Scientists have discovered the oldest known bone at the moment – in fossils of about 400 million years old.
Skeletons of modern vertebrates consist of four types of tissues, which are mineralized during development. Bone is one of the varieties of such tissues: besides it, there are cartilages, as well as dentin and enamel, from which the teeth consist. When and how this skeleton was formed, scientists still can not accurately determine, but a new discovery may help in this.
The fossilized remains referred to belonged to heterostracks, or raznoschitkovym (a subclass of extinct fossils from the class of paranodubital). The animals of this subclass are supposed to have lived during the Silurian and Devonian periods (approximately between 444 and 380 million years ago), and they are considered one of the first vertebrates to develop mineralized skeletons. True, these structures did not include any of the modern types of skeletal tissue.
The skeletons of heterostructures consist of a strange tissue called “aspidin”, says Joseph Keating, the lead author of the study. According to the expert, this tissue is not like any of those that make up the skeletons of modern vertebrates. “For 160 years, scientists have questioned whether aspidin is an intermediate stage in the evolution of mineralized bones,” Keating notes.
According to researchers, the discussion is whether aspidin is a cellular bone, an acellular bone, a dentin, or a transitional stage between dentin and bone. It was especially important to understand the nature of demineralized sites in the matrix of aspidin: it was assumed that they could contain cells, cellular processes or Sharpey’s fibers.
To solve the riddle, the researchers studied the skeletons of heterostructures using synchrotron tomography – a type of computed tomography using high-energy X-rays. As a result, experts determined that these areas contained bundles of collagen fibers and that aspidin is precisely an acellular bone.
Aspidin, thus, is the oldest bone of known science, the researchers note. “The results of the work change our view of the evolution of the skeleton. Aspidin was considered the precursor of mineralized vertebrate tissues. We have shown that it is in essence a type of bone and that all these tissues should thus have developed millions of years earlier than expected, “says Phil Donoghue, co-author of the work.