Whereas placodes in avian and mammalian embryos last long enough that they are easy to see, reptilian placodes exist for a brief period of about 12 hours, and pop up on different locations depending on the species.
Dr. Milinkovitch and his team did not originally set out to find placodes in reptiles; rather they were investigating why certain bearded dragons are born without scales.
By analyzing the genomes of the naked bearded dragons, they narrowed down the culprit to a mutation in a single gene, called EDA, which is also known to disrupt placode development in birds and mammals.
“There must be a link here because this guy doesn’t have scales, and in birds and mammals when that gene is mutated they don’t have hair or feathers.” The clue got the team thinking that maybe reptiles need placodes to make scales and led them to search the microanatomy of reptilian embryos. “When we investigated normal snakes, normal crocodiles and normal lizards we found placodes everywhere,” Dr. Milinkovitch said.
Their findings unite birds, mammals and reptiles as descendants from the same lineage of ancient reptilian creatures that first developed placodes.