There is nothing really quite like the boomerang-head amphibian, Diplocaulus, except, obviously, the other boomerang-head amphibian, Diploceraspis. The question is: Where do these two oddballs fit on the early tetrapod family tree?
The Nectridea had been the traditional nesting clade, and diplocaulids share many nectridean traits. Their caudal vertebrae include fused hemal arches (or ventral extensions of the centra). The nectridean Urocordylus (Figure 1) had posterior skull extensions, but these were extensions of the squamosal, not the supratemporal and tabular, which remained tiny.
Many members of the Keraterpetontidae (within the Nectridea, yellow bar in Figure 2) had posterior supratemporal/tabular “horns” but this clade fused these two bones and the squamosal remained a large bone reaching the margin of the posterior skull. Moreover, these “horn heads” were all elongated forms each with a very long tail.
The exception was Eoserpeton tenuicorne (Moodie 1916), which shared the trait of a supratemporal contacting the quadratojugal, blocking the squamosal from any contact with the posterior margin of the skull (Figure 2), as in Diplocaulus. Eoserpeton also had a wide flat body. There is reason to doubt the presence of transverse processes on the caudal vertebrae of Eoserpeton, because neither the predecessor, Tuditanus, nor the successor, Diplocaulus, had them. The tail was likely longer.
Here Diplocaulus nested with Tuditanus. This hornless microsaur appears to represent an ideal ancestor for Diplocaulus before “the madness” set in. These two are among the very few amphibians with a concave ventral margin of the maxilla. According to available evidence, the lateral expansion of the postparietals and tabulars did not even begin until well after the supratemporals had expanded laterally and they continued growing in Diploceraspis long after the supratemporal had achieved its maximum size in Diplocaulus.
In evolutionary terms, some sort of selective pressure developed posteriorly-oriented “horns” on several elongated keraterpetonids, making them look like little horned lizards (like Phrynosoma). In the wider flatter eoserpetonids other selective pressures expanded these horns laterally. In Diplocaulus and Diploceraspis, the “horns” developed laterally to outlandish proportions as these microsaurs grew to become very much larger.
Diplocaulus may be a nectridean, but just barely. It had more in common with the microsaur Tuditanus.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
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