Four hundred fifty-five million years ago, Earth was a hot place with high sea levels that had flooded many of the landmasses, including what would later become North America. This spreading ocean was exploding with new life, such as the bony-plated fish that still stand as some of the oldest-known vertebrates.
When they died, some became buried in the sediment that covered the ocean floor and fossilized, to be discovered later by paleontologists living in a drier time. One such find, from ancient deposits in Colorado, contains parts of a jawless fish that a new study has scanned with modern X-ray tomography.
