From the Super Mario Wiki
Calamity Clam is a person who only appears in "The Great Gold Coin Rush". He is a sheriff of Lonesome Mushroom, a town that is almost a ghost town. He is also a owner of every building of the town, changing his hat many times. He accidentally shocked Mario, Luigi, Princess Toadstool, and Toad when he first sees them. He tells them who is he and what town it is. He gives Toad a assay with the gold coins he collected from the mine underground. He can't believe that it was a real gold coin, so the ghost town became a boom town. He then talk to Toad, but his talk was interrupted by Claim Jump Koopa. He was forced to escape with Mario, Luigi and Princess Toadstool, leaving Toad and the miners with Claim Jump Koopa.
After Claim Jump Koopa to let Toad and the miners mine for gold, Calamity Clam and the Mario gang moved back to the town, only to find it empty again. The gang fell to the ground, realizing that Claim Jump Koopa is there. When they went down there, they were horrified by the miners digging gold for Claim Jump Koopa. He tells Mario that the Fire Flower are in the Fire Flower Mine. After going through Tomato Sauce Mine and Pasta Mine, they reached the Fire Flower Mine, where Mario and Luigi power up. It is unknown what happen to him after Mario and Luigi grab a Fire Flower to free Toad and the miners.