The external gills and ornamentation of skull roof bones of the Lower Permian tetrapodDiscosauriscus (Kuhn 1933) with remarks to its ontogeny

Until now, the largest specimens ofDiscosauriscus (and alsoUtegenia andAriekanerpeton- the Lower Permian members of the family Discosauriscidae) have been considered to be adult individuals. The largest specimens ofDiscosauriscus, in which the external gills are still present, had skull lengths of about 22-25 mm. Metamorphosis was believed to have commenced at this size in discosauriscids and specimens with skull lengths of about 50-54 mm were considered to be already adult or senile. New finds ofDiscosauriscus demonstrate the presence of external gills in specimens with skull lengths of up to 32 mm. On the base of this, together with the ornamentation or the skull roof bones and osteological features of new large specimens, it is here concluded that 1)Discosauriscus represents the larval, metamorphic and early juvenile stages of a reptiliomorph tetrapod, the adults of which are yet unknown and 2) two other discosauriscids(Ariekanerpeton andUtegenia) also represent early ontogenetic stages.     

The ‘Unicorn’ Dinosaur That Wasn’t: A New Reconstruction of the Crest of Tsintaosaurus and the Early Evolution of the Lambeosaurine Crest and Rostrum

The lambeosaurine Tsintaosaurus spinorhinus has traditionally been reconstructed with an elevated, hollow, spike-like crest composed entirely of the nasal bones, although this has been disputed. Here, we provide a new reconstruction of the skull of this species based on reexamination and reinterpretation of the morphology and articular relationships of the type and Paratype skulls and a fragmentary crest. We confirm the presence of a supracranial crest composed of the elevated nasal bones, but also including the premaxillae. We hypothesize that the crest is a tall, lobate, hollow structure that projects dorsally and slightly caudally a distance greater than the height of the skull along the quadrate. In our reconstruction, the nasal passage passes through the crest, but enters the skull rostral to the tubular process of the nasals, not through it. Tsintaosaurus spinorhinus is rediagnosed on the basis of a suite of cranial autapomorphies including a circumnarial fossa subdivided into three accessory fossae, prefrontal with ascending rostral process and lateral flange, nasals fused sagittally to form elongate tubular process that rises dorsally from skull roof, each nasal being expanded rostrocaudally into a rhomboid distal process, and medial processes of premaxillae at the summit of the cranial crest inserted between rhomboid processes of nasals. Tsintaosaurus spinorhinus lacks characters that are present in more derived lambeosaurines (parasaurolophins and lambeosaurins), such as rotation of the caudal margin of the crest to an acute angle with the skull roof, lateral processes of the nasals that enclose part of the intracranial cavity and participate in the formation of the walls of the common median chamber, and a smooth narial fossa lacking ridges and accessory fossae. We hypothesize that ancestrally the rostrum of lambeosaurines may have been more similar to that in Saurolophinae, and became subsequently reduced in complexity during evolution of the group.     

New observations on the skull of Archaeopteryx

Although skeletal remains of the iconic oldest known avialian Archaeopteryx have been known for almost 150 years, several aspects of the cranial anatomy of this taxon have remained enigmatic, mainly because of the strongly flattened and often fractured and incomplete nature of available skull materials. New investigation of the skulls of the recently described, excellently preserved tenth (Thermopolis) and the seventh (Munich) specimens revealed several previously unrecognized characters and helps to resolve some problematic issues. Thus, the nasal of Archaeopteryx shows a lateral notch for the lacrimal, as is found in many other saurischian dinosaurs, the maxilla clearly participates in the margin of the external nares, and there seems to be a pneumatic foramen in the lacrimal, comparable to the lacrimal fenestra found in many non-avian theropods. In the braincase, Archaeopteryx shows pneumatic features reminiscent of non-avian theropods, including a ventral basisphenoid recess and an anterior tympanic recess that is laterally incised into the basisphenoid/prootic. Most importantly, however, the postorbital process of the jugal shows a facet for the suture with the postorbital, thus resolving the question of whether Archaeopteryx had a closed postorbital bar. A new reconstruction of the skull of Archaeopteryx is presented, making the skull of this taxon even more theropod-like than previously recognized. Furthermore, the closed postorbital bar and the configuration of the bones of the skull roof cast serious doubt on claims that an avian-style cranial kinesis was present in this taxon.     

Wildungenichthys grossi n. gen., n. sp. — a new selenosteid arthrodire (Placodermi, Arthrodira) from the Kellwasserkalk (late Frasnian, Upper Devonian) of Bad Wildungen (Hessen, W-Germany)

A new arthrodire genus and species,Wildungenichthys grossi, is described from the Frasnian Kellwasserkalk of Bad Wildungen (W-Germany) from an incomplete skull with parts of the mandible and shoulder girdle. The new taxon is unique in a number of features, including the loss or probable fusion of the postmarginal, strong reduction of infraorbital sensory line canal on suborbital, firm connection of the cheek and skull roof and an unusual configuration of the cheek bones. It is an advanced pachyosteomorph eubrachythoracan and is referred to the Selenosteidae, showing close resemblances toEnseosteus.     

The sutural pattern of skull-roof bones in Lower Permian Discosauriscus austriacus from Moravia

Sutures between ornamented bones of Discosauriscus austriacus are mostly simple, but there are also more complicated, rarely serrated, sutures between some bones. In small individuals, the sutures are simple, but the same sutures also occur in the largest specimens. The character of the sutures and the incomplete ossification of bones around the pineal foramen indicate the larva type of organization of Discosauriscus The fenestra between premaxillaries and nasals appears to be absent. In the majority of specimens, a squamosal-intertemporal sutural contact is present, althought it is sometimes reduced and in a few cases interrupted by a postorbital and supratemporal contact Therefore the character 'intertemporal-squamosal suture present or absent' cannot be used in this rigorous sense for testing the relationships of early tetrapods. The configuration of the suture between both parietals in osteolepiforms, Discosauriscus , and various early amphibians and reptiles indicates that the bones enclosing the pineal foramen in osteolepifonns are frontals. *** D iscosauriscus . Seymouriamorpha, Lower Permian tetrapod, skull exoskeleton, sutures.     

The identification of the dermal bones of the head

The discovery of the ichthyostegid Amphibia in Upper Devonian rocks by Säve-Söderbergh (1932) introduced further difficulties into the already complex problems of the dermal bones of the skull roof. For some years previously ideas about the origin of the tetrapods had been dominated by Watson's (1926) Croonian Lecture in which he had demonstrated beyond reasonable doubt that the crossopterygian fishes and not the Dipnoi were their ancestors, and had attempted to show that many of the features of the Carboniferous labyrinthodonts were a direct inheritance from these fishes. It was to be expected, therefore, that any Amphibia from the Upper Devonian would be intermediate in their structures between the Middle Devonian osteolepids and the Carboniferous labyrinthodonts, but when discovered the ichthyostegids did not conform at all well to this expectation. While their skulls showed some very primitive features which might have been expected, the pattern of the dermal bones did not conform to plan, for these new animals had lost, it seemed, the intertemporals, bones found in both the osteolepids and nearly all early labyrinthodonts, and had a single postparietal bone in place of the paired bones of all other early Amphibia. The osteolepid skull had many more bones than these earliest Amphibia.     

Hyobranchial skeleton and hypobranchial muscles of rhipidistians

The hyobranchial skeleton of the porolepiform rhipidistian Laccognathus panderi Gross is described. The double composition of the ceratohyal in crossopterygians is proposed. The urohyal of porolepiforms, like that of Latimeria, consists of cartilaginous axial and membranous peripheral portions. The differences between porolepiforms and osteolepiforms in the structure of the hyobranchial skeleton, particularly, in the shape of the urohyal are attributable to different arrangements of the hypobranchial muscles. Porolepiforms and coelacanths have retained the coracomandibularis muscle inherited from early gnathostomes, whereas the same muscle of osteolepiforms was transformed into the geniohyoideus muscle. This transformation is accounted for by functional changes in the hyobranchial apparatus.     

The postparietal shield of the Pragian dipnomorph Arquatichthys and its implications for the rhipidistian cranial anatomy

Rhipidistians comprise dipnomorphs (the lungfish lineage) and tetrapodomorphs (the tetrapod lineage). Arquatichthys porosus Lu and Zhu, 2008 is a Pragian dipnomorph from the Posongchong Formation of Zhaotong, Yunnan, South China (∼409 million years ago, Early Devonian), previously represented by a lower jaw and few scattered scales. Here we describe a newly-discovered postparietal shield of Arquatichthys by means of high-resolution computed tomography. The cranial morphology of Arquatichthys resembles that of the basal dipnomorph Powichthys in having more than two supratemporal bones each side, more than one row of openings for sensory canals on the marginal bones, and a straight posterior margin of the shield. An intricate occipital artery system is present between the skull roof and neurocranium, as in Youngolepis and the tetrapodomorph Eusthenopteron. The discovery of the postparietal shield of Arquatichthys adds new evidence in the cranial evolution of rhipidistians, and helps to improve our understanding of the character transformations during the early diversification of rhipidistians.     

New skull of Schizodelphis sulcatus Gervais, 1861 (Mammalia, Odontoceti, Eurhinodelphinidae) from the Lower Miocene of Pignan (Hérault, France) and its implications for systematics of Eurhinodelphinidae

Schizodelphis sulcatus is the type species of the Eurhinodelphinid genus Schizodelphis and was until now only known from a rather fragmentary specimen, lacking some important parts of the skull (vertex, ear bones and apex of rostrum). The study of a well-preserved skull of S. sulcatus Gervais, discovered in the lower Miocene of Hérault (France), helps to improve the diagnosis of this enigmatic species. Diagnostic features include the low height at the base of the rostrum, the lower position of the paroccipital process compared to the height of the postglenoid process in lateral view, the ventrally deflected postglenoid process and the ovoid-shaped nasals. It also confirms that the type species of Schizodelphis is indeed distinct. For the first time in Schizodelphis, the vertex is preserved and the ear bones are found in anatomic connection with the basicranium. A cladistic analysis of cranial characters of Eurhinodelphinidae, including ear bones, is performed and reveals a close phylogenetic relationship between S. sulcatus and the North American species Schizodelphis barnesi. This analysis also highlights some apomorphies of the genus as the smoothly laterally deflected anterior spine of the bulla in ventral view and the reestablishment of the contact between the falciform process of the squamosal and the lateral lamina of the pterygoid. Conversely, the attribution of Schizodelphis morckhoviensis to the genus Schizodelphis is questioned by this analysis.     

The Parietal Problem: How to Cut This Gordian Knot?

Although no one has disputed that the piscine progenitors of the tetrapods have a homologue of the human parietal bone, opinions differ as to where in the skull roof this homologue is located. One view holds it to be either of two interorbital bones that together surround the foramen neuroepiphysium (and the so-called pineal plates); another, that it is each of the two mesial bones which comes next in order, behind the orbital cavities. Both of these views are untenable because neither of the proposed bones has proved to be amenable to conversion to the parietal bone of man. In seeking a solution to this issue thoughts turn to the tentorium cerebelli, whose topographic relationships and comparative morphology place it in the key position as a plausible derivative of the posterior half of the cranium of the tetrapod forerunners. Following this line of reasoning, it can be suggested that the tentorial exoskeleton, here called the pluteal bones, originally was situated in the dermis superjacent to the synotic tectum. In early therian phylogeny, these skull bones were covered by the backwardly expanding cerebral hemispheres which, concomitantly, became overgrown and thus protected by epiotic exoskeleton. It follows that the likely homologues of the human parietal bones are those parts of the skull roof of the piscine progenitors of the tetrapods that lie dorsolateral to the otic capsules.     

The cranial morphology of Greererpeton burkemorani Romer (Amphibia: Temnospondyli)

The skull of Greererpeton burkemorani Romer, a temnospondyl amphibian from the Upper Mississippian at Greer, West Virginia is described. A detailed account of the stapes of a Mississippian amphibian is given for the first time and its function is discussed. It is suggested that the stapes formed the principal element of support for the back of the braincase and resisted potential dislocation of the otico-occipital region from the skull roof during contraction of the hypaxial musculature.
Greererpeton is included in the Colosteidae and an amended diagnosis of the family is given. Erpetosaurus differs from Colosteus, Greererpeton and Pholidogaster in the pattern of bones in the skull roof and palate, the dentition and the otic region and, consequently, it is removed from the Colosteidae. The Temnospondyli are considered to be a monophyletic group characterized by the development of a connection between the dorsal portion of the occipital arch, the exoccipital bones, and the skull roof. The loxommatids are removed from the Temnospondyli as they retain the plesiomorphic condition of braincase attachment which relies exclusively on derivatives of the auditory capsules.
On the basis of similarities in the structure of the braincase, palate and manus it is suggested that microsaurs are the collateral descendants (sister group) of temnospondyls. This relationship may account for the large number of similarities in the three living groups of Amphibia: Anura are generally believed to have descended from temnospondyls, while the Urodela and Apoda are often considered to have descended from microsaurs. These systematic conclusions endorse the recent suggestions that neither the Lepospondyli nor the Labyrinthodontia are natural groups, and both terms should be abandoned.     

Postcranial morphology and growth of the pachypleurosaur Anarosaurus heterodontus (Sauropterygia) from the Lower Muschelkalk of Winterswijk, The Netherlands

Sauropterygia from the Muschelkalk are only found in lag deposits known as bone beds, and most of the material consists of isolated bones. Alpha taxonomy of Sauropterygia from the Germanic Basin which include Pachypleurosauria is thus based mainly on skull morphology of a few specimens. Articulated or associated postcranial material of pachypleurosaurs, associated with diagnostic skull material, is very rare in the Germanic Basin and currently occurs in larger numbers only in the Lower Muschelkalk of Winterswijk (Gelderland Province, The Netherlands), which continuously produces new material. For the first time, the morphology of several partially articulated skeletons of the pachypleurosaur Anarosaurus heterodontus is described and compared. Some of those specimens have skull material attached; others were identified as pachypleurosaurs on the basis of their long bone histology. The current study revealed that postcranial bones of A.?heterodontus feature a diverse morphology reflecting differences during ontogeny. Thus, A.?heterodontus specimens could be assigned to size classes (I?CIII). However, on the basis of morphology, histology, and maximal known size of isolated skulls and humeri, none of these specimens represent fully grown individuals. Growth mark counts of midshaft-femur samples, morphologically assigned to size class?III, document that this size class was reached within the first year of life. Size class?III continued into the second year of life, and then afterwards skeletal maturity was reached. Thus, a juvenile A.?heterodontus grew very fast, which is also indicated by its bone tissue type, composed of a high number of radial vascular canals and a fast-deposited bone matrix. The assignment of isolated bones from Lower to Middle Muschelkalk localities to A.?heterodontus is now possible with an extensive amended diagnosis of this taxon. This largely contributes to the understanding of taxonomical diversity and distribution. Morphological comparison of the postcranial skeleton of A.?heterodontus with that of the two other valid pachypleurosaurs from the Germanic Basin, Anarosaurus pumilio and Dactylosaurus, supports their close phylogenetic relationship. Furthermore, the skeleton of A.?heterodontus has no morphological or histological aquatic adaptation such as pachyostosis or pachyosteosclerosis and thus represents the least degree of aquatic adaptation within Pachypleurosauria.     

Old and new hypotheses about the homology of the compound bones from the cheek and otico-occipital regions of the anuran skull

We studied the larval development of compound bones from the otico-occipital and cheek regions in species of the neobatrachian genera Batrachyla, Hylorina, Leptodactylus, Odontophrynus and Pleurodema. Comparisons were made using a set of Ambystoma spp. (Caudata) and Ceratophrys ornata (Anura; Ceratophryidae) larvae. As suggested by previous studies, we verified the compound nature of the exoccipital (two centers, anurans only), frontoparietal (one center, most anurans and Ambystoma; three centers, some anurans), and squamosal (two centers, all anurans and Ambystoma) bones. We discuss old and new homology hypotheses for each of the compound bone centers in the context of the most widely accepted scenario of lissamphibian origins and relationships, i.e., monophyletic Lissamphibia that includes the clade Batrachia (Caudata + Anura) and the most divergent Gymnophiona. Our findings have a direct impact on our understanding of the composition of the skull in Lissamphibia. We recognized the presence of the following bones: (i) opisthotic (fused to the exoccipital) and tabular (fused to the squamosal) in Batrachia (Anura + Caudata) and (ii) supratemporal (fused to the parietal portion of the frontoparietal) in Anura. Separate centers of the parietal were found only in Pleurodema.     

Ground penetrating radar and palaeontology: The detection of sirenian fossil bones under a sunflower field in Tuscany (Italy)

The application of Ground Penetrating Radar (GPR) in vertebrate palaeontology is very rare. We describe the discovery of an Early Pliocene sirenian skeleton detected by GPR in a locality near Grosseto (Tuscany, Italy). The specimen represents one of the most complete skeletons of Metaxytherium subapenninum (Mammalia: Sirenia) ever found in the Mediterranean area. Using a monostatic antenna of 200 MHz, this non-invasive technique allowed us to detect most of the bones of the skeleton (skull, mandible, vertebrae and ribs) revealed in a distinct zone reflecting the electromagnetic waves. Other bones were found in correspondence with some smaller reflective zones of high back-scattered energy. Each bone was located in a grid system to compare its position with the spatial distribution of reflective zones. We are confident that the positive outcomes experienced in this work will encourage the use of GPR for future field research in vertebrate palaeontology.     

Morphometric study of allometric skull growth in the temnospondyl Archegosaurus decheni from the Permian/Carboniferous of Germany

The skulls of ninety-six specimens of the temnospondyl Archegosaurus decheni from the Permian/Carboniferous of Germany, ranging in length from almost 20 to almost 280 mm, are used in a morphometric analysis of cranial ontogeny. A Cartesian transformation, a bivariate regression analysis, and a principal component analysis (PCA) reveal changes of general skull proportions and of particular skull roofing bones during growth. The preorbital part of the skull exhibits a much faster growth rate than the postorbital region, whereas the orbits exhibit a considerable decrease in relative size. The skull bones possess low allometry coefficients of growth in width, what leads to a proportionally much more slender skull than in most other temnospondyls, comparable only to other archegosaurids, the cochleosaurid Chenoprosopus, and longirostrine stereospondyls like Australerpeton and trematosaurids. Distinctly negative allometry of the width of the cheek region, where part of the adductor musculature originated, suggests a weaker bite in A. decheni than in temnospondyls with broad-parabolic skulls like Onchiodon and Sclerocephalus. Results from the analysis of skull growth in A. decheni offer a biological interpretation of the life habits of this animal as becoming increasingly more adaptated to piscivory during ontogeny. The continuous growth of the skull with a gradual shift in proportions supports, in conjunction with morphological data, the hypothesis that A. decheni did not undergo a metamorphosis.     

The erectile cheek-spine apparatus in the bristlenose catfish Ancistrus (Loricariidae, Siluriformes), and its relation to the formation of a secondary skull roof

In the South American catfish family Loricariidae, the opercle has been decoupled from the lower jaw, and has also lost its function in expiration. While many loricariid species have a small and slightly mobile opercle with reduced opercular musculature, within the hypostomine subfamily a novel opercular mechanism has developed that erects a tuft of enlarged odontodes anterior to the opercle. This defensive mechanism is examined in Ancistrus cf. triradiatus. The opercle has a prominent anterior process and the orientation of the reinforced articulation hinge to the hyomandibular bone has shifted. The opercular musculature is well developed, with a hypertrophied dilatator operculi that extends deep inside the skull roof bones and toward the midline, over the brain, but below the superficial skull roof. Hence the frontal, sphenotic, parieto-supraoccipital and compound pterotic bones consist of a dorsal, superficial part and a deeper part separating the brain from the muscle: two functional skull roofs are thus formed. The impact on the path of the cranial sensory canals is substantial, moving canals away from the skull surface. Hypertrophy of cranial muscles is known from many teleosts, but the invasion of such large muscles into the skull, which is drastically modified and literally hollowed out, has never been described before. These cranial modifications are greater in males than in females, related to the territorial behavior of the former, in which the erectile spines are usually used.