Ancient marine organisms that excavated holes in shells and other calcium carbonate substrates on the sea bed have not been widely studied by palaeontologists. Together with scrape and scratch marks made by rasping grazers on shell surfaces, they constitute a group of hard–substrate trace fossils that have many modern equivalents. Even the smallest forms can be studied by casting the holes in plastic resin and dissolving the shell. The variety of borers increased in the Mesozoic Era, thereby avoiding the attentions of predatory groups that were radiating at the same time.