Discrete embryonic character variation uncovers hidden ecological adaptations in lacertid lizards.

Embryogenesis is the first step in the ontogenetic life journey of any individual, and is thus a starting point for natural selection to cause evolutionary change. There are slight variations in the timing of embryonic development, known as heterochrony, which may eventually lead to major differences in adult anatomy. To test this hypothesis, the embryonic development of three closely related lizard species, Darevskia armeniaca, Lacerta agilis, and L. viridis, which are adapted to different habitats, was compared by analyzing discrete timing characters. Both intra- and interspecific variation was detected. The latter may be interpreted as embryonic pre-adaptions to later adult lifestyles, demonstrating that developmental penetrance manifests within a few million years. Traits with large intraspecific temporal variation, such as limb-related features, were susceptible to natural selection. In particular, the mountain-dwelling, climbing species D. armeniaca showed embryonic preadaptions by an early developing limb anlagen. This observation demonstrated interspecific variation, which was elusive in a previous comparative study based on purely metric data of developing limb lengths, and highlighted the importance of multiple data sources to draw robust conclusions about evolutionary change. Timing differences indicated unexplored ecological adaptations of the poorly understood lifestyle of these lizards. Thus, embryonic research provides a platform to explore superficially hidden evolutionary adaptations of all organisms on Earth.

Limb reduction in squamate reptiles correlates with the reduction of the chondrocranium: A case study on serpentiform anguids.

BACKGROUND: In vertebrates, the skull evolves from a complex network of dermal bones and cartilage-the latter forming the pharyngeal apparatus and the chondrocranium. Squamates are particularly important in this regard as they maintain at least part of the chondrocranium throughout their whole ontogeny until adulthood. Anguid lizards represent a unique group of squamates, which contains limbed and limbless forms and show conspicuous variation of the adult skull. RESULTS: Based on several emboadryonic stages of the limbless lizards Pseudopus apodus and Anguis fragilis, and by comparing with other squamates, we identified and interpreted major differences in chondrocranial anatomy. Among others, the most important differences are in the orbitotemporal region. P. apodus shows a strikingly similar development of this region to other squamates. Unexpectedly, however, A. fragilis differs considerably in the composition of the orbitotemporal region. In addition, A. fragilis retains a paedomorphic state of the nasal region. CONCLUSIONS: Taxonomic comparisons indicate that even closely related species with reduced limbs show significant differences in chondrocranial anatomy. The Pearson correlation coefficient suggests strong correlation between chondrocranial reduction and limb reduction. We pose the hypothesis that limb reduction could be associated with the reduction in chondrocrania by means of genetic mechanisms.

Light Microscopy of Development of the Olfactory Organ of European Weatherfish Misgurnus fossilis (Тeleostei: Cobitidae).

We studied the formation of the olfactory organ of European weatherfish Misgurnus fossilis (Linnaeus, 1758) from the formation of its placodes to the formation of lamellae using light microscopy. The aim of the present investigation was to determine which morphogenetic features of the olfactory organ ate associated with the demersal lifestyle. The olfactory organ of European weatherfish goes through such developmental stages as olfactory placode, olfactory pit, and olfactory chamber with lamellae. Formation of the olfactory pit occurs in prolarvae, but a considerable increase in size takes place during the switch to exogenous feeding. In late larva, lamellae are formed in the aboral part olfactory chamber as an outward fold at its bottom. At the same time, incurrent and excurrent nostrils start being formed as the result of approximation of lateral and medial margins in the rostal part above the olfactory cavity. In contrast to other fishes, the peculiarity of morphogenesis of olfactory organ of M. fossilis is its late formation followed by intensive postembryonic development.

Chondrification and Character Identification in the Skull Exemplified for the Basicranial Anatomy of Early Squamate Embryos.

The neurocranium of vertebrates is mainly derived from early cartilaginous anlagen, the so-called chondrocranium, the base of the future skull. Two initial bar-shaped and paired chondrifications flank the notochord, the rostral trabecles and the caudal parachordals. In most reptiles, there is an additional component, the transverse acrochordal, which is placed between trabecles and parachordals. All these elements compose the base of the future chondrocranium. There are several drastically different hypotheses concerning the development and interrelationship of these elements. We reexamined the basicranial development in four squamates and found that all species show very similar conditions of early chondrocranial development. The anterior part of the notochord is not embedded into the basal plate as it was previously reported. It remains free. The medial edges of the parachordals form the lateral walls of the basicranial fenestra. Only the posterior portions of the parachordals fuse and form the basal plate. The space in-between the parachordals is filled with a thin layer of cells, which never chondrify. The anterior tips of the parachordals later fuse with the posterior edge of the acrochordal, which ultimately delimitates, as crista sellaris, the basicranial fenestra anteriorly. We consider the observed processes a common development at least in lizards and review a variety of methodological approaches and differences in data interpretation as reasons for the anatomical differences reported in the literature. Moreover, based on our data we argue that the acrochordal is of mesodermal origin, which coincides with results of fate map experimental studies.

Chondrocranial anatomy of Testudo hermanni (Testudinidae, Testudines) with a comparison to other turtles.

Using histological cross-sections, the chondrocranium anatomy was reconstructed for two developmental stages of Hermann's tortoise (Testudo hermanni). The morphology differs from the chondrocrania of most other turtles by a process above the ectochoanal cartilage with Pelodiscus sinensis being the only other known species with such a structure. The anterior and posterior processes of the tectum synoticum are better developed than in most other turtles and an ascending process of the palatoquadrate is missing, which is otherwise only the case in pleurodiran turtles. The nasal region gets proportionally larger during development. We interpret the enlargement of the nasal capsules as an adaption to increase the surface area of the olfactory epithelium for better perception of volant odors. Elongation of the nasal capsules in trionychids, in contrast, is unlikely to be related to olfaction, while it is ambiguous in the case of Sternotherus odoratus. However, we have to conclude that research on chondrocranium anatomy is still at its beginning and more comprehensive detailed descriptions in relation to other parts of the anatomy are needed before providing broad-scale ecological and phylogenetic interpretations.

Morphological association between muscle attachments and ossification sites in the late cartilaginous skull of tuatara embryos.

During development, the embryonic cartilaginous skull in most vertebrates is partially replaced by bones with endochondral and perichondral ossifications. Muscle attachments are thought to influence the patterns of ossification and, hence, the differentiation of the skull. To investigate the association between muscle attachments and early ossifications of reptilian embryos, we conducted digital 3D reconstructions of the cranium, the head, and the neck musculature from a histological section series of a late term embryonic tuatara, Sphenodon punctatus, with a total body length of 52 mm. As the sole living rhynchocephalian species, it is an important outgroup in comparative studies of squamate evolution. We found that head and neck muscles are largely associated with early ossification of the basal plate and the palatoquadrate, and with three other ossifications in an older specimen with a total body length of 72 mm. These results suggest that tensile forces resulting from embryonic muscle contraction are largely, but not exclusively, correlated with the area of endochondral ossification in the chondrocranium and palatoquadrate in tuatara. Beyond little-known genetic factors, the complexity of chondrocranial architecture, the progress of its development, and the effect of multiple muscle transmitting forces in the chondrocranium must be considered to provide a more comprehensive discussion of the mechanical properties of the embryonic skull.

Comparative anatomy and ontogeny of appendicular skeleton of Pseudopus apodus (Pallas, 1775) (Anguimorpha, Anguidae) and a pattern of hindlimb loss in Anguinae.

We present a detailed anatomy of the pectoral girdles, pelvic girdles, and hindlimbs of adult and juvenile specimens of Pseudopus apodus (Pallas, 1775). We compared the individual bones of the appendicular skeleton of P. apodus with those of Anguis fragilis and species of Ophisaurus living in North America, North Africa, and Southeast Asia. We found no anatomical features in P. apodus in common with the species of Ophisaurus living in only North America, North Africa, and Southeast Asia. Additionally, we present the prehatching ontogeny of the pelvic girdle of P. apodus and A. fragilis and the prehatching ontogeny of the hindlimb of P. apodus. In the ontogeny of the pelvic girdle of P. apodus, it is possible to distinguish the ossification centers of ilium, ischium, and pubis. In contrast, in the ontogeny of A. fragilis, no ossification centers of ilium, ischium, and pubis are present, and no hindlimb element was detected. In Stage 1 of ontogeny in Pseudopus, the femur and tibia are present; in Stage 2, the nodule representing the fibula appears; and in Stage 3, in addition to the femur, tibia, and fibula, four tarsal elements are present. This anatomical condition corresponds to the anatomical composition of the hindlimb of the adult O. koellikeri. In Stage 4, the involution of all tarsal elements and fibula begins, and in the last two prehatching stages, only femur and tibia remain; this condition is present not only in the adults of Pseudopus, but also in those of several other species of Ophisaurus.

Vertebral Comparative Anatomy and Morphological Differences in Anguine Lizards With a Special Reference to Pseudopus apodus.

The article reports on the first detailed vertebral and rib morphology of anguine taxon Pseudopus apodus using micro-computed tomography. A comparison shows significant morphological differences of vertebrae of Pseudopus relative to those of Anguis and Ophisaurus. Usually, there are 55 presacral vertebrae, two sacral, and 95-97 caudal vertebrae. Pseudopus apodus can be defined by 23 diagnostic features concerning the vertebral column. Although zygapophyseal articulation between atlas and axis is well developed in limbed anguid gerrhonotine lizards like Abronia or Barisia, it is absent in the extant representatives of the clade Anguinae, which are limbless. Thus, our study brings further support to the hypothesis about the complete reduction of this articulation in forms with reduced or absent limbs. Comparison of adult and juvenile morphology of vertebrae of P. apodus was also analyzed. Heterochrony in the evolution of this taxon was previously confirmed by its skull morphology and it can be also documented on the basis of vertebrae. Our data suggest that a peramorphic heterochronic process played a role in the evolution of this largest extant anguine species. Geometric morphometric analyses revealed a pattern of high vertebral disparity among species. We found a clear separation of limbless forms in morphospace. Pseudopus apodus always clusters within Ophisaurus-species confirming molecular and some morphological phylogenies. Only the first tail vertebra shows a distinct difference to those of other anguids, which might be related to altered locomotion associated to the larger body size in this species. Anat Rec, 302:232-257, 2019. © 2018 Wiley Periodicals, Inc.

Tracing the developmental origin of a lizard skull: Chondrocranial architecture, heterochrony, and variation in lacertids.

The sand lizard, Lacerta agilis, is a classical model species in herpetology. Its adult skull anatomy and its embryonic development are well known. The description of its fully formed primordial skull by Ernst Gaupp, in 1900, was a key publication in vertebrate morphology and influenced many comparative embryologists. Based on recent methodological considerations, we restudied the early cranial development of this species starting as early as the formation of mesenchymal condensations up to the fully formed chondrocranium. We traced the formation of the complex chondrocranial architecture in detail, clarified specific homologies for the first time, and uncovered major differences to old textbook descriptions. Comparison with other lacertid lizards revealed a very similar genesis of the primordial skull. However, we detected shifts in the developmental timing of particular cartilaginous elements, mainly in the nasal region, which may correlate to specific ecological adaptation in the adults. Late timing of nasal elements might be an important innovation for the successful wide range distribution of the well-known sand lizard.

Skull Anatomy and Ontogeny of Legless Lizard Pseudopus apodus (Pallas, 1775): Heterochronic Influences on Form.

Pseudopus apodus (Pallas, 1775) is the largest extant legless species of the subfamily Anguinae (Anguimorpha, Anguidae) living mostly in the sub-arid territories ranging from the Balkan area in Europe to Kazakhstan in Asia. The species of other two genera live in North America, South-East Asia and North Africa (Ophisaurus) and Europe and South-West Asia (Anguis). The interrelationships of Anguinae are unresolved; this is in part the consequence of the insufficient knowledge of the cranial, postcranial and integumentary anatomy of the individual anguine species. The aim of this article is to fulfill this gap in our knowledge of the anguine anatomy. Now, in the first part of the project, the individual bones of the exocranium and visceral endocranium of the anguine legless lizard P. apodus are described in detail. In the present study, P. apodus is revealed to have autoapomorphic features of the skull which clearly distinguish it from Anguis and Ophisaurus. In addition, the study of posthatchling ontogeny of exocranium of P. apodus revealed some features, such as a nasal process of premaxilla being slightly widened in about its mid-length, that are also typical for adults of the Ophisaurus and Anguis species as well as extinct species of Pseudopus. This strongly indicates that peramorphic heterochronic process played role in the evolution of the P. apodus skull. Anat Rec, 2017. © 2017 Wiley Periodicals, Inc. Anat Rec, 300:460-502, 2017. © 2016 Wiley Periodicals, Inc.