We first met Dimetrodon natalis a few posts ago when Romer and Price presented it along with a selection of other Dimetrodon skulls. I don’t know much about Clepsydrops (Cope 1875), a name which seems to have gone out of favor, or to the back of the museum drawer. Romer and Price in their Review of the Pelycosauria (1940), considered it an ophiacodont, close to Varanosaurus, perhaps based on size, but this was before the skull became known. The femur is robust. The data is a little hard to read. In the old days they used to say “image at 1/4 size” rather than applying scale bars.
Shelton et al. 2103 reported, “The Briar Creek Bonebed (Artinskian, Nocona Formation) in Archer County is oneof the richest sources of Dimetrodon bones in the Lower Permian of Texas, USA. Based on size, a small (D. natal is ), an intermediate (D. booneorum), and a large species (D. limbatus) have been described from this locality. It has been proposed that these traditionally recognised species represent an ontogenetic series of only one species.
… The external fundamental systems observed in the largest humerus and the two largest femora confirm that D. natal is is not the juvenile of a larger species. The presence of the EFS in the cortex of their long bones unquestionably indicates that these animals had attained skeletal maturity.
…The results thus partially refute Bakker’s (1982) hypothesis,that the bones of D. natalis, D. booneorum and D. limbatus only represent an ontogenetic series of a single species, which may in turn disprove the juvenile/adult habitat shift hypothe-sis. Juveniles and adults of D. natalis are found in the samebonebed. However, the ﬁndings are insufﬁcient for fully test-ing the habitat shift hypothesis. The results support Brinkman(1988), as well as the morphological classiﬁcation used by Romer & Price (1940).”
Bakker RT 1982. Juvenile–adult habitat shift in Permian fossil reptiles and amphibians. Science 217: 53–55.
Brinkman D 1988. Size-independent criteria for estimating relative age in Ophiacodon and Dimetrodon (Reptilia, Pelycosauria) from the Admiral and Lower Belle Plains formations of West-central Texas. Journal of Vertebrate Paleontology 8, 172–80.
Romer AS and Price LW 1940. Review of the Pelycosauria. Geological Society of American Special Papers 28, 538 pp.
Shelton CD, Sander PM, Stein K, Winkelhorst H 2013. Long bone histology indicates sympatric species of Dimetrodon (Lower Permian, Sphenacodontidae) Earth and Environmental Science Transactions of the Royal Society of Edinburgh 09/2012; 103(3-4). DOI:10.1017/S175569101300025X