Basal Tubera Research Articles

Braincase Anatomy of the Titanosaurian SauropodLirainosaurus astibiaefrom the Late Cretaceous of the Iberian Peninsula

Lirainosaurus is the only titanosaurian sauropod described to date from the Late Cretaceous of the Iberian Peninsula. The type of Lirainosaurus astibiae consists of both cranial and postcranial remains that were found as disarticulated elements in the Lano quarry (Trevino, northern Spain). This taxon was diagnosed originally on the basis of vertebral and appendicular autapomorphic traits. The study of a paratypic skull fragment and a second referred specimen provides information about its braincase morphology. Lirainosaurus is regarded as a derived titanosaur on the basis of the complete fusion between the prootic and the exoccipital-opisthotic complex, the position of the cranial foramina, and the shape and orientation of the occipital condyle. The braincase of L. astibiae appears to be diagnostic in the presence of a foramen distally on each basal tubera. The absence of median subcondylar foramina in the basioccipital may be an autopomorphic trait or be due to ontogenetic growth. A comparison with other...

A reassessment of Mongolosaurus haplodon Gilmore, 1933, a titanosaurian sauropod dinosaur from the Early Cretaceous of Inner Mongolia, People's Republic of China

The Early Cretaceous of China has yielded a diverse sauropod fauna, dominated by basal titanosauriforms. Known from a basicranium, teeth and the first three cervical vertebrae, Mongolosaurus haplodon is a poorly understood taxon whose evolutionary relationship to other sauropods has remained uncertain. Redescription of Mongolosaurus identifies key features that confirm its titanosaurian affinities and demonstrate its validity, based on several autapomorphies and a unique combination of characters. Diagnostic features include: a low neural spine which does not extend above the epipophyses; the dorsal edges of the posterior articular surfaces of anterior cervical vertebrae projecting more posteriorly than the ventral edges; and postaxial spinoprezygapophyseal laminae forming a dorsoventrally elongate X-shape in anterior view. Mongolosaurus is incorporated into three independent cladistic analyses; however, its precise phylogenetic position within Titanosauria remains unclear. This is partly a consequence of the limited material, but also reflects the unusual combination of basal and derived features for this clade and the need for a better understanding of titanosaur intra-relationships. Reassessment of this animal provides new information on the atlas-axis complex, which is extremely poorly known in titanosaurs, and highlights several new characters that may prove useful in future phylogenetic analyses, such as the ratio of the transverse widths of the basal tubera and occipital condyle. A number of features pertaining to both the cranial material and cervical vertebrae are similar to those seen in some diplodocoids (including an anteroventral projection on the atlantal intercentrum and distal bifurcation of anterior cervical neural spines), providing further evidence for convergence between these two major neosauropod radiations. Other remains previously attributed to Mongolosaurus from China and Russia cannot be referred to this genus. In summary, Mongolosaurus represents one of the earliest known titanosaurs from eastern Asia and adds to our understanding of this diverse clade of sauropods.

Redescription of the holotype of Platecarpus tympaniticus Cope, 1869 (Mosasauridae: Plioplatecarpinae), and its implications for the alpha taxonomy of the genus

ABSTRACT The fragmentary holotype and only known material of Platecarpus tympaniticusCope, 1869, is redescribed and compared to other closely related plioplatecarpine mosasaurs. Re-characterization of the holotype reveals the following similarities to Platecarpus ictericus: exoccipital-basioccipital suture transversely oriented in condylar view; basal tubera not inflated and widely separated along ventral midline; cervical intervertebral joint surface strongly curved; seventh cervical vertebra lacking articulating intercentrum (i.e., hypapophyseal peduncle). In addition, the relatively complete right quadrate of the holotype of P. tympaniticus is virtually indistinguishable from those of P. ictericus. Quadrate characters, when combined with the four previously mentioned characters, uniquely diagnose both P. tympaniticus and P. ictericus among known mosasaur taxa. Because P. tympaniticus was established before P. ictericus, the former becomes the specific senior synonym of the latter, and is also the generic type of Platecarpus.

The braincase of a specimen of Proterochampsa Reig (Archosauriformes: Proterochampsidae) from the Late Triassic of Argentina

The proterochampsids are a Triassic group of superficially crocodile-like forms belonging to the Archosauriformes. In the present contribution, we present new information regarding the braincase of the proterochampsid Proterochampsa Reig 1959, from the Ischigualasto Formation (Carnian) of Argentina, and discuss its phylogenetic considerations. Some unique neurocranial features of Proterochampsa are described, including: the prominence and thickness of the V-shaped ridge that surrounds the basisphenoidal fossa; the medially concave lateral arms of the same ridge; and the semilunar depression on the parabasisphenoid ventrolaterally exposed. Other features are only shared with likely unrelated archosauriforms, including: the great lateral development of the basipterygoid processes and caudal development of its distal end; an eight-shaped metotic foramen; laterally directed basipterygoid processes; and rostral boundary of the basisphenoidal recess V-shaped. Proterochampsa differs in many other aspects from the archosauriform Chanaresuchus, including: a proportionally shorter basioccipital basal tubera; cultriform process ovoid in cross-section; longitudinal sulcus dorsal to the basipterygoid process; deep basisphenoidal recess; and the absence of a prominent intertuberal plate. In many braincase features, Proterochampsa is more similar to archosaurs than to Euparkeria, erythrosuchids and Proterosuchus. They include a reduced semilunar depression. A ventral border of the basioccipital forming a wide convexity and a dorsoventrally thin paroccipital process likely represents a feature shared with Chanaresuchus, but not with Doswellia and other basal archosauriforms.

New material of the mosasaur Plioplatecarpus nichollsae Cuthbertson et al., 2007, clarifies problematic features of the holotype specimen

ABSTRACT New material of Plioplatecarpus nichollsae Cuthbertson et al, 2007 (Squamata: Mosasauridae) clarifies many problematic features of the holotype. New observations demonstrate its unique cranial features among plioplate-carpines (=Ectenosaurus, Platecarpus and Plioplatecarpus), such as a parallel-sided frontal shield, a large, oval pineal foramen, and a highly inflated basal tubera of the basioccipital. Although the quadrate morphology is virtually identical to that of Platecarpus ictericus, Plio. nichollsae exhibits more synapomorphies with stratigraphically younger members of the genus than Platecarpus does, including newly recognized characters such as a pair of well-developed parolfactory-bulb recesses and anteriorly diverging ventro-lateral processes found on the frontal. Re-characterization of the species indicates its closest anatomical resemblance to descriptions given for the contemporary Platecarpus somenensis specimens (lower middle Campanian) from North America, which indicates that the current generic assignment for the latter taxon needs to be re-evaluated.

Un nuevo basicráneo de titanosaurio de la Formación Allen (Campaniano–Maastrichtiano), Provincia de Río Negro, Patagonia, Argentina

We describe a new titanosaurian braincase (MML-194), from the Upper Cretaceous (middle Campanian–lower Maastrichtian) of Río Negro Province, Argentina. Among titanosaurs, this specimen resembles Bonatitan reigi, more than any other member of the clade; the similarity is based on the supraoccipital protuberance bearing a median groove (also present in Saltasaurus and Rapetosaurus), the prominent basal tubera, the exit for the nerve VII located on the prootic crest, the occipital condyle and the foramen magnum almost of the same width. This material allows to observe some internal structures that are not appreciable in other titanosaurs, such as the pituitary cavity, the dorsum sellae and the foramen for the passage of the internal carotids, among other characters. The specimen MML-194 and Bonatitan were exhumed of same geological unit, the Allen Formation, from which have also been collected fossil eggs assignable to sauropods (megalooliths), for what is not unlikely that some of these taxa has been responsible of the laying of those eggs.

Description of a sauropod dinosaur braincase (Titanosauridae) from the Late Cretaceous Rio Colorado Subgroup, Patagonia

The fossil record of cranial material from titanosaurid sauropods is very poor and no complete skull has been described so far. Here we describe a new braincase (MUCPv-334) that was recovered from reddish sandstones of the Rio Colorado Subgroup (Late Cretaceous) of the region of Bajo del Añelo, approximately 20 km north of the town Añelo (Neuquén Province, Argentina). This specimen is attributed to the Titanosauridae based on the ventrally projected basipterygoid processes, a common condition shared by other titanosaurids. The robustness of MUCPv-334 together with an unusually expanded crista prootica and the presence of an anterior prolongation of the parasphenoid reaching the basal tubera were not reported in other members of the Titanosauridae, indicating a larger diversity in the braincase morphology of this sauropod clade than previously thought.

Braincase structure of the Middle Jurassic theropod dinosaurPiatnitzkysaurus

The braincase of the Middle Jurassic basal tetanuran theropod Piatnitzkysaurus floresi is described in detail. It exhibits several probable autapomorphic characters of this taxon, including the extremely shortened and narrow basipterygoid processes, the presence of parasphenoid recesses that seem to communicate with each other, and basipterygoid recesses that are approximately as long anteroposteriorly as they are high. The braincase of Piatnitzkysaurus shows two possible synapomorphies with the enigmatic European Middle Jurassic theropod Piveteausaurus, a subrectangular pituitary fossa that is wider than high and the transverse span between the basipterygoid processes being smaller than the width of the basal tubera. A noteworthy aspect of the braincase of Piatnitzkysaurus is the high number of accessory pneumatic recesses, which indicates that the presence of an anterior and dorsal tympanic recess, and possibly also of subcondylar and basipterygoid recesses is plesiomorphic for tetanurans. In contrast, the bones enclosing the brain (e.g., laterosphenoid, prootic, supraoccipital) are very massive and lack internal pneumatic chambers, as they are present in later tetanurans.

Embryos and eggs for the Cretaceous theropod dinosaur Troodon formosus

ABSTRACT Elongate and asymmetric eggs of the oospecies Prismatoolithus levis occur regularly in the Upper Cretaceous Two Medicine Formation of western Montana. These eggs had previously been assigned to the ornithischian Orodromeus makelai, for both juvenile and adult remains are typically associated with these eggs. Reexamination of the embryos shows them to exhibit at least 24 apomorphies of the clades Dinosauria, Theropoda and Paraves. The embryos also display a pneumatic quadrate, closely placed basal tubera, a high tooth count, a metatarsal II much narrower than IV and a strongly constricted metatarsal III, all possible synapomorphies of the Troodontidae. Presence of large basal tubera and a broadly rounded anterior border of the maxillary fenestra permit assignment to Troodon formosus. Most but not all bones appear ossified, suggesting a developmental level comparable to stages 35–38 of avian embryos and a time approaching hatching. Embryos show a consistent level of development from one egg to another indicating synchronous hatching of the clutch. Embryonic Troodon exhibit long distal segments and radically different hindlimb proportions in comparison to adults. Orodromeus and other small vertebrate remains associated with Troodon egg horizons may represent prey of the adults during egg-laying and brooding. Troodon eggs show several aspects either shared or convergent with some birds, and further demonstrate the close relationship of Troodontidae and Aves. These features include: asymmetric egg form, non-branching angusticanaliculate pores, distinct structural differentiation of the mammillary and overlying prismatic layer, barrel-shaped mammillary cones with a blocky calcite cleavage, and prismatic structure visible throughout the second structural layer.

An unusual braincase (?Stokesosaurus clevelandi) from the Cleveland-Lloyd Dinosaur Quarry, Utah (Morrison Formation; Late Jurassic)

ABSTRACT A basicranium from the Cleveland-Lloyd Dinosaur Quarry is the most advanced of any Jurassic theropod, combining features considered synapomorphies for two different Cretaceous theropod families, the Itemiridae and Tyrannosauridae. The basicranium has large pockets on the basipterygoid processes, flat basal tubera, and is extremely shortened, so that the basisphenoidal sinus is visible only in occipital view and the basiptyerygoid processes are positioned at the posterolateral corners of the braincase. CT scanning reveals the basicranium to be extensively pneumatized. A number of lines of evidence suggests that this specimen is referable to the rare Morrison theropod Stokesosaurus clevelandi. Stokesosaurus may be the earliest itemirid, the earliest tyrannosaurid, or a lineage that independently reached a comparable level of basicranial organization. Unfortunately the limited amount of known Stokesosaurus material does not allow for choosing among these alternatives. However, it is clear that Morrison theropods are morphologically much more diverse than suspected.

The phylogeny of varanoid lizards and the affinities of snakes

Evidence that platynotan squamates (living varanoid lizards, snakes and their fossil relatives) are monophyletic is presented. Evolutionary relationships within this group are then ascertained through a cladistic analysis of 144 osteological characters. Mosasauroids (aigialosaurs and mosasaurs), a group of large marine lizards, are identified as the nearest relatives of snakes, thus resolving the long-standing problem of snake affinities. The mosasauroid–snake clade (Pythonomorpha) is corroborated by 40 derived characters, including recumbent replacement teeth, thecodonty, four or fewer premaxillary teeth, supratemporal–prootic contact, free mandibular tips, crista circumfenestralis, straight vertical splenio-angular joint, loss of posterior ramus of the coronoid, reduced basipterygoid processes, reduced interpterygoid vacuity, zygosphene–zygantral articulations, and absence of epiphyses on the axial skeleton and skull. After mosasauroids, the next closest relatives of snakes are varanids (Varanus,SaniwaandSaniwides) and lanthanotids (LanthanotusandCherminotus). Derived features uniting varanids and lanthanotids include nine cervical vertebrae and three or fewer pairs of sternal ribs. The varanid–lanthanotid–pythonomorph clade, here termed Thecoglossa, is supported by features such as the anteriorly positioned basal tubera, and the loss of the second epibranchial. Successive outgroups to thecoglossans areTelmasaurus, an unresolved polytomy (Estesia, Gobidermatidae and Helodermatidae),ParavaranusandProplatynota. The ‘necrosaurs’ are demonstrated to be an artificial (polyphyletic) assemblage of primitive platynotans that are not particularly closely related to each other. Snakes are presumed to have evolved from small, limbless, burrowing lizards and the inability of previous analyses to resolve the affinities of snakes has been attributed to extensive convergence among the numerous lineages of such lizards. The present study contradicts this claim, demonstrating that the problem is due instead to omission of critical fossil taxa. No modern phylogenetic analysis of squamate relationships has simultaneously includedbothmosasauroids and snakes: previous studies have therefore failed to identify the mosasauroid–snake association and the suite of derived characters supporting it. Mosasauroids are large aquatic animals with well-developed appendages, and none of the derived characters uniting mosasauroids and snakes is obviously correlated with miniaturization, limb reduction or fossoriality. Recognition that mosasauroids, followed by varanids and lanthanotids, are the nearest relatives of snakes will also facilitate studies of relationships within snakes, which until now have been hampered by uncertainty over the most appropriate (closely–related) lizard outgroups.

Lesothosaurus, “Fabrosaurids,” and the early evolution of Ornithischia

ABSTRACT New materials of Lesothosaurus diagnosticus permit a detailed understanding of one of the earliest and most primitive ornithischians. Skull proportions and suturai relations can be discerned from several articulated and disarticulated skulls. The snout is proportionately long with a vascularized, horn-covered tip. The premaxillary palate is broad and vomers are long and fused anteriorly. Unlike many later ornithischians, the postpalatine vacuities are broadly open. The basal tubera are short and gently depressed, and the epiotic contributes to the sidewall of the braincase. The mandibular symphysis is spout-shaped, and the dentition is marked by oblique wear facets, in contrast to earlier reports. The tooth-to-tooth wear facets and form of the predentary-dentary articulation suggest long-axis rotation of the mandibular rami during mastication. The forelimb is proportionately very short, with a partially opposable pollex. The ischium lacks an obturator process. The reduced hallux is held well above the substrate during locomotion. L. diagnosticus is diagnosed below on the basis of apomorphic features. Other “fabrosaurids” constitute a heterogeneous assemblage of poorly known ornithischians and hatchling prosauropods that do not share any apomorphic features with L. diagnosticus. Fabrosaurus australis is a nomen dubium, and, as a consequence, the family Fabrosauridae is invalid. Echinodon becklesii may represent a primitive heterodontosaur, and Tawasaurus, Fulengia, and portions of the holotype of Technosaurus represent hatchling prosauropods. Pisanosaurus mertii may be the most primitive ornithischian, as indicated by the form of the distal crus and astragalus, but a more precise phylogenetic assessment will require additional remains.