In the dissertation, the histological structure of the long bones of birds from the fossil order Odontopterygiformes and its connection with their biological, particularly ecological, features were studied for the first time. The study was conducted on the material from the collection of the National Museum of Natural History at the National Academy of Sciences of Ukraine. This material comes from Eocene (lower Lutetian) locality Ikove (Luhansk region) and represents two species: Lutetodontopteryx tethyensis Mаyr еt Zvоnоk, 2012 та cf. Dаsоrnіs sp. Bone microstructure of these species and osteohistological differences between them are described. It was found that in respect to the microstructure of the bones, these birds are generally similar to recent representatives of Neognathae: bulk of the cortex is composed of woven-parallel tissue (as a rule, without growth layers), which is covered externally and internally by layers of lamellar tissue, sometimes with growth layers. This indicates the same pattern of growth as in modern Neognathae (rapid growth and its sudden termination in most cases) and its comparable duration (within one year, contrary to previous assumptions). It was found that the growth layers in the inner circumferential layer of the femur of cf. L. tethyensis were formed with a period of much less than a year, which makes them unsuitable for determining individual age (in recent birds, this method was reported to be suitable for some species and unsuitable for others). The degree of remodeling of the examined bones varies from practically absent to almost complete. The latter is sometimes observed in both species and indicates particularly high individual age and, possibly, repeated oviposition. Some bones of both species show partial remodeling of the inner circumferential layer: a phenomenon, described, but not studied, in birds and mammals. At least in the bones of young individuals, it can suggest lack of food. The relatively fast growth of Odontopterygiformes enables them to specialize in feeding on squid, a resource whose abundance usually varies greatly within the year, and which best corresponds to the morphology of their pseudoteeth. The direction of most bone fibers and vascular canals in the studied humeri of Odontopterygiformes is longitudinal, and in femora they run at a greater angle to the longitudinal axis of the bone. This is consistent with what is observed in recent birds with gliding flight and may have biomechanical reasons (inclination of the fibers to the longitudinal axis increases the strength of the bone with respect to torsional loading, which is weaker in the humerus of birds with gliding flight, than with flapping one), although interpreting the direction of bone fibers as an adaptation to the direction of the loading is controversial. Most of the examined bones show manifestations of microscopic bioerosion, most often represented by Wedl tunnels, the origin of which is associated with fungi or cyanobacteria. Tunnels of atypical morphology are also observed, which indicates taxonomic diversity of the organisms that created the tunnels. The bioerosion of the bones indicates their accessibility to microorganisms shortly after the death of the animal and carries paleoecological and taphonomic information about the bones and Ikove location. A technique for the production of ground sections for microscopic examination is proposed, which does not require hard-to-reach means and is designed to obtain sections, well suited for polarizing microscopy and long-term storage. Equipment for serial sectioning of hard objects and equipment for polarizing microscopy and photographing of microslides has been designed, which is suitable for work on non-specialized microscopes with non-specialized objectives and allows to take pictures at different directions of polarizers without rotating the object, which facilitates comparison and combination of images. Software was written for processing photomicrographs starting from the stage of "raw" data from the photo matrix.
