Vol 46, No 7 (2019)

The jaw apparatus of the parakeet auklet (Aethia psittacula) has been investigated in comparison to that of the crested auklet (Aethia cristatella). The mechanical and biological effects of the distinctive bill structure in the parakeet auklet have been analyzed. The oral slit is concave; therefore, equilibrium is achieved without the need for disabling the posterior or anterior parts of the dorsal adductor of the lower jaw and the adductor can act with the maximal force regardless of the position of the object clamped in the bill. Shearing of large food objects, such as polychaetes and jellyfish, by the parakeet auklet bill is a jaw apparatus specialization unique among birds. Strengthening of the jaw apparatus, which was initially adapted for capturing small planktonic crustaceans, involved the development of additional aponeuroses in the internal adductor, consolidation of the latter, shortening of the bill, and changes of the shape of the oral slit opening.

The works by F.Ya. Dzerzhinsky and his disciples have rendered the jaw apparatus of birds a conventional and thoroughly characterized research object for functional morphology in Russia. The author, who communicated with Dzerzhinsky for many years, attempts to reconsider his scientific heritage in a critical manner and to use it as a foundation for outlining the most promising directions of further research. The analysis shows that underestimation of the problem of muscle mass economy in the jaw apparatus, despite the critical importance of any excess ballast for birds, was a serious drawback of Dzerzhinsky’s method. A new model based on the separation of the kinematics of bill opening and food object compression provides a solution for the problem of economical use of muscle mass in the avian jaw apparatus. The length of the major jaw adductor muscles remains virtually unchanged when the bill is opened, so that these muscles act as inextensible links of the closed multilink kinematic chain of the kinetic skull. Length change upon mouth opening being unnecessary, these muscles can be short-fibered, that is, lightweight but strong. The large force of these muscles is only used by the bird when the object is fixed in the bill and the jaw joint locking mechanism is switched on; a different kinematic chain used for object compression only is formed in this case. If the nonparallel arrangement of the pterygoid muscle and the jugal bar is also taken into account, the avian capacity for using the full force potential of the jaw adductor muscles without careful adjustment to each food object and the specific conditions of food object capturing becomes easy to understand. The main elements of the proposed model, which were found out by Dzerzhinsky himself, acquire a new meaning in light of the concept of economical use of the muscle mass. This enables us to make a major generalization concerning the adaptive significance of the cranial kinesis in birds as a means of reducing the jaw adductors’ mass without decreasing their force. In addition to the new model of jaw mechanics, other promising directions are proposed for the development of Dzerzhinsky’s work based on the expansion of the set of methods. The analysis of the aponeurotic framework of jaw muscles from the standpoint of optimizing the spatial packaging of muscle fibers may be a promising topic for research based on a combination of computer tomography and computational modeling.

Specific features of the sciurid auditory capsule have been analyzed based on 63 species from 20 genera. Its morphological specificity is characterized by a peculiar combination of primitive and advanced features stable within the group. They are (1) a rounded and inflated bulla with internal septa and a primitive attachment of the tympanic ring to the inner tympanic wall; (2) a transverse position of the bulla relative to the axis of the promontorium, resulting in the presence of deep petrosal fossae in front of and above the promontorium which are not covered by the tympanic bone; (3) a fully formed osseus facial canal and a developed bone tube of the stapedial artery; (4) the presence of a meato-cochlear bridge connecting the promontorium to the posterior wall of the acoustic duct which, in a fully developed form, does not occur in any other recent rodents; (5) pneumatization of the mastoid due to the expanding of the epitympanic chamber both anteriorly and posteriorly, with the formation of a premeatal pocket and a large epitympano-mastoid chamber divided into parts. The sciurid auditory capsule can generally be considered as morphologically advanced. Its diversity within the family concerns functionally significant features and is manifested in the structure of the external acoustic meatus, the size of the tympanic membrane, the length of the processes of auditory ossicles, and, most importantly, the degree of pneumatization of the auditory capsule. This is associated with quantitative variations of the characters and does not affect its structural plan, as a rule.

Among ancient and modern marine reptiles, several structural types of the locomotor apparatus were or are present, supporting different styles of swimming. Ichthyosaurs, mosasaurs, saltwater crocodiles, and representatives of many other groups swam or swim with horizontal undulations of the body primarily using the tail with a vertical caudal fin. Sea turtles with a reduced tail and their body completely immobilized by the shell use only limbs transformed into flippers for swimming. Sauropterygians had a substantially immobilized trunk and a horizontal caudal fin. They used a unique type of subaquatic locomotion with the leading role of two pairs of powerful flippers and vertical undulations of the tail, partial analogs of which are found in sea turtles, sirens, cetaceans, and some semi-aquatic mammals.

An electron microscopic study of the microstructure of the primary remiges in owls (Strigiformes) was conducted using 13 species: Nyctea scandiaca, Bubo bubo, Asio otus, Otus scops, O. sunia, Aegolius funereus, Athene noctua, Glaucidium passerinum, Surnia ulula, Strux aluco, S. uralensis, S. nebulosa, and Tyto alba. Owls have a number of species-specific microstructural primary remex characteristics that are taxonomically important. First of all, these are the structural features of the pennaceous barb, which differ markedly not only at the ordinal, but also at the species level: cross-section configuration, pith architectonics in cross- and longitudinal sections, and cuticular barb structures. This work emphasizes that the identified elements are of taxonomic significance only when comparing the specific, strictly analogous sections of the barbs in different bird species. Based on the present study, we conclude that along with the typical elements of feather architectonics characteristic of representatives of other bird orders, the owls show a number of peculiar microstructural features. Thus, as the result of an analysis of the microstructure of the owls primary remex pennaceous barb vanule, the unique features of the distal barbules and the structure of the apical section of the barb with tightly adjacent and elongate proximal and distal barbules could be distinguished. These characteristics are revealed to cause a dense fleecy structure of the vane dorsal surface and the presence of a complex of peculiar “bunches” that form the cleft edge, i.e., the purely specific traits of the owls feather that are not found in representatives of any other bird orders we have investigated.