Takanobu Tsuihiji

Cranial osteology of a juvenile specimen of Tarbosaurus bataar (Theropoda, Tyrannosauridae) from the Nemegt Formation (Upper Cretaceous) of Bugin Tsav, Mongolia

ABSTRACT A juvenile skull of the tyrannosaurid Tarbosaurus bataar found in the Bugjn Tsav locality in the Mongolian Gobi Desert is described. With a total length of 290 mm, the present specimen represents one of the smallest skulls known for this species. Not surprisingly, it shows various characteristics common to juvenile tyrannosaurids, such as the rostral margin of the maxillary fenestra not reaching that of the external antorbital fenestra and the postorbital lacking the cornual process. The nasal bears a small lacrimal process, which disappears in adults. Lacking some of the morphological characteristics that are adapted for bearing great feeding forces in adult individuals, this juvenile specimen suggests that T. bataar would have changed its dietary niches during ontogeny. The numbers of alveoli in the maxilla (13) and dentary (14 and 15) are the same as those in adults, suggesting that they do not change onto genetically in T. bataar and thus are not consistent with the hypothesis that the numbers of alveoli decreases ontogenetically in tyrannosaurids.

An exquisitely preserved troodontid theropod with new information on the palatal structure from the Upper Cretaceous of Mongolia

Troodontidae is a clade of small-bodied theropod dinosaurs. A new troodontid, Gobivenator mongoliensis gen. et sp. nov., is described based on the most complete skeleton of a Late Cretaceous member of this clade presently known, from the Campanian Djadokhta Formation in the central Gobi Desert. G. mongoliensis is different from other troodontids in possessing a pointed anterior end of the fused parietal and a fossa on the surangular in front of the posterior surangular foramen. The skull was superbly preserved in the specimen and provides detailed information of the entire configuration of the palate in Troodontidae. Overall morphology of the palate in Gobivenator resembles those of dromaeosaurids and Archaeopteryx, showing an apparent trend of elongation of the pterygoid process of the palatine and reduction of the pterygopalatine suture toward the basal Avialae. The palatal configuration suggests that the skull of Gobivenator would have been akinetic but had already acquired prerequisites for later evolution of cranial kinesis in birds, such as the loss of the epipterygoid and reduction in contact areas among bones.

First Report of a Pectoral Girdle Muscle in Snakes, with Comments on the Snake Cervico-dorsal Boundary

Abstract The morphology, topological relationships, and innervation patterns of m. cervicoquadratus in various snakes, especially basal taxa, were examined. Nearly all aspects of its anatomy suggest that this muscle is homologous with a part of m. cucullaris, most likely m. episternocleidomastoideus, the muscle that connects the skull and pectoral girdle in non-ophidian squamates. This is the first report of any pectoral girdle muscle persisting in snakes. In snakes examined, the most posterior extent of m. cervicoquadratus does not extend more posteriorly than the level of the 11th precloacal vertebra, suggesting that the length of the neck inferred on this characteristic would be quite short compared to the length of the entire precloacal region. However, different anatomical and developmental characteristics are known to indicate various lengths of the neck in snakes. This suggests that such characteristics potentially evolve independently from one another, and thus no single criterion may be sufficient to delimit the neck in snakes.

The First Discovery of Pterosaurs from the Upper Cretaceous of Mongolia

Cervical vertebrae of azhdarchid pterosaurs were discovered in two Upper Cretaceous (Baynshire Suite) dinosaur localities, Bayshin Tsav and Burkhant, in the Gobi Desert. These are the first discoveries of pterosaur remains in the Upper Cretaceous of Mongolia. The Burkhant specimen includes a nearly complete atlas-axis complex, which has rarely been described in this clade of pterosaurs. Although all elements comprising this complex are fused together, a wing-like atlas neural arch is still discernable. The postzygapophyseal facet of the axis is long anteroposteriorly and convex dorsally, and would likely have allowed a fairly large range of dorsoventral flexion at the axis-third cervical joint unlike in other well-known ornithocheiroids such as Pteranodon and Anhanguera. Both Mongolian localities represent inland, terrestrial environments, which were apparently not typical habitats of pterosaurs, thus adding further evidence for the ubiquity of Azhdarchidae during the Late Cretaceous.

Evolutionary and Developmental Aspects of Avian-Specific Traits in Limb Skeletal Pattern

The two sets of paired appendages, called limbs, are locomotory organs in tetrapods that are used for various functions (e.g., walking, running, crawling, digging, climbing, diving, swimming, and flying). Unlike such organs as the eye, which contain specialized tissues such as the lens and photoreceptor, the limb does not have any specialized cells or tissues, but consists of common tissues, such as bone, cartilage, muscle, blood vessels, and dermis. However, limb morphology is highly specialized and varies to provide species-specific modes of locomotion. As do the vertebrae and skull, the limb skeleton varies in morphology among species. The diversity of limb skeletal morphology provides examples of material for studies on morphogenesis. Avian forelimbs have evolved into wings for flight. The skeletal pattern in the avian limb has many traits that are unique among extant species of vertebrates; some of such traits are avian-specific, others are shared with more basal members of Theropoda, to which Aves belongs. Since such avian traits generally form during ontogenic development, determining when and how they appear in the developing embryonic limbs or limb buds provides important insights into the mechanisms underlying the generation of vertebrate morphological diversity. Here, we present an overview of several features of the skeletal pattern in the avian limb and discuss the developmental mechanisms responsible for their unique and lineage-specific traits.

HOMOLOGY OF THE NEOCERATOPSIAN CERVICAL BAR ELEMENTS

In neoceratopsian dinosaurs, the centra, neural arches, and neural spines of several anteriormost cervical vertebrae are fused together to form a structure called the cervical bar (Langston, 1975) or syncervical (Ostrom and Wellnhofer, 1986). This structure characterizes all neoceratopsians (Dodson and Currie, 1990; Dodson et al., 2004; You and Dodson, 2004) for which the anterior cervicals are known, but it remains controversial how many vertebrae are fused together to produce this structure, namely, whether it consists of the first three (atlas through the third cervical) or four (atlas through the fourth cervical) elements. Analyses of ceratopsian interrelationships have employed from two (Sereno, 1984; Chinnery, 2004) to five (Xu et al., 2002) characters related to the cervical bar; accordingly, sorting out the exact identities of the elements contributing to this structure may have implications for our understanding of support, branch lengths, and rates in ceratopsian evolution. Hatcher in Hatcher et al. (1907) described the first three cervicals as being fused together to produce the cervical bar in YPM 1822, Triceratops prorsus Marsh, 1890 (Fig. 1.1). Hatcher inferred that the atlantal contribution consists of a paired neural arch and a centrum that has a concave anterior surface for articulation with the occipital condyle. Each half of this neural arch was considered as arising from the dorsolateral edge of the centrum and uniting with each other dorsally to produce a neural spine. Hatcher inferred that the neural arch and spine of the atlas were completely fused with those of the axis, leaving no trace of a zygapophyseal articulation. Here we call this the three-element hypothesis. Lull in Hatcher et al. (1907), however, argued that the cervical bar in T. prorsus consists of four cervicals, with the atlas being reduced to just a ringlike bone on the anterior margin of …

New material of a troodontid theropod (Dinosauria: Saurischia) from the Lower Cretaceous of Mongolia

New material of a troodontid theropod from Khamaryn Ar in Mongolia, representing the second troodontid specimen described from the Lower Cretaceous of the Gobi Basin, is reported. This material consists mainly of caudal, manual and pedal bones, and can be assigned to Troodontidae based on the presence of derived features such as distal caudals bearing sulci on neural arches instead of neural spines, asymmetrical pes with slender metatarsal II and robust metatarsal IV, and pedal phalanx II-2 with the distal articular surface less than half the size of the proximal surface. The present specimen is considered merely as Troodontidae gen. et sp. indet. because of the lack of definitive autapomorphies. The present finding suggests that further exploration of the Lower Cretaceous in the Gobi Basin may still provide much new information on the theropod fauna in this region.

Finding the neck-trunk boundary in snakes: anteroposterior dissociation of myological characteristics in snakes and its implications for their neck and trunk body regionalization.

The neck and trunk regionalization of the presacral musculoskeletal system in snakes and other limb‐reduced squamates was assessed based on observations on craniovertebral and body wall muscles. It was confirmed that myological features characterizing the neck in quadrupedal squamates (i.e., squamates with well‐developed limbs) are retained in all examined snakes, contradicting the complete lack of the neck in snakes hypothesized in previous studies. However, the posterior‐most origins of the craniovertebral muscles and the anterior‐most bony attachments of the body wall muscles that are located at around the neck–trunk boundary in quadrupedal squamates were found to be dissociated anteroposteriorly in snakes. Together with results of a recent study that the anterior expression boundaries of Hox genes coinciding with the neck–trunk boundary in quadrupedal amniotes were dissociated anteroposteriorly in a colubrid snake, these observations support the hypothesis that structures usually associated with the neck–trunk boundary in quadrupedal squamates are displaced relative to one another in snakes. Whereas certain craniovertebral muscles are elongated in some snakes, results of optimization on an ophidian cladogram show that the most recent common ancestor of extant snakes would have had the longest craniovertebral muscle, M. rectus capitis anterior, that is elongated only by several segments compared with that of quadrupedal squamates. Therefore, even such a posteriorly displaced “cervical” characteristic plesiomorphically lies fairly anteriorly in the greatly elongated precloacal region of snakes, suggesting that the trunk, not the neck, would have contributed most to the elongation of the snake precloacal region. A similar dissociation of structures usually associated with the neck–trunk boundary in quadrupedal squamates is observed in limb‐reduced squamates, suggesting that these forms and snakes may share a developmental mechanism producing modifications in the anterior–posterior patterning associated with body elongation. J. Morphol. 2012.

New information on the cranial morphology of Avimimus (Theropoda: Oviraptorosauria)

ABSTRACT The cranial morphology of the oviraptorosaurian Avimimus portentosus is described based on a new specimen, one that includes bones such as the nasal and the jugal, which had not been available or only incompletely preserved previously. The left and right nasals are fused together as in oviraptorids. Morphology of the jugal, which is not fused with the quadratojugal, and the postorbital indicate that the infratemporal fenestra is completely separate from the orbit, not confluent with the latter, as inferred previously. The left and right dentaries are fused together without a trace of suture. Such newly available information indicates that the skull of Avimimus is not as ‘avian’-like as inferred in previous studies. Rather, it shows a mixture of plesiomorphic and derived character states observed in Oviraptorosauria, consistent with an intermediate phylogenetic position of this dinosaur between Early Cretaceous basal oviraptorosaurians and the diverse clade of Caenagnathoidea.

Theropod tooth from the Upper Cretaceous Himenoura Group in the Koshikijima Islands, southwestern Japan

Abstract. A theropod tooth found in the Upper Cretaceous Himenoura Group in the Koshikijima Islands, Kagoshima Prefecture, is described. Recent paleontological work in the region suggests a middle Campanian age for the specimen. Lacking a feature diagnostic of any specific theropod clade, this specimen is considered merely as Theropoda incertae sedis. The present finding confirms the potential of the Himenoura Group as a new source of information on the diversity of Theropoda during the Campanian.