Early Views on the Evolution of Mammals

The point of The Pterosaur Heresies is to encourage others to try nesting pterosaurs with lizards (and within that clade the Tritosauria and the Fenestrasauria).

That’s why reading Jenkins (1967) reminded me that other clades (for instance, the Mammalia) had trouble getting it right early on.

Jenkins (1967) reports that the first mammal-like reptile to be described was Dicynodon by Owen (1844). Only the long tusks reminded Owen of mammals. Otherwise the “cranial organization manifested [by] modern lizards.”

In the following three decades the mammal-like aspects of various Karoo fossils, including those of cynodonts went unappreciated and considered convergent. Jenkins writes, “the many reptilian features of these fossils, and the fact that Huxley and others maintained that mammals arose directly from Amphibia, delayed recognition that therapsids were the stock from which mammals arose.”

Thirty years later Owen (1876) compared theriodonts and anomodonts to dinosaurs and living reptiles. Jenkins (1976) writes, “Evidently Owen could not bring himself to accept the possibility that mong them were the reptilian ancestors of mammals.”

Cope (1878) first proposed the order Theromorpha to include Pelycosauria and Anomodontia (at the time representing all South African therapsids). This order he regarded as “approximating the Mammalia more closely than any other division of the Reptilia.”

Seeley (1888) followed suit, that therapsids and mammals were kin, but that therapsids were not the ancestors of mammals. Both were derived from a common parent, “perhaps in Silurian and Devonian strata.” Osborn agreed that no known therapsid could be ancestral to mammals.

Marsh followed Huxley in citing an amphibian origin for mammals (scale-less skin, double occipitial condyle).

Finally Broom (1901) proposed that mammals arose from primitive theriodonts. He included cynodonts in the lineage of mammals. Watson (1913) proposed that Gorgonopsia gave rise to mammals. Broom (1929) reported that mammals arose from ictidosaurs (like Pachygenelus) and these arose from cynodonts. Later discoveries and later reports confirm Broom’s assessments.

See any similarities?
The current situation in pterosaurs in broadly similar. Proponents holding to an archosaur alliance have no idea what archosaur or lineage of archosaurs produces pterosaurs. They rely on suprageneric taxa and avoid testing proven candidates. They cannot demonstrate a gradual acquisition of pterosaurian traits from that side of the tree. No archosaur sisters approximate the morphology of pterosaurs.

We’ve seen this movie before folks (anyone care to discuss the impact of Ostrom 1969? one hundred years after Archaeopteryx?).

Evidence already supports a lizard/pterosaur kinship. Now we’re just waiting for the mood to swing and prejudice to vanish.

References
Jenkins FA Jr. 1967. The Postcranial Skeleton of African Cynodonts. Peabody Museum Bulletin 36. 216 pp.
Ostrom JH 1969. A new theropod dinosaur from the Lower Cretaceous of Montana. Postilla 128:1-17.

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