Cranial Growth and Variation in Edmontosaurs (Dinosauria: Hadrosauridae): Implications for Latest Cretaceous Megaherbivore Diversity in North America

The well-sampled Late Cretaceous fossil record of North America remains the only high-resolution dataset for evaluating patterns of dinosaur diversity leading up to the terminal Cretaceous extinction event. Hadrosaurine hadrosaurids (Dinosauria: Ornithopoda) closely related to Edmontosaurus are among the most common megaherbivores in latest Campanian and Maastrichtian deposits of western North America. However, interpretations of edmontosaur species richness and biostratigraphy have been in constant flux for almost three decades, although the clade is generally thought to have undergone a radiation in the late Maastrichtian. We address the issue of edmontosaur diversity for the first time using rigorous morphometric analyses of virtually all known complete edmontosaur skulls. Results suggest only two valid species, Edmontosaurus regalis from the late Campanian, and E. annectens from the late Maastrichtian, with previously named taxa, including the controversial Anatotitan copei, erected on hypothesized transitional morphologies associated with ontogenetic size increase and allometric growth. A revision of North American hadrosaurid taxa suggests a decrease in both hadrosaurid diversity and disparity from the early to late Maastrichtian, a pattern likely also present in ceratopsid dinosaurs. A decline in the disparity of dominant megaherbivores in the latest Maastrichtian interval supports the hypothesis that dinosaur diversity decreased immediately preceding the end Cretaceous extinction event

On the estimation of body mass in temnospondyls: a case study using the large-bodied Eryops and Paracyclotosaurus

Temnospondyli are a morphologically varied and ecologically diverse clade of tetrapods that survived for over 200 million years. The body mass of temnospondyls is a key variable in inferring their ecological, physiological and biomechanical attributes. However, estimating the body mass of these extinct creatures has proven difficult because the group has no extant descendants. Here we apply a wide range of body mass estimation techniques developed for tetrapods to the iconic temnospondyls Paracyclotosaurus davidi and Eryops megacephalus. These same methods are also applied to a collection of extant organisms that serve as ecological and morphological analogues. These include the giant salamanders Andrias japonicus and Andrias davidianus, the tiger salamander Ambystoma tigrinum, the California newt Taricha torosa and the saltwater crocodile, Crocodylus porosus. We find that several methods can provide accurate mass estimations across this range of living taxa, suggesting their suitability for estimating the body masses of temnospondyls. Based on this, we estimate the mass of Paracyclotosaurus to have been between 159 and 365 kg, and that of Eryops between 102 and 222 kg. These findings provide a basis for examining body size evolution in this clade across their entire temporal span

The dentary of hadrosauroid dinosaurs: evolution through heterochrony

The near-global distribution of hadrosaurid dinosaurs during the Cretaceous has been attributed to mastication, a behaviour commonly recognized as a mammalian adaptation. Its occurrence in a non-mammalian lineage should be accompanied by the evolution of several morphological modifications associated with food acquisition and processing. This study investigated morphological variation in the dentary, a major element of the hadrosauroid lower jaw. Eighty-four hadrosauroid dentaries were subjected to geometric morphometric and statistical analyses to investigate their taxonomic, ontogenetic, and individual variation. Results suggest increased food acquisition and processing efficiency in saurolophids through a complex pattern of evolutionary and growth-related changes. The edentulous region grew longer relative to dentary length, allowing for food acquisition specialization anteriorly and processing posteriorly, and became ventrally directed, possibly associated with foraging low-growing vegetation, especially in younger individuals. The saurolophid coronoid process became anteriorly directed and relatively more elongate, with an expanded apex, increasing moment arm length, with muscles pulling the jaw more posteriorly, increasing mechanical advantage. During growth, all hadrosauroids underwent anteroposterior dental battery elongation by the addition of teeth, and edentulous region ventralization decreased. The dental battery became deeper in saurolophids by increasing the number of teeth per tooth family. The increased coronoid process anterior inclination and relative edentulous region elongation in saurolophids are hypothesized to have evolved through hypermorphosis and/or acceleration, peramorphic heterochronic processes; the development of an anteroposteriorly shorter but dorsoventrally taller saurolophid dentary, is probably due to post-displacement in dental battery elongation and edentulous region decreased ventral orientation, a paedomorphic heterochronic process

Extrapolating body masses in large terrestrial vertebrates

Despite more than a century of interest, body-mass estimation in the fossil record remains contentious, particularly when estimating the body mass of taxa outside the size scope of living animals. One estimation approach uses humeral and femoral (stylopodial) circumferences collected from extant (living) terrestrial vertebrates to infer the body masses of extinct tetrapods through scaling models. When applied to very large extinct taxa, extant-based scaling approaches incur obvious methodological extrapolations leading some to suggest that they may overestimate the body masses of large terrestrial vertebrates. Here, I test the implicit assumption of such assertions: that a quadratic model provides a better fit to the combined humeral and femoral circumferences-to-body mass relationship. I then examine the extrapolation potential of these models through a series of subsetting exercises in which lower body-mass sets are used to estimate larger sets. Model fitting recovered greater support for the original linear model, and a nonsignificant second-degree term indicates that the quadratic relationship is statistically linear. Nevertheless, some statistical support was obtained for the quadratic model, and application of the quadratic model to a series of dinosaurs provides lower mass estimates at larger sizes that are more consistent with recent estimates using a minimum convex-hull (MCH) approach. Given this consistency, a quadratic model may be preferred at this time. Still, caution is advised; extrapolations of quadratic functions are unpredictable compared with linear functions. Further research testing the MCH approach (e.g., the use of a universal upscaling factor) may shed light on the linear versus quadratic nature of the relationship between the combined femoral and humeral circumferences and body mass

Postcranial Anatomy of Edmontosaurus regalis (Hadrosauridae) from the Horseshoe Canyon Formation, Alberta, Canada

Edmontosaurus regalis is common in the latest Campanian deposits of Alberta, Canada, and is known from numerous articulated cranial specimens and isolated bonebed material. Despite its large sample size and long research history, only the cranial skeleton has been described in detail with little reference to postcrania, the majority of which remained unprepared at the time of its original description. This study presents the first full description of the postcrania of E. regalis based on CMN 2289, the paratype of the species. In addition, this study presents the most completely figured postcranial skeleton of a hadrosaurid to date. CMN 2289 preserves an almost complete dorsal vertebral column, along with a complete pelvis and almost complete set of limbs, which allows for comparisons with other hadrosaurids. Quantitative analyses of dorsal vertebral morphology reveal important variations within a single individual, with implications for phylogenetic character construction. There is an overall increase in the centrum height and width along the dorsal series, compared to an overall decrease in centrum length. A high amount of variation is noted in spine height along the dorsal series. Limb proportions in CMN 2289 between the distal and proximal limb segments, as well as between the forelimb and hindlimb, are similar to other hadrosaurids and suggest little ontogenetic variation. A revised body mass estimate of E. regalis is presented at 7963 kg, twice that proposed by previous studies and similar in size to its sister taxon, E. annectens

Data from: Extrapolating body masses in large terrestrial vertebrates

Despite more than a century of interest, body-mass estimation in the fossil record remains contentious, particularly when estimating the body mass of taxa outside the size scope of living animals. One estimation approach uses humeral and femoral (stylopodial) circumferences collected from extant (living) terrestrial vertebrates to infer the body masses of extinct tetrapods through scaling models. When applied to very large extinct taxa, extant-based scaling approaches incur obvious methodological extrapolations leading some to suggest that they may overestimate the body masses of large terrestrial vertebrates. Here, I test the implicit assumption of such assertions: that a quadratic model provides a better fit to the combined humeral and femoral circumferences-to-body mass relationship. I then examine the extrapolation potential of these models through a series of subsetting exercises in which lower body-mass sets are used to estimate larger sets. Model fitting recovered greater support for the original linear model, and a nonsignificant second-degree term indicates that the quadratic relationship is statistically linear. Nevertheless, some statistical support was obtained for the quadratic model, and application of the quadratic model to a series of dinosaurs provides lower mass estimates at larger sizes that are more consistent with recent estimates using a minimum convex-hull (MCH) approach. Given this consistency, a quadratic model may be preferred at this time. Still, caution is advised; extrapolations of quadratic functions are unpredictable compared with linear functions. Further research testing the MCH approach (e.g., the use of a universal upscaling factor) may shed light on the linear versus quadratic nature of the relationship between the combined femoral and humeral circumferences and body mass

MASSTIMATE: Body Mass Estimation Equations for Vertebrates

‘MASSTIMATE’ is an R software package that implements a variety of body mass estimation equations derived from linear models and, generally, used to estimate the body masses of fossil vertebrates. Estimation equations are from a variety of sources but are normally based on regressions between skeletal measurements (e.g., femoral circumference) and body mass in living taxa. The package is freely available on CRAN: https://CRAN.R-project.org/package=MASSTIMATE and is occasionally updated as new research and equations are presented

Body size correlates with discrete-character morphological proxies

Principal coordinates analysis (PCoA) is a statistical ordination technique commonly applied to morphology-based cladistic matrices to study macroevolutionary patterns, morphospace occupation, and disparity. However, PCoA-based morphospaces are dissociated from the original data; therefore, whether such morphospaces accurately reflect body-plan disparity or extrinsic factors, such as body size, remains uncertain. We collated nine character–taxon matrices of dinosaurs together with body-mass estimates for all taxa and tested for relationships between body size and both the principal axis of variation (i.e., PCo1) and the entire set of PCo scores. The possible effects of body size on macroevolutionary hypotheses derived from ordinated matrices were tested by reevaluating evidence for the accelerated accumulation of avian-type traits indicated by a strong directional shift in PCo1 scores in hypothetical ancestors of modern birds. Body mass significantly accounted for, on average, approximately 50% and 16% of the phylogenetically corrected variance in PCo1 and all PCo scores, respectively. Along the avian stem lineage, approximately 30% of the morphological variation is attributed to the reconstructed body masses of each ancestor. When the effects of body size are adjusted, the period of accelerated trait accumulation is replaced by a more gradual, additive process. Our results indicate that even at low proportions of variance, body size can noticeably affect macroevolutionary hypotheses generated from ordinated morphospaces. Future studies should thoroughly explore the nature of their character data in association with PCoA-based morphospaces and use a residual/covariate approach to account for potential correlations with body size

The anatomy and homologies of the ceratopsid syncervical

The Ceratopsidae and all other members of the Ceratopsia, except Psittacosaurus and Hongshanosaurus (Family: Psittacosauridae), comprise the order of Neoceratopsia (sensu Sereno, 2000). All members of this group can be distinguished from psittacosaurids and other members of the Ornithischia by fusion of the anterior-most vertebrae (Fig. 1). The resulting structure is called the syncervical (Ostrom and Wellnhofer, 1986) or cervical bar (Langston, 1975). This fusion probably occurred in order to stabilize the neck region so that it could support the relatively larger heads characteristic of neoceratopsians (Hatcher et al., 1907)

Taphonomy of the Danek Bonebed: a monodominant Edmontosaurus (Hadrosauridae) bonebed from the Horseshoe Canyon Formation, Alberta

The Danek Bonebed (Horsethief Member, Horseshoe Canyon Formation, Late Campanian) is dominated by the remains of at least 12 Edmontosaurus regalis. Skeletal remains of a tyrannosaurid and ceratopsid are also known. The predominantly disarticulated remains were interred on a periodically inundated floodplain and, although the cause of death is unknown, a sudden, catastrophic death explains the demographic spread, faunal diversity, rare greenstick fractures, and homogeneous weathering/abrasion categories of the assemblage. The Danek Bonebed shares a similar taphonomic signature to the Liscomb Bonebed (Prince Creek Formation, Alaska), but it is unique among all other described hadrosaurid bonebeds in the unusually high proportion of bite-marked bones (∼30%), suggesting scavenging played a major role in the reworking of the assemblage. The highest frequency of bite marks is found on small, often unidentifiable (and commonly ignored) bone fragments, underscoring the role that such fragments can play in taphonomic interpretation. Finally, the recognition of E. regalis from central Alberta is an important datum linking contemporaneous occurrences in southern Alberta with slightly older records of this species from the Wapiti Formation in northwestern Alberta