Even before I started counting, it was obvious that the fin-backs’ predator-to-prey ratio was more like the spiders’ than the lions’. Mammalian top predators are always rare, but finbacks were overwhelmingly abundant. Romer noted that in one of his first papers published about Permian faunas in the 1920s, Dimetrodon was the single most common genus in most locales. (Amusement park dinosaur)That went against all the laws of bioenergetics, unless the predator had a very low metabolism and its interaction with the ecosystem permitted it to reach its maximum theoretical abundance. I invested half a year in measuring every specimen Romer possessed, and extended the census to all the other samples of finbacks housed here in the United States and abroad. In nearly every quarry, in every formation, in every habitat, the result was the same: The finback predators were the first or second most common genera among all the fauna.
The theory of predator-to-prey ratios had proved stunningly reliable with relation to the cold-blooded finbacks. If metabolic level was the primary agent determining the abundance of predators, then cold-blooded Dimetrodon should have been highly abundant in all kinds of predator-prey systems, and over species of prey in many different habitats. The hundreds of specimens I catalogued showed that was precisely the case. Dimetrodon was extremely catholic in its choices of prey. In sediments laid down in quiet Permian lakes, the commonest large prey was a fish-eating reptile, Ophiacodon. (Dinosaur Costume)At other sites deposited in swampy flood-plains, Ophiacodon was rare but another semiaquatic species, the big-headed amphibian Eryops, assumed the role of supplying food to the finback.
Dimetrodon had no choice but semiaquatic, fish-eating prey, because at this stage of evolutionary history the large plant-eaters hadn’t developed very far and were only rarely of good size. But could Dimetrodon have maintained its extraordinary abundance in normal ecosystems, where it fed on land-living vegetarians? (dinosaur factory)This was a key question, because ultimately Dimetrodon s predator-to-prey ratio had to be compared to that for meat-eating dinosaurs which fed almost exclusively on plant-eaters. Fortunately, Dimetrodon’s fossil record did include two dry, floodplain habitats where a big plant-eater a buck-toothed reptile called Diadectes was its most common prey. In these two habitats Dimetrodon proved to be as successful and abundant as it was in the systems where fish-eaters were its main fare.