 Dimetrodon may have been sexually dimorphic , meaning that males and females had slightly different body sizes . Some specimens of Dimetrodon have been hypothesized as males because they have thicker bones , larger sails , longer skulls , and more pronounced maxillary " steps " than others . Based on these differences , the mounted skeletons in the American Museum of Natural History ( AMNH 4636 ) and the Field Museum of Natural History may be males and the skeletons in the Denver Museum of Nature and Science ( MCZ 1347 ) and the University of Michigan Museum of Natural History may be females . 
