This all started
a few days ago with some interest by readers in the nesting of dissorophoids (Cacops and kin; Fig. 1) apart from temnospondyls. The large reptile tree (LRT) nested dissorophoids at the base of the lepospondyls, contra traditional studies. I tested this heretical nesting several times over and the nesting is robust. Today we’ll put that nesting to yet another test.
Here’s the problem
Cacops looks like a temnospondyl. It’s big. It has a big head, short torso and tiny tail. It was probably terrestrial, judging by the robust limbs. Even the palate looks like that of a temnospondyl. The question is: can all this be by convergence?
In this case, as in many others…
it’s better not to eyeball it, or play favorites, or follow tradition, but to let the computer decide.
Over the last few days
I’ve been combing the Internet for traditional dissorophid outgroups in the literature. Iberospondylus was one candidate, but it nested only with temnospondyls in the LRT, far from dissorophids.
Another, perhaps better candidate
is Parioxys fericolus (Cope 1878, Carroll 1964; Early Permian). It shares several traits with Cacops, like a big curved squamosal. Cope (1882) later suggested his specimens were actually young Eryops (Fig. 2), but subsequent workers considered Parioxys a separate genus. Moustafa (1955) allied Parioxys with the Dissoroophidae in the super-family Dissorophoidea. Carroll (196) described an earlier and more primitive species (Parioxys bolli, Fig. 3).
“It is primarily on the basis of the configuration of the pelvis and the possession of two pairs of sacral ribs, as well as the lack of a fourth trochanter on the femur, that Moustafa allied Parioxys with the dissorophids.”
Among basal tetrapods, Cacops is atypical in having two sacral ribs, although Eryops has one “true sacral” and another vertebra very much like it. Carroll further notes,
“Since the features that Moustafa used to ally the dissorophids with Parioxys have developed separately within the two groups, these characters cannot be cited to indicate close relationship. The possession of a posterior proximal ramus of the adductor ridge in P. bolli, and the presence of a fourth trochanter, further separate the genus from dissorophids, which do not show these features even in the later Middle Pennsylvanian genera.”
After phylogenetic analysis
the dissorophids remain nested at the base of the lepospondyls. Parioxys nested with Eryops. Only with the removal of ALL intervening taxa do dissorophids nest with temnospondyls, and then there is loss of resolution.
With the removal of Parioxys from the dissorophids, the former clade, Dissorophoidea,
now appears to be paraphyletic
Yet another heresy.
I know the basal tetrapod workers don’t like this new insight into temnospondyl and dissorophid relations, or rather the lack thereof. Maybe this will solve some of the problems they’ve been having on their own in phylogenetic analyses.
And add this discovery to the pile
of pterosaur origins, turtle origins, whale origins, snake origins, dinosaur origins, multituberculate origins, bat origins, diadectid origins, reptile origins and many more that the large reptile tree brings insight to. I never thought it would go this far.
if anyone can produce a taxon or a set of taxa that can attract Cacops and the dissorophids to the temnospondyls, please send them over. I am more than willing to test any serious candidates.
Carroll RL 1964. The relationships of the Rhachitomous amphibian Parioxys. American Museum Novitates 2167:1-11.
Cope ED 1878. Descriptions of extinct Batrachia and Reptilia from the Permian formation of Texas. Proc. Amer. Phil. Soc., vol. 17, pp. 505-530.
Cope ED 1882. Third contribution to the history of the Vertebrata of the Permian formation of Texas. Ibid., vol. 20, pp. 447-461.
Moustafa YS 1955. The skeletal structure of Parioxys ferricolus, Cope. Bull. Inst. d’Egypte 36: 41-76.