[D] Caenagnathus [sG]
Cretaceous Late Campanian
Saurischia Theropoda Tetanurae Coelurosauria Oviraptorosauria Caenagnathidae
Caenagnathus (Sternberg, 1940) > Caenagnathus collinsi (Sternberg, 1940) > Caenagnathus sternbergi (Cracraft, 1971)
NMC P-3605/8776; partially damaged lower jaw; Upper Cretaceous, Oldman Formation, Alberta, Canada.
Caenagnathus collinsi (Sternberg, 1940) Upper Cretaceous, Judith River Group (Wedge), Oldman Formation, Alberta, Canada.
Holotype. Description: Dentary occupies at least two thirds of the length of the lower jaw. Its occlusal edge is equally concave throughout. The symphysis occupies a little less than half of the length of the dentary. The rostral part is flared, slightly concave, with longitudinal convex ridge-like crests, supposedly connected with the development of the thickened ramphothec 17 (?).
The processes of the dentary are horizontally directed posteriorly. The top of the low adductoral projection is displaced posteriorly and is located on the middle of the length of the oblong lateral opening, the antero-ventral limited area the edges of which are formed by the dentary, upper edge almost completely by the surangular, the posterior by the surangular and angular. The articular joint surface has a rounded outline, with a longitudinal projection, which divides the articular processes, of which the medial is wider than the lateral, with a massive retroarticular process, slanted downwards.
In the framework of the general features of similarity of the lower jaw of caenagnathids and oviraptoris the basic differences between them are as follows: Caenagnathids Oviraptors Gently sloping concavity along Slight concavity only in the the entire occlusal edge. extreme anterior part of the edge, the rest is convex. Symphysis elongated. Symphysis shortened. Lateral opening is displaced Opening, displaced forward, posteriorly, rounded, wide, narrow, elongated, undivided. consists of two sections. Upper edge of the opening is Upper edge is formed by the formed by the surangular dentary, posterior by the and angular surangular. Adductoral projection is located Projection is located posteriorly above the middle of the lateral to the opening. opening.
Upper process of the dentary is Process is directed along an directed horizontally to the arch dorsally and posteriorly. Posterior. 18 It is important to note, that in contrast to the original material on oviraptorids the study of Caenagnathus in this case was based only on written material, in many areas highly limited. This is why only the most basic, documented features were taken into consideration in the characterization of its lower jaw, in the background of similarity with the oviraptor characterization forming also considerable differences. In all likelihood, the detailed study of the original material will considerably broaden the basis of both the similarities and the differences between these animals.
Upper Cretaceous, Oldman Formation. Location: Alberta, Canada. Caenagnathus sternbergi (Cracraft, 1971).
NMC #2690; posterior end of the right lower jaw branch; Upper Cretaceous, Oldman Formation, Steveville, Alberta, Canada.
Holotype. The extreme limitedness of the existing material makes it difficult to examine. We can only note that the configuration of the articular surface is somewhat different from that of the species type.
Upper Cretaceous, Judith River Group (Wedge), Oldman Formation. Location: Steveville, Alberta, Canada. Development of Toothless Theropods Despite the high degree of specificity, the toothless carnivorous dinosaurs share the common theropod structure, which is especially easy to observe in the postcranial skeleton. At the same time, such peculiar features of the skull 19 anatomy, as the recently discovered basisphenoidal capsule, the blockage of the basipterygoidal articulation, the development of the parabasal canals, were not exclusive to the ornithomimosaurs.
Among other groups, the most remarkable in this sense are the saurornithoidids (Barsbold, 1974) and the segnosaurs (Perle, 1979). The establishment of the primitive ornithomimosaurs with the developed first toe and the specific differentiation from the better known progressive forms confirms the theory of development of this whole group from a basic theropod branch. The primitive ornithomimosaurs apparently developed for a long time in the zone of adaptation, fairly close to the basic progressive branch.
We can allow the possibility that, that in the representatives of the latter occurred a displacement, which only deepened the “ornithomimid” specialization. The parallel evolution of the two lines occurred already in the Early Cretaceous of Mongolia, judging by the undescribed fine in the Southeast of the most ancient progressive ornithomimids (Hurenduh, upper Dzunbaysk suite; Aptian-Albian; another progressive ornithomimid is known from approximately the same time in the Cloverly Formation, USA (see Ostrom, 1970).
The development of oviraptorosaurs went by a path that demanded different adaptations, which are the most clearly expressed in the structure of the peculiar toothless jaw apparatus. The evolution of oviraptorosaurs was directed toward the strengthening compressing possibilities of their jaws, adapted for the squashing of hard objects (Barsbold, 1977). We must particularly note the general principal similarity of the lower jaw of the oviraptorids and the caenagnathids , which suggests the possibility of 20 kinship. Apparently, caenagnathids developed in the adaptive zone, different from that of the oviraptorids, but on the basis of a single original structural plan.
The absence of the clavicle and sternum (the remains of the poorly preserved one not taken into consideration) among the carnivorous dinosaurs always in the past served as negative proof of a possible connection with birds. Presently the oviraptorids—the first group of theropods, where the clavicles were not only present, but had bird structure, and the sternum was similar to the analogous structure in the keel-less birds. If one is to add to this the pneumatization of some of the skull bones (Osmolska, 1976) and the development of the crest, in general similar with the casuary’s crest covered by a horny shield, then oviraptorids occupy one of the most interesting positions in the comparative morphology of birds and carnivorous dinosaurs, which is actively being developed in recent years (Ostrom, 1973, 1976)
Source: Polyglot Paleontologist