 Cope was the first to describe a sail @-@ backed synapsid with the naming of Clepsydrops natalis in his 1878 paper , although he called the sail a fin and compared it to the crests of the modern basilisk lizard ( <unk> ) . Sails were not preserved in the specimens of D. <unk> and D. gigas that Cope described in his 1878 paper , but elongated spines were present in the D. <unk> specimen he described . Cope commented on the purpose of the sail in 1886 , writing , " The utility is difficult to imagine . Unless the animal had aquatic habits , and swam on its back , the crest or fin must have been in the way of active movements ... The limbs are not long enough nor the claws acute enough to demonstrate arboreal habits , as in the existing genus <unk> , where a similar crest exists . " 
