New X-ray images of ancient fish fossils have helped scientists solve a 160-year-old mastery about the origins of the vertebrate skeleton.

Heterostracans are a group of fossil fishes that lived 400 million years ago. The heterostracan fossil record has offered the oldest evidence of mineralized skeletons among vertebrates. But scientists have struggled to determine what type of tissue heterostracan skeleton's were made of.

Bone, cartilage, dentine and enamel all mineralize as they develop, gaining strength and rigidity. But millions of years later, these fossilized tissues are difficult to distinguish -- until now.

Scientists at the universities of Manchester and Bristol used CT scanning technology, featuring high energy X-rays, to image the internal structure of heterostracan skeletons. The ancient fish built their skeletons using an entirely unique tissue.

"Heterostracan skeletons are made of a really strange tissue called aspidin," Manchester researcher Joseph Keating said in a news release. "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."

The hi-def images -- detailed in the journal Nature Ecology and Evolution -- suggest the aspidin skeleton's tiny tubes were once filled with bundles of collagen fibers.

"These findings change our view on the evolution of the skeleton," said Bristol researcher Phil Donoghue. "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."

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