Marrella was the first Burgess Shale fossil that Whittington re-examined, and gave the first indication that surprises were on the way.[33] 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.[34] A fossil of Marrella from the Burgess Shale has also provided the earliest clear evidence of molting.[35]