A 508-million-year-old critter — one that looks like a weird lobster with 50 legs, two claws and a tent-like shell — is the oldest known arthropod with mandibles on record, a new study finds.Arthropods are a group of invertebrates that includes spiders, insects and crustaceans. Many arthropods, including flies, ants, crayfish and centipedes, have mandibles...