An Osteological Review of Alioramus

Abstract - The Late Cretaceous tyrannosaurid theropod Alioramus has long been one of the most puzzling large carnivorous dinosaur taxa, largely because for several decades it has been represented only by a single, fragmentary specimen that seems to represent a long-snouted and gracile individual but is difficult to interpret. The discovery of a substantially complete skeleton of Alioramus at the Tsaagan Khuushu locality in the Maastrichtian Nemegt Formation of Mongolia, recovered during the 2001 American Museum–Mongolian Academy of Sciences expedition and described as a new species (Alioramus altai) in 2009, definitively shows that this mysterious taxon is a distinct form of longirostrine tyrannosaurid that lived alongside the larger and more robust Tarbosaurus. Here we describe and figure this remarkably preserved skeleton in detail. We provide exhaustive descriptions and photographs of individual bones, and make extensive comparisons with other tyrannosauroids. This monographic description provides further evidence that Alioramus is an unusual long-snouted, gracile, and slender-limbed taxon with an unpredecented degree of cranial ornamentation among tyrannosaurids and an extremely pneumatized skeleton.

Anatomical comparisons indicate that the long skull of Alioramus is an autapomorphic feature that is proportionally longer (relative to femur length) than in any other known tyrannosaurid specimen, including juveniles, and that Alioramus is morphologically distinctive relative to similarly sized individuals of the contemporary and sympatric Tarbosaurus. The holotype specimen of A. altai belongs to a young individual, and many differences between it and the other known specimen of Alioramus (the holotype of A. remotus) may represent ontogenetic variation. The unusual longirostrine skull of Alioramus was largely produced by lengthening of the snout bones (maxilla, nasal, dentary, lacrimal, jugal), rather than the orbiotemporal bones (frontal, postorbital, squamosal, quadratojugal). The long snout, gracile skull bones, comparatively small attachment sites for jaw muscles, and lack of interlocking sutures and a robust orbital brow would have precluded the holotype individual from employing the characteristic “puncture-pull” feeding style of large-bodied adult tyrannosaurids, in which the muscular jaws, thick teeth, and interlocking sutures enabled individuals to bite with enough force to fracture bone. Whether adult Alioramus could utilize “puncture-pull” feeding awaits discovery of mature individuals of the genus. The coexistence of the long-snouted Alioramus and robust and deep-snouted Tarbosaurus, which are found together at the Tsaagan Khuushu locality, demonstrate that multiple large tyrannosaurids were able to live in sympatry, likely because of niche partitioning due to differences in craniofacial morphology and functional behavior.

Reference - Brusatte, S.L., Carr, T.D., Norell, M.A. (2012). "The Osteology of Alioramus, A Gracile and Long-Snouted Tyrannosaurid (Dinosauria: Theropoda) from the Late Cretaceous of Mongolia". Bulletin of the American Museum of Natural History, 366: 1-197. doi: 10.1206/770.1.

Characteristic 'Death Poses' in Theropods May Have Been the Result of a Cerebral Disorder Producing Muscle Spasms After Death

Opisthotonic postures, more colloquially known as "death poses" are a common occurrence in theropod and bird fossils. They are characterized by, in life, what would be a certainly impossible feat: the neck arching backward at an incredibly sharp angle, to the point of where the head is even touching the back; the tail arching forward in many ways the neck does, even to when the tail and the head are close to touching; and the hindlimbs appear rigid and stiff.

An explanation for such phenomena has not yet reached the surface, however, paleontologists do know that these are muscle spasms creating the "death pose", not the geological/geographical conditions of its site or the way it was buried or fossilized. Conversely, the exact cause of the spasms are unknown. Or are they? A recent paper in the journal Palaeobiodiversity and Palaeoenvironments may have found the answer. Mental disorders having to do with the cerebral hemisphere of the brain may have caused the spasms postmortem,* according to the article, which was written by paleontologists Achim G. Reisdorf and Michael Wuttke.

To test this theory, they analyzed two basal coelurosaurs, Compsognathus longipes (below) and Juravenator starki, of which are both known for their "death poses". By analyzing the biomechanical constraints of the two dinosaurs plus experiments concerning decomposition tested using deceased domestic fowls, it has been found that muscle spasms are postmortem phenomena.

* The cerebrum has many functions, of which one is the complete control of your physical body, including via muscles.

Compsognathus longipes in the opisthotonic
posture, also known as the "death pose".
Image taken by Wikipedia user Ballista, here.

Abstract - More or less complete and articulated skeletons of fossil air-breathing vertebrates with a long neck and tail often exhibit a body posture in which the head and neck are recurved over the back of the animal. Additionally, the tail is typically drawn over the body, while the limbs have a rigid appearance. In palaeontological literature, this “opisthotonic posture” of such fossils still requires a causal interpretation in an etiological context. According to this hypothesis, there is a presumption of a cerebral disorder generating perimortem muscle spasms that are preserved by rapid burial or other sequestration of a skeleton in the fossil record. We re-evaluate this “opisthotonic posture hypothesis” by analysing the non-avian theropods Compsognathus longipes and Juravenator starki from the famous South Franconian plattenkalks of the Upper Jurassic Solnhofen Archipelago. Decay experiments with the extant domestic fowl Gallus gallus L. and analysis of the theropods’ constructional morphological constraints reveal that the opisthotonic posture is not a peri- but a postmortem phenomenon. By analysing the timeline of decomposition, it is possible to recognise different stages of decay, depending on the varying decay resistance of soft tissues. Adipocere formation must have blocked further decay until embedding was completed by minimal sedimentation. Analyses of the palaeoenvironment of the basins of the Solnhofen Archipelago show that the conditions of deposition of individual basins cannot be considered to be similar, even inside the same time frame. Therefore, a generalised approach of looking at the depositional setting must be excluded. Assumptions by Faux and Padian (2007) that the accepted palaeoenvironmental reconstruction of the Solnhofen Fossillagerstätte has to be questioned in the light of the opisthotonic posture hypothesis enforce the need for a review of palaeoecological factors of the Franconian Plattenkalks from a taphonomic perspective.

Reference - Reisdorf, A.G., Wuttke, M. (2012). "Re-evaluating Moodie’s Opisthotonic-Posture Hypothesis in Fossil Vertebrates Part I: Reptiles—the taphonomy of the bipedal dinosaurs Compsognathus longipes and Juravenator starki from the Solnhofen Archipelago (Jurassic, Germany)". Palaeobiodiversity and Palaeoenvironments, in press. doi: 10.1007/s12549-011-0068-y.

Two New Specimens of Nemegtomaia Preserving Fossilized Eggs and Nests

Nemegtomaia barsboldi is a species of avian-like oviraptorine manirap- toran theropod, a taxon of which its entire infraorder (Oviraptorosauria) is famous for. All caenagnathoids (let alone oviraptorids) are avire- migians, meaning they possess pennaceous or contour feathers (feathers with a stalk or quill that have a series of "branches" called barbs and barbules) like that of modern birds. As well, fossil evidence of Citipati in brooding position, warming up its eggs, found in 2001 has thereafter led to the discovery of numerous oviraptorosaur specimens taking care of their offspring, even in nests (which went on to infer that the eggs associated with Oviraptor were not those of Protoceratops, but its own).

Recently described from the Gobi Desert's Nemegt Basin of Mongolia are the skeletons of two new Nemegtomaia individuals, which lived there approximately seventy million years ago. The first specimen, MPC-D 107/15, is that of a nest filled with eggs with the assumed parent taking care of them. This specimen consists of the skull, the fore- and hindlimbs (including a hand) and unattached portions of the feet. About two miles to the east of MPC-D 107/15 is another specimen and is represented by both hands and the upper leg bones. It has been cataloged as MPC-D 107/16. It is smaller than the former and may symbolize that this spec- imen was juvenile.

Abstract - Two new specimens of the oviraptorid theropod Nemegtomaia barsboldi from the Nemegt Basin of southern Mongolia are described. Specimen MPC-D 107/15 was collected from the upper beds of the Baruungoyot Formation (Campanian-Maastrichtian), and is a nest of eggs with the skeleton of the assumed parent of Nemegtomaia on top in brooding position. Much of the skeleton was damaged by colonies of dermestid coleopterans prior to its complete burial. However, diagnostic characters are recovered from the parts preserved, including the skull, partial forelimbs (including the left hand), legs, and distal portions of both feet. Nemegtomaia represents the fourth known genus of oviraptorid for which individuals have been found on nests of eggs. The second new specimen, MPC-D 107/16, was collected a few kilometers to the east in basal deposits of the Nemegt Formation, and includes both hands and femora of a smaller Nemegtomaia individual. The two formations and their diverse fossil assemblages have been considered to represent sequential time periods and different environments, but data presented here indicate partial overlap across the Baruungoyot-Nemegt transition. All other known oviraptorids from Mongolia and China are known exclusively from xeric or semi-arid environments. However, this study documents that Nemegtomaia is found in both arid/aeolian (Baruungoyot Formation) and more humid/fluvial (Nemegt Formation) facies.

The two new specimens of Nemegtomaia barsboldi. This diagram represents the fossil material associated with the new fossils MPC-D/107/15-16 in grey. From Fanti, Currie and Badamgarav et al. 2012. Credited to Marco Auditore.

Reference - Fanti, F., Currie, P.J., Badamgarav, D. (2012). "New Specimens of Nemegtomaia from the Baruungoyot and Nemegt Formations (Late Cretaceous) of Mongolia". PLoS ONE, 7(2): e31330. doi: 10.1371/journal.pone.0031330.