The outcrops of Late Triassic rock in the village of Krasiejow in western Poland have been the subject of palaeontological investigations since the 1990s, yielding a large number of important Tetrapod fossils, including the Temnospondyls Metoposaurus krasiejowensi and Cyclotosaurus intermedius, the Phytosaur Paleorhinus cf. arenaceus, and the Dinosauroform Silesaurus opolensis. One notable Krasiejow Vertebrate is the Diapsid Ozimek volans, which has been interpreted as a potential gliding Animal on the basis of its very long and graceful forelimb and hind-limb bones.
Ozimek volans was originally interpreted as a Sharovipterygid, grouping it with the Middle-Late Triassic Sharovipteryx mirabilis from Kyrgyzstan, also thought to have been as a glider, but more recently has been considered to be a member of the Tanystropheidae, a group of early Archosauromorphs often exhibiting long necks and tails, at least some of which may have been aquatic.
No close relative of Ozimek volans has a similar limb arrangement; a similar pattern is seen in Sharovipteryx mirabilis, but this can be ruled out as a close relative on the basis of other parts of its anatomy, suggesting the similarity is the result of similar ecological adaptations, rather than common ancestry. Interestingly, Sharovipteryx mirabilis has a preserved membrane reaching the end of its hind limbs, leading weight to the idea that this species was adapted to gliding, and therefore that the similar morphological adaptations seen in Ozimek volans could also be adaptations to aerial behaviour.
In Birds and Pterosaurs, flight was achieved not just by the elongation of the limbs, but by numerous adaptations to reduce the weight of the skeleton. The most notable of these is the appearance of pneumatic bones, in which the dense marrow filling of the bone is replaced with a hollow, air-filled cavity. This is known in living Birds, and has been demonstrated in Mesozoic Birds, as well as closely related Theropod groups, and Sauropods, although it is only inferred in Pterosaurs, on the basis of the large size these Animals achieved. Both Birds and Pterosaurs have extremely thin bone walls, made of dense fibrolamellar bone tissue, which is very strong and better able to withstand the stresses of flight.
Notably, neither the presence or absence of pneumatic bone nor fibrolamellar bone tissue can be used as absolute determiners of flying behaviour. Fibrolamellar bone tissue is found in a number of Archosaur groups which show no other adaptations to flying, but absent in both small Birds and Bats, while Bats also lack pneumatic bone, something found in the (clearly non-areal) Sauropods. Thus, to establish the flying capacity of an extinct Animal it is necessary to look at its whole morphology, including skeletal and muscular adaptations and the possible presence of flight membranes and/or feathers.
Gliding is a form of flight in which the Animal has active control of aerodynamic forces, but is unable to gain altitude by muscular activity. Such behaviour can be hard to determine in an Animal simply from its morphology, as it is achieved in different ways in a very wide range of organisms, with no single set of adaptations common to all. Indeed, many gliding organisms are morphologically little different to their closest non-gliding relatives, achieving flight purely through behavioural changes. Examination of living gliding Mammals suggests that long humeri and femora improve the aspect ratio of gliding Animals, and that this becomes more important as the Animal becomes larger. Little is currently known about the bone histology of gliding Mammals, although some Flying Squirrels have a light-weight humerus and more circular diaphysis in cross-section compared to non-gliding taxa, which helps to resist torsional loads and provide resistance to multidirectional bending.
Ozimek volans has been noted to have had very thin bone walls, an adaptation associated with weight-reduction and flight, but its histology has not, to date, been examined, limiting our understanding of the living Animal's growth and life history.
In a paper published in the journal Palaontology on 26 June 2024, Dorota Konietzko-Meier of the Institute of Organismic Biology at the University of Bonn, Elżbieta Teschner, also of the Institute of Organismic Biology at the University of Bonn, and of the Institute of Biology at the University of Opole, Agnieszka Tańczuk of the Department of Zoology and Nature Protection at the Maria Curie-Skłodowska University in Lublin, and Martin Sander, again of the Institute of Organismic Biology at the University of Bonn, present the results of a study of the bone histology of Ozimek volans, and the implications of this for the life history and behaviour of the living Animal.
Most of the material from which Ozimek volans was described is held in the collection of the Institute of Paleobiology of the Polish Academy of Sciences in Warsaw, though two blocks containing bones assigned to the species are held in the collection of the Institute of Biology at Opole University, and it is material from one of these blocks which is used in Konietzko-Meier et al.'s study. The block contains a cluster of bones interpreted as a partial skeleton, including articulated cervical vertebrae, possible coracoids and a pes, as well as a humerus and a femur, although Konietzko-Meier et al. note that the humerus is only 46% of the size of the largest known humerus, and therefore have been presumed to have come from a juvenile, while the femur is the largest known femur assigned to the species, and at least twice as long as the humerus, making it unlikely that they were from the same Animal.
Konietzko-Meier et al. took three thin sections, each about 40μm thick, from the midshaft of the humerus, one transverse, one longitudinal and one oblique tangential, and two from the femur, the bone here being too thin for an oblique section to be taken. These were then examined under both light and scanning electron microscopes at the University of Bonn.
The humerus has a thickness of 3.4 mm at the site where it was sampled, with a cortex (outer layer of bone) thickness of 0.5 mm. This cortex is made up of two layers, the periosteal portion, which is laid down on the exterior as the bone grows, and a compact endosteal layer, formed from the inner surface outwards as a secondary structure, by reworking of the original bone. The outer periosteal is more heavily mineralized, and made up of fibrous lamellae with simple vascular canals, with several lines of arrested growth (marks left in growing bone by slowed growth at one time of year, typically winter in a temperate climate). The fibres which make up this layer are not arranged at random, but have a longitudinal orientation. No sign of pneumosteum can be seen anywhere on the bone.
The femur is 2.7 mm in width at the point where it was sampled, with a cortex thickness of 0.2 mm. The structure of this bone is less complex, with signs of bone resorption on the inner surface, but no endosteal cortex layer. The periosteal layer is again lamellar and highly mineralized, with several lines of arrested growth. Vascular canals are more tightly backed than in the humerus.
Konietzko-Meier et al. recognise that their sample size is small, but propose thar some simple observations about bone growth in Ozimek volans can be made. Bone growth is typically driven from the periosteal surface, where lamellae of new cortex tissue are laid down. A line of bone reworking called the Haversian substitution front progresses outwards from the medullary (inner) edge of the bone, producing a layer of secondary tissue including Haversian bone. At the same time, a resorption line also works outwards from the inner surface, removing bone tissue, and sometimes overtaking the Haversian substitution front.
This three-front model of bone growth was first observed in Sauropods, but has now been found in a wide range of Tetrapod groups. However, the compact endosteal layer is a tissue not seen in Sauropods (or most other Tetrapods) and may represent a fourth growth front (i.e. lamellar bone/Haversian bone/compact endostreal bone/resorbtion line). Notably, the Hacersian growth front in Ozimek volans appears to be very slow, generally being obliterated by the endostreal growth front and resorption line.
Lamellar bone is common in Tetrapods, particularly non-endothermic ones, although it is also found in many small Mammals. However, the lamellar structure of Ozimek volans is exceptionally well-developed, unlike anything seen in any other fossil group. However, something similar has been observed in living Bats, and is thought to be an adaptation enabling the formation of lone, lightweight, and strong bones needed for flight in a group which has not evolved pneumatic bone (is has also been suggested that the adoption of strong lamellar bone rather than pneumatic bone has placed a restriction on the size to which bones can grow). It is therefore quite possible that this adaptation in Ozimek volans was also an adaptation to flight (although in this case unpowered).
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