You've probably never really wondered where your skeleton came from, but that has puzzled paleontologists for over a century. In particular the question mark was hovering over the skeletons of a strange, ancient fish family called heterostracan, but now UK scientists say they've cracked it, declaring the 400 million-year-old fossils to be the oldest examples of bone ever found.

The skeletons of modern vertebrates are made up of four different types of tissue that mineralize as we develop to become strong and rigid. Bone is obviously the main one, but there's also cartilage in there, as well as the dentine and enamel that make up teeth.

Exactly when and how this vital piece of biology came to be is still unclear, but the answer may lie in the fossils of heterostracan. These weird old fish lived during the Silurian and Devonian periods, between about 444 and 380 million years ago, and they were some of the first vertebrates to evolve mineralized skeletons. That said, those structures don't appear to be made up of any of the four modern skeletal tissue types.

"Heterostracan skeletons are made of a really strange tissue called 'aspidin'," says Joseph Keating, lead researcher on the study. "It is crisscrossed by tiny tubes and does not closely resemble any of the tissues found in vertebrates today. For 160 years, scientists have wondered if aspidin is a transitional stage in the evolution of mineralized tissues."

The controversy, according to the researchers, is whether aspidin is a type of cellular or acellular bone, dentine or some kind of evolutionary middle ground. The key seems to be the strange spaces in the middle of the aspidin, which have been suggested to have contained cells, cell processes or bundles of fibers, each of which would indicate a different kind of tissue.

To peek closer than ever before, the scientists used Synchrotron Tomography, a detailed form of CT scan that uses very high energy X-rays. The researchers found that the spaces had a linear shape, which they say leaves only one option – aspidin is acellular bone, making it the earliest evidence of bone found so far in the fossil record.

"These findings change our view on the evolution of the skeleton," says Phil Donoghue, co-author of the study. "Aspidin was once thought to be the precursor of vertebrate mineralized tissues. We show that it is, in fact, a type of bone, and that all these tissues must have evolved millions of years earlier."

The research was published in the journal Nature Ecology & Evolution.

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