Skull Proportions Research Articles

Osteology and ontogeny of Early Cretaceous Philydrosaurus (Diapsida: Choristodera) based on new specimens from Liaoning Province, China

Choristoderes are a group of extinct diapsid reptiles that once occupied the freshwater systems in the Northern Hemisphere from the Middle Jurassic through the Miocene. The Early Cretaceous monjurosuchid Philydrosaurus from western Liaoning, China, represents a transitional morphotype between a broad-snouted (crocodile-like) and a narrow-snouted (gavial-like) skull during the evolution of Choristodera. New specimens of the taxon from the Early Cretaceous Jiufotang Formation include an incomplete braincase and a nearly complete juvenile skeleton; the study of these specimens provides new information on the braincase and ontogenetic change of the skeleton of this transitional monjurosuchid. In the braincase floor, the foramen internus canalis caroticus externus opens in a groove lateral to the constricted lateral aspect of the parasphenoid, and ventral exposure of this foramen differs from Champsosaurus, in which parasphenoid/pterygoid fusion completely encloses the canal. On the occiput, the vagus nerve foramen penetrates the basioccipital, while the two foramina for the hypoglossal nerve open at the exoccipital/basioccipital suture. Comparative study of the new juvenile specimen with adult Philydrosaurus reveals previously unknown developmental changes of the cranial and postcranial skeleton of this monjurosuchid. Ontogenetically, Philydrosaurus underwent a dramatic change of skull proportions, including elongation of the antorbital and postorbital regions, and elongation of the jaws, with a great increase of the number of marginal teeth. The lower temporal fenestra is entirely closed in early ontogeny, as in large, fully-grown adults.

A new neosuchian with Asian affinities from the Jurassic of northeastern Brazil

Phylogenetic relationships near the origin of extant crocodylians are weakly supported, and this lack of resolution makes for poor estimates of taxonomic and morphological diversity. Previously known taxa are found throughout the Cretaceous in Laurasia and at a few sites from Brazil, Australia, and northern Africa. Here, we report Batrachomimus pastosbonensis gen. et sp. nov. from the Late Jurassic of northeastern Brazil, which is deeply nested within Neosuchia and associated to the Cretaceous Asian paralligatorids. The new taxon predates all other members of Paralligatoridae and its immediate sister group (including Eusuchia) by 30 million years. A nearly complete skull, osteoderms, and limb bones were recovered, and autapomorphies of B. pastosbonensis include a scalloped lateral margin of the rostrum in dorsal view, unsculpted alveolar margin at the caudalmost portion of the maxilla, blunt lateral prongs on the jugal at the base of the postorbital bar, hourglass shaped choanae, and pterygoid choanal septum extended between the palatal shelves of the palatines. The crocodyloid-like skull proportions and the slender, conical teeth suggest an amphibious and piscivorous life history to this 1 m long animal.

An early juvenile specimen of Bolong yixianensis (Ornithopoda: Iguanodontia) from the Lower Cretaceous of Ningcheng County, Nei Mongol, China

We describe an early juvenile specimen (ZMNH M8812) of Bolong yixianensis from the Yixian Formation (Lower Cretaceous) of Ningcheng County, Nei Mongol, China. The specimen consists of an almost complete skeleton preserved two-dimensionally on a slab. The short and deep skull proportions and unfused neurocentral sutures in most preserved vertebrae suggest that the ZMNH M8812 is a juvenile individual. Osteohistological study confirms a very early developmental stage. The study reveals the ontogenetic changes of Bolong for the first time. The specimen revealed one additional autapomorphy for Bolong yixianensis: the lingual face of the maxillary crown is bounded by thickened mesial and distal margins and bisected by a prominent median principal ridge. The study revealed the following ontogenetic trends of Bolong: increased tooth rows in both maxilla and dentary, increased robustness of the jugal and scapula, the radius and ulna become more robust and shorter relative to the hindlimb and the metatarsals become proportionally shorter. ZMNH M8812 represents the first juvenile non-hadrosaurid iguanodontian specimen described from the Lower Cretaceous of eastern Asia.

Lake Tana large barbs diversity: Developmental and hormonal bases

Helerochronies caused by changes in the hormonal status are proposed to play a major role in the diversification of the Lake Tana barbs of Labeobarbus (=Barbus) genus (Dgebuadze and Mina, 2008). To test this hypothesis, we (i) compared temporal patterns of cranial development in L. intermedius, likely ancestral for Tana species flock, and L. brevicephalus and L. megastoma proposed to be paedomoiphic and peramoiphic, respectively; (ii) studied the role of thyroid hormone (TH) in the regulation of L. intermedius development; (iii) evaluated the effect of TH changes on the definitive morphology, and (iv) evaluated the possible role of TH in diversification of Tana barbs. Skull development was found to be heterochronicaily changed in L. brevicephalus and L. megastoma as compared to L. intermedius; revealed heteroclironies may account for differences in skull proportions distinguishing these barbs, TH was shown to play a major role in the regulation of ontogeny. Changes in TH level cause heterochronies in the development of a skull, axial skeleton, fins and fin girdles, squamation, and dentition. Heteroclironies were shown to influence adult morphology of these structures. These findings support the hypothesis of the involvement of developmental and hormonal factors in the diversification of the Lake Tana large barbs.

The Roots of Amphibian Morphospace: A Geometric Morphometric Analysis of Paleozoic Temnospondyls

Abstract Temnospondyls—a major component of Permian and Carboniferous terrestrial ecosystems—display great diversity in skull shapes and proportions. To quantify and interpret this diversity, we conducted a geometric morphometric analysis using 45 landmarks on the dorsal skull surface of 90 species with well-represented cranial material. Results show a correlation between morphospace occupation and phylogenetic proximity of taxa for trees in which dvinosaurs and dissorophoids are sister groups and join an edopoid-–eryopoid–basal archegosauriform clade. Most large groups of Carboniferous and Permian temnospondyls occupy specific areas of morphospace. Nearest-neighbor analyses reveal significantly greater taxon clustering than expected under either uniform or Gaussian null models. Size correlates strongly with shape across the whole data set, highlighting the association of some features (short, broad snout; large orbits) with small size. A significant relationship between size and shape is not observed in ...

Heterochronies in skull development of Lake Tana large African barbs (Labeobarbus; Cyprinidae; Teleostei)

Summary Large African barbs (genus Labeobarbus) of Lake Tana (Ethiopia) form a species flock, which includes 15 species differing in biology, ecology, and shape features, including skull proportions. Early craniogenesis was studied in three species of Lake Tana barbs: L. intermedius, a proposed ancestor of Tana species flock, L. brevicephalus, a small-sized and early-maturating barb, and L. megastoma, a large-sized, piscivorous species. The general sequence of cranial cartilage and bone appearance is the same for the barb species studied. However, they differ greatly in temporal pattern of craniogenesis. As compared to L. intermedius, bony skull development is accelerated relative to chondrocranial development in L. brevicephalus and retarded in L. megastoma. These differences in temporal pattern of craniogenesis are proposed to account for the different cranial proportions displayed by these barbs.

Morphometric study of the skull of a Late Pleistocene mummy of the Bilibino horse from the western Chukchi Peninsula

The skull of a mummy horse from the Late Pleistocene of the western Chukchi Peninsula is described. This is the seventh horse mummy recorded during the past 150 years in the Pleistocene permafrost of Siberia. Because of unique preservation (the skeleton is covered by soft tissues and skin) and young individual age (1–1.5 years of age), it is presently impossible to provide its correct species allocation of this specimen. Morphological features of the skull proportions and dentition of the Bilibino horse apparently reflect both species and individual characteristics of the structure and development.

Craniometrical variability in the cave bears (Carnivora, Ursidae): Multivariate comparative analysis

Morphological and molecular data suggest the existence of several taxa of cave bears, which were found to belong to three major mitochondrial haplogroups: kudarensis ( kudarensis), spelaeus ( ladinicus, eremus, spelaeus), and ingressus ( uralensis, ingressus, kanivetz). An analysis of craniometrical variability was carried out based on 20 measurements of 279 skulls from 40 European, Ural, and Caucasian localities, in order to investigate morphological similarity/dissimilarity of these taxa in a multivariate approach. The craniometrical analysis divides cave bears into two groups: small cave bears and large cave bears. The group of small bears consists of rossicus/ uralensis and ladinicus. In some skull proportions, these taxa display intermediate position between brown and cave bears, i.e. presumably possessing archaic characters of their common ancestor. The group of large bears includes taxa with more specialized skulls. An early radiation within this group is demonstrated by kudarensis which probably ranged across Asia, and deningeri which occupied Europe east to the Ural Mountains. In its craniometrical characters, kanivetz from the Late Pleistocene of the Urals resembles deningeri. Other taxa of large cave bears ( spelaeus, ingressus and eremus) reveal further evolution of cranial characters, being similar in the skull proportions. The level of difference between spelaeus and ingressus does not exceed that between subspecies of the recent brown bear, such as Ursus arctos beringianus and U. a. piscator. The examined isolated population of large cave bears from Volga River region (Zhiguli Hills) is similar to ingressus. Thus, based on the craniometrical data, the following species of cave bear are recognized: Ursus kudarensis (with the subspecies U. k. praekudarensis and U. k. kudarensis), U. deningeri (several subspecies), U. rossicus (with subspecies U. r. rossicus and U. r. uralensis), U. ladinicus, U. spelaeus (with subspecies U. s. spelaeus, U. s. eremus, U. s. ingressus, and, provisionally, U. s. kanivetz,).

A mid-Pliocene shearwater skull (Aves: Procellariidae: Puffinus) from the Taihape Mudstone, central North Island, New Zealand

A fossil skull of Puffinus sp. (Aves: Procellariidae), 85 mm long, is reported from the mid-Pliocene Taihape Mudstone (3.220–3.110 Ma; Waipipian Stage) at Mataroa, near Taihape, central North Island, New Zealand. This is the first described evidence for Puffinus in New Zealand's pre-Pleistocene fossil record. The skull belonged to a shearwater of a species close in skull size and proportions to the present-day wedge-tailed shearwater (P. pacificus). It could be conspecific with P. pacificus, or of the ancestral species from which P. pacificus evolved. Certainty on that point is difficult with a sample of one and less than perfect preservation and exposure, however. The Mataroa fossil, and undescribed fossil evidence from Hawera, Taranaki, establish the presence of a shearwater fauna of at least several species in Pliocene coastal waters off the North Island of New Zealand. This is unsurprising given the importance of procellariiforms in New Zealand's current avifauna.

A model for the bite mechanics in the herbivorous dinosaur Stegosaurus (Ornithischia, Stegosauridae)

Although the herbivorous dinosaur Stegosaurus (Ornithischia, Stegosauridae) is a well-described Late Jurassic taxon, little is known about the feeding habits and biomechanics of its homodont dentition. The presence of a rhamphotheca has been suggested, but it is still unknown how much such structure would have participated in the foraging behaviour of Stegosaurus. To better understand the feeding mechanism of this taxon, three-dimensional models of a Stegosaurus tooth were created, using the software ZBrush®. One model was simple and lacked serrations, whereas the other model included serrations. Those models were then transferred to the software Strand7®, where finite element analyses took place. The models were given material properties of enamel, based on studies done with crocodilian and mammalian teeth. In addition to that, bite forces were calculated for Stegosaurus, based on skull proportions. The results show little difference between the force distributions on the serrated and non-serrated models, indicating an efficient mechanism of stress dissipation that avoids high stresses being transferred to the jaw bones during biting. Digital plant models were also created to test the calculated bite forces in Stegosaurus, which suggests this animal was capable of biting through smaller branches. Computer modelling and analyses provide additional information about feeding habits and plant preferences for Stegosaurus, and can be adapted for studying other comparable herbivorous taxa.

A New Species of the Sauropsid ReptileNothosaurusfrom the Lower Muschelkalk of the Western Germanic Basin, Winterswijk, the Netherlands

A nothosaur skull recently discovered from the Lower Muschelkalk (early Anisian) locality of Winterswijk, The Netherlands, represents at only 46 mm in length the smallest nothosaur skull known today. It resembles largely the skull morphology of Nothosaurus marchicus. Differences concern beside the size, the straight rectangular and relative broad parietals, the short posterior extent of the maxilla, the skull proportions, and the overall low number of maxillary teeth. In spite of its small size, the skull can not unequivocally be interpreted as juvenile. It shows fused premaxillae, nasals, frontals, and parietals, a nearly co-ossified jugal, and fully developed braincase elements, such as a basisphenoid and massive epipterygoids. Adding the specimen to an existing phylogenetic analysis shows that it should be assigned to a new species, Nothosaurus winkelhorsti sp. nov., at least until its juvenile status can be unequivocally verified. Nothosaurus winkelhorsti sp. nov. represents, together with Nothosaurus...

Patterns of morphological and genetic variation in the endemic Malagasy batMiniopterus gleni(Chiroptera: Miniopteridae), with the description of a new species,M. griffithsi

Over the past decade, major advances have been made concerning the systematics and species diversity of Malagasy bats, largely based on specimens collected during inventories and associated morphological and molecular genetic studies. Herein we describe a new species of endemic bat from southern Madagascar, Miniopterus griffithsi sp. n., which is the sister taxa to Miniopterus gleni, a taxon described in 1995 (holotype from Sarodrano, just north of the Onilahy River in the southwest). Based on current information, M. griffithsi is found in the sub-arid bioclimatic zone, south of the Onilahy River, and M. gleni occurs in a variety of different bioclimatic zones, north of the Onilahy River to the northern portion of the island and on the near shore island of Ile Sainte Marie. The realization that M. griffithsi was a separate entity was first based on phylogeographic studies of the M. gleni complex. Comparisons using 397 bp of mitochondrial cytochrome b found a divergence of 1.2% within animals occurring across much of Madagascar north of the Onilahy River, 0.07% in those south of the Onilahy River, and 7.4% in populations separated by this river. Subsequently, morphological characters were identified that supported the specific separation of populations occurring south (M. griffithsi) and north of the Onilahy River (M. gleni), which include tragus shape, pelage coloration, and skull proportions.

A new specimen ofBaphetesfrom Nýřany, Czech Republic and the intrinsic relationships of the Baphetidae

Abstract‘Loxomma’bohemicumfrom the Upper Carboniferous assemblage from Nýřany, Czech Republic, is a nomen dubium restricted to the type and only specimen. The new binomenBaphetes orientalisis created for a skull referred toBaphetes bohemicusby later authors. A previously undescribed baphetid specimen from Nýřany is referred toB. orientalisdespite differences in skull proportions. It comprises a skull in dorsal aspect, mandibles and some associated postcranial elements. The skull possesses sclerotic ring elements within the orbital region of the dorsal fenestration of the skull, confirming the eye location. The elongate gastralia are arranged perpendicular to the interclavicle edge in contrast to the condition in temnospondyls and colosteids. Cladistic analysis of 24 characters of 11 baphetoids was carried out usingAcanthostegaandCrassigyrinusas outgroups.Eucrittawas the most primitive baphetoid, withSpathicephalusbeing the sister‐taxon to the remaining taxa, justifying a monotypic Spathicephalidae as a sister‐taxon to the Baphetidae. The Baphetidae are divided into a subfamily Baphetinae nom.nov. containing twoBaphetesspecies; and a subfamily Loxommatinae withLoxommaas a paraphyletic grade leading to aMegalocephalus +Kyrinionclade. The Linton taxon ‘Baphetes’lintonensisis transferred to the genusLoxommato give the new combinationLoxomma lintonensis.

Head kinematics and feeding adaptations of the Permian and Triassic dicynodonts

ABSTRACT The distribution of neck muscles and the shape of the skull shows that Permian and Triassic dicynodontsmay be classified into three categories which probably reflect feeding on low, medium and high vegetation. These are distinguished on the basis of the occipital index, the difference between the relative width and height of the occiput, which gives a measure of the relative importance of the lateral and the dorsoventral clusters of neck muscles. The basicranium is relatively shortened in Triassic forms, except in Vinceria, Shansiodon, Tetragonias, and Jachaleria, as has been noted before. Data on skull proportions indicates that the height of the parietal crest may be of little taxonomic use, and that the genus Tetragonias is not a clade. Dicynodonts with dietary preferences at the intermediate level correspondto major branching points in dicynodont phylogeny. High-level feeding adaptations among dicynodonts arose by the middle of the Tatarian (latest Permian) and lasted until the Late Triassic. Specialized low feeders existed only in the Middle Triassic.

A Giant Skull of the TemnospondylXenotosuchus africanusfrom the Middle Triassic of South Africa and Its Ontogenetic Implications

A previously unreported large skull of the mastodonsaurid temnospondyl Xenotosuchus africanus, from the Cynognathus Assemblage Zone (Early to Middle Triassic) of the Beaufort Group, Karoo Basin of South Africa, is described. The species is plesiomorphic in most aspects of its cranial anatomy, and only one autapomorphy is tentatively identified herein, namely contact between the left and right exoccipitals posterior to the parasphenoid. The new specimen permits recognition of a growth series for X. africanus from sub-adult (all previously known specimens) to fully adult (new specimen) stages. Ontogenetic changes associated with this include changes in skull proportions, and changes in the morphology of the lacrimal flexure, frontal, postfrontal, cultriform process and body of the parasphenoid, and the transvomerine tooth row.

A Juvenile Specimen of Anshunsaurus (Reptilia: Thalattosauria)

A marine reptile from the Ladinian deposits near Xingyi is described and identified as a juvenile of Anshunsaurus wushaensis on the basis of similar skull proportions and many postcranial characters. Based on this specimen and observations of the holotype of A. wushaensis, there is no distinct ontogenetic differentiation in the length of the jugal. The absence of an astragalus in the holotype, and the greater length of metacarpal V relative to metacarpal IV, could be due to intraspecific variation. Ossification is not synchronous for corresponding elements on both sides of the body.

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.

Deja vu: the evolution of feeding morphologies in the Carnivora

The fossil record of the order Carnivora extends back at least 60 million years and documents a remarkable history of adaptive radiation characterized by the repeated, independent evolution of similar feeding morphologies in distinct clades. Within the order, convergence is apparent in the iterative appearance of a variety of ecomorphs, including cat-like, hyena-like, and wolf-like hypercarnivores, as well as a variety of less carnivorous forms, such as foxes, raccoons, and ursids. The iteration of similar forms has multiple causes. First, there are a limited number of ways to ecologically partition the carnivore niche, and second, the material properties of animal tissues (muscle, skin, bone) have not changed over the Cenozoic. Consequently, similar craniodental adaptations for feeding on different proportions of animal versus plant tissues evolve repeatedly. The extent of convergence in craniodental form can be striking, affecting skull proportions and overall shape, as well as dental morphology. The tendency to evolve highly convergent ecomorphs is most apparent among feeding extremes, such as sabertooths and bone-crackers where performance requirements tend to be more acute. A survey of the fossil record indicates that large hypercarnivores evolve frequently, often in response to ecological opportunity afforded by the decline or extinction of previously dominant hypercarnivorous taxa. While the evolution of large size and carnivory may be favored at the individual level, it can lead to a macroevolutionary ratchet, wherein dietary specialization and reduced population densities result in a greater vulnerability to extinction. As a result of these opposing forces, the fossil record of Carnivora is dominated by successive clades of hypercarnivores that diversify and decline, only to be replaced by new hypercarnivorous clades. This has produced a marvelous set of natural experiments in the evolution of similar ecomorphs, each of which start from phylogenetically and morphologically unique positions.

Growth, proportions and variation of the skull of harbour porpoises (Phocoena phocoena) from the Sea of Azov

Skulls of 153 porpoises from the Sea of Azov and the Black Sea were studied. Thirty-seven skull measurements were examined. Skull growth in porpoises can be divided into five periods. The greatest changes in growth patterns occur after the first weeks of life. A ‘relay effect’ in growth of some structures (e.g. rostrum) was observed. Individual variation usually increases when a given structure grows intensively; however, some structures are highly variable over the entire lifespan. Sexual dimorphism is demonstrated in most of the measurements in adult animals. Temporal variation is minor. Skull structures are characterized by six types of allometry; three of them demonstrate drastic changes of allometry during lifespan, thus requiring two allometric equations to describe them. Skull proportions change critically during the first weeks of life. Six groups of correlated measurements differing in the pattern of correlation links during ontogeny were identified. Three factors play the most important roles in forming the definitive skull proportions: correlation between adjacent structures, impact of total skull size and inclusion in a certain functional complex.

Cranial ontogeny, diet, and ecogeographic variation in African lorises

A series of 22 craniodental measurements were obtained for the three subspecies of potto (Perodicticus) and angwantibos (Arctocebus). To describe patterns of variation in Perodicticus, a discriminant function analysis (DFA) was performed with adult data. To investigate the ecogeographic correlates of size variation in Perodicticus, adult cranial dimensions were compared with field data on latitudinal and longitudinal coordinates for available specimens as well as altitudinal data for a more limited sample. Ontogenetic series for larger-bodied Perodicticus and smaller-bodied Arctocebus were compared to test the hypothesis that inter- and intrageneric variation in skull form results from the differential extension/truncation of shared patterns of relative growth, and to assess morphological variation in the masticatory complex of sister taxa with differing dietary habits. Analyses of relative growth indicate that skull proportions in Perodicticus subspecies are largely ontogenetically scaled. In comparisons between Perodicticus and Arctocebus, most facial dimensions also are ontogenetically scaled, with all but one of the seven divergent comparisons (interorbital breadth) representing a feature of the masticatory apparatus. The DFA provided independent support for prior classifications of Perodicticus into three taxa. Size differentiation in African lorises appears to be correlated with altitudinal variation (Bergmann's Rule) as well as character displacement. The smallest pottos, P. p. potto, occupy low-lying coastal habitats in western Africa, whereas the larger, eastern forms inhabit higher, presumably colder elevations. The largest potto, P. p. edwardsi, is sympatric throughout most of its range with the smallest and most insectivorous African lorises (Arctocebus). A basis for intrageneric taxonomic variation in Perodicticus is supported by such nonclinal size variation, as well as divergences in the ontogeny of masticatory proportions corresponding to interspecific variation in dietary proclivities.