https://phys.org/news/2018-07-year-old-mystery-skeletons.html

Evidence for the early evolution of our skeletons can be found in a group of fossil fishes called heterostracans, which lived over 400 million years ago. These fishes include some of the oldest vertebrates with a mineralised skeleton that have ever been discovered. Exactly what tissue heterostracan skeletons were made from has long puzzled scientists.

Now a team of researchers from the University of Manchester, the University of Bristol and the Paul Scherrer Institute in Switzerland have taken a detailed look inside heterostracan skeletons using Synchrotron Tomography: a special type of CT scanning using very high energy X-rays produced by a particle accelerator. Using this technique, the team have identified this mystery tissue.