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What do birds and dinosaurs have in common? Tendon ossification

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A team of Polish paleontologists proved that the soft tissues ossified in the tendons of dinosaurs. Such a phenomenon is currently observed in birds, when the soft tissues of the tendons are mineralized during the lifetime of the individual. The research findings were published in the Zoological Journal of the Linnean Society.

The soft parts of extinct vertebrates, such as collagen, blood vessels and cells, are rarely preserved in fossils. Polish paleontologists were the first to identify these soft elements in the tendons of dinosaurs, including those of the Homalocephalus living in the Late Cretaceous (about 80 million years ago). The team also found that the soft tissues ossified in dinosaur tendons. Such a phenomenon is observed today in birds, when the soft tissues of the tendons are mineralized during the life of an individual. – The results of our research prove that the pattern of the tendon mineralization process and the process of cell and tissue differentiation are shared by dinosaurs with modern birds – explained the team leader, Dr. Dawid Surmik from the Institute of Earth Sciences of the University of Silesia in Katowice.

In the tendons of dinosaurs, the process of ossification took place

Tendons are the elements that connect muscles to bones. In most vertebrates, they are flexible and flexible, but in dinosaurs, some tendons were ossified. Dr. Surmik explained that in dinosaurs, as in birds, ossification of tendons led to the formation of structures typical of bones, for example, secondary osteons (the osteon is the basic structural and functional unit of compact bone tissue) and bone-like cells (responsible for the exchange of nutrients in bones). ‘This was despite the fact that tendons before ossification are made of fibrous connective tissue, not cartilage, as is the case with most bones before ossification. Previously, it was suggested that only bone-like structures were present in ossified tendons, but this view can now be rejected, he added. Therefore, tendons, although they are usually made of flexible and durable connective tissue, can ossify, i.e. change into bone tissue. Such ossified tendons occur, among others, in modern birds, for example in turkeys, in those parts of the musculoskeletal system that are exposed to heavy loads. “Tendon ossification may have occurred early in the evolution of certain groups of archosaurs (primates), which include pterosaurs, dinosaurs and modern birds, as an adaptation for more efficient locomotion or weight distribution,” explained the paleontologist.

Reconstruction of the Late Cretaceous Homalocephale dinosaur from the Gobi Desert in Mongolia. One of the dinosaurs whose tendons were studied by Polish scientists Jakub Zalewski

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The challenge of Polish paleontologists

As described by Dr. Surmik, the biggest challenge during the research was the limited number of samples and preparation (obtaining, extracting and purifying elements and chemical compounds in laboratories) of very small fossilized structures. The conducted analyzes were destructive, which was associated with irreversible damage to the tested material. “Therefore, the minimum number of samples was used, and the sequence of analyzes was planned very meticulously to use as little valuable material as possible, achieving the most objective results” – said Surmik. Samples of fossilized tendons were dissolved in a special chemical solution to remove phosphate, which is the main mineral component of bone. As a result of the dissolution of the bones, numerous, although very small, only a tenth of a millimeter, petrified fragments of blood vessels were obtained. They were accompanied by cells attached to them, with an average size not exceeding 0.01 millimeter. Precise studies of such tiny structures required the use of microscopes, including a scanning electron microscope and an atomic force microscope. – Fossilized soft parts (e.g. vessels, cells, collagen fibers) in fossil vertebrate bones from the Mesozoic (252-66 million years ago) have been described since the 1960s, and the very fact of the preservation of bone cell-like structures in cross-sections of fossilized bones studied under the microscope was noted by researchers as early as the 19th century. Recently, thanks mainly to the spectacularly preserved vertebrate fossils from China, cells have been identified in fossilized cartilage tissues. Never before, however, had fossilized tissues been searched for in the ossified tendons of non-avian dinosaurs. We searched and managed to find them, and they are preserved in excellent condition – concluded the scientist.

Main photo source: Jakub Zalewski

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