Ianthodon: a basal edaphosaur without tall neural spines

A new paper
by Spindler, Scott and Reisz (2015) brings us new data on the basal pelycosaur Ianthodon schultzei (Fig. 1; Garnet locality, Missourian Age; 305-306 mya, Middle Pennsylvanian, Late Carboniferous). The authors reported that Ianthodon represented a more basal sphenacodontid than Haptodus. In the large reptile tree Ianthodon was derived from a sister to Haptodus and nested at the base of Edaphosaurus + Ianthasaurus + Glaucosaurus, all edaphosaurids.

Figure 1. Ianthodon schultzei was considered a basal pelycosaur, and it is, but here nests as a basal edaphosaur. And it has no tall neural spines. So pelycosaur sails were convergent, not homologous.

Figure 1. Ianthodon schultzei (image modified from Spindler, Scott and Reisz 2015) was considered a basal pelycosaur, and it is, but here nests as a basal edaphosaur. And it has no tall neural spines. So pelycosaur sails were convergent, not homologous. Spindler, Scott and Reisz considered this specimen a juvenile due to its incomplete ossification.

Notably Ianthodon does not have tall neural spines. Earlier we wondered whether the tall neural spines of Edaphosaurus and Dimetrodon were convergent or homologous. Now it is clear, via Ianthodon, and Sphenacodon (sorry I did not notice this yesterday) that the tall neural spines of Edaphosaurus and Dimetrodon were convergent.

Most well-known pelycosaurs
were Early Permian in age. Ianthodon demonstrates an earlier origin for their carnivore/ herbivore split. And it retains carnivore teeth! Therapsids likewise originated in the Late Carboniferous according to this new data.

Phylogenetic history
Spindler, Scott and Reisz (2015) report, “In the original description and phylogenetic analysis of Kissel and Reisz (2004), Ianthodon was found to nest surprisingly high within Sphenacodontia, as a sister taxon to the clade that included Pantelosaurus, Cutleria and sphenacodontids. In a subsequent, large-scale analysis, Ianthodon was found to be more basal, near the edaphosaurid–sphenacodont node (Benson, 2012), but its exact position remained poorly resolved. In the latter analysis, Benson (2012) extensively revised the character list and included all known “pelycosaur” grade synapsids, while Kissel and Reisz (2004) used data and taxa derived from Laurin (1993), which mainly followed Reisz et al. (1992). Another recent analysis of sphenacodont synapsids by Fröbisch et al. (2011), as part of a description of a new taxon, recovered Ianthodon, Palaeohatteria and Pantelosaurus in an unresolved polygamy.”

The Spindler, Scott and Reisz (2015) analysis
used 122 characters (vs. 228 in the large reptile tree). Their tree shows 12 taxa, 4 of which are suprageneric. In their tree Ianthodon nested between Edaphosauridae and Haptodus. (So close, but no cigar.) Their tree also nested two therapsid taxa (Biarmosuchus and Dinocephalia) with Cutleria, Sphenacodon, Ctenospondylus and Dimetrodon. Thus Spindler, Scott and Reisz appear to be excluding several key taxa and their tree topology differs significantly from the large reptile tree at the base of the Therapsida, with or without Ianthodon.

References
Spindler F, Scott D. and Reisz RR 2015. New information on the cranial and postcranial anatomy of the early synapsid Ianthodon schultzei (Sphenacomorpha: Sphenacodontia), and its evolutionary significance. Fossil Record 18:17–30.

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