Dimetrodon

Name Meaning:  Two Measures of Teeth

Location Found:  Red Beds, Texas

Geologic Era:  Permian

Estimated Range:  Texas, Oklahoma, Germany

Size:  Species range from .6 to 4.6 meters long (2 to 15.1 feet)

Extinction:  272 million years ago

There are actually several species of Dimetrodon, ranging in size from 60 centimeters in length to 4.6 meters in length.  Our first specimen, a partial femur, comes from one of the middle sized species, Dimetrodon limbatus.  It was recovered in the Red Beds of Texas, a Permian deposit.  This is the same formation that our Eryops and Captorhinus specimen were recovered from.  Dimetrodon and organisms like it are often shown in the same books and other media as dinosaurs, leading to the misconception that Dimetrodon was a dinosaur.  

The second specimen is a cast of the cranium of a large Dimetrodon. It is made of ceramic, and would have come from one of the larger species. Unfortunately, the cranium was damaged during shipping, and had to have several teeth repaired as a result. 

The larger Dimetrodons would have been the apex predators of their environment,  There were few terrestrial animals that matched them in size, though some of the larger marine sharks would have been bigger.  Dimetrodon’s environment was probably warm, wet, and crossed by small rivers and lakes.  A fair comparison in modern terms might be a Louisiana bayou or the Everglades in Florida.  There would have been plenty for Dimetrodon to eat, ranging from amphibians to smaller reptiles, even freshwater sharks.  

In these terms, Dimetrodon would be considered a generalist predator, eating most anything that it could catch.  There is little evidence of dietary specialization as seen in animals like the much later Spinosaurus.  Though the two animals share the same kinked upper jaw, this does not indicate a close relationship, as this trait shows up again in other dinosaurs.  Indeed, Dimetrodon and its kin are not closely related to dinosaurs at all; rather, they are often classified as reptile-like mammals.  This group of animals was largely wiped out before the dinosaurs really took over, but in a sense still are among us in a distant fashion as mammals.   

The skeletons pictured above and below may represent some of the earlier examples of sexual dimorphism in the fossil record.  Sexual dimorphism is the phenomenon where males and females of a species look markedly different.  Good examples of this in modern animals are peafowl.  Males are larger and much more colorful than females, and this carries over in many avian species.  The general trend with sexual dimorphism is that the male is larger than the female, though this is not always the case.  While pigmented cells do not readily leave fossil traces behind for us to easily tell what color ancient animals were, skeletal differences are usually present.  

In Dimetrodon’s case, two “morphs” consistently show up.  One is overall larger, with a heavier skull, taller spines, and more robust bones (see above).  This is thought to the the “male” form.  The other morph is smaller, lighter boned, smaller spined, and with a somewhat more delicate looking skull (insomuch as a skull like Dimetrodon’s could be considered delicate).  This is thought to represent the female (see below). 

Dimetrodon’s large dorsal sail, while an interesting feature, is actually not unique in the fossil record.  This structure shows up several times in other animals, ranging from amphibians to reptiles to some dinosaurs.  The function of the sail is uncertain, though thermoregulation is certainly a possibility.  A thermoregulatory function would have given Dimetrodon a bit more control over its body temperature than the average ectotherm would have.  

Another possibility is that the sail functioned in display, helping with species identification and identifying worthy mates.  A third possibility is that it performed both functions.  The only thing the sail would not have been great for would be defense, and the spines were very thin and would not have stood up to stress very well.  Also, Dimetrodon was one of the largest (if not the largest) animal in its environment, so such a defense would not really be necessary.

Image Credits:

Full Skeleton (male):  By Smokeybjb – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=22506395

Full Skeleton (female):  By Daderot – I took this photograph., Public Domain, https://commons.wikimedia.org/w/index.php?curid=12872149

Life Reconstruction:  By DiBgd – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=42203983