Phylogeny of basal iguanodonts (Dinosauria: Ornithischia): an update

The precise phylogenetic relationships of many non-hadrosaurid members of Iguanodontia, i.e., basal iguanodonts, have been unclear. Therefore, to investigate the global phylogeny of basal iguanodonts a comprehensive data matrix was assembled, including nearly every valid taxon of basal iguanodont. The matrix was analyzed in the program TNT, and the maximum agreement subtree of the resulting most parsimonious trees was then calculated in PAUP. Ordering certain multistate characters and omitting taxa through safe taxonomic reduction did not markedly improve resolution. The results provide some new information on the phylogeny of basal iguanodonts, pertaining especially to obscure or recently described taxa, and support some recent taxonomic revisions, such as the splitting of traditional "Camptosaurus" and "Iguanodon". The maximum agreement subtree also shows a close relationship between the Asian Probactrosaurus gobiensis and the North American Eolambia, supporting the previous hypothesis of faunal interchange between Asia and North America in the early Late Cretaceous. Nevertheless, the phylogenetic relationships of many basal iguanodonts remain ambiguous due to the high number of taxa removed from the maximum agreement subtree and poor resolution of consensus trees.     

A new phylogeny of Stegosauria (Dinosauria,Ornithischia)

The stegosaurs are some of the most easily recognizable dinosaurs, but are surprisingly rare as fossils. Consequently much remains unknown about their palaeobiology, and every new stegosaurian find contributes to our understanding of the evolution of the clade. Since the last attempt to examine the evolutionary relationships of Stegosauria, new specimens have come to light, including the most complete individual of Stegosaurus ever found, new taxa have been described and, perhaps most importantly, new methods for analysis of cladistic datasets have been produced. In the light of these new data and technological advances, the phylogenetic relationships of the stegosaurs and basal armoured dinosaurs are investigated. The inclusion of continuous data results in much better resolution than was previously obtained, and the resulting single most parsimonious tree supports re‐erection of the genera Miragaia and Hesperosaurus, which had previously been synonymized with Dacentrurus and Stegosaurus respectively. The recently described genus Alcovasaurus is resolved as a basal thyreophoran, but this is most likely a consequence of a very high degree of missing data and the questionable ontogenetic stage of the specimen. Examination of the effects of continuous data on the analysis suggest that while it contains a phylogenetic signal congruent with that of discrete data and provides better resolution than discrete data alone, it can affect topologies in unpredictable ways, particularly in areas of the tree where there are large amounts of missing data. The phylogeny presented here will form the basis for future work on the palaeobiology of the plated dinosaurs.     

Taphonomy and habitat preference of North American pachycephalosaurids (Dinosauria,Ornithischia)

The traditional view of North American pachycephalosaurids holds that their domes are typically worn, as though they had undergone extensive fluvial transport, and that these animals therefore inhabited upland environments (e.g. piedmont/intermontane settings) relative to where their remains are typically found. Using a statistically robust sample of domes from Alberta, Canada, we subject these hypotheses to various forms of quantitative testing and show that: (1) domes typically exhibit minimal rounding; (2) dome roundness does not positively correlate with distance from the presumed intermontane origin; and (3) pachycephalosaurid remains are not relatively more abundant in intermontane than in alluvial or coastal plain palaeoenvironments. These findings contradict the traditional view of North American pachycephalosaurid dome taphonomy and support the argument that pachycephalosaurids frequented alluvial and coastal lowlands.     

The Caribou Migration Model for Arctic Hadrosaurs (Dinosauria: Ornithischia): A Reassessment

The recovery of abundant dinosaur fossils from high paleolatitudes of northern Alaska has raised some hard questions in relation to any available model of dinosaur physiology. To explain the occurrence of hadrosaurs at high ancient latitudes, a model involving long-distance migration analogous to that of the modern caribou has been proposed. The model calls for seasonal movements over great latitudinal distances by these Cretaceous hadrosaurs. This model is reassessed in terms of the growth, body sizes, and inferred physiological ecology of the hadrosaurs and the caribou. Histological data suggest that juvenile hadrosaurs obtained from northern Alaska were greater than 1 year in age. Comparison of the relative sizes of juvenile and adult hadrosaurs with juvenile and adult caribou suggests, based on qualitative energetics, that the juvenile hadrosaurs were too small to participate in long-distance migration. The "hadrosaurs as caribou" model provides clues to the feeding ecology of North Slope hadrosaurs, if they are reinterpreted as year-round residents of high latitudes. However, it does not constitute a satisfactory basis on which to infer long-distance seasonal migrations by these animals.     

Cranial ontogeny in Stegoceras validum (Dinosauria: Pachycephalosauria): a quantitative model of pachycephalosaur dome growth and variation

Historically, studies of pachycephalosaurs have recognized plesiomorphically flat-headed taxa and apomorphically domed taxa. More recently, it has been suggested that the expression of the frontoparietal dome is ontogenetic and derived from a flat-headed juvenile morphology. However, strong evidence to support this hypothesis has been lacking. Here we test this hypothesis in a large, stratigraphically constrained sample of specimens assigned to Stegoceras validum, the best known pachycephalosaur, using multiple independent lines of evidence including conserved morphology of ornamentation, landmark-based allometric analyses of frontoparietal shape, and cranial bone histology. New specimens show that the diagnostic ornamentation of the parietosquamosal bar is conserved throughout the size range of the sample, which links flat-headed specimens to domed S. validum. High-resolution CT scans of three frontoparietals reveal that vascularity decreases with size and document a pattern that is consistent with previously proposed histological changes during growth. Furthermore, aspects of dome shape and size are strongly correlated and indicative of ontogenetic growth. These results are complementary and strongly support the hypothesis that the sample represents a growth series of a single taxon. Cranial dome growth is positively allometric, proceeds from a flat-headed to a domed state, and confirms the synonymy of Ornatotholus browni as a juvenile Stegoceras. This dataset serves as the first detailed model of growth and variation in a pachycephalosaur. Flat-headed juveniles possess three characters (externally open cranial sutures, tuberculate dorsal surface texture, and open supratemporal fenestrae) that are reduced or eliminated during ontogeny. These characters also occur in putative flat-headed taxa, suggesting that they may also represent juveniles of domed taxa. However, open cranial sutures and supratemporal fenestrae are plesiomorphic within Ornithischia, and thus should be expected in the adult stage of a primitive pachycephalosaur. Additional lines of evidence will be needed to resolve the taxonomic validity of flat-headed pachycephalosaur taxa.     

Diversity and convergences in the evolution of feeding adaptations in ankylosaurs (Dinosauria: Ornithischia)*

Ankylosaurian dinosaurs were low-browsing quadrupeds that were traditionally thought of as simple orthal pulpers exhibiting minimal tooth occlusion during feeding, as in many extant lizards. Recent studies, however, have demonstrated that effective chewing with tooth occlusion and palinal jaw movement was present in some members of this group. Qualitative and quantitative analysis of feeding characters (i.e. craniodental features, tooth wear patterns, origin and insertion of jaw adductors) reveal at least three different jaw mechanisms during the evolution of Ankylosauria. Whereas, in basal members, food processing was restricted to simple orthal pulping, in late Early and Late Cretaceous North American and European forms a precise tooth occlusion evolved convergently in many lineages (including nodosaurids and ankylosaurids) complemented by palinal power stroke. In contrast, Asian forms retained the primitive mode of feeding without any biphasal chewing, a phenomenon that might relate to the different types of vegetation consumed by these low-level feeders in different habitats on different landmasses. Further, a progressive widening of the muzzle is demonstrated both in Late Cretaceous North American and Asian ankylosaurs, and the width and general shape of the muzzle probably correlates with foraging time and food type, as in herbivorous mammals.     

A stegosaur vertebra (Dinosauria: Ornithischia) from the Callovian (Middle Jurassic) of Sarthe, western France

A dinosaur vertebra found in the course of road works in the “Chevain Marls” (marine Callovian) of the Vermont outlier, near the village of Béthon (Sarthe, Pays-de-la-Loire, western France) is described and referred to a stegosaur (cf. Lexovisaurus). It is the first record of a dinosaur in the Middle Jurassic of that area. The specimen was probably derived from the floating carcass of an animal that had lived on the emerged areas of the Armorican Massif, near which the Chevain Marls were deposited. This fossil is an addition to the short list of stegosaur remains reported from France.     

Structural features of neural spines of the caudal vertebrae of protoceratopoids (Ornithischia: Neoceratopsia)

The structure of caudal neural spines of protoceratopoids displays adaptation for aquatic and terrestrial mode of life. The increasing height of caudal neural spines in the series Leptoceratops, Udanoceratops, Protoceratops, Bagaceratops is connected with the extent of adaptation for swimming and changes in inclination of neural spines are connected with the mechanical balance of the lever. Thus, the anterior caudal vertebrae (1cd–15cd) of Protoceratops and Bagaceratops show an anticliny, which promotes extension (rise) of a heavy tail in terrestrial conditions. In the middle part of the tail (16cd–23cd), with the greatest height of neural spines, a decrease in width and increase in thickness counteract transverse loads accompanying movements on land. At the same time, the supraspinal ligament prevents divergence of neural spines caused by curvature of the tail as it is raised above the ground.     

On Asian ornithopods (Dinosauria: Ornithischia). 4. Probactrosaurus Rozhdestvensky, 1966

The genus Probactrosaurus was first established for material discovered by a joint Russian/Chinese expedition to the Chinese autonomous region of Neimongol (Inner Mongolia). Fossils were collected at a site named Maorty (= Maortu). Material attributable to ornithopod dinosaurs was considered sufficiently distinct to permit the definition of two species of the new genus: Probactrosaurus gobiensis and P. alashanicus. The former species was based on a considerable quantity of skeletal material, much of which is still to be found in the collections of the Palaeontological Institute (PIN), Moscow. The latter was based on far less well‐preserved specimens, including a holotype (a posterior skull roof) that can no longer be found in the collections of the PIN and which, along with other materials, was reportedly returned to the Institute of Vertebrate Palaeontology and Palaeoanthropology, Beijing. Some remnants of the original material attributed to P. alashanicus have been located in the PIN collections. Both taxa, established by A. K. Rozhdestvensky, are re‐described using all of the available material collected during the early Sino‐Soviet expeditions. Probactrosaurus alashanicus is considered to be a junior subjective synonym of P. gobiensis. Further comparisons are made with the recently described species Probactrosaurus mazongshanensis Lu, 1997. The latter does not appear to be referable to the genus Probactrosaurus. Probactrosaurus is a gracile ornithopod (ranging between 4 and 6 m in length). The skull is unadorned by any form of cranial crest; however, the premaxillary beak is deflected ventrally and the dentition is similar to that seen in more derived hadrosaurid ornithopods. The postcranial skeleton is notable for its gracility, in particular the elongate forearm and manus, and the retention of a small, conical pollex spine. Systematic analysis suggests that P. gobiensis is a derived non‐hadrosaurid iguanodontian ornithopod and the basal sister‐taxon to the clade Hadrosauridae. The phylogeny of currently known iguanodontians is reviewed. © 2002 The Linnean Society of London, Zoological Journal of the Linnean Society, 2002, 136 , 113–144.     

Disparity vs. diversity in Stegosauria (Dinosauria,Ornithischia): cranial and post-cranial sub-dataset provide different signals

The Stegosauria represents an iconic group of ornithischian dinosaurs, with a fossil record spanning the Middle Jurassic to the Late Cretaceous. In this contribution I present the first detailed analysis of the relationship between disparity and diversity through the evolutionary history of the group. The analysis has been performed on a recently published cladistic dataset, allowing the separate study of the signals deriving from discrete characters and from continuous morphometric characters. Whereas the disparity as sum of variance is decoupled with respect to diversity, the sum of ranges provides a signal fairly consistent with the trend in the number of taxa. Both sub-data sets show that evolution of stegosaurs can be considered essentially as symmetrical, i.e. the maximum exploration of the possible morphospace takes place about half way through the history of the group, with subsequent significant decline until extinction in the Upper Cretaceous. An interesting result is a decoupling of the tempo and mode of evolution of the cranium and postcranium in stegosaurs. Specifically, the evolutionary radiation with maximum saturation of morphospace is anticipated in the cranial skeleton, with maximum peak in the Oxfordian; in contrast, the postcranium explores the largest number of morphotypes subsequently during the Kimmeridgian.     

On Mongolian ornithopods (Dinosauria: Ornithischia). 1. Iguanodon orientalis Rozhdestvensky 1952

The type material of the Cretaceous ornithopod Iguanodon orientalis Rozhdestvensky, 1952 is redescribed. The characters which were ori0ginally used to establish a new species are shown to be either incorrect or indistinguishable from Iguanodon bernissartensis Boulenger, 1881. Additional material, previously unrecognized, further confirms the similarity of I. orientalis to I. bernissartensis. It is proposed that I. orientalis should be relegated in synonymy as a junior subjective synonym of I. bernissartensis Boulenger, 1881. The identification of the Barremian species I. bernissartensis in Asia indicates an unusually wide geographic spread for this species, and provides a potential biostratigraphic indicator for beds in Mongolia which had been previously dated variously between the Cenomanian and Maastrichtian stages.     

First evidence of dinosaurian secondary cartilage in the post-hatching skull of Hypacrosaurus stebingeri (Dinosauria, Ornithischia)

Bone and calcified cartilage can be fossilized and preserved for hundreds of millions of years. While primary cartilage is fairly well studied in extant and fossilized organisms, nothing is known about secondary cartilage in fossils. In extant birds, secondary cartilage arises after bone formation during embryonic life at articulations, sutures and muscular attachments in order to accommodate mechanical stress. Considering the phylogenetic inclusion of birds within the Dinosauria, we hypothesized a dinosaurian origin for this "avian" tissue. Therefore, histological thin sectioning was used to investigate secondary chondrogenesis in disarticulated craniofacial elements of several post-hatching specimens of the non-avian dinosaur Hypacrosaurus stebingeri (Ornithischia, Lambeosaurinae). Secondary cartilage was found on three membrane bones directly involved with masticatory function: (1) as nodules on the dorso-caudal face of a surangular; and (2) on the bucco-caudal face of a maxilla; and (3) between teeth as islets in the alveolar processes of a dentary. Secondary chondrogenesis at these sites is consistent with the locations of secondary cartilage in extant birds and with the induction of the cartilage by different mechanical factors - stress generated by the articulation of the quadrate, stress of a ligamentous or muscular insertion, and stress of tooth formation. Thus, our study reveals the first evidence of "avian" secondary cartilage in a non-avian dinosaur. It pushes the origin of this "avian" tissue deep into dinosaurian ancestry, suggesting the creation of the more appropriate term "dinosaurian" secondary cartilage.     

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.     

New skull of Lesothosaurus (Dinosauria: Ornithischia) from the Upper Elliot Formation (Lower Jurassic) of southern Africa

A new ornithischian skull from the Elliot Formation of southern Africa is described. The specimen is compared in detail with the fabrosaurid Lesothosaurus diagnosticus. It actually shares many characters with specimens of the syntypes of this species or specimens referred to it. It is nevertheless not identical to any of these specimens and it is, moreover, remarkably larger than them. The possibility of attributing this specimen to a so far undescribed ‘large fabrosaur’ from the same formation is discussed. It is concluded that the specimen in question in this paper, while being ascribable to the genus Lesothosaurus, cannot be determined to a specific level until the existence of two fabrosaurid species in the ‘Stormberg Group’ is demonstrated and their range of morphological and size variation is properly appraised.     

A New Basal Ankylosaurid (Dinosauria: Ornithischia) from the Lower Cretaceous Jiufotang Formation of Liaoning Province,China

A new ankylosaurid, Chuanqilong chaoyangensis gen. et sp. nov., is described here based on a nearly complete skeleton from the Lower Cretaceous Jiufotang Formation of Baishizui Village, Lingyuan City, Liaoning Province, China. Chuanqilong chaoyangensis can be diagnosed on the basis of two autapomorphies (glenoid fossa for quadrate at same level as the dentary tooth row; distally tapering ischium with constricted midshaft) and also a unique combination of character states (slender, wedge-like lacrimal; long retroarticular process; humerus with strongly expanded proximal end; ratio of humerus to femur length  = 0.88). Although a phylogenetic analysis places Chuanqilong chaoyangensis as the sister taxon of the sympatric Liaoningosaurus near the base of the Ankylosauridae, the two taxa can be distinguished on the basis of many features, such as tooth morphology and ischial shape, which are not ontogeny-related. Chuanqilong chaoyangensis represents the fourth ankylosaurid species reported from the Cretaceous of Liaoning, China, suggesting a relatively high diversity in Cretaceous Liaoning.     

Cranial anatomy and systematics of Hypacrosaurus altispinus,and a comparative analysis of skull growth in lambeosaurine hadrosaurids (Dinosauria: Ornithischia)

The cranial anatomy of the helmet‐crested lambeosaurine Hypacrosaurus altispinus (Ornithischia: Hadrosauridae) is described, with a focus on ontogenetic and individual variation in phylogenetically significant characters of the cranial crest, braincase, and facial skeleton. Cranial material of H. altispinus represents a relatively complete growth series that includes crestless juveniles of less than half the size of large individuals with fully developed crests. Cranial ontogeny is compared with other lambeosaurines using bivariate morphometrics and through qualitative comparison of a size‐standardized cranial growth series. Bivariate analyses reveal that the relative growth of the skull and cranial crest of H. altispinus and H. stebingeri are similar, and that Hypacrosaurus more closely resembles Corythosaurus than Lambeosaurus. Hypacrosaurus altispinus is systematically revised. The taxon is characterized by five autapomorphies, most of which are concentrated in the skull, as well as an enlarged terminal ischial foot. Maximum parsimony and Bayesian likelihood (Mk+gamma) phylogenetic analyses were conducted to test the monophyly of the genus. Hypacrosaurus monophyly is corroborated in light of new anatomical data. Although H. stebingeri and H. altispinus share few derived characters of the skull, the hypothesis that H. stebingeri is a metaspecies that represents the ancestor of H. altispinus cannot be rejected. © 2010 The Linnean Society of London, Zoological Journal of the Linnean Society, 2010, 159 , 398–434.     

A Re-Evaluation of the Chasmosaurine Ceratopsid Genus Chasmosaurus (Dinosauria: Ornithischia) from the Upper Cretaceous (Campanian) Dinosaur Park Formation of Western Canada

Background

The chasmosaurine ceratopsid Chasmosaurus is known from the Upper Cretaceous (Campanian) Dinosaur Park Formation of southern Alberta and Saskatchewan. Two valid species, Chasmosaurus belli and C. russelli, have been diagnosed by differences in cranial ornamentation. Their validity has been supported, in part, by the reported stratigraphic segregation of chasmosaurines in the Dinosaur Park Formation, with C. belli and C. russelli occurring in discrete, successive zones within the formation.

Results/Conclusions

An analysis of every potentially taxonomically informative chasmosaurine specimen from the Dinosaur Park Formation indicates that C. belli and C. russelli have indistinguishable ontogenetic histories and overlapping stratigraphic intervals. Neither taxon exhibits autapomorphies, nor a unique set of apomorphies, but they can be separated and diagnosed by a single phylogenetically informative character—the embayment angle formed by the posterior parietal bars relative to the parietal midline. Although relatively deeply embayed specimens (C. russelli) generally have relatively longer postorbital horncores than specimens with more shallow embayments (C. belli), neither this horncore character nor epiparietal morphology can be used to consistently distinguish every specimen of C. belli from C. russelli.

Status of Kosmoceratops in the Dinosaur Park Formation

Kosmoceratops is purportedly represented in the Dinosaur Park Formation by a specimen previously referred to Chasmosaurus. The reassignment of this specimen to Kosmoceratops is unsupported here, as it is based on features that are either influenced by taphonomy or within the realm of individual variation for Chasmosaurus. Therefore, we conclude that Kosmoceratops is not present in the Dinosaur Park Formation, but is instead restricted to southern Laramidia, as originally posited.     

A New Brachylophosaurin Hadrosaur (Dinosauria: Ornithischia) with an Intermediate Nasal Crest from the Campanian Judith River Formation of Northcentral Montana

Background

Brachylophosaurini is a clade of hadrosaurine dinosaurs currently known from the Campanian (Late Cretaceous) of North America. Its members include: Acristavus gagslarsoni, which lacks a nasal crest; Brachylophosaurus canadensis, which possesses a flat paddle-shaped nasal crest projecting posteriorly over the dorsal skull roof; and Maiasaura peeblesorum, which possesses a dorsally-projecting nasofrontal crest. Acristavus, from the lower Two Medicine Formation of Montana (~81–80 Ma), is hypothesized to be the ancestral member of the clade. Brachylophosaurus specimens are from the middle Oldman Formation of Alberta and equivalent beds in the Judith River Formation of Montana; the upper Oldman Formation is dated 77.8 Ma.

Methodology/Principal Findings

A new brachylophosaurin hadrosaur, Probrachylophosaurus bergei (gen. et sp. nov.) is described and phylogenetically analyzed based on the skull and postcranium of a large individual from the Judith River Formation of northcentral Montana (79.8–79.5 Ma); the horizon is equivalent to the lower Oldman Formation of Alberta. Cranial morphology of Probrachylophosaurus, most notably the nasal crest, is intermediate between Acristavus and Brachylophosaurus. In Brachylophosaurus, the nasal crest lengthens and flattens ontogenetically, covering the supratemporal fenestrae in large adults. The smaller nasal crest of Probrachylophosaurus is strongly triangular in cross section and only minimally overhangs the supratemporal fenestrae, similar to an ontogenetically earlier stage of Brachylophosaurus. Sutural fusion and tibial osteohistology reveal that the holotype of Probrachylophosaurus was relatively more mature than a similarly large Brachylophosaurus specimen; thus, Probrachylophosaurus is not simply an immature Brachylophosaurus.

Conclusions/Significance

The small triangular posteriorly oriented nasal crest of Probrachylophosaurus is proposed to represent a transitional nasal morphology between that of a non-crested ancestor such as Acristavus and the large flat posteriorly oriented nasal crest of adult Brachylophosaurus. Because Probrachylophosaurus is stratigraphically and morphologically intermediate between these taxa, Probrachylophosaurus is hypothesized to be an intermediate member of the Acristavus-Brachylophosaurus evolutionary lineage.     

Oldest occurrence of the genusStegoceras (Dinosauria: Pachycephalosauridae) from the Foremost Formation (Campanian) of South Alberta, Canada

Three pachycephalosaurid frontoparietals from the Foremost Formation (Campanian) of South Alberta, Canada, are described and assigned to the genusStegoceras Lambe 1902 on the basis of measurements. Measurements and indices suggest that all of the specimens originate from male animals. The specimens are the oldest fossil record of this genus. The possibility is discussed, that they represent a species that could be an ancestor of the North AmericanStegoceras validum and/or the MongolianPrenocephale prenes.