Early Permian Sail-back Synapsids

Everyone knows
about Dimetrodon and Edaphosaurus, the two Early Permian sail back synapsid reptiles (Figs. 1, 2). Ianthasaurus was a more primitive sister to Edaphosaurus. Secodontosaurus was a sister to Dimetrodon. A taxon without a sail, Haptodus, was basal to both clades.

Figure 1. Dimetrodon, a sailback pelycosaur synapsid reptile of the Early Permian.

Figure 1. Dimetrodon, a sailback pelycosaur synapsid reptile of the Early Permian.

Dimetrodon 
was a meat-eater. Edaphosaurus was a plant-eater. Every grade-schooler knows this. Skull size and sail design readily distinguish these two iconic taxa. Other traits, from teeth to toes also distinguish them.

Figure 2. Edaphosaurus, a sailback pelycosaur synapsid reptile of the Early Permian.

Figure 2. Edaphosaurus, a sailback pelycosaur synapsid reptile of the Early Permian. Note the tall caudal neural spines, distinct from Dimetrodon (figure 1).

Several specimens
of Dimetrodon are known (Fig. 3). Several attempts at reconstructing the skull of Edaphosaurus have been made (Fig. 2). I have the impression that there is not yet a single complete skull known for this taxon.

Figure 2. Click to enlarge. Sphenacodont skulls to scale. Figure 2. Click to enlarge. Sphenacodont skulls to scale.

Figure 3. Click to enlarge. Sphenacodont skulls to scale. See Figure 2 for Edaphosaurus skulls. Not sure why Sphenacodon is not considered a species of Dimetrodon. The skulls are nearly identical.

The two sails
are either convergent or homologous. At this point, we don’t know. They both have individual designs with Edaphosaurus having curved neural spines with short spars on each “mast”. If they are homologous, Ianthsaurus (Fig. 4) is close to that common ancestor. At present, sail-less Haptodus is the last common ancestor.

Figure 4. Ianthasaurus, a basal edaphosaur.

Figure 4. Ianthasaurus, a basal edaphosaur not far from the common ancestor to all tailback pelycosaurs.

Interestingly,
at the same time that sails were developing in one synapsid clade, another clade, the Therapsida, led by Cutleria and Stenocybus was developing in different ways. At present only skulls are known, but more derived therapsids had longer legs and apparently a more active lifestyle, again dividing at their origin into meat-eaters, like Biarmosuchus, and plant-eaters, like Niaftasuchus and the Dromasauria.

The Early Permian
reminds me of the Early Triassic with regard to the great amount of evolutionary novelty appearing then, likely in response to new environs, weather patterns, predators and experiments in raising the metabolism in several clades. At this time basal diapsids and basal lepidosaurs were diversifying as well.

References
Case ED 1878. Descriptions of extinct Batrachia and Reptilia from the Permian formation of Texas. Proceedings of the American Philosophical Society xvii pp. 505-530.
Cope ED 1882. Third contribution to the history of the Vertebrata of the Permian formation of Texas. Proceedings of the American Philosophical Society (20): 447–461.
Marsh OC 1878. Introduction and succession of vertebrate life in America: Popular Science Monthly, v. 12, p. 513-527, 672-697.
Modesto SP 1994. The Lower Permian Synapsid Glaucosaurus from Texas. Palaeontology 37:51-60
Reisz RR and Berman DS 1986. Ianthasaurus hardestii n. sp., a primitive edaphosaur (Reptilia, Pelycosauria) from the Upper Pennsylvanian Rock Lake Shale near Garnett, Kansas. Canadian Journal of Earth Sciences 23(1): 77–91.
Reisz R R, Berman DS and Scott D 1992. The cranial anatomy and relationships of Secodontosaurus, an unusual mammal-like reptile (Pelycosauria: Sphenacodontidae) from the early Permian of Texas. Zoological Journal of the Linnean Society 104: 127–184.
Romer, AS 1936. Studies on American Permo-Carboniferous tetrapods. Problems of Paleontology, USSR 1: 85–93.
Romer AS and Price LW 1940. Review of the Pelycosauria. Geological Society of America Special Papers 28: 1-538.

wiki/Ianthasaurus
wiki/Edaphosaurus
wiki/Secodontosaurus
wiki/Dimetrodon
wiki/Sphenacodon

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