Eric Snively

On the Fossil Record of the Gekkota

Gekkota is often interpreted as sister to all remaining squamates, exclusive of dibamids, or as sister to Autarchoglossa. It is the only diverse lineage of primarily nocturnal lizards and includes some of the smallest amniotes. The skeleton of geckos has often been interpreted as paedomorphic and/or “primitive” but these lizards also display a wide range of structural specializations of the postcranium, including modifications associated with both scansorial locomotion and limb reduction. Although the concept of “Gekkota” has been variously applied by different authors, we here apply a rigorous apomorphy based definition, recent advances in gekkotan morphology and phylogenetics, and diverse comparative material to provide a comprehensive assessment of 28 known pre‐Quaternary geckos, updating the last such review, published three decades ago. Fossils evaluated include both sedimentary fossils and amber‐embedded specimens. Known Cretaceous geckos are exclusively Asian and exhibit character combinations not seen in any living forms. Cenozoic gekkotans derive from sites around the world, although Europe is especially well represented. Paleogene geckos are largely known from disarticulated remains and show similarities to Sphaerodactylidae and Diplodactylidae, although resemblances may be plesiomorphic in some cases. Many Neogene gekkotans are referable to living families or even genera, but their geographic occurrences are often extralimital to those of modern groups, as is consistent with paleoclimatic conditions. The phylogenetic placement of fossil gekkotans has important repercusions for timetree calibration, but at present only a small number of fossils can be confidently assigned to even family level groupings, limiting their utility in this regard. Anat Rec, 297:433–462, 2014.

Common Functional Correlates of Head-Strike Behavior in the Pachycephalosaur Stegoceras validum (Ornithischia, Dinosauria) and Combative Artiodactyls

Background Pachycephalosaurs were bipedal herbivorous dinosaurs with bony domes on their heads, suggestive of head-butting as seen in bighorn sheep and musk oxen. Previous biomechanical studies indicate potential for pachycephalosaur head-butting, but bone histology appears to contradict the behavior in young and old individuals. Comparing pachycephalosaurs with fighting artiodactyls tests for common correlates of head-butting in their cranial structure and mechanics. Methods/Principal Findings Computed tomographic (CT) scans and physical sectioning revealed internal cranial structure of ten artiodactyls and pachycephalosaurs Stegoceras validum and Prenocephale prenes. Finite element analyses (FEA), incorporating bone and keratin tissue types, determined cranial stress and strain from simulated head impacts. Recursive partition analysis quantified strengths of correlation between functional morphology and actual or hypothesized behavior. Strong head-strike correlates include a dome-like cephalic morphology, neurovascular canals exiting onto the cranium surface, large neck muscle attachments, and dense cortical bone above a sparse cancellous layer in line with the force of impact. The head-butting duiker Cephalophus leucogaster is the closest morphological analog to Stegoceras, with a smaller yet similarly rounded dome. Crania of the duiker, pachycephalosaurs, and bighorn sheep Ovis canadensis share stratification of thick cortical and cancellous layers. Stegoceras, Cephalophus, and musk ox crania experience lower stress and higher safety factors for a given impact force than giraffe, pronghorn, or the non-combative llama. Conclusions/Significance Anatomy, biomechanics, and statistical correlation suggest that some pachycephalosaurs were as competent at head-to-head impacts as extant analogs displaying such combat. Large-scale comparisons and recursive partitioning can greatly refine inference of behavioral capability for fossil animals.

Multibody dynamics model of head and neck function in Allosaurus (Dinosauria, Theropoda)

We present a multibody dynamics model of the feeding apparatus of the large Jurassic theropod dinosaur Allosaurus that enables testing of hypotheses about the animal’s feeding behavior and about how anatomical parameters influence function. We created CTand anatomical-inference-based models of bone, soft tissue, and air spaces which we use to provide inertial properties for musculoskeletal dynamics. Estimates of bone density have a surprisingly large effect on head inertial properties, and trachea diameter strongly affects moments of inertia of neck segments for dorsoventral movements. The ventrally-placed insertion of m. longissimus capitis superficialis in Allosaurus imparted over twice the ventroflexive accelerations of a proxy control insertion lateral to the occipital condyle, the latter being its position in nearly all other theropods. A feeding style that involved defleshing a carcass by avian-raptor-like retraction of the head in Allosaurus is more probable than is lateroflexive shake-feeding, such as that seen in crocodilians and inferred for tyrannosaurids. Eric Snively. Department of Mechanical Engineering, Russ College of Engineering, 249 Stocker Center, Ohio University, Athens, OH 45701, USA es180210@ohio.edu John R. Cotton. Department of Mechanical Engineering, Russ College of Engineering, 249 Stocker Center, Ohio University, Athens, OH 45701, USA cotton@ohio.edu Ryan Ridgely. Department of Biomedical Sciences, Heritage College of Osteopathic Medicine, Ohio University, Athens, OH 45701, USA ridgely@ohio.edu Lawrence M. Witmer. Department of Biomedical Sciences, Heritage College of Osteopathic Medicine, Ohio University, Athens, OH 45701, USA witmerl@ohio.edu

Bone-Breaking Bite Force of Basilosaurus isis (Mammalia, Cetacea) from the Late Eocene of Egypt Estimated by Finite Element Analysis

Bite marks suggest that the late Eocence archaeocete whale Basilosaurus isis (Birket Qarun Formation, Egypt) fed upon juveniles of the contemporary basilosaurid Dorudon atrox. Finite element analysis (FEA) of a nearly complete adult cranium of B. isis enables estimates of its bite force and tests the animal’s capabilities for crushing bone. Two loadcases reflect different biting scenarios: 1) an intitial closing phase, with all adductors active and a full condylar reaction force; and 2) a shearing phase, with the posterior temporalis active and minimized condylar force. The latter is considered probable when the jaws were nearly closed because the preserved jaws do not articulate as the molariform teeth come into occulusion. Reaction forces with all muscles active indicate that B. isis maintained relatively greater bite force anteriorly than seen in large crocodilians, and exerted a maximum bite force of at least 16,400 N at its upper P3. Under the shearing scenario with minimized condylar forces, tooth reaction forces could exceed 20,000 N despite lower magnitudes of muscle force. These bite forces at the teeth are consistent with bone indentations on Dorudon crania, reatract-and-shear hypotheses of Basilosaurus bite function, and seizure of prey by anterior teeth as proposed for other archaeocetes. The whale’s bite forces match those estimated for pliosaurus when skull lengths are equalized, suggesting similar tradeoffs of bite function and hydrodynamics. Reaction forces in B. isis were lower than maxima estimated for large crocodylians and carnivorous dinosaurs. However, comparison of force estimates from FEA and regression data indicate that B. isis exerted the largest bite forces yet estimated for any mammal, and greater force than expected from its skull width. Cephalic feeding biomechanics of Basilosaurus isis are thus consistent with habitual predation.

The Tarsometatarsus of the Ostrich Struthio camelus: Anatomy, Bone Densities, and Structural Mechanics.

Background The ostrich Struthio camelus reaches the highest speeds of any extant biped, and has been an extraordinary subject for studies of soft-tissue anatomy and dynamics of locomotion. An elongate tarsometatarsus in adult ostriches contributes to their speed. The internal osteology of the tarsometatarsus, and its mechanical response to forces of running, are potentially revealing about ostrich foot function. Methods/Principal Findings Computed tomography (CT) reveals anatomy and bone densities in tarsometatarsi of an adult and a young juvenile ostrich. A finite element (FE) model for the adult was constructed with properties of compact and cancellous bone where these respective tissues predominate in the original specimen. The model was subjected to a quasi-static analysis under the midstance ground reaction and muscular forces of a fast run. Anatomy–Metatarsals are divided proximally and distally and unify around a single internal cavity in most adult tarsometatarsus shafts, but the juvenile retains an internal three-part division of metatarsals throughout the element. The juvenile has a sparsely ossified hypotarsus for insertion of the m. fibularis longus, as part of a proximally separate third metatarsal. Bone is denser in all regions of the adult tarsometatarsus, with cancellous bone concentrated at proximal and distal articulations, and highly dense compact bone throughout the shaft. Biomechanics–FE simulations show stress and strain are much greater at midshaft than at force applications, suggesting that shaft bending is the most important stressor of the tarsometatarsus. Contraction of digital flexors, inducing a posterior force at the TMT distal condyles, likely reduces buildup of tensile stresses in the bone by inducing compression at these locations, and counteracts bending loads. Safety factors are high for von Mises stress, consistent with faster running speeds known for ostriches. Conclusions/Significance High safety factors suggest that bone densities and anatomy of the ostrich tarsometatarsus confer strength for selectively critical activities, such as fleeing and kicking predators. Anatomical results and FE modeling of the ostrich tarsometatarsus are a useful baseline for testing the structure’s capabilities and constraints for locomotion, through ontogeny and the full step cycle. With this foundation, future analyses can incorporate behaviorally realistic strain rates and distal joint forces, experimental validation, and proximal elements of the ostrich hind limb.

FINITE ELEMENT COMPARISON OF CRANIAL SINUS FUNCTION IN THE DINOSAUR MAJUNGASAURUS AND HEAD-CLUBBING GIRAFFES

Majungasaurus crenatissimus is a spectacularly preserved carnivorous dinosaur from latest Cretaceous Madagascar. Computed tomographic (CT) scans reveal unusual internal anatomy of the dinosaur’s cranium [1,2; Figure 1]: the nasals form a large hollow chamber traversed with bony struts, and a unicorn-like projection of the frontals is also hollow. The wall thickness and struts within these sinuses recall sinuses of giraffes, which strike each other with a median projection (ossicone) above a frontal sinus and lateral ossicones of the parietals [3]. Giraffe-like cranial sinuses, and large attachments for neck muscles [4], raise the hypothesis that Majungasaurus could engage in giraffe-like head strikes to each other’s necks and flanks.Copyright

Unexpected behavior in the Cretaceous: tooth-marked bones attributable to tyrannosaur play. A comment by E. Snively & T. Samman

Tyrannosaur bite marks on several bones, including ceratopsian (horned dinosaur) occipital condyles scattered in bone beds, have recently been attributed to play behavior. In the cited cases, play was considered a more likely behavior than feeding, because the bones had been isolated and dissociated from other skeletal elements on the surface before final burial, and in this condition there would be little if any remaining meat. Although the hypothesis of play is difficult to falsify or to support specifically, the evidence is reasonable that the tyrannosaurs were engaged in activity without immediate selective consequences (common in their modern relatives). We explore alternate hypotheses, and further questions that this research raises. Are the bite marks from juvenile or adult tyrannosaurs? The latter’s mouths appear large for manipulating ceratopsian condyles, and neck biomechanics suggests that maximal-effort play (throwing the condyles) would require hardly noticeable exertion. How strongly can we infer play in extinct dinosaurs by activities in their extant phylogenetic bracket (birds and crocodilians), considering that play behaviors are taxon-specific? We suggest that physiological bracketing of neurotransmitter activity would bolster the general inference of possible play in large dinosaurs. Results for humans reporting ‘fun’ during play would be a baseline for evident activity of neurochemical reward systems in extant reptiles. Such testing is perhaps impossible, but points to the heuristic value of intriguing interpretations from behavioral trace evidence in fossils. Literature on animal play documents evident play-like behavior in birds and crocodilians, the extant phylogenetic bracket for inferring behavior in Mesozoic dinosaurs (BRYANT & RUSSELL 1992; WITMER 1995; ISLES 2009). There are three kinds of play – locomotor/activity, object and social (BURGHART 2005). Birds exhibit all three types (FICKEN 1977), and crocodilians engage in object play (LAZELL & SPITZER 1977). Parsimoniously, we can infer the possibility of object play in extinct dinosaurs possessing the brain sophistication of crocodilians. ROTHSCHILD (2015) infers object play in tyrannosaurids based on tooth marks. This inference is abductive (i.e., generating an hypothesis that accounts better than others for the reliable data or observation, perhaps more than deductive sensu ROTHSCHILD [2015]): contradiction of alternative hypotheses winnows interpretations down to play. Bites on ostensibly nutrition-poor bones such as phalanges and ceratopsian occipital condyles argue against feeding behavior. Play, although a high-order inference, appears to be the logical conclusion. This intellectual exercise is valuable, necessary and rigorous because it stimulates the formulation of both (1) alternate answers and, as importantly, (2) alternate questions and means of testing inferences of dinosaur play. Ethology Ecology & Evolution, 2015 Vol. 27, No. 4, 422–427, http://dx.doi.org/10.1080/03949370.2014.984346