Circumorbital Bones Research Articles

Osteology and phylogenetic position of Plagiosaurus depressus (Temnospondyli: Plagiosauridae) from the Late Triassic of Germany and the repeated loss of dermal bones in plagiosaurids

Abstract The skull and postcranium of the Late Triassic plagiosaurid temnospondyl Plagiosaurus depressus from Halberstadt (Germany) are redescribed in detail. Plagiosaurus possesses two autapomorphies, the abbreviated tabular and the broad contact between the postorbital and parietal. A comprehensive phylogenetic analysis of Plagiosauridae finds a clade Plagiosaurinae consisting of Plagiosaurus and Gerrothorax. Among other characters, both taxa share the solid box-like sides of the low pectoral girdle, the short but robust humerus with small deltopectoral crest and well-developed supinator process, and an anterodorsally directed lateral line sulcus close to the mandibular symphysis. Compared with Gerrothorax, the skull of Plagiosaurus is deeper and more slender, and the postfrontal is absent, meaning that the parietal forms the posteromedial margin of the enlarged orbit. This new interpretation of the bone configuration shows that reduction of circumorbital bones associated with orbital enlargement occurred three times independently within Plagiosauridae, albeit probably in different functional contexts. Our phylogenetic analysis further reveals the Plagiosuchinae (Plagioscutum + Plagiosuchus) as the most basal plagiosaurid clade, which, in turn, forms the sister group to Plagiosaurinae and Plagiosterninae (Plagiorophus + (Plagiosternum + Megalophthalma)). The mentioned shared derived characters of Plagiosaurinae in the pectoral girdle, forelimbs, and mandibular lateral lines suggest a similar benthonic lifestyle.

The predatory fish Saurichthys reflects a complex underwater ecosystem of the Late Triassic Junggar Basin, Xinjiang, China

ABSTRACT The iconic saurichthyid fishes (‘lizard fish’) are specialised predators in the aquatic realm of Early Mesozoic era. After a rapid diversification in the Middle Triassic, they seemingly shrank their geographical distribution from a global pattern to that mainly confined to Euramerican region and exhibited increasing rarity in the freshwater environment since the Late Triassic. Here, we report a new species of Saurichthys from the Late Triassic freshwater deposits of the Junggar Basin, northwestern China. This new species Saurichthys sui sp. nov. is characterised by a mosaic of primitive and derived features in Saurichthys: large circumorbital bones and spiracular plate, a vestigial opercle and pectoral fins devoid of fringing fulcra. This feature combination supports the hypothesis that the freshwater system might have served as the refugium of some basal saurichthyid species. Saurichthys sui represents the first record of large predatory fishes in the Triassic Junggar Basin and indicates the establishment of a trophically complex lacustrine ecosystem there. Together with the co-occurring fossil insects and plants, Saurichthys sui, showing some affinities to its congeners from central Asia and North China, implies some biogeographical links of the fossil localities of non-marine Saurichthys on the continents north to PalaeoTethys during the Middle-Late Triassic.

Nusaviichthys nerivelai gen. et sp. nov., an Albian crossognathiform fish from the Tlayúa lagerstätte, Mexico

The fish Nusaviichthys nerivelai gen. et sp. nov. is described here based on well-preserved fossils from the Albian deposits of the Tlayúa quarry, near Tepexi de Rodríguez, Puebla, central Mexico. The occurrence of this new fish from Tlayúa expands the taxonomical and morphological diversity of the crossognathiformes of the Caribbean region of America during the Early Cretaceous. The inclusion of the osteological features of this species into comparative and phylogenetic analyses reveal its belonging to the order Crossognathiformes, suborder Pachyrhizodontoidei, and family Notelopidae; some of the most significant of these features include the presence of a large extrascapula bone; circumorbital bones forming a completely closed ring; medioparietal condition of the skull; posterior infraorbitals hypertrophied, covering the entire cheek, and extended up to the anterior part of the opercular bones; the relative short uroneural 1 anteriorly restricted to the preural 1; and the cycloid scales with a crenulated posterior edge. This new species differs from its relatives in the presence of numerous and small conic teeth in both jaws and vomer; the large triangular gular plate, and the numerous radiating grooves ridges present in the supraorbital, dermosphenotic, skull roof bones, opercular bones, and jaws; and the almost horizontal position of the neural and hemal spines of the last caudal centra. Despite the weak support, the naturalness of this extinct teleost order is recovered in the phylogenetic analysis performed here, which includes this new Mexican fossil fish as well as other crossognathiform genera not included in previous studies.

Comparative analysis of the parasphenoid bone development in Mexican tetra ( Astyanax mexicanus)

Astyanax mexicanus is a teleost fish that belongs to the order Characiform. Ithas two distinct eye morphs as surface-dwelling, pigmented eyed surface fish population, and an eyeless cavefish population. Accumulation of functional mutations in the genes which are important for eye development led to the loss of eyes in cavefish. Some cranial changes have occurred from the direct effect of eye loss in cavefish including size and position of the circumorbital bones, ossified sclera, and the shape of both suborbital 3 and the supraorbital bone. The maxillary teeth, the positions of suborbital 4 to suborbital 6, and the shape of the opercular bone were found to be unaffected by the eye loss. Unlike higher vertebrates, the teleost eyeball does not cover by a bony socket and it has direct communication with the inner neurocranium. However, the influence of eye development on the inner neurocranial bones has gained little attention in research. In A. mexicanus palate bones are mainly made of a series of bones such as parasphenoid bone, basioccipital, vomer, and ethmoid. Among them, parasphenoid bone makes the largest part of the palate. This study was designed to understand the effect of eye loss in cavefish on parasphenoid bone development, shape determination, and bone remodeling activity. A. mexicanus populations; Surface fish and Pachon cavefish at 5dpf, 10 dpf, 20 dpf, 35 dpf, and 60 dpf subjected to alizarin red bone staining. Adult surface fish and two cave populations Pachon and Tinaja cavefish were subjected to micro-CT imaging and morphometric analysis was carried out using those images to compare the parasphenoid bone anatomy amongst these three morphs. TRAP and ALP staining were used to determine the bone remodeling activity in 60 dpf, A. mexicanus populations of surface and Pachon cavefish. A. mexicanus cavefish and surface are similar to each other in bone development, articulation, and ossification with certain exceptions such as late protrusion of lateral wings in cavefish. The straightened parasphenoid bone observed in cavefish and curved parasphenoid bone observed in surface fish beneath the eye orbit. Bone remodeling activity in cavefish was found to be increased compared to the surface fish. Cavefish have a direct or indirect effect on eye loss on the development of bones in the cranial region. Differential bone remodeling activity observed in cave and surface fish may result from the limited resources in the cave environment and as a long-term adaptation into troglomorphic cave environment.

Cranial morphology of the tanystropheid Macrocnemus bassanii unveiled using synchrotron microtomography

The genus Macrocnemus is a member of the Tanystropheidae, a clade of non-archosauriform archosauromorphs well known for their very characteristic, elongated cervical vertebrae. Articulated specimens are known from the Middle Triassic of Alpine Europe and China. Although multiple articulated specimens are known, description of the cranial morphology has proven challenging due to the crushed preservation of the specimens. Here we use synchrotron micro computed tomography to analyse the cranial morphology of a specimen of the type species Macrocnemus bassanii from the Besano Formation of Monte San Giorgio, Ticino, Switzerland. The skull is virtually complete and we identify and describe the braincase and palatal elements as well the atlas-axis complex for the first time. Moreover, we add to the knowledge of the morphology of the skull roof, rostrum and hemimandible, and reconstruct the cranium of M. bassanii in 3D using the rendered models of the elements. The circumorbital bones were found to be similar in morphology to those of the archosauromorphs Prolacerta broomi and Protorosaurus speneri. In addition, we confirm the palatine, vomer and pterygoid to be tooth-bearing palatal bones, but also observed heterodonty on the pterygoid and the palatine.

The skull osteology of the blind cave cyprinid Phreatichthys andruzzii Vinciguerra, 1924

The present article describes the morphology of the skull of Phreatichthys andruzzii Vinciguerra, 1924, blind cave fish from central Somalia. The main findings concern a general flattening of the skull and the loss of several bones (coronomeckelian, prefrontal, supratemporal, intercalar, supraopercle) belonging to the various regions of the head skeleton; but the more important modifications are shown by the orbital region, consisting in the complete disappearance of lachrymal, suborbital, sclerotics, and the substantial reduction of the other circumorbital bones. Usually the cranial lateral line canals run through the dermal bones; on the contrary in P. andruzzii the canals are nearly exclusively sheathed by thin bony muffs. These are sometimes fused to the underlying dermal bones, but in several regions they remain fully separated and independent, running through the adjoining soft tissues. The considerable degree of osteological regression shown by the skull appears to be related to the high level of a...

New Miocene Caribbean gavialoids and patterns of longirostry in crocodylians

Gavialoidea is a clade of slender- and long-snouted crocodylomorphs with a single living species, the Indian gharial Gavialis gangeticus. Because elongated snouts (longirostry) have evolved independently in several crocodylomorph clades, this head shape has been interpreted as an ecological adaptation. How this condition affected patterns of diversification and how longirostrine-associated cranial features changed through adaptive radiations remain poorly understood. Two new small gryposuchine gavialoids, Dadagavialis gunai gen. et sp. nov. (early Miocene, Panama) and Aktiogavialis caribesi sp. nov. (late Miocene, Venezuela), evidence remarkable Miocene diversification of longirostrine forms in the Neotropics and support transmarine biogeographical relations between northern South America, the Caribbean, and southernmost North America before the Isthmus of Panama was fully established. By integrating phylogenetics and geometric morphometrics, we focus on this gavialoid diversity to investigate patterns of longirostry across the crown group of crocodylomorphs (Crocodylia). Analyses revealed that the snout shape of gavialoids has occupied a small, distinct and almost invariable morphospace since the Cretaceous, in contrast with the morphologically labile snout shape of other crocodylians (crocodyloids and alligatoroids). Our results suggest iterative environmental shift occupations throughout gavialoid evolution without major changes in snout proportions, but involving conspicuous rearrangements of the circumorbital bones. The longirostrine gavialoid morphotype is a distinct adaptation for seizing small prey and typically includes short and wide premaxillae and enlarged ‘caniniform’ teeth only at the tip of the snout. In longirostrine crocodyloids (Tomistoma, Crocodylus intermedius), the conservation of powerful bites and ‘caniniforms’ closer to the jaw joints allowed them to exploit a wider range of prey sizes, which could explain their snout shape plasticity. Therefore, the Mio–Pliocene extirpation of gryposuchine gavialoids from the Caribbean by the arrival of Crocodylus is quite unlikely. The last gryposuchine survived throughout the Pliocene in the south-eastern Pacific, where Crocodylus has never been documented.http://zoobank.org/urn:lsid:zoobank.org:pub:34CDBC8B-98E1-4770-8FC3-A77D3DBF4057

X‐Ray Computed Tomography Reveals Hidden Morphology in an Extinct Mammalian Relative

High Resolution X‐ray computed tomography is commonly used to study internal structures of fossil skulls. CT allows inference about the structure of brain surface morphology, the inner ear, and internal bony elements such as turbinates. Computed tomography data is frequently published in figures as isosurface renderings of either the segmented endocranial volume or of the surface of the specimen. Here I demonstrate how CT improves upon superficial anatomical understanding of fossils in a Late Triassic early mammal relative from the Ischigualasto Formation in Argentina. When the specimen, PVSJ 882, was first discovered and initially described, it was assumed to be closely related to mammals on account of the perceived absence of two circumorbital bones, the prefrontal and postorbital. It was not until the specimen could be viewed non‐destructively in cross section that the prefrontal and postorbital were identified to be present. However, other derived early mammalian features grew apparent, including a fusion of the prootic and opisthotic to form the petrosal, as well as cochlea housed within a bony promontorium. The discovery of primitive orbital elements in a specimen with such a derived ear is new to mammal paleontology. The superficial morphology of other derived mammal relatives is encouraged to be studied with CT. Furthermore, I demonstrate how adjusted volume renderings can more accurately communicate the morphology of the specimen in question, while isosurfaces can be misleading for the same reasons that photos of the specimen in rock matrix can be misleading. While isosurface renderings have advantages in figures depicting CT data, they should not be used to the exclusion of volume renderings when volume renderings can depict subtle sutures, unprepared foramina, and deep, delicate bone buried under rock matrix.Support or Funding InformationThe Jackson School of Geosciences at the University of Texas at Austin

A redescription of the Middle Devonian dipnoanPentlandia macropteraTraquair, 1889, and an assessment of the Phaneropleuridae

During the Devonian Period, lungfishes underwent rapid diversification. Such rapid diversification has marred phylogenetic analyses and is suspected as being responsible for reducing phylogenetic hypotheses to large polytomies. This problem is exacerbated by poor morphological understanding of key Devonian taxa comprising loosely defined, paraphyletic lungfish families. Pentlandia macroptera Traquair, 1888, is one such taxon that has not received a detailed assessment since it was first discovered nearly 130 years ago. A detailed redescription of P. macroptera demonstrates that it resides in the monophyletic crown group of Devonian lungfish comprising phaneropleurids, fleurantiids, and scaumenacids but, at this stage, establishment of a new family based on this clade is not warranted. It is considered to be the most derived Middle Devonian lungfish. Several previously unrecognized structures for this taxon are described including the circumorbital bones, the cheek bones, cranial ribs, the lower jaw, and tooth plates, all of which show the derived condition. Pentlandia macroptera is also considered derived in possessing an unossified snout and neurocranium and represents the first occurrence of these conditions that were later to become prevalent in Carboniferous dipnoans. Of greatest significance, P. macroptera demonstrates the presence of the oldest pelvic girdle seen in the Dipnoi, which is conserved in form through to the Upper Devonian and even modern lungfish. The current work indicates that many of the derived characteristics of Late Devonian and Carboniferous dipnoans had their origin in the Middle Devonian.

Complex Craniofacial Changes in Blind Cave-Dwelling Fish Are Mediated by Genetically Symmetric and Asymmetric Loci

The genetic regulators of regressive craniofacial morphologies are poorly understood. To shed light on this problem, we examined the freshwater fish Astyanax mexicanus, a species with surface-dwelling and multiple independent eyeless cave-dwelling forms. Changes affecting the skull in cavefish include morphological alterations to the intramembranous circumorbital bones encircling the eye. Many of these modifications, however, have evolved separately from eye loss, such as fragmentation of the third suborbital bone. To understand the genetic architecture of these eye-independent craniofacial alterations, we developed and scored 33 phenotypes in the context of an F2 hybrid mapping pedigree bred from Pachón cavefish and surface fish. We discovered several individuals exhibiting dramatic left-right differences in bone formation, such as extensive fragmentation on the right side only. This observation, along with well-known eye size asymmetry in natural cave-dwelling animals, led us to further evaluate left-right genetic differences for the craniofacial complex. We discovered three phenotypes, inclusive of bone fragmentation and fusion, which demonstrated a directional heritable basis only on one side. Interestingly, the overall areas of affected bones were genetically symmetric. Phenotypic effect plots of these novel craniofacial QTL revealed that cave alleles are associated with abnormal conditions such as bony fusion and fragmentation. Moreover, many linked loci overlapped with other cave-associated traits, suggesting regressive craniofacial changes may evolve through linkage or as antagonistic pleiotropic consequences of cave-associated adaptations. These novel findings illuminate significant craniofacial changes accompanying evolution in complete darkness and reveal complex changes to the skull differentially influenced by genetic changes affecting the left and right sides.

Primary homologies of the circumorbital bones of snakes

Some snakes have two circumorbital ossifications that in the current literature are usually referred to as the postorbital and supraorbital. We review the arguments that have been proposed to justify this interpretation and provide counter-arguments that reject those conjectures of primary homology based on the observation of 32 species of lizards and 81 species of snakes (both extant and fossil). We present similarity arguments, both topological and structural, for reinterpretation of the primary homologies of the dorsal and posterior orbital ossifications of snakes. Applying the test of similarity, we conclude that the posterior orbital ossification of snakes is topologically consistent as the homolog of the lacertilian jugal, and that the dorsal orbital ossification present in some snakes (e.g., pythons, Loxocemus, and Calabaria) is the homolog of the lacertilian postfrontal. We therefore propose that the terms postorbital and supraorbital should be abandoned as reference language for the circumorbital bones of snakes, and be replaced with the terms jugal and postfrontal, respectively. The primary homology claim for the snake "postorbital" fails the test of similarity, while the term "supraorbital" is an unnecessary and inaccurate application of the concept of a neomorphic ossification, for an element that passes the test of similarity as a postfrontal. This reinterpretation of the circumorbital bones of snakes is bound to have important repercussions for future phylogenetic analyses and consequently for our understanding of the origin and evolution of snakes.

New information about the anatomy of a peculiar fish of the genus Hipposyngnathus Daniltshenko, 1960

An extraordinary pipefish genus Hipposyngnathus has been described from Paleogene and Neogene sediments from several localities in the world. This fish is clearly recognizable by its unpaired abdominal ventral ledge, but, unfortunately, no authors have been able to provide a detailed description of the skull until today. The newly discovered specimen from the Oligocene locality Hermanowa (Poland) is sufficiently preserved and allow for the first time the description and reconstruction of the skull morphology in detail. The skull shows a typical elongation of the preorbital area, tiny terminal jaws, a reduced number of circumorbital bones and a typical syngnathid cranial architecture. The snout is relatively short, but low. Separate skull bones have been identified.

Development of the Dorsal Circumorbital Bones in the Leopard Gecko (Eublepharis macularius) and Its Bearing on the Homology of These Elements in the Gekkota

Five nominal elements comprise the circumorbital series of bones in gekkotans: prefrontal, postfrontal, postorbital, jugal, and lacrimal. Determination of the homology of two of these, the postfrontal and postorbital, has been particularly problematic. Two conflicting hypothesis exist relating to these: either the postorbital is lost and the postfrontal remains or they fuse during development to form a combined element, the postorbitofrontal. Such a combined element apparently occurs in at least some members of all lizard clades. There is, however, no direct developmental evidence that supports either theory. To overcome that, we investigate the sequence and pattern of ossification in the circumorbital region in a developmental series of the Leopard gecko. We posit that both the postfrontal and postorbital appear during development. Contrary to previous predictions they neither fuses to each other, nor do either degenerate. Instead, the postfrontal shifts anteriorly and fuses with the frontal to become indistinguishable from it by the time of hatching, and the postorbital persists as a robust independent element bounding the frontoparietal suture. These observations accord, in part, with both hypotheses of homology of these elements and result in the recognition of a new pattern, placing in doubt the existence of the composite postorbitofrontal. The phylogenetic implications of these findings may prove to be far reaching if similar and conserved patterns of development are encountered in other clades.

Sclerotic plates or circumorbital bones in early jawed fishes?

Abstract: Circumorbital dermal bones are found in most groups of early vertebrates that have dermal bony plates on the head. Taxonomic distribution of dermal sclerotic plates on the eye itself is less clear, partly because the eyeball is rarely preserved and sometimes because sclerotic bones have been misinterpreted as circumorbital bones. Based on the examination of climatiid Climatius plus mesacanthid, cheiracanthid and acanthodid acanthodiform acanthodians, we conclude that most, if not all, acanthodiforms and climatiids had sclerotic rings. Presence and number of these elements should be included as a character in phylogenetic analyses of early jawed vertebrates.

The Circumorbital Bones of the Gekkota (Reptilia: Squamata)

The enormous variation of the orbit in lepidosaurs is better conceptualized in terms of composition and configuration. Broadly, the orbit varies from having totally closed rim to being open posteriorly. Two processes are responsible for changes in the components of the circumorbital series, element loss and fusion. The resulting contacts among elements are the main factors determining orbital configuration. Here, we present a revision of the gekkotan circumorbital bones in the general context of the Lepidosauria. From observations of a sample of 105 species of gekkotans prepared using different techniques, we describe the main changes in the orbit and corroborate the presence or absence of some of the ambiguous elements such as the lacrimal and the jugal. The supraorbital bones of squamates are reviewed and some problems of homology are evaluated using recent phylogenenetic hypothesis.

Nannaroter mckinziei, a new ostodolepid ‘microsaur’ (Tetrapoda, Lepospondyli, Recumbirostra) from the Early Permian of Richards Spur (Ft. Sill), Oklahoma

ABSTRACT A newly discovered, small lepospondyl from the Early Permian of Oklahoma is demonstrated to be a new genus and species of ostodolepid ‘microsaur’ and is given the name Nannaroter mckinziei. The holotype and only known specimen comes from the Permian-aged fissure fill deposits in the Ordovician limestone from the Dolese Brothers Limestone Quarry, the richest, most diverse Paleozoic assemblage of exclusively terrestrial vertebrates. The superbly preserved skull is identified as the smallest known ostodolepid for its characteristic cranial architecture. Like all ostodolepids, it has a wedge-shaped skull with a high posterior profile and broad occipital surface, and a lower temporal emargination. Like ostodolepids, gymnarthrids, pantylids, and Rhynchonkos, the anterior rostrum is overturned, producing a recumbent snout. Despite its small size, Nannaroter has a massively ossified skull with deeply interlocking sutures designed to tightly resist anteroposteriorly directed forces. Medial laminae are present on all circumorbital bones producing thick orbital walls, and there is a well-developed ventral process of the frontal that is suturally attached to the sphenethmoid portion of the braincase anterodorsally. In addition, a massive, triangular pterygoid-epipterygoid complex reinforces the posterolateral wall of the braincase and contacts the tabular and parietal portion of the dermatocranium. This description is the first part of a revision of the ostodolepids, which will explore adaptations in amphibians to a fossorial lifestyle.

The plagiosaurid temnospondylPlagiosuchus pustuliferus(Amphibia: Temnospondyli) from the Middle Triassic of Germany: anatomy and functional morphology of the skull

The cranial anatomy of the plagiosaurid temnospondyl Plagiosuchus pustuliferus, from the Middle Triassic of Germany, is described in detail on the basis of a newly discovered skull and mandibular material. The highly derived skull is characterized by huge orbitotemporal fenestrae, a reduction of the circumorbital bones – the prefrontal, postfrontal and (probably) postorbital are lost – and the expansion of the jugal to occupy most of the lateral skull margin. Ventrally the extremely long subtemporal vacuities correlate with the elongate adductor fossa of the mandible. The dentition is feebly developed on both skull and mandible. Ossified ?ceratobranchials and ‘branchial denticles’ indicate the presence of open gills clefts in life. The remarkably divergent cranial morphology of P. pustuliferus highlights the extraordinary cranial diversity within the Plagiosauridae, probably unsurpassed within the Temnospondyli. Specific structural aspects of the skull – including an extremely short marginal tooth row, feeble dentition and an elongated chamber for adductor musculature – together with evidence for a hyobranchial skeleton, suggests that P. pustuliferus utilized directed suction feeding for prey capture. © 2009 The Linnean Society of London, Zoological Journal of the Linnean Society, 2009, 155, 348–373.

A new genus of ionoscopiform fish (Halecomorphi) from the Lower Cretaceous (Albian) lithographic limestones of the Tlayúa Quarry, Puebla, Mexico

Quetzalichthys perrilliataen. gen. and sp. is described based on five specimens from Lower Cretaceous (Middle-Upper Albian) limestones of the Tlayúa Quarry, near Tepexi de Rodríguez, Puebla, Mexico. This new taxon displays the diagnostic characters of Ionoscopiformes, which consequently designate it as a new member of this order of Mesozoic fishes. Additionally,Quetzalichthysn. gen. shares several derived characters withIonoscopusandOshunia, including the presence of more than 15 supraneurals, solid and well-ossified monospondylous vertebrae with two lateral longitudinal oval fossae, thin scales of the amioid type, and the ventral surface of some circumorbital bones being intensely pitted.Quetzalichthys perrilliataeis characterized by three unique characters: 26 abdominal centra, 21 principal dorsal fin rays, and two supraorbitals. Additionally, its unpaired fins and tail show conditions intermediate to those found inIonoscopusandOshunia.A cladistic analysis of Ionoscopiformes, including both taxa found in the Tlayúa Quarry (QuetzalichthysandTeoichthys), identifies two monophyletic families within the order, Ophiopsidae (includingOphiopsis, Macrepistius, Teoichthys) and Ionoscopidae (involvingIonoscopus, Oshunia, andQuetzalichthys).

Skull ontogeny: developmental patterns of fishes conserved across major tetrapod clades

In vertebrates, the ontogeny of the bony skull forms a particularly complex part of embryonic development. Although this area used to be restricted to neontology, recent discoveries of fossil ontogenies provide an additional source of data. One of the most detailed ossification sequences is known from Permo-Carboniferous amphibians, the branchiosaurids. These temnospondyls form a near-perfect link between the piscine osteichthyans and the various clades of extant tetrapods, retaining a full complement of dermal bones in the skull. For the first time, the broader evolutionary significance of these event sequences is analyzed, focusing on the identification of sequence heterochronies. A set of 120 event pairs was analyzed by event pair cracking, which helped identify active movers. A cladistic analysis of the event pair data was also carried out, highlighting some shared patterns between widely divergent clades of tetrapods. The analyses revealed an unexpected degree of similarity between the widely divergent taxa. Most interesting is the apparently modular composition of the cranial sequence: five clusters of bones were discovered in each of which the elements form in the same time window: (1) jaw bones, (2) marginal palatal elements, (3) circumorbital bones, (4) skull roof elements, and (5) neurocranial ossifications. In the studied taxa, these "modules" have in most cases been shifted fore and back on the trajectory relative to the Amia sequence, but did not disintegrate. Such "modules" might indicate a high degree of evolutionary limitation (constraint).

Skeletal elements in the vertebrate eye and adnexa: Morphological and developmental perspectives

Although poorly appreciated, the vertebrate eye and adnexa are relatively common sites for skeletogenesis. In many taxa, the skeleton contributes to internal reinforcement in addition to the external housing of the eye (e.g., the circumorbital bones and eyelids). Eyeball elements such as scleral cartilage and scleral ossicles are present within a broad diversity of vertebrates, albeit not therian mammals, and have been used as important models for the study of condensations and epithelial-mesenchymal interactions. In contrast, other elements invested within the eye or its close surroundings remain largely unexplored. The onset and mode of development of these skeletal elements are often variable (early versus late; involving chondrogenesis, osteogenesis, or both), and most (if not all) of these elements appear to share a common neural crest origin. This review discusses the development and distribution of the skeletal elements within and associated with the developing eye and comments on homology of the elements where these are questionable.