Antorbital Fenestra Research Articles

The skull of Rapetosaurus krausei (Sauropoda: Titanosauria) from the Late Cretaceous of Madagascar

ABSTRACT Rapetosaurus krausei (Sauropoda: Titanosauria) from the Upper Cretaceous Maevarano Formation of Madagascar is the best-preserved and most complete titanosaur yet described. The skull of Rapetosaurus is particularly significant because most titanosaurs are diagnosed solely on the basis of fragmentary postcranial material, and knowledge of the titanosaur skull has remained incomplete. Material referred to Rapetosaurus includes the type skull from an adult that preserves the basicranium, rostrum, mandible, and palate. A second, juvenile skull preserves most of the braincase and cranial vault, as well as some of the palate and lower jaw. Here we provide a detailed description of Rapetosaurus cranial anatomy and highlight comparative relationships among known titanosaur and other neosauropod skulls. The Rapetosaurus skull is similar to those of diplodocoids in its overall shape, with retracted external nares and an elongated snout. However, extensive tooth distribution and bone articulations surrounding the external narial region and orbit are more similar to those of macronarians like Camarasaurus and Brachiosaurus. The maxilla, basicranium, paroccipital process, and pterygoid are among the most diagnostic elements of the Rapetosaurus skull, along with the enlarged antorbital fenestra, anteroventrally oriented braincase, and mandible. Titanosaur crania exhibit a greater diversity than previously recognized and, in light of Rapetosaurus, it is apparent that there is not a narrowly constrained bauplan for the skull of titanosaurs. Broad generalizations about evolution based on previously known, fragmentary fossils require re-evaluation. Ultimately, Rapetosaurus will be key in resolving titanosaur higher-level and ingroup phylogeny.

A New Protoceratopsid (Dinosauria: Neoceratopsia) from the Late Cretaceous of Inner Mongolia, China

Abstract An almost complete skull, which was collected from the Upper Cretaceous of the Bayan Mandahu area in Inner Mongolia, China by the Sino‐Canadian Dinosaur Project, is described and assigned to a new genus of protoceratopsid dinosaur, Magnirostris dodsoni gen. et sp. nov. This new taxon is distinguished from other protoceratopsids by the robust rostral bone and the existence of incipient orbital horn cores. The existence of an additional antorbital fenestra indicates a close relationship between Magnirostris and Bagaceratops.

New information on cranial and dental features of the Triassic archosauriform reptile Euparkeria capensis

AbstractThe course of the nasolacrimal duct, interdental plate morphology, and most details of tooth and denticle morphology have not previously been described in non–archosauriform reptilkes. Here I describe these details in the Triassic archosauriform Euparkeria capensis. The nasolacrimal canal opens orbitally via a pair of foramina between the lacrimal and prefrontal. The canal arches over the antorbital fenestra, as in archosaurs. The term ‘interdental unit’ is introduced for the unit composed of an interdental septum and its accompanying interdental plate. There is no demarcation between interdental plate and septum in E. capensis. The interdental units are heavily pitted on exposed surfaces. Like teeth, they are implanted in the dental groove and are separate from the surrounding bone and from each other. They are well positioned to serve as spacers between teeth, and to resist sagittal forces on teeth during prey capture. The teeth of E. capensis are labiolingually compressed, except for the nearly conical premaxillary teeth and mesialmost dentary tooth. Lateral teeth are serrated on mesial and distal keels. The denticles are low, rounded, and separated by grooves, and are slightly larger on the distal keel. Tooth morphology suggests carnivorous habits for Euparkeria.

Erpetosuchus, a crocodile-like basal archosaur from the Late Triassic of Elgin, Scotland

Erpetosuchus, a small archosaurian reptile from the Late Triassic of Scotland and North America, hasoften been implicated in the ancestry of crocodilians. A restudy of the type specimen, using new high-fidelity casts, as well as examination of new, hitherto undescribed material, allows a detailed description and restoration of Erpetosuchus granti from the Lossiemouth Sandstone Formation (late Carnian, Late Triassic). This small reptile is known only from the front end of its body; a complete skull, cervical vertebral column, anterior dorsals and ribs, shoulder girdle, and forelimb. The skull shows a number of unusual features: a reducedrow of only 4–5 teeth on the anterior part of the maxilla, a large antorbital fenestra set in a deep fossa whose margins aremarked by distinct sharply angled ridges, a jugal that is divided into a lateral and a ventral portion by a sharp ridge, a deeplyrecessed tympanic area, the angular and surangular marked by a strong ridge running back from the ventral margin of the mandibular fenestra, and teeth oval in cross-section and lacking anterior and posterior carinae and marginal serrations. The remains suggest that Erpetosuchus wasa light, cursorial animal that may have fed on insects. A cladistic analysis of crurotarsan archosaurs indicates that Erpetosuchus isthe closest sister group of Crocodylomorpha among known basal archosaurs. It shares with them a deep recess in the cheek region framed bythe quadrate and quadratojugal which slope forward side-by-side at an angle of 45° above horizontal, and reach the uppermargin of the lower temporal fenestra. In Erpetosuchus the recess is entirely lateral, while in crocodylomorphs, the recesspenetrates medially as well, since the quadrate/quadratojugal bar meets the side wall of the braincase.

Cranial morphology of Sinornithosaurus millenii Xu et al. 1999 (Dinosauria: Theropoda: Dromaeosauridae) from the Yixian Formation of Liaoning, China

The recent discovery of the early dromaeosaurid Sinornithosaurus millenii Xu et al. 1999 is especially important for our understanding of maniraptoran evolution, particularly of avian origins. This paper presents a thorough description of the skull of this species, revises the diagnosis based on cranial features, and discusses similarities to those of other dromaeosaurids and Archaeopteryx. Distinctive cranial characteristics of S. millenii include the presence of a deep excavation on the posteroventral margin of the premaxilla, a diastema between the premaxillary and maxillary teeth, a number of pits and ridges on the anterolateral surface of the antorbital fossa, long posterolateral process of the parietal that are sharply posteriorly directed, the column-like margin of the pterygoid process of the quadrate, the bifurcated posterior end of the dentary, and a distinctive groove posterior to the anterior carina on the lingual surface of the premaxillary tooth crowns. The present study on the skull of S. millenii provides new information on the poorly known cranial anatomy of dromaeosaurid dinosaurs, revealing that dromaeosaurids share more similarities with Archaeopteryx than previously thought. It also provides evidence suggesting that early dromaeosaurids are more bird-like than the later, more derived ones, thus contradicting claims that more bird-like dinosaurs appeared later in fossil record than the earliest birds.

A new iguanodontid (Jinzhousaurus yangi gen. et sp. nov.) from the Yixian Formation of western Liaoning, China

A new iguanodontid, Jinzhousaurus yangi gen. et sp. nov., is erected based on the cranial and dental morphology of a specimen from the Yixian Formation of western Liaoning. Although a few primitive characteristics remain, Jinzhousaurus yangi display a number of characteristics similar to those seen in derived iguanodontids. Other interesting characteristics include the absence of antorbital fenestra and the frontal excluded from the participation in orbit, which are closer to the condition in hadrosaurids. The unusual combination of the characteristics provides important data for assessing morphological evolution toward hadrosaurs. The new discovery also represents the first reported large-sized dinosaur from Jehol fauna in western Liaoning, and increases the diversity of this famous fauna. Furthermore, the discovery of a derived iguanodontid from the Yixian Formation provides further evidence supporting a Cretaceous age of the Jehol fauna, which is consistent with an isotopic age.

A skull of Proargyrolagus, the oldest argyrolagid (Late Oligocene Salla Beds, Bolivia), with brief comments concerning its paleobiology

ABSTRACT A skull of the oldest described argyrolagid, Proargyrolagus bolivianus (late Oligocene of the Salla Beds, Bolivia) allows an assessment of the phylogeny of this group and a reconstruction of its paleobiology. Several aspects of the cranial anatomy of the Monodelphis-sized Proargyrolagus serve to support the hypothesis that argyrolagids are marsupials: the palate has large vacuities, the angular process is medially inflected, the auditory bulla is composed of a wing of the alisphenoid, and the dental formula includes four molars. Like Argentine Plio-Pleistocene argyrolagids, Proargyrolagus has an unfused symphysis, and a phaneric and almost vertically oriented ectotympanic in the shape of a flattened ring. Proargyrolagus is more primitive in having a less globular braincase, smaller orbits, and a shallower mandible. The dental formula is 4/3.1/1.3/2.4/4 or 4/4.1/0.3/2.4/4 (vs. 2/2.0/0.1/1.4/4 for Argyrolagus). Large infraorbital foramina transmitted nerves and vessels of the snout. The snout is long with the nasals projecting well forward of the incisors and with a strong antorbital fossa for facial muscles. It can be inferred that this animal had well-developed vibrissae and a mobile proboscis, and relied heavily on its rostrum to gather tactile information. Food manipulation was assisted by an elongate, procumbent lower incisor that occluded with three sharply-edged upper incisors, as in living phalangeriform marsupials. Like Plio-Pleistocene argyrolagids and extant elephant shrews, Proargyrolagus has prismatic and high crowned cheek teeth suggesting an abrasive diet consisting perhaps of seeds.

Archaeopteryx -like skull in Enantiornithine bird

The bird Cathayornis from the Early Cretaceous period gives the first evidence for a post-Jurassic survival of an Archaeopteryx -like skull in birds. This skull combines short, toothed premaxillaries, nasals meeting at the midline and submaxillary fossae in the antorbital fenestra.

Sichuanosuchus shuhanensis, a new ?Early Cretaceous protosuchian (Archosauria: Crocodyliformes) from Sichuan (China), and the monophyly of Protosuchia

ABSTRACT A new protosuchian, Sichuanosuchus shuhanensis, sp. nov. from the ?Lower Cretaceous of Sichuan (China), is distinguished from other known basal crocodyliform archosaurs primarily by the subdivision of the small antorbital fenestra into anterior and posterior openings, the presence of a large, caniniform first maxillary tooth, followed by seven teeth with low, chisel-like crowns, and the posterior palpebrai being larger than the anterior one. Phylogenetic analysis using 128 characters for 29 crocodyliform taxa supports the monophyly of Protosuchia. Sichuanosuchus is most closely related to Shantungosuchus from the Upper Jurassic and Lower Cretaceous of China. These two taxa, together with Gobiosuchus from the Upper Cretaceous of Mongolia, appear to form a clade of late Mesozoic basal crocodyliform reptiles.

On the taxonomic status of Cycnorhamphus and Gallodactylus (Pterosauria: Pterodactyloidea)

One of the rarest pterosaurs from the Upper Jurassic Solnhofen Limestone of southern Germany is Pterodactylus suevicus. Only two specimens are known: the holotype, consisting of a virtually complete skeleton described by Quenstedt (1855); and a second specimen, consisting of only postcranial elements described by Wagner (1858) and lost during World War II (Wellnhofer, 1970). Seeley (1870) noted that P. suevicus differed from P. antiquus, the type species of Pterodactylus, in a number of significant points. Therefore, he placed P. suevicus in a new genus, Cycnorhamphus. Seeley (1870:111) gave the following diagnosis of Cycnorhamphus:“Nares very small, looking upward from a swan-like beak. The middle hole of the skull very large and elongated and lateral. Neck long. Wing-metacarpal long. Four joints in wing-finger. Ilium widening in front. Epipubic bones meeting mesially. The type is Pterodactylus suevicus (Quenstedt).”The character “nares very small” was the result of a misinterpretation of depressions on the premaxillae that probably resulted from crushing, and “middle hole of the skull” referred to the confluent naris and antorbital fenestra, which Seeley apparently considered the antorbital fenestra alone.

Homology of facial structures in extant archosaurs (birds and crocodilians), with special reference to paranasal pneumaticity and nasal conchae.

Homology of virtually all major components of facial anatomy is assessed in Archosauria in order to address the function of the antorbital cavity, an engimatic structure that is diagnostic for the group. Proposed functions center on its being a housing for a gland, a muscle, or a paranasal air sinus. Homology is approached in the context of the Extant Phylogenetic Bracket method of reconstructing unpreserved aspects of extinct organisms. Facial anatomy and its ontogeny was studied in extant archosaurs (birds and crocodilians) to determine the osteological correlates of each soft-tissue component; resemblances between birds and crocodilians comprised the similarity test of homology. The congruence test of homology involved surveying phylogenetically relevant fossil archosaurs for these bony signatures. The facial anatomy of extant birds and crocodilians is examined in detail to provide background and to discover those apomorphic aspects that contribute to the divergent specialization of these two groups and thus obscure homologies. Birds apomorphically show enlarged eyeballs, expanded nasal vestibules, and reduced maxillae, whereas crocodilian faces are dorsoventrally flattened (due to nasal rotation) and elongated. Most facial attributes of archosaurs are demonstrably homologous and in fact characterize much more inclusive groups. Special emphasis has been placed on the nasal conchae and paranasal air sinuses. Within Amniota, the following conchal structures are homologous, and all others are neomorphs: Avian caudal concha, crocodilian concha + preconcha, Sphenodon caudal concha, squamate concha, and probably the mammalian crista semicircularis. The avian antorbital paranasal air sinus is homologous with the crocodilian caviconchal sinus; the maxillary sinus of placental mammals is not homologous with the archosaurian paranasal sinus. With regard to the function of the antorbital cavity, archosaurs possess homologous nasal glands, dorsal pterygoideus muscles, and paranasal air sinuses, but the osteological correlates of only the paranasal sinus involve the antorbital fenestrae and fossae. Thus, the antorbital cavity is best interpreted as principally a pneumatic structure. © 1995 Wiley-Liss, Inc.

The skull and neck of the basal theropodHerrerasaurus ischigualastensis

ABSTRACT We describe the skull and neck of Herrerasaurus ischigualastensis from specimens discovered recently in the Upper Triassic Ischigualasto Formation of northwestern Argentina. The skull has a rectangular profile and a transversely narrow snout. Marked supratemporal depressions for jaw adductor musculature on the skull roof and a well-developed, sliding intra-mandibular joint suggest that Herrerasaurus ischigualastensis was an active predator. The neck is relatively slender with prominent epipophyses on all of the cervical vertebrae. Diagnostic features of the species include the semicircular shape of the antorbital fossa and the long, slender posterodorsal process of the dentary. Other features of the skull, such as the loss of the postfrontal and small post-temporal opening, constitute dinosaurian synapomorphies. These synapomorphies, however, remain equivocal at the level of Dinosauria because they cannot be evaluated in immediate dinosaurian outgroups for which cranial information is lacking. Co...

A large crested theropod from the Jurassic of Xinjiang, People's Republic of China

A skull and partial skeleton of a new genus and species, Monolophosaurus jiangi n.gen., n.sp., of large theropod has been recovered from the Middle Jurassic beds in the Junggar Basin of Xinjiang, China. The most striking characteristic is a crest, formed primarily by the nasal and lacrimal bones, on the midline of the skull. Several foramina connect the hollow interior of the crest with the antorbital fossa. The function of the crest is unknown, although it most likely was used for visual recognition by other members of the same species. It is difficult to make a precise taxonomic assignment in the absence of information on the appendicular skeleton, but it appears to be more closely related to Allosaurus than to other megalosaur grade theropods.

The skull of the basal stegosaurHuayangosaurus taibaiiand a cladistic diagnosis of stegosauria

ABSTRACT The recent discovery of the Middle Jurassic stegosaur, Huayangosaurus taibaii has shed new light on the early phylogeny of Stegosauria. The skull of this taxon is described in detail and compared to those of other thyreophorans. Cranial and postcranial autapomorphies of Huayangosaurus include high premaxillary and maxillary tooth counts (7 and 25–30, respectively), small postorbital horn, intercostal articular flanges, and a carpus composed of a single coossified block. Nineteen synapomorphies unite Huayangosaurus and other stegosaurs and constitute a revised cladistic diagnosis for Stegosauria. Huayangosaurus is the sister-taxon to all other stegosaurs, which have lost premaxillary teeth, reduced the size of the antorbital fossa, increased the length of the prepubic process of the pubis, and lost lateral scute rows along the trunk.

The Jordan theropod (Maastrichtian, Montana, U.S.A.) referred to the genus Aublysodon

The Jordan theropod skull is shown to belong to Aublysodon mirandus,, a theropod dinosaur previously known only from isolated teeth. Although the skull is incomplete, it retains numerous primitive characters that set it apart from all other known Late Cretaceous theropods. These features include a long, low muzzle, relatively large teeth, frontals longer than wide, smooth nasals, V-shaped anterior margin of antorbital fossa, premaxillary- maxillary suture acutely inclined, and low angle of symphysis. Aublysodon is characterized by premaxillary teeth D-shaped in cross section and with posteriorly directed carinae, a V-shaped frontoparietal suture, and a peculiar first dentary tooth. It probably possessed small premaxillaries. Aublysodon is an aberrant theropod which at this time is best retained in its own family, the Aublysodontidae.

A new trematochampsid crocodile from the Early Cretaceous of north-western Patagonia, Argentina and its palaeobiogeographical and phylogenetic implications

Amargasuchus minor is described from the Early Cretaceous (Neocomian, of the La Amarga Formation of northwest Patagonia (Argentina). This species possesses a moderately high and narrow snout, strong festooning, a relatively large number of maxillary teeth and a well developed antorbital fenestra, and it lacks hypertrophied teeth, a combination of characters which suggests that it belongs to the crocodile family Trematochampsidae. Amargasuchus differs, however, from Trematochampsa in several respects. These include an almost straight lateral edge of the maxilla in dorsal aspect, a different distribution of the largest teeth, a longer snout, and laterally compressed alveoli. The new material supports a Gondwanan origin of the Trematochampsidae and agrees with a new phylogenetic hypothesis which considers trematochampsids to be the ancestral group of certain lineages of ziphodont mesosuschian crocodiles.

Aerosaurus wellesi, new species, a varanopseid mammal-like reptile (Synapsida : Pelycosauria) from the Lower Permian of New Mexico

ABSTRACT Aerosaurus wellesi n. sp. is based upon two nearly complete articulated skeletons from the Lower Permian deposits near Arroyo del Agua, New Mexico. Study of these superbly preserved specimens indicates that the genus Aerosaurus is distinguishable from most other pelycosaurs, including Varanops and Varanodon, in having fewer maxillary and dentary teeth, all of which are more laterally compressed and more strongly recurved. The parasphenoid is uniquely expanded laterally and posteriorly, and bears rows of teeth on ridges. Aerosaurus also differs from all other pelycosaurs in which the braincase is adequately known in the formation of the dorsum sellae mainly from the basisphenoid. Aerosaurus was probably an aggressive predator of lizard-like habitus, with an unusually long tail for a pelycosaur. Aerosaurus, Varanops, and Varanodon are the only pelycosaurs that can be included in the family Varanopseidae with certainty; however, Basicranodon may also be a varanopseid. The possibility that the alleged antorbital fenestra in Varanodon may be artificial casts further doubt on the suggestion that Varanodon, and by extension the Varanopseidae, was involved in the ancestry of archosaurs.

The anatomy of the thecodont reptile Euparkeria capensis Broom

The South African Museum specimens of Euparkeria originally described by Broom (1913 a, b ) and later studied by Haughton (1922) have been further prepared using the dental mallet. In addition, material in Professor D. M. S. Watson’s collection has been prepared and studied. As a result of inquiries made at Aliwal North, it is suggested that all the material may represent a single find and a small exposure of Cynognathus zone shale close to the town is considered to be the most probable location of the site. On the basis of the specimens already mentioned a detailed description of most of the skeleton is given, the only parts remaining unknown, or incompletely known, being the carpus and manus, the braincase and the caudal vertebrae. The following features are discussed in functional terms: skull architecture; the limbs and their girdles; the vertebral column and ribs. The skull, although showing some adaptation to predacious habits, is relatively unspecialized and a subsequent reversal of trend towards a more omnivorous or even vegetarian diet is not impossible. A consideration of the probable arrangement of the jaw muscles suggests that the antorbital fenestra of the archosaurs was not originally evolved in relation to the development of the pterygoideus D musculature. A study of the limbs and their girdles indicates that Euparkeria was facultatively bipedal and that when this gait was adopted the femora were brought into a vertical position, although they must have been held horizontally while at rest or moving slowly. It is suggested that the alteration in position of the rib articulations along the length of the vertebral column is related to efficiency of pulmonary ventilation. Expansion of the tips of the neural spines is shown to be related not to the development of dermal armour but to the arrangement of the muscles of the transversospinalis system. The term ‘scute tables’ used to designate these expansions is therefore abandoned and they are referred to simply as spine tables. The relationships of Euparkeria to other groups is briefly discussed. Euparkeria shows a number of progressive features related to a highly carnivorous habit and increased locomotor efficiency combined with retention of several primitive characters. Previous authors have tended to lay emphasis on one or other of these types of characteristic and accordingly classify Euparkeria either as an advanced form or as a very primitive one: the consequent variation in proposed systematic position ranges from inclusion in the Sphenosuchidae (v. Huene 1962) to the Erythrosuchidae (Hughes 1963). It is concluded that Euparkeria , although a progressive form, is closely related to the latter family with which it may be convenient, for the time being, to classify it. It could well have been directly ancestral to advanced pseudosuchians such as Ornithosuchus and Hesperosuchus and possibly to the prosauropods and sauropods but not to the ornithischians or the birds and was a trifle too advanced to have been directly ancestral to the Aëtosauridae, to which it is, however, closely related.

The genera of phytosaurs

The long-snouted, crocodile-like, amphibious reptiles of the Late Triassic known as phytosaurs are assigned to 5 genera on the basis of cranial characters. The most primitive genus, Paleorhinus, has external nares well forward of the antorbital fenestrae, the posttemporal arch at level of the skull roof, a slender rostrum without crests, homodont dentition, a low quadrate, and no trace of a posterior squamosal process. Mystriosuchus, which has highly specialized posterior nares, tall quadrate, and depressed posttemporal bar, retains the primitive form of the squamosal, homodont dentition, and slender (much longer) rostrum of Paleorhinus from which it may be derived through Francosuchus (the European subgenus of Paleorhinus). The remaining phytosaur genera are characterized by rounded processes of the squamosal bone projecting well behind the occipital surface of the skull, and by the development of heterodont dentition and generally more robust skulls. Angistorhinus, the most primitive member of this lineage, retains a primitive posttemporal arch at the level of the skull deck, but is advanced over Paleorhinus in its posteriorly placed external nares. Its rostrum is slender and lacks crests; the dentition is only slightly differentiated. Phytosaurus and Rutiodon, the more specialized members of this group, have depressed posttemporal arches and tall quadrates. Rostra of Rutiodon vary from slender and uncrested in small skulls to moderately robust with high dorsal crests in the posterior half in the largest individuals; teeth vary from moderately to strongly heterodont. The squamosal processes are slender and show progressive phyletic elongation. The ratio of rostral to postrostral length varies inversely with size. Phytosaurus skulls are characterized by massive rostra with continuous dorsal crests and have a relatively constant rostral ratio independent of size; the squamosal processes are short and deep; dentition is strongly heterodont; considerable evidence suggests that the dorsal armor plates differed from other phytosaurs.

The Genera of Phytosaurs

The long-snouted, crocodile-like, amphibious reptiles of the Late Triassic known as phytosaurs are assigned to 5 genera on the basis of cranial characters. The most primitive genus, Paleorhinus, has external nares well forward of the antorbital fenestrae, the posttemporal arch at level of the skull roof, a slender rostrum without crests, homodont dentition, a low quadrate, and no trace of a posterior squamosal process. Mystriosuchus, which has highly specialized posterior nares, tall quadrate, and depressed posttemporal bar, retains the primitive form of the squamosal, homodont dentition, and slender (much longer) rostrum of Paleorhinus from which it may be derived through Francosuchus (the European subgenus of Paleorhinus). The remaining phytosaur genera are characterized by rounded processes of the squamosal bone projecting well behind the occipital surface of the skull, and by the development of heterodont dentition and generally more robust skulls. Angistorhinus, the most primitive member of this lineage, retains a primitive posttemporal arch at the level of the skull deck, but is advanced over Paleorhinus in its posteriorly placed external nares. Its rostrum is slender and lacks crests; the dentition is only slightly differentiated. Phytosaurus and Rutiodon, the more specialized members of this group, have depressed posttemporal arches and tall quadrates. Rostra of Rutiodon vary from slender and uncrested in small skulls to moderately robust with high dorsal crests in the posterior half in the largest individuals; teeth vary from moderately to strongly heterodont. The squamosal processes are slender and show progressive phyletic elongation. The ratio of rostral to postrostral length varies inversely with size. Phytosaurus skulls are characterized by massive rostra with continuous dorsal crests and have a relatively constant rostral ratio independent of size; the squamosal processes are short and deep; dentition is strongly heterodont; considerable evidence suggests that the dorsal armor plates differed from other phytosaurs.