Development of craniofacial musculature in Monodelphis domestica (Marsupialia, Didelphidae)

Sequence of chondrocranial development in basal anurans-Let's make a cranium

Background

The craniofacial skeleton is an evolutionary innovation of vertebrates. Due to its complexity and importance to protect the brain and aid in essential functions (e.g., feeding), its development requires a precisely tuned sequence of chondrification and/or ossification events. The comparison of sequential patterns of cartilage formation bears important insights into the evolution of development. Discoglossus scovazzi is a basal anuran species. The comparison of its chondrocranium (cartilaginous neuro- & viscerocranium) development with other basal anurans (Xenopus laevis, Bombina orientalis) will help establishing the ancestral pattern of chondrification sequences in anurans and will serve as basis for further studies to reconstruct ancestral conditions in amphibians, tetrapods, and vertebrates. Furthermore, evolutionary patterns in anurans can be studied in the light of adaptations once the ancestral sequence is established.

Results

We present a comprehensive overview on the chondrocranium development of D. scovazzi. With clearing and staining, histology and 3D reconstructions we tracked the chondrification of 44 elements from the first mesenchymal Anlagen to the premetamorphic cartilaginous head skeleton and illustrate the sequential changes of the skull. We identified several anuran and discoglossoid traits of cartilage development. In D. scovazzi the mandibular, hyoid, and first branchial arch Anlagen develop first followed by stepwise addition of the branchial arches II, III, and IV. Nonetheless, there is no strict anterior to posterior chondrification pattern within the viscerocranium of D. scovazzi. Single hyoid arch elements chondrify after elements of the branchial arch and mandibular arch elements chondrify after elements of the branchial arch I.

Conclusions

In Osteichthyes, neurocranial elements develop in anterior to posterior direction. In the anurans investigated so far, as well as in D. scovazzi, the posterior parts of the neurocranium extend anteriorly, while the anterior parts of the neurocranium, extend posteriorly until both parts meet and fuse. Anuran cartilaginous development differs in at least two crucial traits from other gnathostomes which further supports the urgent need for more developmental investigations among this clade to understand the evolution of cartilage development in vertebrates.

The influence of divergent reproductive strategies in shaping modularity and morphological evolution in mammalian jaws

Marsupial neonates are born at an earlier developmental stage than placental mammals, but the rapid development of their forelimbs and cranial skeleton allows them to climb to the pouch, begin suckling and complete their development ex utero. The mechanical environment in which marsupial neonates develop is vastly different from that of placental neonates, which exhibit a more protracted development of oral muscles and bones. This difference in reproductive strategy has been theorized to constrain morphological evolution in the oral region of marsupials. Here, we use 3D morphometrics to characterize one of these oral bones, the lower jaw (dentary), and assess modularity (pattern of covariation among traits), morphological disparity and rates of morphological evolution in two clades of carnivorous mammals: the marsupial Dasyuromorphia and placental fissiped Carnivora. We find that dasyuromorph dentaries have fewer modules than carnivorans and exhibit tight covariation between the angular and coronoid processes, the primary attachment sites for jaw-closing muscles. This pattern of modularity may result from the uniform action of muscles on the developing mandible during suckling. Carnivorans are free from this constraint and exhibit a pattern of modularity that more strongly reflects genetic and developmental signals of trait covariation. Alongside differences in modularity, carnivorans exhibit greater disparity and faster rates of morphological evolution compared with dasyuromorphs. Taken together, this suggests dasyuromorphs have retained a signal of trait covariation that reflects the outsized influence of muscular force during early development, a feature that may have impacted the ability of marsupial carnivores to explore specialized regions of morphospace.

Anatomical comparison across heads, fore- and hindlimbs in mammals using network models

Animal body parts evolve with variable degrees of integration that nonetheless yield functional adult phenotypes: but, how? The analysis of modularity with Anatomical Network Analysis (AnNA) is used to quantitatively determine phenotypic modules based on the physical connection among anatomical elements, an approach that is valuable to understand developmental and evolutionary constraints. We created anatomical network models of the head, forelimb, and hindlimb of two taxa considered to represent a 'generalized' eutherian (placental: mouse) and metatherian (marsupial: opossum) anatomical configuration and compared them with our species, which has a derived eutherian configuration. In these models, nodes represent anatomical units and links represent their physical connection. Here, we aimed to identify: (1) the commonalities and differences in modularity between species, (2) whether modules present a potential phylogenetic character, and (3) whether modules preferentially reflect either developmental or functional aspects of anatomy, or a mix of both. We predicted differences between networks of metatherian and eutherian mammals that would best be explained by functional constraints, versus by constraints of development and/or phylogeny. The topology of contacts between bones, muscles, and bones + muscles showed that, among all three species, skeletal networks were more similar than musculoskeletal networks. There was no clear indication that humans and mice are more alike when compared to the opossum overall, even though their musculoskeletal and skeletal networks of fore- and hindlimbs are slightly more similar. Differences were greatest among musculoskeletal networks of heads and next of forelimbs, which showed more variation than hindlimbs, supporting previous anatomical studies indicating that in general the configuration of the hindlimbs changes less across evolutionary history. Most observations regarding the anatomical networks seem to be best explained by function, but an exception is the adult opossum ear ossicles. These ear bones might form an independent module because the incus and malleus are involved in forming a functional primary jaw that enables the neonate to attach to the teat, where this newborn will complete its development. Additionally, the human data show a specialized digit 1 module (thumb/big toe) in both limb types, likely the result of functional and evolutionary pressures, as our ape ancestors had highly movable big toes and thumbs.

Transient role of the middle ear as a lower jaw support across mammals

Mammals articulate their jaws using a novel joint between the dentary and squamosal bones. In eutherian mammals, this joint forms in the embryo, supporting feeding and vocalisation from birth. In contrast, marsupials and monotremes exhibit extreme altriciality and are born before the bones of the novel mammalian jaw joint form. These mammals need to rely on other mechanisms to allow them to feed. Here, we show that this vital function is carried out by the earlier developing, cartilaginous incus of the middle ear, abutting the cranial base to form a cranio-mandibular articulation. The nature of this articulation varies between monotremes and marsupials, with juvenile monotremes retaining a double articulation, similar to that of the fossil mammaliaform Morganucodon, while marsupials use a versican-rich matrix to stabilise the jaw against the cranial base. These findings provide novel insight into the evolution of mammals and the changing relationship between the jaw and ear.

Major evolutionary transitions and innovations: the tympanic middle ear

One of the most amazing transitions and innovations during the evolution of mammals was the formation of a novel jaw joint and the incorporation of the original jaw joint into the middle ear to create the unique mammalian three bone/ossicle ear. In this review, we look at the key steps that led to this change and other unusual features of the middle ear and how developmental biology has been providing an understanding of the mechanisms involved. This starts with an overview of the tympanic (air-filled) middle ear, and how the ear drum (tympanic membrane) and the cavity itself form during development in amniotes. This is followed by an investigation of how the ear is connected to the pharynx and the relationship of the ear to the bony bulla in which it sits. Finally, the novel mammalian jaw joint and versatile dentary bone will be discussed with respect to evolution of the mammalian middle ear.This article is part of the themed issue 'Evo-devo in the genomics era, and the origins of morphological diversity'.

Muscle development in the shark Scyliorhinus canicula: implications for the evolution of the gnathostome head and paired appendage musculature

BACKGROUND

The origin of jawed vertebrates was marked by profound reconfigurations of the skeleton and muscles of the head and by the acquisition of two sets of paired appendages. Extant cartilaginous fish retained numerous plesiomorphic characters of jawed vertebrates, which include several aspects of their musculature. Therefore, myogenic studies on sharks are essential in yielding clues on the developmental processes involved in the origin of the muscular anatomy.

RESULTS

Here we provide a detailed description of the development of specific muscular units integrating the cephalic and appendicular musculature of the shark model, Scyliorhinus canicula. In addition, we analyze the muscle development across gnathostomes by comparing the developmental onset of muscle groups in distinct taxa. Our data reveal that appendicular myogenesis occurs earlier in the pectoral than in the pelvic appendages. Additionally, the pectoral musculature includes muscles that have their primordial developmental origin in the head. This culminates in a tight muscular connection between the pectoral girdle and the cranium, which founds no parallel in the pelvic fins. Moreover, we identified a lateral to ventral pattern of formation of the cephalic muscles, that has been equally documented in osteichthyans but, in contrast with these gnathostomes, the hyoid muscles develop earlier than mandibular muscle in S. canicula.

CONCLUSION

Our analyses reveal considerable differences in the formation of the pectoral and pelvic musculatures in S. canicula, reinforcing the idea that head tissues have contributed to the formation of the pectoral appendages in the common ancestor of extant gnathostomes. In addition, temporal differences in the formation of some cranial muscles between chondrichthyans and osteichthyans might support the hypothesis that the similarity between the musculature of the mandibular arch and of the other pharyngeal arches represents a derived feature of jawed vertebrates.

On the development of the chondrocranium and the histological anatomy of the head in perinatal stages of marsupial mammals

An overview of the literature on the chondrocranium of marsupial mammals reveals a relative conservatism in shape and structures. We document the histological cranial anatomy of individuals representing Monodelphis domestica, Dromiciops gliroides, Perameles sp. and Macropus eugenii. The marsupial chondrocranium is generally characterized by the great breadth of the lamina basalis, absence of pila metoptica and large otic capsules. Its most anterior portion (cupula nasi anterior) is robust, and anterior to it there are well-developed tactile sensory structures, functionally important in the neonate. Investigations of ossification centers at and around the nasal septum are needed to trace the presence of certain bones (e.g., mesethmoid, parasphenoid) across marsupial taxa. In many adult marsupials, the tympanic floor is formed by at least three bones: alisphenoid (alisphenoid tympanic process), ectotympanic and petrosal (rostral and caudal tympanic processes); the squamosal also contributes in some diprotodontians. The presence of an entotympanic in marsupials has not been convincingly demonstrated. The tubal element surrounding the auditory tube in most marsupials is fibrous connective tissue rather than cartilage; the latter is the case in most placentals recorded to date. However, we detected fibrocartilage in a late juvenile of Dromiciops, and a similar tissue has been reported for Tarsipes. Contradictory reports on the presence of the tegmen tympani can be found in the literature. We describe a small tegmen tympani in Macropus. Several heterochronic shifts in the timing of development of the chondocranium and associated structures (e.g., nerves, muscles) and in the ossification sequence have been interpreted as largely being influenced by functional requirements related to the altriciality of the newborn marsupial during early postnatal life. Comparative studies of chondocranial development of mammals can benefit from a solid phylogenetic framework, research on non-classical model organisms, and integration with imaging and sectional data derived from computer-tomography.

Evolution of the Genotype-to-Phenotype Map and the Cost of Pleiotropy in Mammals

Evolutionary studies have long emphasized that the genetic architecture of traits holds important microevolutionary consequences. Yet, studies comparing the genetic architecture of traits across species are rare, and discussions of the evolution of genetic systems are made on theoretical arguments rather than on empirical evidence. Here, we compared the genetic architecture of cranial traits in two different mammalian model organisms: the gray short-tailed opossum, Monodelphis domestica, and the laboratory mouse, Mus musculus We show that both organisms share a highly polygenic genetic architecture for craniofacial traits, with many loci of small effect. However, these two model species differ significantly in the overall degree of pleiotropy, N, of the genotype-to-phenotype map, with opossums presenting a higher average N They also diverge in their degree of genetic modularity, with opossums presenting less modular patterns of genetic association among traits. We argue that such differences highlight the context dependency of gene effects, with developmental systems shaping the variational properties of genetic systems. Finally, we also demonstrate based on the opossum data that current measurements for the relationship between the mutational effect size and N need to be re-evaluated in relation to the importance of the cost of pleiotropy for mammals.

Heterochrony and developmental timing mechanisms: changing ontogenies in evolution

Heterochrony, or a change in developmental timing, is an important mechanism of evolutionary change. Historically the concept of heterochrony has focused alternatively on changes in size and shape or changes in developmental sequence, but most have focused on the pattern of change. Few studies have examined changes in the mechanisms that embryos use to actually measure time during development. Recently, evolutionary studies focused on changes in distinct timekeeping mechanisms have appeared, and this review examines two such case studies: the evolution of increased segment number in snakes and the extreme rostral to caudal gradient of developmental maturation in marsupials. In both examples, heterochronic modifications of the somite clock have been important drivers of evolutionary change.

Statistical support for the hypothesis of developmental constraint in marsupial skull evolution

Background

In contrast to placental neonates, in which all cranial bones are ossified, marsupial young have only the bones of the oral region and the exoccipital ossified at birth, in order to facilitate suckling at an early stage of development. In this study, we investigated whether this heterochronic shift in the timing of cranial ossification constrains cranial disparity in marsupials relative to placentals.

Methods

We collected three-dimensional (3D) landmark data about the crania of a wide range of extant placentals and marsupials, and from six fossil metatherians (the clade including extant marsupials and their stem relatives), using a laser scanner and a 3D digitizer. Principal components analysis and delta variance tests were used to investigate the distribution and disparity of cranial morphology between different landmark sets (optimizing either number of landmarks or number of taxa) of the whole skull and of individual developmental or functional regions (neurocranium, viscerocranium, oral region) for extant placentals and marsupials. Marsupial and placental data was also compared based on shared ecological aspects including diet, habitat, and time of peak activity.

Results

We found that the extant marsupial taxa investigated here occupy a much smaller area of morphospace than the placental taxa, with a significantly (P<0.01) smaller overall variance. Inclusion of fossil taxa did not significantly increase the variance of metatherian cranial shape. Fossil forms generally plotted close to or within the realm of their extant marsupial relatives. When the disparities of cranial regions were investigated separately, significant differences between placentals and marsupials were seen for the viscerocranial and oral regions, but not for the neurocranial region.

Conclusion

These results support the hypothesis of developmental constraint limiting the evolution of the marsupial skull, and further suggest that the marsupial viscerocranium as a whole, rather than just the early-ossifying oral region, is developmentally constrained.

Ontogeny of the cranial system in Laonastes aenigmamus

Rodents, together with bats, are among the ecologically most diverse and most speciose groups of mammals. Moreover, rodents show elaborate specializations of the feeding apparatus in response to the predominantly fore-aft movements of the lower jaw. The Laotian rock rat Laonastes aenigmamus was recently discovered and originally thought to belong to a new family. The difficulties in classifying L. aenigmamus based on morphological characters stem from the fact that it presents a mixture of sciurognathous and hystricognathous characteristics, including the morphology of the jaw adductors. The origin of the unusual muscular organization in this species remains, however, unclear. Here, we investigate the development of the masticatory system in Laonastes to better understand the origin of its derived morphology relative to other rodents. Our analyses show that skull and mandible development is characterized by an overall elongation of the snout region. Muscle mass increases with positive allometry during development and growth, and so does the force-generating capacity of the jaw adductor muscles (i.e. physiological cross-sectional area). Whereas fetal crania and musculature are more similar to those of typical rodents, adults diverge in the elongation of the rostral part of the skull and the disproportionate development of the zygomaticomandibularis. Our data suggest a functional signal in the development of the unusual cranial morphology, possibly associated with the folivorous trophic ecology of the species.

Evolution of the mammalian middle ear and jaw: adaptations and novel structures

Having three ossicles in the middle ear is one of the defining features of mammals. All reptiles and birds have only one middle ear ossicle, the stapes or columella. How these two additional ossicles came to reside and function in the middle ear of mammals has been studied for the last 200 years and represents one of the classic example of how structures can change during evolution to function in new and novel ways. From fossil data, comparative anatomy and developmental biology it is now clear that the two new bones in the mammalian middle ear, the malleus and incus, are homologous to the quadrate and articular, which form the articulation for the upper and lower jaws in non-mammalian jawed vertebrates. The incorporation of the primary jaw joint into the mammalian middle ear was only possible due to the evolution of a new way to articulate the upper and lower jaws, with the formation of the dentary-squamosal joint, or TMJ in humans. The evolution of the three-ossicle ear in mammals is thus intricately connected with the evolution of a novel jaw joint, the two structures evolving together to create the distinctive mammalian skull.

Developmental origins of precocial forelimbs in marsupial neonates

Marsupial mammals are born in an embryonic state, as compared with their eutherian counterparts, yet certain features are accelerated. The most conspicuous of these features are the precocial forelimbs, which the newborns use to climb unaided from the opening of the birth canal to the teat. The developmental mechanisms that produce this acceleration are unknown. Here we show that heterochronic and heterotopic changes early in limb development contribute to forelimb acceleration. Using Tbx5 and Tbx4 as fore- and hindlimb field markers, respectively, we have found that, compared with mouse, both limb fields arise notably early during opossum development. Patterning of the forelimb buds is also accelerated, as Shh expression appears early relative to the outgrowth of the bud itself. In addition, the forelimb fields and forelimb myocyte allocation are increased in size and number, respectively, and migration of the spinal nerves into the forelimb bud has been modified. This shift in the extent of the forelimb field is accompanied by shifts in Hox gene expression along the anterior-posterior axis. Furthermore, we found that both fore- and hindlimb fields arise gradually during gastrulation and extension of the embryonic axis, in contrast to the appearance of the limb fields in their entirety in all other known cases. Our results show a surprising evolutionary flexibility in the early limb development program of amniotes and rule out the induction of the limb fields by mature structures such as the somites or mesonephros.

Developmental origins of species-specific muscle pattern

Vertebrate jaw muscle anatomy is conspicuously diverse but developmental processes that generate such variation remain relatively obscure. To identify mechanisms that produce species-specific jaw muscle pattern we conducted transplant experiments using Japanese quail and White Pekin duck, which exhibit considerably different jaw morphologies in association with their particular modes of feeding. Previous work indicates that cranial muscle formation requires interactions with adjacent skeletal and muscular connective tissues, which arise from neural crest mesenchyme. We transplanted neural crest mesenchyme from quail to duck embryos, to test if quail donor-derived skeletal and muscular connective tissues could confer species-specific identity to duck host jaw muscles. Our results show that duck host jaw muscles acquire quail-like shape and attachment sites due to the presence of quail donor neural crest-derived skeletal and muscular connective tissues. Further, we find that these species-specific transformations are preceded by spatiotemporal changes in expression of genes within skeletal and muscular connective tissues including Sox9, Runx2, Scx, and Tcf4, but not by alterations to histogenic or molecular programs underlying muscle differentiation or specification. Thus, neural crest mesenchyme plays an essential role in generating species-specific jaw muscle pattern and in promoting structural and functional integration of the musculoskeletal system during evolution.

Petrosal anatomy in the fossil mammal Necrolestes: evidence for metatherian affinities and comparisons with the extant marsupial mole

We present reconstructions of petrosal anatomy based on high-resolution X-ray computed tomography scans for the fossil mammal Necrolestes and for the marsupial mole Notoryctes sp. Compared with other mammals, Necrolestes exhibits a mosaic of plesiomorphic and derived characters, but most of the evidence supports its metatherian status. We revised previous descriptions and report on features of phylogenetic or functional significance. Necrolestes exhibits features that support metatherian affinities, such as the presence of a short and lateral prootic canal, and the loss of the stapedial artery in adults. A deep groove at the anterior pole of the promontorium is present in front of the cochlear housing, a variant on the extrabullar pathway of the internal carotid artery. The promontorium is laterally bordered by a large bony projection resembling the eutherian tegmen tympani [De Beer GR (1937) The Development of the Vertebrate Skull, Oxford, Clarendon Press, p. 391]. Posteromedial to the secondary facial foramen and anterolateral to the fenestra vestibuli is a pronounced fossa for the tensor tympani muscle. On the medial part of the pars canalicularis there is a great inflation of the medial side of the caudal tympanic process, a structure of unknown function. The internal acoustic meatus exhibits a broad transverse septum and is bordered laterally by a broad prefacial commissure. The cochleae of Necrolestes and of Notoryctes have fewer spiral turns (1.1 and 1.6, respectively) than most marsupials. The lateral semicircular canal is more expanded than the posterior semicircular canal in Necrolestes but not in Notoryctes. Both Necrolestes and Notoryctes have a second crus commune, i.e. the lateral semicircular canal opens into the ampulla of the posterior semicircular canal. A stylomastoid foramen enclosed anterodorsally by both the pars cochlearis and pars canalicularis is present in Dasyuridae, Dromiciops gliroides and Notoryctes.

Opossum (Monodelphis domestica): A Marsupial Development Model

INTRODUCTIONMonodelphis domestica is the most commonly used laboratory marsupial. In addition to the many factors that make it a convenient laboratory animal (small size, ease of care, nonseasonal breeding), it is the first marsupial whose genome has been sequenced. In this article, we present an overview of aspects of its biology and its use as a model organism. We also discuss basic care, breeding, embryo manipulation, and modifications of common techniques for the study of the development of this species.

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.

Craniofacial development in marsupial mammals: Developmental origins of evolutionary change

Biologists have long studied the evolutionary consequences of the differences in reproductive and life history strategies of marsupial and eutherian mammals. Over the past few decades, the impact of these strategies on the development of the marsupial embryo and neonate has received attention. In this review, the differences in development in the craniofacial region in marsupial and eutherian mammals will be discussed. The review will highlight differences at the organogenic and cellular levels, and discuss hypotheses for shifts in the expression of important regulatory genes. The major difference in the organogenic period is a whole-scale shift in the relative timing of central nervous system structures, in particular those of the forebrain, which are delayed in marsupials, relative to the structures of the oral-facial apparatus. Correlated with the delay in development of nervous system structures, the ossification of the bones of the neurocranium are delayed, while those of the face are accelerated. This study will also review work showing that the neural crest, which provides much of the cellular material to the facial skeleton and may also carry important patterning information, is notably accelerated in its development in marsupials. Potential consequences of these observations for hypotheses on constraint, evolutionary integration, and the existence of developmental modules is discussed. Finally, the implications of these results for hypotheses on the genetic modulation of craniofacial patterning are presented.

Patterns of spatial and temporal visceral arch muscle development in the Mexican axolotl (Ambystoma mexicanum)

Vertebrate head development is a classical topic that has received renewed attention during the last decade. Most reports use one of a few model organisms (chicken, mouse, zebrafish) and have focused on molecular mechanisms and the role of the neural crest, while cranial muscle development has received less attention. Here we describe cranial muscle differentiation and morphogenesis in the Mexican axolotl, Ambystoma mexicanum. To determine the onset of differentiation we use antibodies against desmin and optical sectioning using confocal laser scanning microscopy on whole-mount immunostained embryos. This technique makes it possible to document the cranial muscle in three dimensions while keeping the specimens intact. Desmin expression starts almost simultaneously in the first, second, and third visceral arch muscles (as in other amphibians studied). Muscle anlagen divide up early into the different elements which constitute the larval cranial musculature. We extend and refine earlier findings, e.g., by documenting a clear division between interhyoideus and interhyoideus posterior. The timing of cranial muscle differentiation differs among vertebrate groups, but seems to be constant within each group. This study provides a morphological foundation for further studies of muscle cell fate and early differentiation.

Skeletal development ofMacrochelys temminckii (Reptilia: Testudines: Chelydridae)

Few descriptions of the development and sequence of chondrification and ossification of the entire skeleton of turtles exist, particularly compared to other groups of reptiles. In this study, the embryonic skeleton and its ontogenesis are described for the Alligator Snapping Turtle, Macrochelys temminckii (Chelydridae). Morphological descriptions utilize cleared and double-stained embryonic specimens and form the basis of comparison of the ontogenesis of the skeleton between this species and its extant sister taxon, Chelydra serpentina. The embryonic chondrocranium, as well as the sequences of formation and ossification of the entire skeleton, are compared between these closely related species, and afford a unique opportunity to examine differences in their patterns of skeletal formation. In M. temminckii, the first elements to ossify (Stage 17) are associated with the dermatocranium and upper jaw, followed by elements of the palate, lower jaw, and long bones of the limbs. In both species the majority of endochondral braincase elements (prootic, opisthotic, supraoccipital, and exoccipital) ossify after the majority of dermal elements of the skull. The sequences of formation of the chondral primordia of the limb elements, as well as ossification of autopodial elements, are generally congruent between these species.

Ontogeny of feeding function in the gray short-tailed opossum Monodelphis domestica: empirical support for the constrained model of jaw biomechanics

The constrained model of masticatory function enables specific predictions of bite force potentials in skulls of differing craniodental configurations. In this study, empirical support for the constrained model is provided using maximum voluntary bite force data along Region I and II of the jaws of gray short-tailed opossums Monodelphis domestica. Then, growth series of M. domestica are used to assess how bite force potential changes with growth by evaluating craniodental changes using longitudinal sets of dorsoventral radiographs and by assessing maximal bite force potential at the Region I-II boundary of the jaw in juveniles (aged 70-80 days) and adults. Our findings show that, while juveniles and adults alike enclose at least three molariform teeth within Region II (the area of highest bite force potential along the jaw), age-dependent elongation of the masticatory muscle resultant lever arm and narrowing of the palate relative to jaw length especially enhance the mechanical advantage of the adductor muscle resultant in adults. While maximal bite forces at the Region I-II boundary are absolutely greater in adults, these bite forces scale isometrically with body mass, which suggests that mass-specific forces exerted by jaw adductor muscles of larger (adult) opossums are disproportionately smaller than those exerted by smaller (juvenile) opossums.

Early differentiation and migration of cranial neural crest in the opossum, Monodelphis domestica

Marsupial mammals are born at a highly altricial state. Nonetheless, the neonate must be capable of considerable functional independence. Comparative studies have shown that in marsupials the morphogenesis of many structures critical to independent function are advanced relative to overall development. Many skeletal and muscular elements in the facial region show particular heterochrony. Because neural crest cells are crucial to forming and patterning much of the face, this study investigates whether the timing of cranial neural crest differentiation is also advanced. Histology and scanning electron microscopy of Monodelphis domestica embryos show that many aspects of cranial neural crest differentiation and migration are conserved in marsupials. For example, as in other vertebrates, cranial neural crest differentiates at the neural ectoderm/epidermal boundary and migrates as three major streams. However, when compared with other vertebrates, a number of timing differences exist. The onset of cranial neural crest migration is early relative to both neural tube development and somite formation in Monodelphis. First arch neural crest cell migration is particularly advanced and begins before any somites appear or regional differentiation exists in the neural tube. Our study provides the first published description of cranial neural crest differentiation and migration in marsupials and offers insight into how shifts in early developmental processes can lead to morphological change.

Sequence heterochrony and the evolution of development

One of the most persistent questions in comparative developmental biology concerns whether there are general rules by which ontogeny and phylogeny are related. Answering this question requires conceptual and analytic approaches that allow biologists to examine a wide range of developmental events in well-structured phylogenetic contexts. For evolutionary biologists, one of the most dominant approaches to comparative developmental biology has centered around the concept of heterochrony. However, in recent years the focus of studies of heterochrony largely has been limited to one aspect, changes in size and shape. I argue that this focus has restricted the kinds of questions that have been asked about the patterns of developmental change in phylogeny, which has narrowed our ability to address some of the most fundamental questions about development and evolution. Here I contrast the approaches of growth heterochrony with a broader view of heterochrony that concentrates on changes in developmental sequence. I discuss a general approach to sequence heterochrony and summarize newly emerging methods to analyze a variety of kinds of developmental change in explicit phylogenetic contexts. Finally, I summarize a series of studies on the evolution of development in mammals that use these new approaches.

Early development of the neural plate, neural crest and facial region of marsupials

Marsupial mammals have a distinctive reproductive strategy. The young are born after an exceptionally short period of organogenesis and are consequently extremely altricial. Yet because they must be functionally independent in an essentially embryonic condition, the marsupial neonate exhibits a unique suite of adaptations. In particular, certain bones of the facial region, most cranial musculature and a few additional structures are accelerated in their development. In contrast, central nervous system structures, especially the forebrain, are markedly premature at birth, resembling an embryonic d 11 or 12 mouse. This review examines the developmental processes that are modified to produce these evolutionary changes. The focus is on the early development of the neural plate, neural crest and facial region in the marsupial, Monodelphis domestica, compared with patterns reported for rodents. Neural crest begins differentiation and migration at the neural plate stage, which results in large accumulations of neural crest in the facial region at an early stage of development. The early accumulation of neural crest provides the material for the accelerated development of oral and facial structures. The first arch region is massive in the early embryo, and the development of the olfactory placode and frontonasal region is advanced relative to the forebrain region. The development of the forebrain is delayed in marsupials relative to the hindbrain or facial region. These observations illustrate how development may be modified to produce evolutionary changes that distinguish taxa. Further, they suggest that development is not necessarily highly conserved, but instead may be quite plastic.

Skeletal fiber types and spindle distribution in limb and jaw muscles of the adult and neonatal opossum, Monodelphis domestica

The South American opossum, Monodelphis domestica, is very immature at birth, and we wished to assess its potential for studies of jaw muscle development. Given the lack of prior information about any Monodelphis fiber types or spindles, our study aimed to identify for the first time fiber types in both adult and neonatal muscles and the location of spindles in the jaw muscles. Fiber types were identified in frozen sections of adult and 6-day-old jaw and limb muscles by using myosin ATPase and metabolic enzyme histochemistry and by immunostaining for myosin isoforms. The distribution of fiber types and muscle spindles throughout the jaw-closer muscles was identified by immunostaining of sections of methacarnoy-fixed, wax-embedded heads. Most muscles contained one slow (type I) and two fast fiber types (equivalent to types IIA and IIX), which were similar to those in eutherian muscle, and an additional (non-IIB) fast type. In jaw-closer muscles, the main extrafusal fiber type was IIM (characteristic of these muscles in some eutherians), and almost all spindles were concentrated in four restricted areas: one in masseter and three in temporalis. Six-day neonatal muscles were very immature, but future spindle-rich areas were revealed by immunostaining and corresponded in position to the adult areas. Extrafusal and spindle fiber types in Monodelphis share many similarities with eutherian mammalian muscle. This finding, along with the immaturity of myosin isoform expression observed 6 days postnatally, indicates that Monodelphis could provide a valuable model for studying early developmental events in the jaw-closer muscles and their spindles.

The platypus is not a rodent: DNA hybridization, amniote phylogeny and the palimpsest theory

We present DNA-hybridization data on 21 amniotes and two anurans showing that discrimination is obtained among most of these at the class and lower levels. Trees generated from these data largely agree with conventional views, for example in not associating birds and mammals. However, the sister relationships found here of the monotremes to marsupials, and of turtles to the alligator, are surprising results which are nonetheless consistent with the results of some other studies. The Marsupionta hypothesis of Gregory is reviewed, as are opinions about the placement of chelonians. Anatomical and reproductive data considered by Gregory do not unequivocally preclude a marsupial-monotreme special relationship, and there is other recent evidence for placing turtles within the Diapsida. We conclude that the evidential meaning of the molecular data is as shown in the trees, but that the topologies may be influenced by a base-compositional bias producing a seemingly slow evolutionary rate in monotremes, or by algorithmic artefacts (in the case of turtles as well).