Mark B. Goodwin

Extreme Cranial Ontogeny in the Upper Cretaceous Dinosaur Pachycephalosaurus

Background Extended neoteny and late stage allometric growth increase morphological disparity between growth stages in at least some dinosaurs. Coupled with relatively low dinosaur density in the Upper Cretaceous of North America, ontogenetic transformational representatives are often difficult to distinguish. For example, many hadrosaurids previously reported to represent relatively small lambeosaurine species were demonstrated to be juveniles of the larger taxa. Marginocephalians (pachycephalosaurids + ceratopsids) undergo comparable and extreme cranial morphological change during ontogeny. Methodology/Principal Findings Cranial histology, morphology and computer tomography reveal patterns of internal skull development that show the purported diagnostic characters for the pachycephalosaurids Dracorex hogwartsia and Stygimoloch spinifer are ontogenetically derived features. Coronal histological sections of the frontoparietal dome of an adult Pachycephalosaurus wyomingensis reveal a dense structure composed of metaplastic bone with a variety of extremely fibrous and acellular tissue. Coronal histological sections and computer tomography of a skull and frontoparietal dome of Stygimoloch spinifer reveal an open intrafrontal suture indicative of a subadult stage of development. These dinosaurs employed metaplasia to rapidly grow and change the size and shape of their horns, cranial ornaments and frontoparietal domes, resulting in extreme cranial alterations during late stages of growth. We propose that Dracorex hogwartsia, Stygimoloch spinifer and Pachycephalosaurus wyomingensis are the same taxon and represent an ontogenetic series united by shared morphology and increasing skull length. Conclusions/Significance Dracorex hogwartsia (juvenile) and Stygimoloch spinifer (subadult) are reinterpreted as younger growth stages of Pachycephalosaurus wyomingensis (adult). This synonymy reduces the number of pachycephalosaurid taxa from the Upper Cretaceous of North America and demonstrates the importance of cranial ontogeny in evaluating dinosaur diversity and taxonomy. These growth stages reflect a continuum rather than specific developmental steps defined by “known” terminal morphologies.

Texture analysis from synchrotron diffraction images with the Rietveld method: dinosaur tendon and salmon scale.

A Rietveld method is described which extracts information on crystal structure, texture and microstructure directly from two-dimensional synchrotron diffraction images. This is advantageous over conventional texture analysis that relies on individual diffraction peaks, particularly for low-symmetry materials with many overlapping peaks and images with a poor peak-to-background ratio. The method is applied to two mineralized biological samples with hydroxylapatite fabrics: an ossified pachycephalosaurid dinosaur tendon and an Atlantic salmon scale. Both are measured using monochromatic synchrotron X-rays. The dinosaur tendon has very strongly oriented crystals with c-axes parallel to the tendon direction. The salmon scale displays a weak texture.

Cretaceous Extinctions: Multiple Causes

![Figure][1] Deccan plateau basalts. Lava from Deccan volcanism formed distinct layering. CREDIT: GSFC/NASA In the Review “The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene boundary” (P. Schulte et al. , 5 March, p. [1214][2]), the terminal Cretaceous

Major cranial changes during Triceratops ontogeny

This is the first cranial ontogenetic assessment of Triceratops, the well-known Late Cretaceous dinosaur distinguished by three horns and a massive parietal–squamosal frill. Our analysis is based on a growth series of 10 skulls, ranging from a 38 cm long baby skull to about 2 m long adult skulls. Four growth stages correspond to a suite of ontogenetic characters expressed in the postorbital horns, frill, nasal, epinasal horn and epoccipitals. Postorbital horns are straight stubs in early ontogeny, curve posteriorly in juveniles, straighten in subadults and recurve anteriorly in adults. The posterior margin of the baby frill is deeply scalloped. In early juveniles, the frill margin becomes ornamented by 17–19 delta-shaped epoccipitals. Epoccipitals are dorsoventrally compressed in subadults, strongly compressed and elongated in adults and ultimately merge onto the posterior frill margin in older adults. Ontogenetic trends within and between growth stages include: posterior frill margin transitions from scalloped to wavy and smooth; progressive exclusion of the supraoccipital from the foramen magnum; internal hollowing at the base of the postorbital horns; closure of the midline nasal suture; fusion of the epinasal onto the nasals; and epinasal expansion into a morphologically variable nasal horn. We hypothesize that the changes in horn orientation and epoccipital shape function to allow visual identity of juveniles, and signal their attainment of sexual maturity.

Cranial histology of pachycephalosaurs (Ornithischia: Marginocephalia) reveals transitory structures inconsistent with head-butting behavior

Abstract Modern histological techniques allow paleontologists to investigate the internal microstructure of bone tissue. We apply high resolution images of histological thin sections from an ontogenetic series (not conspecific) of pachycephalosaurid frontoparietal domes to test the hypothesis that these Late Cretaceous dinosaurs used their heads as battering rams, analogous to the behavior of the bighorn sheep, Ovis canadensis, or as a thermoregulatory device. Our analysis reveals that the internal structure of the pachycephalosaur dome is a dynamic tissue that reflects the changeable expansion and vascularity of the dome throughout ontogeny. The radiating structures within the frontoparietal dome, used previously to support “head-butting” hypotheses, are unexpectedly transitory, diminishing in mature individuals and nearly absent in adult skulls where head-butting behavior is presumed to occur. The unique architecture of the pachycephalosaurid dome is dividable into three distinct Zones. We demonstrate that the relative vascularity, associated tissue structures, and orientation and density of Sharpeys fibers within these Zones are modified during growth. Evidence for an external dome covering in vivo precludes the determination of the final shape of the pachycephalosaur skull. On the basis of these new observations, we propose that cranial display in support of species recognition and communication is a more parsimonious interpretation of the function of the pachycephalosaurid dome. Sexual display behaviors were probably secondary.

Mesozoic continental vertebrates with associated palynostratigraphic dates from the northwestern Ethiopian plateau

The East African Rift separates the northwestern and southeastern Ethiopian high plateaus, which are capped by massive Cenozoic volcanics overlying thick deposits of marine and nonmarine Mesozoic sediments. During geological mapping projects of the 1920s-1930s, a few Mesozoic vertebrate fossils were found on the southeastern plateau. in contrast, beginning in 1976, and then from 1993 to the present, paleontological field work in the Abay (Blue Nile) River gorge along the eastern edge of the northwestern plateau resulted in the discovery of fossil chondrichthyans (Priohybodus, Hybodus, Rhinobatos), osteichthyans (Lepidotes, cf. Pycnodus), dipnoans (Asiatoceratodus), chelonians (Pelomedusidae, Plesiochelyidae, Pleurosternidae), crocodylians (Goniopholis), dinosaurs, (cf. Acrocanthosaurus Hypsilophodontidae), pollen and other microfossils documenting a coastal biota in part, if not entirely, of latest Jurassic (Tithonian) age. These fossils include new biogeographic records for Africa and document biostratigraphic range extensions. The Ethiopian Mesozoic fauna adds to the growing evidence of limited interchange of vertebrates between Africa and Western Europe during the transition from the Jurassic into the Cretaceous.

THE SMALLEST KNOWN TRICERATOPS SKULL: NEW OBSERVATIONS ON CERATOPSID CRANIAL ANATOMY AND ONTOGENY

Abstract The discovery of the smallest Triceratops skull (UCMP 154452) provides a new ontogenetic end member for the earliest stage of ceratopsid (Centrosaurinae plus Chasmosaurinae) cranial development. The lack of co-ossification among the parietal, squamosals, postorbitals, quadratojugal arch, and the braincase preserves sutural contacts and bone surfaces that later become obscured in adults. The ability to document the early development and morphology of the horns and frill in Triceratops allows a reevaluation of their functional roles. UCMP 154452 shows that the cranial ornamentation of the frill and the postorbital horns were not restricted to adults, but began at an early age in this species. This evidence supports the hypothesis that the function of ceratopsid horns and frills was potentially important for visual communication and species recognition because in this young form it could not have functioned in sexual display. Although some features of UCMP 154452 anticipate or mimic the adult character states, some braincase characters recapitulate the juvenile and adult stages of more basal neoceratopsians.

ONTOGENY OF CRANIAL EPI-OSSIFICATIONS IN TRICERATOPS

Abstract Historically, the scarcity of non-adult Triceratops fossils collected from Upper Cretaceous sediments of North America limited our understanding and promoted controversy with regard to the morphology, and presence or absence of cranial epi-ossifications in this widely known horned dinosaur. The recent discovery of several exceptionally well preserved juvenile and subadult Triceratops skulls and numerous juvenile, subadult, and adult cranial elements, from the Hell Creek Formation of eastern Montana, confirms the ontogeny and morphology of epi-ossifications in this study. We propose to standardize the terminology for these four cranial epi-ossifications: epinasal, epijugal, epiparietal, and episquamosal. We describe the ontogeny and timing of the fusion of each of these epi-ossifications and the rostral from a cranial growth series. Although the timing is variable, the epinasal fuses first, followed by the rostal, the epijugals, the episquamosals, and lastly by the epiparietals. Co-ossification of the epinasal, rostral and epijugals unites several of the anterior (rostral-nasal-premaxillae) and lateral (jugal-quadratojugal) skull elements. In combination with forward directed postorbital horns and a massive fan-shaped frill, cranial epi-ossifications may have enhanced visual display and species communication in Triceratops.

Dinosaur Census Reveals Abundant Tyrannosaurus and Rare Ontogenetic Stages in the Upper Cretaceous Hell Creek Formation (Maastrichtian), Montana, USA

BACKGROUND A dinosaur census recorded during the Hell Creek Project (1999-2009) incorporates multiple lines of evidence from geography, taphohistory, stratigraphy, phylogeny and ontogeny to investigate the relative abundance of large dinosaurs preserved in the Upper Cretaceous Hell Creek Formation of northeastern Montana, USA. Overall, the dinosaur skeletal assemblages in the Hell Creek Formation (excluding lag-influenced records) consist primarily of subadult or small adult size individuals. Small juveniles and large adults are both extremely rare, whereas subadult individuals are relatively common. We propose that mature individuals of at least some dinosaur taxa either lived in a separate geographic locale analogous to younger individuals inhabiting an upland environment where sedimentation rates were relatively less, or these taxa experienced high mortality before reaching terminal size where late stage and often extreme cranial morphology is expressed. METHODOLOGY/PRINCIPAL FINDINGS Tyrannosaurus skeletons are as abundant as Edmontosaurus, an herbivore, in the upper Hell Creek Formation and nearly twice as common in the lower third of the formation. Smaller, predatory dinosaurs (e.g., Troodon and dromaeosaurids) are primarily represented by teeth found in microvertebrate localities and their skeletons or identifiable lag specimens were conspicuously absent. This relative abundance suggests Tyrannosaurus was not a typical predator and likely benefited from much wider food choice opportunities than exclusively live prey and/or specific taxa. Tyrannosaurus adults may not have competed with Tyrannosaurus juveniles if the potential for selecting carrion increased with size during ontogeny. CONCLUSIONS/SIGNIFICANCE Triceratops is the most common dinosaur and isolated skulls contribute to a significant portion of this census. Associated specimens of Triceratops consisting of both cranial and postcranial elements remain relatively rare. This rarity may be explained by a historical collecting bias influenced by facies and taphonomic factors. The limited discovery of postcranial elements may also depend on how extensive a fossil quarry is expanded after a skull is collected.

Cranial anatomy and diagnosis of Stygimoloch spinifer (Ornithischia: Pachycephalosauria) with comments on cranial display structures in agonistic behavior

ABSTRACT A new skull of Stygimoloch spinifer (MPM 8111) from the Upper Cretaceous Hell Creek Formation of North Dakota is the most complete specimen discovered to date. It allows much of the skull and braincase of this unusual pachycephalosaurid to be described for the first time and confirms a suite of diagnostic characters for the species. The skull is long with a vaulted, transversely narrow frontoparietal dome and a robust squamosal forming a prominent posterior shelf. The shelf is ornamented by three to four large, low-angle horns and multiple clusters of smaller bony nodes. The orientation of the squamosal is preserved along an unambiguous contact with the frontoparietal suture, allowing definitive determination of the orientation of the squamosal horns. These cranial features indicate a different mode of agonistic behavior than previously suggested for Stegoceras and Pachycephalosaurus. The high, narrow dome of S. spinifer is not suited for head-butting, and the orientation of its squamosal horns a...