Fish fossils shed light on origin of skeletons

Avatar By Joseph Scalise | 2 years ago

Ancient fish-like animals shed light on the evolution of modern vertebrates and reveal how bones came to be, according to a new study published in Nature Ecology & Evolution.

All living vertebrates’ skeletons are made up of four distinct tissue types: bone, cartilage, dentine, and enamel. Each of those materials are special because they mineralize as they develop in a way that makes the skeleton rigid, durable, and strong.

While that is a well-known fact, there are many questions about the way skeletons first came about that scientists cannot answer.

The earliest known evidence of skeleton evolution comes from a group of fish known as heterostracans. The group existed more than 400 million years ago and it had some of the oldest vertebrates with a mineralized skeleton on record.

Even so, researchers have never been able to figure out exactly what tissues made up heterostracan skeletons.

To shed light on that mystery, a team of scientists from the University of Manchester and the University of Bristol used CT scanning to look inside heterostracan skeletons and identify their makeup.

“Heterostracan skeletons are made of a really strange tissue called aspidin,” said lead author Joseph Keating, a scientist at the University of Manchester, according to Phys.org. “It is crisscrossed by tiny tubes and does not closely resemble any of the tissues found in vertebrates today. For a 160 years, scientists have wondered if aspidin is a transitional stage in the evolution of mineralized tissues.”

Though empty now, the tubes once housed fiber-bundles of collagen — a protein found in skin and bones.

That is important because it shows that aspidin is in the earliest evidence of bone in the fossil record. Such a discovery could alter the common view of skeleton evolution and greatly impact the way scientists analyze bones moving forward. It also alters the current fossil timeline as well.

“We show that [aspidin] is, in fact, a type of bone, and that all these tissues must have evolved millions of years earlier,” said study co-author Phil Donoghue, a researcher at the University of Bristol, according to Cosmos.