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