Calvarial suture interdigitation in hadrosaurids (Ornithischia: Ornithopoda): Perspectives through ontogeny and evolution

Lambeosaurine hadrosaurids exhibited extreme modifications to the skull, where the premaxillae, nasals, and prefrontals were modified to form their iconic supracranial crests. This morphology contrasts with their sister group, Hadrosaurinae, which possessed the plesiomorphic arrangement of bones. Although studies have discussed differences between lambeosaurine and hadrosaurine skull morphology and ontogeny, there is little information detailing suture modifications through ontogeny and evolution. Suture morphology is of particular interest due to its correlation with the mechanical loading of the skull in extant vertebrates. We quantify and contrast the morphology of calvarial sutures in iguanodontians and ontogenetic series of Corythosaurus and Gryposaurus to test whether the evolution of lambeosaurine crests impacted the mechanical loading of the skull. We found that suture interdigitation (SI) increases through ontogeny in hadrosaurids, although this increase is more extreme in Corythosaurus than Gryposaurus, and overall suture complexity (i.e., overall shape) remained constant. Lambeosaurines also have higher SI than other iguanodontians, even in crestless juveniles, suggesting that increased sinuosity is unrelated to the structural support of the crest. Hadrosaurines and basal iguanodontians did not differ. Similarly, lambeosaurines have more complexly shaped sutures than hadrosaurines and basal iguanodontians, while the latter two groups do not differ. Taken together, these results suggest that lambeosaurine calvarial sutures are more interdigitated than other iguanodontians, and although suture sinuosity increased through ontogeny, the suture shape remained constant. These ontogenetic and evolutionary patterns suggest that increased suture complexity in lambeosaurines coincided with crest evolution, and corresponding modifications to their facial skeleton altered the distribution of stress while feeding.

Endocasts of ornithopod dinosaurs: Comparative anatomy

Ornithopod dinosaurs were a successful group before they became extinct at the end of the Cretaceous. They were present on every continent, though they were rare in the Southern Hemisphere. We present the results of our work on the brain of these dinosaurs as an attempt to determine which evolutionary trends affected it. Old and new technologies allow us to peer into the skull of long extinct animals and retrieve information about their brain. First we provide a short description of the brain of ornithopod dinosaurs from Europe and Asia, then we sum up the characteristics that can be gathered from it. The presence of valleculae helps us to assess the actual size of the brain with more confidence. The olfactory peduncles are large and these animals had a good sense of smell. There is a trend toward an increase in the size of the cerebral hemispheres, and a more straight-lined brain. The latter can be the result of the ontogeny and the size achieved by the adult animal on the development of the brain. Other characteristics, like the development of the cerebral hemispheres and the encephalization quotient, allude to Hadrosauridae having had cognitive abilities more developed than previously assumed. This is in adequacy with other data from the physical characteristics (e.g., crests) and the social life (e.g., living in herds, communal nests) of these dinosaurs, which denote high and complex behaviors like care for their young, sexual courtship, and gregariousness.

Rare evidence for 'gnawing-like' behavior in a small-bodied theropod dinosaur

Mammalian carnivores show a higher degree of prey bone utilization relative to non-avian theropod dinosaurs, with this major ecological difference reflected in the frequency and morphology of tooth marks in modern and Cenozoic assemblages relative to Mesozoic ones. As such, prey bone utilization (i.e., gnawing, bone-breaking, osteophagy) may represent a key ecological strategy repeatedly exploited by mammalian carnivores but rarely in theropod dinosaurs. Here we describe an isolated adult-sized hadrosaurid pedal ungual (III-4) from the Dinosaur Park Formation (Campanian) of southern Alberta which shows a unique pattern of bite marks from a small- to medium-sized theropod dinosaur. Thirteen distinct tooth marks occur in a restricted area of the ungual, and the pattern suggests up to six repeated, high-power bites delivered to the bone. The tracemaker cannot be definitively identified, but was likely a dromaeosaurid or very young tyrannosaurid. Tooth marks on at least four other Dinosaur Park Formation hadrosaur pedal unguals are reported, but the overall frequency of occurrence in unguals (< 1%) is much lower than that reported for other bones. The pattern of tooth marks on this specimen deviates from most described theropods tooth marks, and given the low volume of meat associated with the ungual, may represent theropod prey bone utilization as part of late-stage carcass consumption, and a behavior similar to mammalian gnawing.

Deep-time biodiversity patterns and the dinosaurian fossil record of the Late Cretaceous Western Interior, North America

In order for palaeontological data to be informative to ecologists seeking to understand the causes of today's diversity patterns, palaeontologists must demonstrate that actual biodiversity patterns are preserved in our reconstructions of past ecosystems. During the Late Cretaceous, North America was divided into two landmasses, Laramidia and Appalachia. Previous work has suggested strong faunal provinciality on Laramidia at this time, but these arguments are almost entirely qualitative. We quantitatively investigated faunal provinciality in ceratopsid and hadrosaurid dinosaurs using a biogeographic network approach and investigated sampling biases by examining correlations between dinosaur occurrences and collections. We carried out a model-fitting approach using generalized least-squares regression to investigate the sources of sampling bias we identified. We find that while the raw data strongly support faunal provinciality, this result is driven by sampling bias. The data quality of ceratopsids and hadrosaurids is currently too poor to enable fair tests of provincialism, even in this intensively sampled region, which probably represents the best-known Late Cretaceous terrestrial ecosystem on Earth. To accurately reconstruct biodiversity patterns in deep time, future work should focus on smaller scale, higher resolution case studies in which the effects of sampling bias can be better controlled.

Taphonomy and taxonomy of a juvenile lambeosaurine (Ornithischia: Hadrosauridae) bonebed from the late Campanian Wapiti Formation of northwestern Alberta, Canada

Hadrosaurid (duck-billed) dinosaur bonebeds are exceedingly prevalent in upper Cretaceous (Campanian–Maastrichtian) strata from the Midwest of North America (especially Alberta, Canada, and Montana, U.S.A) but are less frequently documented from more northern regions. The Wapiti Formation (Campanian–Maastrichtian) of northwestern Alberta is a largely untapped resource of terrestrial palaeontological information missing from southern Alberta due to the deposition of the marine Bearpaw Formation. In 2018, the Boreal Alberta Dinosaur Project rediscovered the Spring Creek Bonebed, which had been lost since 2002, along the northern bank of the Wapiti River, southwest of Grande Prairie. Earlier excavations and observations of the Spring Creek Bonebed suggested that the site yielded young hadrosaurines. Continued work in 2018 and 2019 recovered ~300 specimens that included a minimum of eight individuals, based on the number of right humeri. The morphology of several recovered cranial elements unequivocally supports lambeosaurine affinities, making the Spring Creek sample the first documented occurrence of lambeosaurines in the Wapiti Formation. The overall size range and histology of the bones found at the site indicate that these animals were uniformly late juveniles, suggesting that age segregation was a life history strategy among hadrosaurids. Given the considerable size attained by the Spring Creek lambeosaurines, they were probably segregated from the breeding population during nesting or caring for young, rather than due to different diet and locomotory requirements. Dynamic aspects of life history, such as age segregation, may well have contributed to the highly diverse and cosmopolitan nature of Late Cretaceous hadrosaurids.

A new brachylophosaurin (Dinosauria: Hadrosauridae) from the Upper Cretaceous Menefee Formation of New Mexico

Brachylophosaurini is a clade of hadrosaurid dinosaurs from the Campanian of western North America. Although well-known from northern localities in Montana and Alberta, including abundant material of Brachylophosaurus canadensis and Maiasaura peeblesorum and the holotypes of Acristavus gagslarsoni and Probrachylophosaurus bergei, material from southern localities in Utah and Colorado is restricted to a partial skull referred to A. gagslarsoni and several indeterminate specimens. Here we describe Ornatops incantatus gen. et sp. nov., a new brachylophosaurin known from a partial skeleton from the Allison Member of the Menefee Formation in New Mexico. Ornatops is the first brachylophosaurin reported from New Mexico and the southernmost occurrence of the clade. Ornatops shares with Probrachylophosaurus and Brachylophosaurus a caudally expanded nasofrontal suture on the frontals, but also exhibits an autapomorphic nasofrontal suture morphology, with a horizontal rostral region and elevated caudal region with two prominent parasagittal bumps, which is different from other brachylophosaurin specimens, including juvenile and adult Brachylophosaurus. A phylogenetic analysis places Ornatops in a trichotomy with Probrachylophosaurus and Brachylophosaurus, with Maiasaura and Acristavus as successive outgroups.

Ontogeny in the tube-crested dinosaur Parasaurolophus (Hadrosauridae) and heterochrony in hadrosaurids

The tube-crested hadrosaurid dinosaur Parasaurolophus is remarkable for its unusual cranial ornamentation, but little is known about its growth and development, particularly relative to well-documented ontogenetic series for lambeosaurin hadrosaurids (such as Corythosaurus, Lambeosaurus, and Hypacrosaurus). The skull and skeleton of a juvenile Parasaurolophus from the late Campanian-aged (∼75.5 Ma) Kaiparowits Formation of southern Utah, USA, represents the smallest and most complete specimen yet described for this taxon. The individual was approximately 2.5 m in body length (∼25% maximum adult body length) at death, with a skull measuring 246 mm long and a femur 329 mm long. A histological section of the tibia shows well-vascularized, woven and parallel-fibered primary cortical bone typical of juvenile ornithopods. The histological section revealed no lines of arrested growth or annuli, suggesting the animal may have still been in its first year at the time of death. Impressions of the upper rhamphotheca are preserved in association with the skull, showing that the soft tissue component for the beak extended for some distance beyond the limits of the oral margin of the premaxilla. In marked contrast with the lengthy tube-like crest in adult Parasaurolophus, the crest of the juvenile specimen is low and hemicircular in profile, with an open premaxilla-nasal fontanelle. Unlike juvenile lambeosaurins, the nasal passages occupy nearly the entirety of the crest in juvenile Parasaurolophus. Furthermore, Parasaurolophus initiated development of the crest at less than 25% maximum skull size, contrasting with 50% of maximum skull size in hadrosaurs such as Corythosaurus. This early development may correspond with the larger and more derived form of the crest in Parasaurolophus, as well as the close relationship between the crest and the respiratory system. In general, ornithischian dinosaurs formed bony cranial ornamentation at a relatively younger age and smaller size than seen in extant birds. This may reflect, at least in part, that ornithischians probably reached sexual maturity prior to somatic maturity, whereas birds become reproductively mature after reaching adult size.