 <unk> <unk> were often the dominant <unk> animals in their environments . Large <unk> of <unk> 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 <unk> 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 <unk> . 
