 Marrella was the first Burgess Shale fossil that Whittington re @-@ examined , and gave the first indication that surprises were on the way . Although clearly an arthropod that walked on the sea @-@ floor , Marella was very different from the known marine arthropod groups ( trilobites , crustaceans and chelicerates ) in the structure of its legs and gills , and above all in the number and positions of the appendages on its head , which are the main feature used to classify arthropods . A fossil of Marrella from the Burgess Shale has also provided the earliest clear evidence of molting . 
