THE DINOSAURIA
The University of California Press
ISBN: 0-520-24209-2
Chapter One
Basal Ceratopsia 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.
Anatomy
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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