R. Barsbold CARNIVOROUS DINOSAURS FROM THE CRETACEOUS OF MONGOLIA THE JOINT SOVIET-MONGOLIAN PALEONTOLOGICAL EXPEDITION (Transactions, vol. 19) EDITORIAL BOARD: R. Barsbold, E. I. Vorobjeva, Academician MPR B. Luwsandanzan, Academician L. P. Tatarinov (editor-in-chief), B. A. Trofimov, V. Yu. Reshetov, M. A. Shishkin Transactions of the Joint Soviet-Mongolian Paleontological Expedition 19: 5-119, 30 figs. QE 756.T7 translated by C. Siskron and S. P. Welles translation edited and emended by M. Carrano: [text in brackets] has been added or altered from original translation, or represents an unclear original translation. ??? SECTION CHAPTER I COMPARATIVE MORPHOLOGY OF CARNIVOROUS DINOSAURS SKULL The skull of carnivorous dinosaurs, in spite of a series of detailed studies, has been largely known in general terms. As a rule, its structure has been illustrated in external view, and often the external appearance and proportions were given too great a significance. In many cases a combination of the completeness and preservation of the skull material limited the [ability to perform] a more detailed study to a great extent. Until recently, cranial features were based on the study of hardly more than ten kinds of carnivorous dinosaurs. In addition, only single specimens displayed a more or less high degree of preservation and, as a result, a corresponding wealth of information. It is apparent that a series of basal elements in the skull construction of carnivorous dinosaurs have not been sufficiently illustrated, particularly with the plan of revealing certain features common to entire lineages of carnivorous dinosaurs. In many ways, this concerns the construction of the neurocranium and other skull structures and complexes, without sufficient knowledge of which it has been impossible to discuss the details of morphological groups. Recently a wealth of new information has become known regarding the construction of the skull in carnivorous dinosaurs. The facial crest of oviraptorids, a characteristic sphenoid capsule in the base of the skull, and the inner ear bones of Mongolian ornithomimids and saurornithoidids were noted among the most interesting features (Osmólska, Roniewicz, and Barsbold, 1972; Barsbold, 1974). A pocket was found in the ear region of the central Asiatic Itemirus that was originally described as "inner auditory" (Kurzanov, 1976a). The question of the development of the semicircular canals of the inner ear was raised, discussed controversially until recently (Rozhdestvensky, 1968; Kurzanov, 1976a; Ostrom, 1964), and now has been decided negatively. Foremost, the study of these listed features on the whole helps [to develop] a deeper knowledge of skull anatomy in the carnivorous dinosaurs. On this basis an attempt was undertaken to sort out new groups of carnivorous dinosaurs that revealed themselves more clearly upon study of their skull and that of the aforementioned Itemirus (Kurzanov, 1976a). Some of the features had been acknowledged as unique (as it turned out, without sufficient basis), and this was reflected in the characteristics of the taxon of the corresponding class. The material presently accumulated of Mongolian carnivorous dinosaurs clearly reveals a series of peculiarities in the construction of the basicranium and ear region. This justifies the need for addressing a series of questions on the skull anatomy of carnivorous dinosaurs in greater detail, and subsequent interpretations of the general aspects of their morphological evolution follow from their conclusions. Facial Region. The facial region (Fig. 1) is considerably elongate and fairly low in most small carnivorous dinosaurs, especially in a series of "narrow-skulled" forms such as dromaeosaurids (Velociraptor), saurornithoidids, ornithomimids, etc. At the same time, the facial region often becomes fairly tall as the skull becomes larger, which is characteristic primarily of carnosaurs and "high-skulled" dromaeosaurs. Small-sized oviraptorids are characterized mostly by their shortened facial region. The upper jaw was dentigerous in the majority of forms, or edentulous, as in garudimimids, the first group of carnivorous dinosaurs to become well known as being equipped with a horny beak (Marsh, 1881). Later, oviraptorids were added to this group (Barsbold, 1976a, b; Osborn, 1924), and most recently caenagnathids (Osmólska, 1976), of which only the lower jaw is known (Sternberg, 1940; Cracraft, 1971). Erlicosaurus is a special case in which the front part of the jaw was toothless. The upper jaw in ornithomimids, which comprises the bony base of the upper beak covering, is elongated and slightly widened, and on the whole preserves the proportions known in the toothed forms. The upper jaw of oviraptorids is completely opposite – it was extremely tall, yet the main bone comprising it is the premaxilla, with a wide lateral surface that serves as the basis for the horny beak. The jaw bones are highly shortened, which is uncharacteristic for carnivorous dinosaurs. In segnosaurids, the general proportions of the upper jaw are approximately the same as in toothed forms, however the premaxilla and the anterior region of the mandibular bones were edentulous, whereas the greater part of the dentary in the lower jaw bore fine teeth of a "carnivorous" nature. The dimensions and shape of the toothless portion of the upper jaw reveal the shortening and slight widening of the horny covering. The external nares are usually located on the anterior end of the facial region. Only in oviraptorids were they placed posteriorly and sharply dorsally, higher than the antorbital fenestra, which is an essential deviation from the norm in carnivorous dinosaurs. The nasal sinuses in segnosaurids are more unusual, extending for more than half of the facial region. The jaw bones anterior to the antorbital fenestra are perforated by one or two small openings that are usually called accessory antorbital fenestrae (Osborn, 1912). However, the medial construction of the jaw bones turns out to be fairly complex. Isolated regions are [present internal to] the nasal process of the maxilla that are interconnected but divided by a partition, the maxillary septum (Madsen, 1976a). The thinness of the medial wall does not match their preservation in the excavated condition, and this has prevented a more detailed understanding of their construction until now. A similar structure has been observed in ceratosaurids under the name of the jaw region or maxillary sinus (Hay, 1908) and, [subseuquently], in Allosaurus and Marshosaurus (Madsen, 1976a), apparently in Deinonychus (Ostrom, 1969a, b), and also in the Mongolian ankylosaur Saichania (Maryanska, 1977). Apparently this region was connected with the nasal, and so they should be called maxillary sinuses. They begin immediately behind the external nares and open laterally to the antorbital fenestra. In oviraptorids, maxillary sinuses are located above and behind the posterodorsal displacement of the external nares. In allosaurids (Madsen, 1976a), the sinuses are perforated anteriorly and posteriorly, and consist of two chambers divided by the maxillary septum. Each of these chambers has a separate medial and dorsal opening. The sinuses stretch along the inner surface of the maxilla along the occupied space corresponding to the palatal processes contacting each other, which can be clearly observed in tyrannosaurids. The dorsal surface of the facial region in some groups is highly sculptured. Among some ceratosaurs there is a large projection with the appearance of a widened nasal horn, and in others paired sculptured crests extend along the edges of the nasal and prefrontal to the lacrimal. The greatest expression is reached in Dilophosaurus, and to a lesser degree in allosaurids (Marsh, 1884; Hay, 1908; Gilmore, 1920; Madsen, 1976a, b). However, these are all Early Jurassic forms. Later lineages are known only by the later development of characteristic bone extensions. The unpaired, laterally compressed, tall, elongate crests in oviraptorids (Barsbold, 1981) extend from the upper part of the premaxilla to the anterior edge of the frontal, inclusively (Fig. 2). The premaxilla carries one more extension besides the usual nasal processes, that being the most anterior one, which rises from the anterioventral edge of the bony crest. The nasal and maxillary processes bound the large, elongated external nares. The large posterior part of the crest, apparently formed from the nasal bone, has very narrow and elongate anterior processes that form the dorsal edge of the external nares, above which passes the highest part of the crest. The frontals participate in its posterior edge via their massive extensions, which the dorsal edge of the crest passes as it falls obliquely. The lateral surface is not solid, but is composed of thin, braiding bone. The lateral walls of the crest are [appressed] almost to the point of contacting one another. The anterodorsal, more or less solid bony edge of the crest bears narrow, elongate paired opening or pockets, not clearly delineated and divided by a lateral projection. The depth of the opening could be limited by the width of the solid bony edge, [but] they freely connect with the narrow space between the walls of the crest. Its open structure testifies to the fact that the crest could have had a horny covering (as in cassowaries) that demanded a great deal of blood. In the cassowary, the bony base of the horny crest occupies the roof of the skull all the way to the parietal, and is also considerably distinguished by the porosity of its bone. It should be noted that the bony crests are particularly notable in the skulls of larger oviraptoroids, and absent in other forms – Ingenia. In this, the only skull of smaller size and somewhat doubtfully classified among oviraptoroids, the crest is absent, allowing us to note age or sex variations. The Anterior Neurocranium. It is sufficient to limit the study of this part to the laterosphenoid and orbitosphenoid, rather than other elements that are more subject to modification in different types of carnivorous dinosaurs. Laterosphenoids are present in all the studied groups. Often they form a large part of the lateral wall of the neurocranium (Fig.