Ever wonder how creatures on Earth grew heads? Probably not, but they seem to have started with a hard plate in front of their brains. A recent study in Current Biology says fossils dating back hundreds of millions of years show the first signs of this plate, known as the anterior sclerite, which came with tiny eye-like sensors, Smithsonian reports. It's visible in two fossils—of a squishy creature and a harder one—that show how the plate began growing before prehistoric creatures had developed a skeleton, the study says. The fossils reveal "one of the major transitional steps between soft-bodied worm-like creatures and arthropods with hard exoskeletons and jointed limbs—this is a period of crucial transformation," says lead author Javier Ortega-Hernández.
Both fossils date from the Cambrian Period roughly 500 million years ago: the soft trilobite and weird, submarine-like Odaraia alata, which had eyes on stalks and is animated in this video, reports Discovery. The study also spotted an anterior sclerite in the fossil of another Cambrian creature, the Anomalocaridid, which shared an ancestor with the Odaraia alata. The hard plate likely fused into parts of the modern head over time, the study says, as these animals evolved from soft to hard and became today's spiders, crustaceans, and insects, Eureka Alert reports. "Heads have become more complex over time," says Ortega-Hernández. His study "gives us an improved understanding of the origins and complex evolutionary history of this highly successful group." (Read about chicken embryos grown with dinosaur snouts.)