 Very little is known of the soft tissue of <unk> . A block of sandstone , described in 2007 from the Early Carboniferous Mauch Chunk Formation of Pennsylvania , included impressions of the bodies of three <unk> . These impressions show , when alive , they had smooth skin , robust limbs with webbed feet , and a ridge of skin on their undersides . Trackways referable to small <unk> have also been found in Carboniferous and Permian rocks . The trackways , called <unk> , are usually found in strata deposited around freshwater environments , suggesting the animals had some ties to the water . 
