence of higher primates and modern humans. The number of hypobranchial muscles is relatively constant within the therian mammals we examined, although in this case modern humans have more muscles than other mammals. The number of laryngeal and facial muscles in modern humans is greater than that found in most other therian taxa. Interestingly, modern humans possess peculiar laryngeal and facial muscles that are not present in the majority of the other mammalian taxa; this seems to corroborate the crucial role played by vocal communication and by facial expressions in primate and especially in human evolution. It is hoped that by compiling, in one paper, data about the head and neck muscles of a wide range of sarcopterygians, the present work could be useful to comparative anatomists, evolutionary biologists and functional morphologists and to researchers working in other fields such as developmental biology, genetics and/or evolutionary developmental biology.