"I would say that the patterning of the skin with placodes is an evolutionary developmental innovation that allowed the ancestor of amniotes to develop some sort of bumps distributed across the skin and providing some additional mechanical protection," Milinkovitch said.
He said modern amphibians could even sport this ultra-ancient look.
"These animals are supposed to be 'naked,' but quite a few species exhibit bumps and scaly appendages that might have a link with the placodes of amniotes," he said. "After all, this could be an older innovation that occurred in the ancestor of all terrestrial vertebrates (animals with a backbone)."
RELATED: Why Do Peacocks Have Spots on Their Feathers?
Marcelo Sánchez-Villagra, a professor at the University of Zurich's Paleontological Institute and Museum, told Discovery News that the common ancestor of birds, mammals and reptiles "would not be a reptile by definition, but an animal that lived around 330 million years ago."
Sánchez-Villagra's colleague, Torsten Scheyer, added that the common ancestor was likely a "reptiliomorph," named as such "because they combine amphibian features with reptilian features."
Scheyer says reptiliomorphs were mostly aquatic predatory animals that inhabited a variety of different habitats ranging from coal swamps and forests to warm Savanna-like regions. Their fossils have been found in Germany, Texas and at other locations.
Although it appears that birds, mammals and reptiles inherited their placodes from this reptiliomorph common ancestor, clearly a lot of evolution has taken place since that early time.
To better determine how placodes can result in so many different looks, Milinkovitch and Di-Poï investigated bearded dragons. These lizards vary in appearance, with one type of bearded dragon having a lot of scales, another featuring reduced-size scales, and a third with no visible scales whatsoever.