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.
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.
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.
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.
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.
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