Trilobite

Trilobites were a species of arthropods.

In some universes, including that of teen genius Professor Awesome, they were extinct by the modern age, but “showed up as fossils” a lot. In another, however, they survived into the modern day in great numbers, with some becoming terrestrial; Jenny Everywhere said that there were “beaches and jungles full of them if you know where to look”, as, indeed, she herself did.

After Mr Hernandez, the young Professor Awesome's science teacher, told the class to bring trilobites to school if they had any (not specifying “fossils”), the boy got Jenny to take him to the aforementioned other universe, where they collected a box full of live trilobites. After the boy brought them to class and went to show them to Hernandez, he foolishly opened the box and then dropped it, allowing the trilobites to skitter free across the classroom, terrifying the class and teacher. (PROSE: Pit Stop)