Many have seen this cast (fig. 1) entitled, “Baby (or Juvenile) Dimetrodon.” It’s a common piece of plaster merchandise sold at fossil fairs, etc.
Is it a complete fake?
The specimen has not been illustrated in the literature (that I know of), but it has been described (Sternberg 1942) and the description is a perfect match. Parts (Fig. 1 in gray) have been added to the cast to make it more interesting and complete. Sternberg (1942) reported the specimen was originally at the Walker Museum in Chicago, but in 1953 most Walker paleo exhibits, perhaps including this one, were moved to the Field Museum.
Figure 1. Sternberg 1942 described this specimen he attributed to a baby or juvenile Dimetrodon. Parts added by artisans are in gray.
On the plus side
One of the more complete Permian fossils is this baby/juvenile Dimetrodon (Sternberg 1942, Figs. 1, 2), less than a quarter the size of an adult with a much shorter sail and much longer legs. If this is a juvenile Dimetrodon, these proportions change allometrically during growth. The mandible was slightly shorter, compared to the adult, indicating the skull was likewise not larger relative to the body.
Figure 2. Click to enlarge. Figure 1. Baby Dimetrodon (above) compared to adult (below) to the same scale and to different scales. This is the first reconstruction of this specimen that I am aware of. Restored parts in light red. Note the smaller sail and longer legs and tail in the juvenile. Regressing the baby to egg size suggests the sail developed after hatching. I’m curious about the rib length from front to back on the juvenile, different from the adult.
So, longer legs on a juvenile synapsid?
That’s not the pattern we see in mammals or Heleosaurus, a varanopid(?) protodiapsid in which adults have the longer legs. In Dimetrodon the juveniles didn’t have marginally longer legs. Juveniles had legs relatively twice as long as those on adults. Generally longer legs provide more speed to attack prey or avoid predators.
Is this really a baby Dimetrodon?
Or is it a different smaller species? Bakker(1982) suggested different habitats for Permian juveniles would help them avoid adult predation. Brinkman (1988) cast doubt on Bakker’s idea by showing that the specimens found in floodplain and swamp sediments represented two different species, not adult and juvenile populations of the same species.
Do we need more tiny specimens and a few teenage specimens to help determine what the situation is here? Both sides make sense.
Sternberg (1942) wrote,
“The preservation of the bone is poor: It is probable that the bony elements were never well ossified.” He also wrote that three or four partial skeletons of Dimetrodon grandis were found in the same pocket, which lies in the breaks of Coffee Creek in Baylor County, Texas. If we assume those were adults, there goes Bakker’s and Brinkman’s hypotheses. Brinkman did not reference the Sternberg paper, but noted that poor ossification attended smaller Dimetrodon specimens.
Only parts are fake
Just because parts of this specimen have been added with restoration, doesn’t mean the rest of the skeleton is useless or should be labeled “a fake.” In this case, we should use what is real and avoid what is fake. The size and proportion relationships are still good data that make a good story.
Bakker RT 1982. Juvenile-Adult Habitat Shift in Permian Fossil Reptiles and Amphibians. Science 217 (4554): 53–55. doi:10.1126/science.217.4554.53.PMID 17739981.
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 (2): 172–180.
Sternberg CW 1942. The skeleton of an immature pelycosaur, Dimetrodon cf. grandis, from the Permian of Texas. Journal of Paleontology 16 (4): 485–486.
A paper written on fossil fakes is online here.