Phylogeny of frogs as inferred from primarily larval characters (Amphibia:Anura)★

Developmental sequence of the chondrocranium in the obligate carnivorous larvae of Lepidobatrachus laevis (Amphibia: Ceratophryidae)

The vertebrate head and its skull represent a significant innovation that has played a key role in the evolutionary and ecological success of vertebrates. For a global and integral understanding of the evolution of the head skeleton, it is essential to have reliable information on the development of chondrocranium in a wide range of vertebrate species. Therefore, we studied the cranial chondrogenesis of the larva of the Budgett frog, Lepidobatrachus laevis (Ceratophryidae, Neobatrachia). We studied the development using several methods, including histological preparation of transverse sections of the chondrocranium, morphological analysis of three different states of development (mesenchymal aggregation, differentiation, and chondrification), and three-dimensional digital reconstructions. As a result, we observed that the Anlage of the chondrocranium at Gosner stage 19 is laterally compressed, that is, it is initially higher than wide. It gradually flattens, enlarges, and differentiates until reaching a very wide and flat shape at Gosner stage 26. Furthermore, we show that the chondrocranial development of L. laevis takes place in a mosaic pattern, which differs to nontetrapod vertebrates in which an anterior to posterior gradient of chondrification is observed. We identified 19 developmental stages in L. laevis according to the chondrification state of its cranial structures. The first element reaching the differentiation-into-chondroblast stage is the hypobranchial plate of the branchial basket, and chondrification, that is, final differentiation, occurs simultaneously in several structures of the neurocranium and viscerocranium. We hypothesize that the rapid chondrification of L. laevis' chondrocranium is an adaptation to the semi-arid climate conditions from its type locality, El Gran Chaco in South America. Due to the only temporary availability of suitable water accumulations this apparent accelerated development would make sense to ensure the tadpoles are froglets by the time the water body disappears. Here, we provide novel information on chondrocranial development in L. laevis. We compare our results to the developmental sequences, previously known from other vertebrate taxa, particularly within the amphibian phylogeny, in an attempt to understand developmental variability and the evolutionary history of the vertebrate head skeleton.

Gastrotheca Fitzinger, 1843 tadpole morphology: Larval cranium description and its evolutionary implications (Amphibia: Anura: Hemiphractidae)

Hemiphractids have a singular mode of reproduction that involves maternal care. The Andean-endemic Gastrotheca marsupiata species group includes direct-developing and tadpole-bearing species, the latter trait being unique among Gastrotheca. Larval morphology has proven to be a valuable source of evidence to understand the taxonomy and evolution of frogs but remains understudied in Hemiphractids. Herein, we redescribe the larval cranium of G. espeletia, G. gracilis, G. marsupiata, G. peruana, G. pseustes, and G. riobambae, and describe those of G. aureomaculata, G. chrysosticta, G. litonedis, G. monticola and G. psychrophila. Additionally, based on the data gathered, we explore their phylogenetic significance, expanding the knowledge regarding Gastrotheca larval internal morphology. We suggest that the presence of the posterolateral process of crista parotica, the concave palatoquadrate, the quadratoorbital commissure, and the proximal commissures II and III are putative synapomorphies for Gastrotheca. Furthermore, we suggest the long pseudopterygoid process as a putative synapomorphy for Hemiphractyidae.

A hundred-year-old mystery-the reproductive mode and larval morphology of the enigmatic frog genus Allophryne (Amphibia; Anura; Allophrynidae)

Frogs of the Allophrynidae are an enigmatic family from South America. To date, published information is lacking regarding this group's reproductive biology and larval morphology. Here, we provide the first detailed description of the reproductive mode, developmental mode, and tadpole morphology for Allophryne ruthveni. We developed a captive breeding and rearing protocol for this species and then conducted a series of observations to describe aspects of its reproductive biology. In captivity, this species exhibits aquatic oviposition, where single eggs are laid ungrouped within a simple jelly capsule and are scattered free in the water column before sinking to develop on benthic substrates. We did not observe parental care nor any parental interactions with eggs post-fertilization. Tadpoles are characterized by an oval body, anteroventral oral disc, a labial tooth row formula of 2(2)/3, and a dextral vent tube. The buccopharyngeal cavity is marked by the presence of two pairs of infralabial papilla and four lingual papillae. Cranial morphology is characterized by the presence of the commissura quadratoorbital. This species possesses an additional slip of the m. rectus cervicis and of the m. levator arcuum branchialium III. We discuss our results in comparison with glassfrogs (Centrolenidae).

First description of buccopharyngeal anatomy in Pelodryadinae larvae: Morphological comparison and systematic implications (Anura: Hylidae: Pelodryadinae: Litoria rubella and Ranoidea caerulea)

Pelodryadinae, the Australian tree frogs, is a monophyletic group endemic to the Australo-Papuan region. Although we have a relatively good knowledge about tadpoles' phenotypic diversity in terms of external morphology, information about internal anatomy is rare for the subfamily; for instance, their buccopharyngeal cavity is completely unknown. Herein I describe for the first time the buccopharyngeal anatomy of two pelodryadins: Litoria rubella and Ranoidea caerulea. I compare my results with available evidence from Phyllomedusidae, that is, the sister clade to Pelodryadinae, and briefly comment on buccopharyngeal cavity within Hylidae. Both species can be readily distinguished based on lateral ridge, postnarial, buccal roof arena, infralabial papillae, and lingual papillae. Variation between the two species may suggest a large diversity within Pelodryadinae. Pelodryadinae and Phyllomedusinae present similar buccopharyngeal morphologies, although Agalychnis callidryas has a unique morphology and putative apomorphic transformations can be observed in Pithecopus + Phyllomedusa, Ranoidea, and Phasmahyla.

Embryonic pattern of cartilaginous head development in the European toad, Bufo bufo

The craniofacial skeleton of vertebrates is a major innovation of the whole clade. Its development and composition requires a precisely orchestrated sequence of chondrification events which lead to a fully functional skeleton. Sequential information on the precise timing and sequence of embryonic cartilaginous head development are available for a growing number of vertebrates. This enables a more and more comprehensive comparison of the evolutionary trends within and among different vertebrate clades. This comparison of sequential patterns of cartilage formation enables insights into the evolution of development of the cartilaginous head skeleton. The cartilaginous sequence of head formation of three basal anurans (Xenopus laevis, Bombina orientalis, Discoglossus scovazzi) was investigated so far. This study investigates the sequence and timing of larval cartilaginous development of the head skeleton from the appearance of mesenchymal Anlagen until the premetamorphic larvae in the neobatrachian species Bufo bufo. Clearing and staining, histology, and 3D reconstruction enabled the tracking of 75 cartilaginous structures and the illustration of the sequential changes of the skull as well as the identification of evolutionary trends of sequential cartilage formation in the anuran head. The anuran viscerocranium does not chondrify in the ancestral anterior to posterior direction and the neurocranial elements do not chondrify in posterior to anterior direction. Instead, the viscerocranial and neurocranial development is mosaic-like and differs greatly from the gnathostome sequence. Strict ancestral anterior to posterior developmental sequences can be observed within the branchial basket. Thus, this data is the basis for further comparative developmental studies of anuran skeletal development.

The missing piece of the puzzle: larval morphology of Xenohyla truncata (Anura: Hylidae: Dendropsophini) and its implication to the evolution of Dendropsophini tadpoles

Dendropsophini is a highly diverse clade with a controversial phylogenetic and taxonomic history. Different generic arrangements have been proposed and the monophyly of several clades supported or rejected. Previous evidence suggested that larval morphology could play an important role in our understanding of the evolution and diversification of Dendropsophini, although data are missing for most lineages, including the sister group of Dendropsophus, Xenohyla. Herein we describe the internal morphology of the tadpoles of X. truncata and compare our results with available information for members of Dendropsophini and closely related lineages. We propose that the presence of a fan-like papilla in the buccopharyngeal cavity, a single element suprarostral, and a triangular process at the base of the muscular process are synapomorphies for Dendropsophini; moreover, the presence of a divided m. subarcualis rectus II–IV seems to be a synapomorphy for Pseudini and, the nasal sac insertion of the m. levator lateralis could be a synapomorphy of Dendropsophini + Pseudini.

Larval morphology of Amazonia foam-nesting frogs of the genus Engystomops (Anura: Leptodactylidae: Leiuperinae)

The highly differentiated anuran larvae make them an interesting and complementary source of information to understand anuran evolution. Among neotropical foam-nesting frogs, the available information on tadpole morphology for the subfamily Leiuperinae remains largely incomplete and variably reported among genera; in the monophyletic genus Engystomops it is still incipient. Herein, we summarize available information on larval morphology for five of the nine known species of Engystomops, three of them for the first time, reporting external morphology, buccopharyngeal cavity, and skeleton. The tadpoles of the genus have an overall generalized morphology and many traits are conserved across species. Nevertheless, many characters are systematically informative and some are diagnostic for some species, as the paravertebral gland in E. petersi and the dorsally directed spiracle in E. puyango. Other characters provide support for some subclades within the genus. Moreover, some traits, such as the direction of the vent tube, supports the close relationship between Engystomops and Physalaemus, whereas other support the existence of these two as distinct genera, such as the overall shape of the lateral ridge papillae and the presence of a processus pseudopterygoideus. This article is protected by copyright. All rights reserved.

Skull Sutures and Cranial Mechanics in the Permian Reptile Captorhinus aguti and the Evolution of the Temporal Region in Early Amniotes

While most early limbed vertebrates possessed a fully-roofed dermatocranium in their temporal skull region, temporal fenestrae and excavations evolved independently at least twice in the earliest amniotes, with several different variations in shape and position of the openings. Yet, the specific drivers behind this evolution have been only barely understood. It has been mostly explained by adaptations of the feeding apparatus as a response to new functional demands in the terrestrial realm, including a rearrangement of the jaw musculature as well as changes in strain distribution. Temporal fenestrae have been retained in most extant amniotes but have also been lost again, notably in turtles. However, even turtles do not represent an optimal analog for the condition in the ancestral amniote, highlighting the necessity to examine Paleozoic fossil material. Here, we describe in detail the sutures in the dermatocranium of the Permian reptile Captorhinus aguti (Amniota, Captorhinidae) to illustrate bone integrity in an early non-fenestrated amniote skull. We reconstruct the jaw adductor musculature and discuss its relation to intracranial articulations and bone flexibility within the temporal region. Lastly, we examine whether the reconstructed cranial mechanics in C. aguti could be treated as a model for the ancestor of fenestrated amniotes. We show that C. aguti likely exhibited a reduced loading in the areas at the intersection of jugal, squamosal, and postorbital, as well as at the contact between parietal and postorbital. We argue that these “weak” areas are prone for the development of temporal openings and may be treated as the possible precursors for infratemporal and supratemporal fenestrae in early amniotes. These findings provide a good basis for future studies on other non-fenestrated taxa close to the amniote base, for example diadectomorphs or other non-diapsid reptiles.

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.

Osteological development of a small and fast metamorphic frog, Microhyla fissipes (Anura, Neobatrachia, Microhylidae)

Describing osteological development is of great importance for understanding vertebrate phenotypic variations, form-functional transitions and ecological adaptations. Anurans exhibit dramatic changes in their morphology, habitat preferences, diet and behaviour between the tadpole and frog stages. However, the anatomical details of their cranial and postcranial development have not been extensively studied, especially in Microhylidae. In this work, we studied the microhylid Microhyla fissipes, commonly known as the ornamented pygmy frog, a small-sized frog with fast metamorphosis. Its osteological development was comprehensively described based on 120 cleared and stained specimens, including six tadpoles for each stage between 28 and 45, six juveniles and six adults. Additionally, 22 osteological traits of these specimens involved in food acquisition, respiration, audition and locomotion were selected and measured to reflect the changes in tadpole ecological functions during metamorphosis. Our study provides the first detailed qualitative and quantitative developmental information about these structures. Our results have confirmed that skeletal elements (viz., neopalatines, omosternum, clavicles and procoracoids) absent in adults are not detected during development. Our data reveal that morphologically, radical transformations of the cranial structures related to feeding and breathing are completed within stages 42-45 (72 h), but the relative length and width of these skeletons have changed in earlier stages. The postcranial skeletons correlated with locomotion are well developed before stage 42 and approach the adult morphology at stage 45. Indeed, the relative length of the pectoral girdle and forelimb reaches the adult level at stage 42 and stage 45, respectively, whereas that of the vertebral column, pelvic girdle and hind limbs increases from their appearance until reaching adulthood. Based on published accounts of 19 species from Neobatrachia, Mesobatrachia and Archaeobatrachia, cranial elements are among the first ossified skeletons in most studied species, whereas sphenethmoids, neopalatines, quadratojugals, mentomeckelians, carpals and tarsals tend to ossify after metamorphosis. These results will help us to better understand the ecomorphological transformations of anurans from aquatic to terrestrial life. Meanwhile, detailed morphological and quantitative accounts of the osteological development of Microhyla fissipes will provide a foundation for further study.

The conundrum of pharyngeal teeth origin: the role of germ layers, pouches, and gill slits

There are several competing hypotheses on tooth origins, with discussions eventually settling in favour of an 'outside-in' scenario, in which internal odontodes (teeth) derived from external odontodes (skin denticles) in jawless vertebrates. The evolution of oral teeth from skin denticles can be intuitively understood from their location at the mouth entrance. However, the basal condition for jawed vertebrates is arguably to possess teeth distributed throughout the oropharynx (i.e. oral and pharyngeal teeth). As skin denticle development requires the presence of ectoderm-derived epithelium and of mesenchyme, it remains to be answered how odontode-forming skin epithelium, or its competence, were 'transferred' deep into the endoderm-covered oropharynx. The 'modified outside-in' hypothesis for tooth origins proposed that this transfer was accomplished through displacement of odontogenic epithelium, that is ectoderm, not only through the mouth, but also via any opening (e.g. gill slits) that connects the ectoderm to the epithelial lining of the pharynx (endoderm). This review explores from an evolutionary and from a developmental perspective whether ectoderm plays a role in (pharyngeal) tooth and denticle formation. Historic and recent studies on tooth development show that the odontogenic epithelium (enamel organ) of oral or pharyngeal teeth can be of ectodermal, endodermal, or of mixed ecto-endodermal origin. Comprehensive data are, however, only available for a few taxa. Interestingly, in these taxa, the enamel organ always develops from the basal layer of a stratified epithelium that is at least bilayered. In zebrafish, a miniaturised teleost that only retains pharyngeal teeth, an epithelial surface layer with ectoderm-like characters is required to initiate the formation of an enamel organ from the basal, endodermal epithelium. In urodele amphibians, the bilayered epithelium is endodermal, but the surface layer acquires ectodermal characters, here termed 'epidermalised endoderm'. Furthermore, ectoderm-endoderm contacts at pouch-cleft boundaries (i.e. the prospective gill slits) are important for pharyngeal tooth initiation, even if the influx of ectoderm via these routes is limited. A balance between sonic hedgehog and retinoic acid signalling could operate to assign tooth-initiating competence to the endoderm at the level of any particular pouch. In summary, three characters are identified as being required for pharyngeal tooth formation: (i) pouch-cleft contact, (ii) a stratified epithelium, of which (iii) the apical layer adopts ectodermal features. These characters delimit the area in which teeth can form, yet cannot alone explain the distribution of teeth over the different pharyngeal arches. The review concludes with a hypothetical evolutionary scenario regarding the persisting influence of ectoderm on pharyngeal tooth formation. Studies on basal osteichthyans with less-specialised types of early embryonic development will provide a crucial test for the potential role of ectoderm in pharyngeal tooth formation and for the 'modified outside-in' hypothesis of tooth origins.

Larval chondrocranial and internal oral morphology of the neotropical treefrog Boana crepitans (Wied-Neuwied, 1824; Amphibia, Anura, Hylidae)

We describe the internal oral morphology and chondrocranial anatomy for Boana crepitans tadpoles, and compare them with available descriptions for other species in the subfamily Cophomantinae. Among species of the Boana faber group, the chondrocranial anatomy has been reported only for one species internal oral morphology and cranial anatomy are similar to other described species of Boana and Cophomantinae. B. crepitans lacks unique features in the oral cavity and chondrocranium that would distinguish it from other congeneric species. We identify six characters from the internal oral anatomy of tadpoles unique for Cophomantinae. In addition, Boana has infralabial papillae projections, buccal floor arena papillae, and lateral ridge papillae projections shorter than those described for Aplastodiscus and Bokermannohyla.

Anatomical features of the phytotelma dwelling, egg-eating, fanged tadpoles of Rhacophorus vampyrus (Anura: Rhacophoridae)

Tadpoles of the Vampire tree frog Rhacophorus vampyrus differ substantially from other rhacophorid tadpoles, by having profound modifications in external morphology. The morphological peculiarities of this species likely correlate with their arboreal microhabitat and strict oophagous diet. In this work, we examine buccal and musculoskeletal anatomy and compare them to other rhacophorid and egg-eating larvae. The shape and arrangement of cartilages of the lower jaw are unique among tadpoles, and the lack of a palatoquadrate suspensorium is only known in the distantly related macrophagous tadpoles of the dicroglossid Occidozyga baluensis. The cranial musculature is massive, and the morphology of several mandibular, hyoid, and abdominal muscles could be related to the ingestion and transit of large eggs. In the buccal cavity, conspicuous aspects are the absence of ridges and papillae, and the development of a unique glandular zone in the buccal floor. Finally, observations of the skeletal support of keratinized mouthparts allow us to present a topography-based hypothesis of homology of the conspicuous fangs of these tadpoles.

The remarkable larval anatomy of Proceratophrys minuta Napoli, Cruz, Abreu and Del-Grande, 2011 (Amphibia: Anura: Odontophrynidae)

The free living larvae of anurans (i.e., tadpoles) are a key element in the evolution and diversification of this group, and as such, their morphology is an important element to understand the phylogenetic relationships of frogs. However, the lack of data on larval morphology prevents us from fully understanding larval evolution in several lineages. The Neotropical genus Proceratophrys currently comprises 39 species, but descriptions of the internal morphology of larvae in this group are rare and restricted to few aspects of their buccopharyngeal cavity, chondrocranium, and muscles. In the present study, I describe the internal anatomy of the tadpole of P. minuta and report a new remarkable myological character state for the species. Given the rarity of this material, the description of this species' buccopharyngeal and musculo-skeletal elements is based on two tadpoles in developmental stages 30 and 31. Several new apomorphic character states are described: (a) the presence of a conical papilla in the interior of the nostril; (b) a row of five short, conical papilla preceding the tall, postnarial papilla; and (c) the m. mandibulolabialis inserting in the gular skin. This latter feature is a remarkable, newly discovered character state that had never been reported in the literature before and is probably related to a particular feeding habit of the tadpoles of this species. The function of the m. mandibulolabialis in P. minuta is unknown.

A survey of the external morphology, internal oral morphology, chondrocranium and hyobranchial apparatus of Elachistocleis larvae Parker, 1927 (Anura, Microhylidae)

The external morphology, internal oral morphology, the chondrocranium and the hyobranchial apparatus of Elachistocleis bumbameuboi, E. cf. piauiensis, E. cesarii and E. bicolor are described and compared with each other and with other species of microhylids using available descriptions from the literature. The general morphology of Elachistocleis species is conservative in many aspects. Differences between species are subtle and are found in the body shape, the edge of the snout, fin height, if the lateral line is evident, the presence of regular pustules in the buccal roof arena, the posterolateral edge of the cartilago suprarostralis, the shape of the fenestra in the occipital region, presence or absence of fenestra hypophysea, the margin of the processus antorbitalis, expansions in the ventrolateral process, the shape and inclination of the fenestra subocularis, whether the subotic process is single or slightly bifid, and the inclination of the processus anterolateralis hyalis.

Tadpole of the Amazonia frog Edalorhina perezi (Anura: Leptodactylidae) with description of oral internal and chondrocranial morphology

The genus Edalorhina consists of two species of small forest-floor frogs inhabiting the Amazon basin. The tadpole of Edalorhina perezi, the most widely distributed species, was previously described based on a single and early stage (Gosner 25) individual. Herein, we provide a description of the tadpole in Gosner stages 35-36 including internal morphology data (i.e., buccopharyngeal cavity and larval skeleton) based on samples from two populations from Ecuador. Edalorhina shares a generalized morphology with most members of its closely related taxa; however, it is distinguished from the other species by having an almost terminal oral disc. The presence of a dextral vent tube is considered a synapomorphy for the clade consisting of Edalorhina, Engystomops, and Physalaemus. Within this clade, the combination of two lingual papillae, a filiform median ridge, and the lack of buccal roof papillae are diagnostic of E. perezi and putative autapomorphies of Edalorhina. Chondrocranial anatomy provides characteristics, that is, presence of and uniquely shaped processus pseudopterygoideus and cartilago suprarostralis with corpora and alae joined by dorsal and ventral connections that readily differentiates the genus from other Leiuperinae.

Sequence of chondrocranial development in the oriental fire bellied toad Bombina orientalis

The vertebrate head as a major novelty is directly linked to the evolutionary success of the vertebrates. Sequential information on the embryonic pattern of cartilaginous head development are scarce, but important for the understanding of its evolution. In this study, we use the oriental fire bellied toad, Bombina orientalis, a basal anuran to investigate the sequence and timing of larval cartilaginous development of the head skeleton from the appearance of mesenchymal Anlagen in post-neurulation stages until the premetamorphic larvae. We use different methodological approaches like classic histology, clearing and staining, and antibody staining to examine the larval skeletal morphology. Our results show that in contrast to other vertebrates, the ceratohyals are the first centers of chondrification. They are followed by the palatoquadrate and the basihyal. The latter later fuses to the ceratohyal and the branchial basket. Anterior elements like Meckel's cartilage and the rostralia are delayed in development and alter the ancestral anterior posterior pattern observed in other vertebrates. The ceratobranchials I-IV, components of the branchial basket, follow this strict anterior-posterior pattern of chondrification as reported in other amphibians. Chondrification of different skeletal elements follows a distinct pattern and the larval skeleton is nearly fully developed at Gosner Stage 28. We provide baseline data on the pattern and timing of early cartilage development in a basal anuran species, which may serve as guidance for further experimental studies in this species as well as an important basis for the understanding of the evolutionary changes in head development among amphibians and vertebrates.

Comparative description and ossification patterns of Dendropsophus labialis (Peters, 1863) and Scinax ruber (Laurenti, 1758) (Anura: Hylidae)

Although comparative studies of anuran ontogeny have provided new data on heterochrony in the life cycles of frogs, most of them have not included ossification sequences. Using differential staining techniques, we observe and describe differences and similarities of cranial and postcranial development in two hylid species, Scinax ruber (Scinaxinae) and Dendropsophus labialis (Hylinae), providing new data of ontogenetic studies in these Colombian species. We examined tadpoles raining from Gosner Stages 25 to 45. We found differences between species in the infrarostral and suprarostral cartilages, optic foramen, planum ethmoidale, and gill apparatus. In both species, the first elements to ossify were the atlas and transverse processes of the vertebral column and the parasphenoid. Both species exhibited suprascapular processes as described in other hylids. Although the hylids comprise a large group (over 700 species), postcranial ossification sequence is only known for 15 species. Therefore, the descriptions of the skeletal development and ossification sequences provided herein will be useful for future analyses of heterochrony in the group.

Chromosome banding in Amphibia. XXXI. The neotropical anuran families Centrolenidae and Allophrynidae

The mitotic chromosomes of 11 species from the anuran families Centrolenidae and Allophrynidae were analyzed by means of conventional staining, banding techniques, and in situ hybridization. The amount, location, and fluorochrome affinities of constitutive heterochromatin, the number and positions of nucleolus organizer regions, and the patterns of telomeric DNA sequences were determined for most of the species. The karyotypes were found to be highly conserved with a low diploid chromosome number of 2n = 20 and morphologically similar chromosomes. The sister group relationship between the Centrolenidae and Allophrynidae (unranked taxon Allocentroleniae) is clearly corroborated by the cytogenetic data. The existence of heteromorphic XY♂/XX♀ sex chromosomes in an initial stage of morphological differentiation was confirmed in Vitreorana antisthenesi. The genome sizes of 4 centrolenid species were determined using flow cytometry. For completeness and for comparative purposes, all previously published cytogenetic data on centrolenids are included.

Anuran radiations and the evolution of tadpole morphospace

Anurans (frogs and toads) are unique among land vertebrates in possessing a free-living larval stage that, parallel to adult frogs, diversified into an impressive range of ecomorphs. The tempo and mode at which tadpole morphology evolved through anuran history as well as its relationship to lineage diversification remain elusive. We used a molecular phylogenetic framework to examine patterns of morphological evolution in tadpoles in light of observed episodes of accelerated lineage diversification. Our reconstructions show that the expansion of tadpole morphospace during the basal anuran radiation in the Triassic/Early Jurassic was unparalleled by the basal neobatrachian radiation in the Late Jurassic/Early Cretaceous or any subsequent radiation in the Late Cretaceous/Early Tertiary. Comparative analyses of radiation episodes indicate that the slowdown of morphospace expansion was caused not only by a drop in evolutionary rate after the basal anuran radiation but also by an overall increase in homoplasy in the characters that did evolve during later radiations. The overlapping sets of evolving characters among more recent radiations may have enhanced tadpole diversity by creating unique combinations of homoplastic traits, but the lack of innovative character changes prevented the exploration of fundamental regions in morphospace. These complex patterns transcend the four traditionally recognized tadpole morphotypes and apply to most tissue types and body parts.