The hardest bit of your body is the enamel coating your teeth. But new analyses of fish fossils, as well as genetic analyses of a living fish species, suggest that this specialized material once served a very different function: to toughen some bones and scales of ancient fish. The findings bolster earlier suggestions that ancient fish had enamel-armored scales, and they point to a new scenario for exactly how the substance ended up on teeth.
Enamel—an almost pure layer of a mineral called hydroxyapatite—coats the teeth of almost all tetrapods (four-limbed creatures) and lobe-finned fish such as coelacanths. Most living fish do not produce it, but Per Ahlberg, a paleontologist at Uppsala University in Sweden, found an ancient exception. Well-preserved fossils of an ancient fish called Psaroepis romeri reveal that this 20-centimeter-long minipredator, which prowled the seas between 410 million and 415 million years ago, had enamel in its scales and its skull—but not its teeth, according to a paper by Ahlberg and colleagues in the 24 September issue of Nature.
Other teams had found partial fossils of fish with enamel on their scales. But those fragments might not have belonged to the same individual, Ahlberg says, so researchers couldn’t be sure just how the enameled bits were distributed across the body, or if they came from individuals at different ages or developmental stages.
Ahlberg’s team instead looked at a single specimen of Psarolepis, slicing through the jawbone, skull bones, and scales to get a microscopic peek at their internal structure and so identify what they were made of. The teeth were naked dentine, the same material that underlies the enamel in your teeth and those of most modern tetrapods. But the scales and skull bones of this ancient fish included some enamel.
Researchers had suggested that over millions of years of evolution, hardened structures such as external scales gradually migrated into the mouth and changed shape to become teeth. But the patchy distribution of enamel in Psarolepis may suggest a different scenario, in which the pattern of enamel production, rather than the of shape and location of already enameled structures, shifted over time.
The team also analyzed the genome of the spotted gar (Lepisosteus oculatus), a modern-day species that produces a hard enamel-like material called ganoine that covers its scales. The genome shows that gar can produce two of the three proteins needed to make enamel, and suggests that ganoine is essentially a scale-coating version of enamel. Thus, it offers genetic support for the fossil evidence.
These findings “are very interesting,” says Zerina Johanson, a paleobiologist at the Natural History Museum in London. In contrast to previous ideas, the work suggests that hardened structures such as scales may not have physically moved from one place in the body to another as species evolved. Instead, evolution may have shifted the activity of enamelmaking proteins to new body parts.
“This may provide a better understanding of what was going on inside primitive vertebrates,” she says.