Triassic temnospondyls were often the dominant semiaquatic animals in their environments . Large assemblages of metoposaurs with hundreds of individuals preserved together have been found in the southwestern United States . They have often been interpreted as mass death events caused by droughts in floodplain environments . Recent studies show these dense assemblages were instead probably the result of currents accumulating dead individuals in certain areas . These environments seem to have had little diversity , as they were inhabited almost exclusively by metoposaurs .