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Author Biography: David B. Weishampel is Associate Professor of Anatomy at The Johns Hopkins University School of Medicine. Peter Dodson is Associate Professor of Anatomy at the University of Pennsylvania School of Veterinary Medicine. Halszka Osmólska is Professor of Paleontology at the Paleobiological Institute of the Polish Academy of Sciences in Warsaw.
Origin and Relationships of Dinosauria
MICHAEL J. BENTON
Dinosauria is a well-diagnosed clade, and since birds are included within it the group is clearly significant among terrestrial vertebrates. Dinosaurs belong within Archosauria, a broader clade that also includes crocodilians as well as pterosaurs and various basal groups of Triassic age. Over the past thirty years considerable effort has been devoted to disentangling the phylogeny of archosaurs: some relationships have been firmly established, while others have yet to be discovered. This chapter considers the origin of the dinosaurs in terms of phylogeny and the timing of events. The primary evidence comes from a cladistic analysis of the Triassic archosaurs, a paraphyletic group formerly known as thecodontians. That analysis is followed by an account of the evolutionary events that led to the radiation of the most astonishing animals of all time, the dinosaurs themselves.
Previous Cladistic Analyses of Dinosauria
Paleontologists have been interested in the phylogeny of the dinosaurs since their discovery in the 1820s. However, confusion reigned over many aspects of dinosaurian phylogeny until the 1980s, when cladistics began to be applied. The use of cladistic methods led to the solution of many phylogenetic conundrums, but others have continued to defy resolution.
Cladistics as applied to dinosaurs and their close relatives began in the early 1980s with a number of basic studies in which cladograms were compiled by hand from incomplete lists of sometimes poorly delimited characters. In the late 1980s and 1990s data matrices were analyzed by computer algorithms, and evidence was often given of the goodness of fit of particular parts of the tree to the data.
Further developments in cladistic practice have occurred since the publication of the first edition of The Dinosauria in 1990. Some workers have advocated the use of formalized systems for naming and diagnosing taxa. Both before 1990 and since, names have been applied to nodes in cladograms according to the whim of the systematists. Some workers name every node in a cladogram, while others prefer to name only those nodes that are robust (i.e., supported by a great deal of character evidence). In addition, there are differences in the choice of names for clades: some systematists prefer to retain well-known group names that most closely match the traditional understanding of contents and/or diagnostic characters, while others advocate the migration of widely used names to clades subtended by extant groups, the crown-clade concept.
There have also been important developments in the definition and diagnoses of taxa. Until 1990 most cladists equated taxa with their characters (e.g., Aves is diagnosed as those organisms that possess primary flight feathers and wings). Since then, a clear distinction has been proposed between the definitions and diagnoses of clades (de Queiroz and Gauthier 1992, 1994). Clades are diagnosed by characters that evolve at or immediately prior to their origin but do not make individual clades "what they are." Instead, individual clades are named on the basis of their membership. Named taxa are then fixed on the basis of their ancestry.
There are three kinds of definitions of taxon names: apomorphy-based, stem-based, and node-based. An apomorphy-based definition is founded on one or more derived characters (e.g., "Aves consists of all archosaurs with feathered wings used for powered flight"). Apomorphy-based taxa combine definition and diagnosis. Stem- and node-based definitions include ancestors as the basal member of the clade (fig. 1.1). A stembased taxon name refers to a clade that includes all the descendants from a particular cladogenetic event. An example of stem-based definition for Ornithischia is "all dinosaurs more closely related to Triceratops than to Tyrannosaurus." A node-based taxon name is one that is defined by a basal node. For example, a node-based definition for Aves takes the form "Archaeopteryx, Neornithes, their most recent common ancestor, and all descendants." Because we do not know all of the stem taxa in stembased taxa, their diagnoses are not possible. Node-based taxa can be diagnosed on the basis of the derived features that have evolved in or immediately prior to the ancestor of the clade (this practice has been applied to stem-based clades as well). A crown clade is a node-based clade defined solely on the basis of extant forms. For example, the crown group Mammalia is defined as the common ancestor of Ornithorhynchus (platypus) and Homo (humans) and all the descendants of this common ancestor.
The crown-group issue affects the discussion of basal archosaurs and the origin of dinosaurs since certain names have different meanings for different people. Cope (1869a) introduced the name Archosauria for a wide group of extinct and extant reptiles, including anomodonts and rhynchosaurs, and no one has proposed a strict return to his view. However, for most of the twentieth century the name was used to refer to a group that includes modern crocodilians, as well as dinosaurs, pterosaurs, and a variety of Triassic forms, back to the Early Triassic proterosuchids and erythrosuchids, the earliest representative being Archosaurus, from the Late Permian (Tatarian) of Russia (e.g., Romer 1945, 1956, 1966; Hughes 1963; Charig and Reig 1970; Cruickshank 1972; Bonaparte 1982; Carroll 1988; Juul 1994; Gower and Wilkinson 1996; Benton 1997). Archosauria in this sense turns out to include those diapsids that possess an antorbital fenestra—an apomorphy-based taxon.
Gauthier (1986), urging use of the crown-clade concept, defined Archosauria as the clade subtended by living archosaurs (birds and crocodilians), in other words, excluding proterosuchids, erythrosuchids, proterochampsids, and euparkeriids. He argued that this realignment of the name had the advantage that all archosaurs, including fossil forms, would then have predictable soft-part characters (based on extrapolation from living forms). The apomorphy-based Archosauria was then renamed Archosauriformes (Gauthier 1986). Archosauria could also be defined as a stem-based taxon by extending membership down the stem to the next known node, but this approach has not yet been proposed.
Archosauria is treated as an apomorphy-based clade here, with note made to node- or stem-based definitions where relevant. Phylogenetic studies of dinosaurs in their archosaur context have led to the following generally accepted conclusions:
1. Archosauria is monophyletic. This view has been held generally for a long time, although the clade has been regarded as hard to diagnose anatomically (e.g., Romer 1956, 1966) and hence possibly polyphyletic.
2. Archosauria includes a number of basal Triassic forms that are sister groups to Avesuchia sensu Benton (1999), also known as crown group Archosauria sensu Gauthier (1986). The clade consists of two lines, one leading to crocodilians, the other to birds. This split was hinted at by Bonaparte (1975a), Krebs (1976), Cruickshank (1979), and Chatterjee (1982), and it has been confirmed in all subsequent cladistic analyses.
3. The crocodile line, Crurotarsi (Sereno 1991a), consists of Phytosauridae, Ornithosuchidae, Prestosuchidae, Rauisuchidae, Poposauridae, and Crocodylomorpha, but the relationships among those groups are contentious (Gauthier 1986; Benton and Clark 1988; Sereno 1991a; Parrish 1993; Juul 1994; Benton 1999).
4. The bird line, Avemetatarsalia (Benton 1999), consists of Scleromochlus, Pterosauria, and Dinosauromorpha. The South American forms, Lagerpeton and Marasuchus, are close outgroups of Dinosauria (Gauthier 1986; Benton and Clark 1988; Sereno 1991a; Sereno and Arcucci 1993, 1994; Juul 1994; Novas 1996a; Benton 1999). Avemetatarsalia is similar but not equivalent to Ornithosuchia sensu Gauthier (1986) because the former does not include Ornithosuchidae, a clade thought to be included in Crurotarsi (Benton 1999).
5. Dinosauria is monophyletic. Up to 1985, dinosaurs were usually seen as polyphyletic, with as many as three to six ancestors (e.g., Romer 1966; Charig and Reig 1970; Charig 1976a; Krebs 1976; Cruickshank 1979; Thulborn 1980; Chatterjee 1982). Bakker and Galton (1974) and Bonaparte (1976) argued for dinosaurian monophyly before such views became generally accepted (e.g., Gauthier 1986; Benton and Clark 1988; Sereno 1991b, 1997; Sereno and Arcucci 1993, 1994; Juul 1994; Novas 1996a; Benton 1999).
6. Dinosauria includes two clades, Saurischia and Ornithischia. This view has been generally held since Seeley's (1887a) invention of the two names, although he based Saurischia on what would now be seen as a plesiomorphy, namely, the "reptilian" pelvic arrangement. Apomorphies for both clades were given by Gauthier (1986).
7. Saurischia comprises several basal taxa, as well as Theropoda and Sauropodomorpha. These two clades were formerly regarded as having arisen from two or more independent sources among the basal archosaurs (e.g., Charig et al. 1965; Charig 1976b; Cruickshank 1979; Chatterjee 1982), a pattern for which there is no evidence (Gauthier 1986).
8. Ornithischia comprises several basal forms, as well as Thyreophora and Cerapoda (Sereno 1986, 1997, 1998; Weishampel and Witmer 1990a). Thyreophora includes the armored Stegosauria and Ankylosauria, as well as several basal taxa. Cerapoda splits into Marginocephalia (Pachycephalosauria and Ceratopsia) and Ornithopoda (Heterodontosauridae and Euornithopoda).
Archosauria and Its Evolutionary Context
Among living vertebrates, birds and crocodilians are linked as sister groups within the Avesuchia/crown group Archosauria (fig. 1.2). Although seemingly different kinds of animals, these two groups share numerous derived characters of the skull, postcranial skeleton, and soft parts that are absent in other living vertebrates (Gauthier 1986; Benton and Clark 1988). Early molecular studies of the phylogeny of tetrapods were equivocal regarding the nature of Archosauria, and many analyses of protein sequences supported a close pairing of Aves and Mammalia (e.g., Bishop and Friday 1988; Hedges et al. 1990; reviewed in Benton 1990b). More recently, the validity of Archosauria has been accepted based on sequencing of nucleic acids (e.g., Janke and Arnason 1997; Hedges and Poling 1999), although this has been challenged by the suggestion that turtles belong to Archosauria.
Archosauria is included in the larger clade Diapsida (living crocodilians, birds, Sphenodon, lizards, snakes, and their extinct relatives), one of the major clades of tetrapods. The crown group Tetrapoda, four-limbed vertebrates, includes Amphibia and various basal groups, as well as Amniota (Reptilia + Aves + Mammalia). Amniota comprises Anapsida, Diapsida, and Synapsida, diagnosed by the nature of their temporal openings, among other characters (Laurin and Reisz 1995; Benton 1997). The stability of this division of Amniota has been challenged by some morphological and molecular results that suggest that Anapsida should be subsumed within Diapsida: Rieppel and de Braga found that turtles may have been close relatives of lepidosaurs (Sphenodon, lizards and snakes) based on morphological characters (Rieppel and de Braga 1996; de Braga and Rieppel 1997), although their results have been queried (Wilkinson et al. 1997), while complete mitochondrial DNA sequencing suggests that turtles are the sister group to Archosauria (Zardoya and Meyer 1998; Hedges and Poling 1999; Kumazawa and Nishida 1999). These dramatic proposals have not yet been fully tested (reviewed in Rieppel 2000).
Archosauria was established by Cope (1869a) for a broad grouping of amniotes: Crocodylia, Thecodontia, Dinosauria, Anomodontia (i.e., dicynodonts + dinocephalians), and Rhynchocephalia (i.e., sphenodontids + rhynchosaurs). Cope (1869b) then restricted Archosauria to include Dinosauria, Phytosauria, Crocodylia, and Rhynchocephalia, excluding anomodonts. In the 1890s Cope and Baur independently developed the "theory of fenestration," according to which the major lines of amniote evolution could be identified by the numbers of temporal fenestrae, whether none (Anapsida), one (Synapsida), or two (Diapsida), the last two group names introduced by Osborn (1903) to reflect the new phylogenetic ideas.
Osborn (1903) rejected Cope's Archosauria, instead referring archosaur groups to a number of separate sections within Diapsida. This view was followed by many workers until the 1930s, with Broom, von Huene, Haughton, and others referring Triassic archosaurs to Pseudosuchia, Parasuchia, and Protorosauria. Romer (1933) resurrected Archosauria for Thecodontia, Crocodylia, Pterosauria, Saurischia, and Ornithischia but included proterosuchids in Eosuchia within Lepidosauria. Proterosuchids and erythrosuchids were replaced in Archosauria in later works (Romer 1945, 1956, 1966), and this has been the generally accepted viewpoint since (e.g., Hughes 1963; Charig and Reig 1970; Cruickshank 1972; Bonaparte 1982; Carroll 1988; Juul 1994; Gower and Wilkinson 1996; Benton 1997).
Archosauria sensu Benton 1999 is nested within larger clades, the most significant of which are Archosauromorpha and Diapsida. Diapsida comprises some basal taxa, principally Araeoscelidia (Petrolacosaurus, Araeoscelis, and relatives) from the Late Carboniferous and Early Permian, and two major clades, Lepidosauromorpha and Archosauromorpha, which diverged presumably early in the Permian (Benton 1983a, 1984b, 1985; Evans 1984, 1988; Gauthier 1986; Benton and Clark 1988; Laurin 1991; Laurin and Reisz 1995; Dilkes 1997).
Archosauromorpha has been given node- and stem-based definitions. According to the former, Archosauromorpha is defined as the most recent common ancestor of Neornithes (extant birds), Squamata (extant lizards and snakes), and all of the descendants of this common ancestor. Accordingly, the clade comprises Trilophosaurus, Rhynchosauria, Prolacertiformes, and Archosauria, as well as probably thalattosaurs, choristoderans, and drepanosaurids (Benton 1985; Benton and Clark 1988; Laurin 1991; de Braga and Rieppel 1997; Dilkes 1997). Trilophosaurus is an unusual Late Triassic herbivore with no infratemporal openings. Rhynchosaurs are a distinctive clade of Triassic herbivores that had multiple tooth rows and beaklike premaxillary projections. Prolacertiforms are long-necked insectivores and carnivores known from the mid-Permian to the Late Triassic that may comprise an unnatural grouping of diverse archosauromorph taxa (Dilkes 1997). Most cladistic analyses agree that among these archosauromorphs Prolacerta, a prolacertiform, is the sister group of Archosauria and that Rhynchosauria is a more distant outgroup. Other probable archosauromorphs include thalattosaurs, marine Triassic forms, choristoderans, superficially crocodilian-like aquatic animals known from the Triassic to the mid-Tertiary, and drepanosaurids, Late Triassic swimmers.
A stem-based definition for Archosauromorpha (the most recent common ancestor of Prolacerta, Trilophosaurus, Hyperodapedon, and archosaurs and all its descendants) was provided by Laurin (1991:90). However, this definition excludes Protorosaurus, Drepanosauridae, and Tanystropheidae, according to Dilkes's (1997) cladogram. Dilkes (1997:528) gave a revised stembased definition for Archosauromorpha, namely, Protorosaurus and all other saurians that are related more closely to Protorosaurus than to Lepidosauria, but this definition is rejected here since it would refer to a much more restricted grouping according to other cladograms of basal diapsids: it is debated whether "Protorosauria," Rhynchosauria, or Trilophosaurus is the basal archosauromorph taxon (e.g., Benton 1985; Chatterjee 1986a; Evans 1988; Laurin 1991). The present definition can cope with all the competing cladograms and still refers to the same clade contents (Trilophosaurus + Rhynchosauria + "Protorosauria" + Prolacertiformes + Archosauria).
Excerpted from The Dinosauria by David B. Weishampel, Peter Dodson, Halszka Osmólska. Copyright © 2004 the Regents of the University of California. Excerpted by permission of UNIVERSITY OF CALIFORNIA PRESS.
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Rinchen Barsbold, Mongolian People's Republic
Michael J. Benton, Northern Ireland
Walter P. Coombs, Jr., U.S.A.
Philip J. Currie, Canada
Dong Zhiming, People's Republic ofChina
James O. Farlow, U.S.A.
Peter M. Galton, U.S.A.
Jacques A. Gauthier, U.S.A.
John R. Horner, U.S.A.
Seriozha M. Kurzanov, U.S.S.R.
Teresa Maryanska, Poland
John S. McIntosh, U.S.A.
Ralph E. Molnar, Australia
David B. Norman, England
John H. Ostrom, U.S.A.
Timothy Rowe, U.S.A.
Paul C. Sereno, U.S.A.
Hans-Dieter Sues, U.S.A.
Leonid P. Tatarinov, U.S.S.R.
Lawrence M. Witmer, U.S.A.
YOU HAILU PETER DODSON
Ceratopsia consists of Psittacosauridae and Neoceratopsia, the latter formed by numerous basal taxa and Ceratopsidae. Consequently, this chapter on basal ceratopsians includes psittacosaurids and nonceratopsid neoceratopsians. Psittacosauridae is a monogeneric (Psittacosaurus) clade consisting of 10 species, while basal Neoceratopsia is formed by 11 genera, with 12 species of basal Neoceratopsia being recognized (table 22.1). Psittacosaurids are known from the Early Cretaceous of Asia, whereas basal neoceratopsians come from the latest Jurassic (Chaoyangsaurus youngi, Zhao et al. 1999; Swisher et al. 2002) to the latest Cretaceous in Asia and North America. Basal ceratopsians are small (1-3 m long), bipedal or quadrupedal herbivores (figs. 22.1, 22.2). Several taxa are extremely abundant and are represented by growth series from hatchlings to adults. Sexual dimorphism in Protoceratops is well supported (Dodson 1976; Lambert et al. 2001; Tereshchenko 2001). Basal neoceratopsians evolved larger skulls relative to their postcranial skeletons and more developed frills than psittacosaurids.
Definition and Diagnosis
Ceratopsia is defined as all Marginocephalia closer to Triceratops than to Pachycephalosaurus. Autapomorphies of this clade include a high external naris separated from the ventral border of the premaxilla by a flat area, a rostralbone, an enlarged premaxilla, well-developed lateral flaring of the jugal, wide dorsoventral length of the infraorbital ramus of the jugal, and contact of palatal extensions of the maxillae rostral to the choana.
The description below is based mainly on Psittacosaurus mongoliensis (Sereno 1990b), Archaeoceratops oshimai (Dong and Azuma 1997; You 2002; You and Dodson 2003), Protoceratops andrewsi (Brown and Schlaikjer 1940b; Dodson and Currie 1990), and Leptoceratops gracilis (Sternberg 1951; Dodson and Currie 1990), as they include the best preserved and described specimens representing the major subgroups among basal ceratopsians. Additional comments on other taxa are also included and are discussed further in the "Systematics and Evolution" section below.
Skull and Mandible
The skull of basal ceratopsians (figs. 22.3-22.5) is pentangular in dorsal view, with a narrow beak, a strong laterally flaring jugal, and a caudally extended frill. The beak is round in psittacosaurids and pointed in basal neoceratopsians. The jugal horn is more pronounced in psittacosaurids than in basal neoceratopsians. The frill is incipient in psittacosaurids and small but variably developed in basal neoceratopsians. The preorbital portion of the skull is dorsoventrally deep, especially in psittacosaurids, and rostrocaudally short in both psittacosaurids and the most basal members of neoceratopsians such as Archaeoceratops.
The external naris is highly positioned, especially in psittacosaurids, bounded by the premaxilla ventrally and the nasal dorsally. Its shape is subrounded in psittacosaurids and Leptoceratops, but elliptical in Archaeoceratops and Protoceratops. The antorbital fossa or fenestra is not present in psittacosaurids, but a small opening is enclosed between the premaxilla and the lacrimal. In basal neoceratopsians, the shape of the antorbital fossa varies from subtriangular (Archaeoceratops) to elliptical (Protoceratops and Leptoceratops), usually with a small fenestra on the caudodorsal part of the wall. An additional antorbital fenestra is present in Bagaceratops between the premaxilla and the maxilla. The orbit is smaller than the infratemporal fenestra in psittacosaurids and the two are subequal in size in most neoceratopsians, but the orbit is larger than the infratemporal fenestra in Archaeoceratops.
The rostral in psittacosaurids is thin in sagittal section and forms a convex shield that caps a triangular surface on the conjoined premaxillae. Its rostroventral end is round in dorsal view and does not curve ventrally to form a pointed beak. The rostral also contacts the slender rostroventral processes of the nasals. In basal neoceratopsians, the rostral is strongly compressed transversely and extends rostrally beyond the rostral tip of the lower jaw. A sutural contact between the rostral and nasal is absent. In Archaeoceratops and Protoceratops, the ventral edge of the rostral curves strongly rostroventrally to a point, while in Leptoceratops, the ventral edge is horizontal with a well-developed caudolateral process as long as it is high.
The premaxilla is a characteristic element in psittacosaurids. The tall, parrotlike face of psittacosaurids is formed almost entirely from the expansive caudolateral process of the premaxilla, which contacts the lacrimal and the prefrontal caudally. In palatal view, the palatal process of the premaxilla arches from the lateral margin of the beak to the midline. The caudal extension of the palatal process of the premaxilla does not reach the rostral margin of the choana and is separated from the vomer by the palatal extension of the maxillae in between. In basal neoceratopsians, the premaxilla is not as large as in psittacosaurids and is bounded by the rostral rostrally, the nasal dorsally, and the maxilla caudally. In Archaeoceratops, the premaxilla is nearly square in lateral view, but higher than long in Protoceratops and longer than high in Leptoceratops. The caudolateral process of the premaxilla is not developed in Archaeoceratops, but is prominent in both Protoceratops and Leptoceratops. Unlike the flat ventral edge of Archaeoceratops and Protoceratops, the premaxilla is ventrally convex in Leptoceratops. In Bagaceratops, the caudal edge of the premaxilla surrounds an additional antorbital fenestra together with the maxilla caudally.
In psittacosaurids, the maxilla is triangular in lateral view and situated with its caudal half underneath the orbit. It is largely bounded by the premaxilla rostrally and the jugal caudally, has a small contact with the lacrimal dorsally, and does not reach the nasal. In basal neoceratopsians, the maxilla is tall and forms about two-thirds of the height of the face. It sutures with the premaxilla rostrally and the lacrimal and the jugal caudally, and it has a small contact with the nasal dorsally. The premaxillamaxilla suture is vertical in Archaeoceratops, but inclined in both Protoceratops and Leptoceratops. A prominent elliptical antorbital fossa with a small antorbital fenestra exists in most basal neoceratopsians, except in Archaeoceratops, in which the antorbital fossa is triangular and the antorbital fenestra is not evident. In Protoceratops and Bagaceratops, there is a prominent maxillary sinus that communicates with the antorbital fossa but not the nasal cavity (Osmólska 1986).
The nasal of psittacosaurids is unusual in that a slender rostral process extends ventral to the external naris and reaches the rostral. In basal neoceratopsians, the nasal is long and narrow, but never extends beyond the rostral end of the external naris to contact the rostral bone. No horn-core is present in Archaeoceratops and Leptoceratops (You, pers. obs.). An incipient nasal horn core is evident in Protoceratops, Bagaceratops (Maryanska and Osmólska 1975), and Udanoceratops (Kurzanov 1992), which is located caudodorsal to the external naris.
The lacrimal of psittacosaurids is bounded by the prefrontal dorsally, the premaxilla rostrally, the maxilla ventrally, and the jugal caudoventrally. The lateral wall of the lacrimal canal remains only partially ossified, and the canal opens externally in a small pore about halfway along its passage from the margin of the orbit to the nasal cavity, which is bounded rostrally by the premaxilla. In basal neoceratopsians, the lacrimal is largely bounded by the prefrontal dorsally and the maxilla rostroventrally, and has a small contact with the nasal rostrally and the jugal caudally. Unlike in psittacosaurids, premaxilla-lacrimal contact is prevented because the expanded maxilla contacts the nasal dorsally. The rostroventral corner of the lacrimal usually contributes a small portion to the caudodorsal wall of the antorbital fossa. The lacrimal of Leptoceratops is larger than those of other basal neoceratopsians.
The prefrontal borders the rostrodorsal rim of the orbit in both psittacosaurids and basal neoceratopsians. It is surrounded by the frontal, the nasal, the premaxilla, and the lacrimal in psittacosaurids, whereas the premaxilla is excluded in basal neoceratopsians.
The palpebral usually attaches to the caudal rim of the prefrontal. In psittacosaurids, it is a short, dorsally arched rod associated solely with the prefrontal. In Archaeoceratops, the prominent palpebral is triangular with a caudally pointed end, and it articulates with both the prefrontal and the lacrimal.
In both psittacosaurids and basal neoceratopsians, the dorsoventral length of the jugal below the orbit is at least as long as that underneath the infratemporal fenestra. Although the flaring of the jugal characterizes both psittacosaurids and basal neoceratopsians, it shows different configurations in these two groups. In psittacosaurids, the breadth of the skull across the flaring jugal horns can be as long as or longer than the skull length, and the flaring projects from the midsections of the jugals; in basal neoceratopsians, the width of the skull across the flaring jugal horns never exceeds the basal skull length, and the flaring is usually directed caudolaterally from the caudal end of the jugal. The postorbital process of the jugal is short in psittacosaurids, but long and stout in basal neoceratopsians. In Protoceratops and Leptoceratops, there is an incipient jugalsquamosal contact around the rostrodorsal rim of the infratemporal fenestra. The caudolateral end of the jugal is often thickened in basal neoceratopsians, and this thickening is usually accentuated by an epidermal ossification, the epijugal, in Protoceratops and Leptoceratops, but not in Archaeoceratops.
The large quadratojugal of psittacosaurids is located on the caudoventral corner of the skull. The rostral portion passes medial to the jugal, and the caudal portion covers the ventral half of the quadrate shaft in lateral view. In basal neoceratopsians, the quadratojugal is largely excluded from lateral view by the caudal extension of the jugal. It is a transversely thin element inserted between the jugal laterally and the quadrate medially. In Archaeoceratops, a trace of the quadratojugal is still visible laterally at the caudoventral end of the skull.
The postorbital in psittacosaurids is a restricted element, with two rodlike, elongated processes, the jugal process and the squamosal process. In basal neoceratopsians, the jugal process is shortened and the squamosal process is stout. The enlargement of the jugal and the reduction of the infratemporal fenestra exclude contact between the postorbital and the infratemporal fenestra in Protoceratops and Leptoceratops.
In basal ceratopsians, the paired frontals form a major portion of the cranium, border the orbit laterally, and constitute the rostral limit of the supratemporal fenestrae. In Psittacosaurus, the dorsal surface of the frontal is restricted to the flat interorbital portion of the skull roof. In Protoceratops and Leptoceratops, a pair of modest frontoparietal depressions in adult specimens is associated with the rostral borders of the supratemporal fenestrae, reflecting expansion of the attachments of the jaw adductor musculature.
In psittacosaurids, the parietal roofs the braincase and forms the medial border of the supratemporal fenestra. It extends caudally over the occiput as a transversely broad shelf. In basal neoceratopsians, an incipient parietosquamosal frill extends behind the skull, which is simple and lacks the various decorations seen in ceratopsids. In Archaeoceratops, the frill is short, as indicated by the short squamosal, while in Leptoceratops it is short and unfenestrated. The frill of Protoceratops is moderately developed and fan-shaped, tilting caudodorsally with a pair of parietal fenestrae near the caudal end. A median keel develops on the dorsal surface in both Protoceratops and Leptoceratops.
In psittacosaurids, the squamosal forms a simple bar with the postorbital that separates the infra- and supratemporal fenestrae and provides a cotylus for the head of the quadrate. A postquadrate extension of the squamosal developed in basal neoceratopsians. In Protoceratops, it runs caudodorsally along the ventral margin of the parietal, with which it forms the lateral edge of the frill. In Leptoceratops, the postquadrate extension is not well developed, but extends ventrally to hook the head of the quadrate caudally.
In psittacosaurids, the ventral half of the quadrate is erect and largely covered by the quadratojugal laterally. The dorsal half bends caudodorsally to contact the squamosal. In basal neoceratopsians, there is a progressive reorientation of the cheek. The ventral end of the quadrate is rotated forward, the infratemporal fenestra is compressed, and the jugal, quadratojugal, and ventral quadrate are telescoped to lie side by side rather than in series rostral to caudal (Dodson 1993).
In ventral view of the skull of psittacosaurids, a transversely arched secondary palate is present rostrally, formed principally by the premaxillae. Caudal to the choana, the remainder of the palate is composed of the palatine, pterygoid, and ectopterygoid. The vomers, which fuse rostrally, arch in the midline from the secondary palate rostrally to the palatine and pterygoid caudally. The suborbital opening persists as a foramen between the palatal bones and the maxilla. An elongate flange of the pterygoid, the mandibular ramus, is directed caudoventrally toward the adductor fossa of the lower jaw.
The palate of basal neoceratopsians is strongly vaulted both transversely and, in Bagaceratops at least, longitudinally as well. The choana is positioned far forward and oblique to the palatal plane due to the narrowness and vaulting of the snout (Osmólska 1986). The secondary palate is short. The vomer is a straight median bar running between the pterygoids caudally and the palatal processes of the maxillae rostrally. It rises steeply caudodorsally to meet the rostrodorsally inclined longitudinal process of the palatine.
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Posted March 4, 2010
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