 Many <unk> are much larger than living amphibians , and superficially resemble crocodiles . Others are smaller and resemble salamanders . Most have broad , flat heads that are either blunt ( <unk> ) or elongated ( <unk> ) . The skulls are rounded or triangular in shape when viewed from above , and are usually covered in pits and ridges . The rugged surfaces of bones may have supported blood vessels , which could transfer carbon dioxide to the bones to neutralize acidic build up in the blood ( early semiaquatic tetrapods would have had difficulty expelling carbon dioxide from their bodies while on land , and these dermal bones may have been an early solution to the problem ) . Many <unk> also have canal @-@ like grooves in their skulls called sensory sulci . The sulci , which usually run around the nostrils and eye sockets , are part of a lateral line system used to detect vibrations in water . As semiaquatic animals , most <unk> have small limbs with four toes on each front foot and five on each hind foot . Terrestrial <unk> have larger , thicker limbs , and some even have claws . One unusual terrestrial temnospondyl , <unk> , has relatively long limbs for its body , and probably lived as an active runner able to chase prey . 
