Tiktaalik and Spathicephalus now united

Among short-snouted stem tetrapods
long-snouted Tiktaalik (Fig. 1) stood alone… so did long-snouted Spathicephalus (Fig. 2), previously nesting just a few nodes away. A reexamination of both revealed overlooked sutures in Tiktaalik that more or less matched those presented in a Spathicephalus diagram. Those traits were re-scored in the large reptile tree (LRT) and now the two loners nest together… along with little Koliops, which has big eyes and a small snout.

Figure 1. Revised skull sutures in Tiktaalik. Compare these to the sutures in Spathicephalus, figure 2.

Figure 1. Revised skull sutures in Tiktaalik. Compare these to the sutures in Spathicephalus, figure 2.

So phylogenetic bracketing indicates
Spathicephalus had fins, not feet. If the post-crania has been published, please let me know. Same for Koliops.

Considering the Late Devonian appearance of Tiktaalik,
Spathicephalus is what the ancestors of Tiktaalik evolved to become by the Late Carboniferous. Despite its traditional transitional status, Tiktaalik had already evolved traits not found in the main lineage that produced frogs, salamanders and reptiles. The branching point must have been much earlier, in the Mid-Devonian.

Figure 1. Spathicephalus, a filter feeding temnospondyl with elongate orbits now nests with Koilops.

Figure 2. Spathicephalus, a filter feeding stem tetrapod close to Tiktaalik, figure 1.

Tiktaalik roseae (Daeschler, Shubin and Jenkins 2006; Late Devonian, 375mya; 4-9 meters in length) nests between Pandericthys and Tetrapoda in the large reptile tree. Distinct from Pandericthys the opercular bones are absent, the orbits are further back and higher on the skull, ribs, a pelvis and large bones within the four digit-less finned limbs are present.

Spathicephalus mirus (Watson 1926; Late Carboniferous, 320 mya) was described, “unlike that of any other early tetrapod, with a flattened, square-shaped skull and jaws lined with hundreds of very small chisel-like teeth.” The extended orbit shape traditionally allied Spathicephalus with Baphetes, but here it nests with the stem tetrapod, Tiktaalik, which is just beginning to show that orbit shape.

The Spathicephalus fossil does not show tooth replacement. Rather every tooth is present without gaps. Tiktaalik also has jaws rimmed with tiny teeth. Distinct from derived temnospondyls, but like basal forms, the palate is closed on this bottom-feeder.

Figure 3. Koliops with bones colorized, nests as a big-eyed, short-snouted, smaller, (perhaps juvenile?) relative of Tiktaalik and Spathicephalus in the LRT.

Figure 3. Koliops with bones colorized, nests as a big-eyed, short-snouted, smaller, (perhaps juvenile?) relative of Tiktaalik and Spathicephalus in the LRT.

Koilops herma (Clack et al. 2016; NMS G. 2013.39/14) Tournasian, early Carboniferous ~375 mya) is a basal tetrapod with a flat skull and large orbits nesting with Spathicephlaus and Tiktaalik. The nares were close to the rim of the short rorstrum. The pineal foramen was enormous. The maxillary teeth were small and sharp. The premaxillary teeth were much larger, distinct from other basal tetrapods. The nasals were broader anteriorly.

This was low-hanging fruit
that apparently escaped everyone’s view, including my own, until now. Let me know if this relationship was published elsewhere. I would like to credit the authors, if so.

Clack et al. (14 other authors) 2016. Phylogenetic and environmental context of a Tournaisian tetrapod fauna. Nature ecology & evolution 1(0002):1-11.
Daeschler EB, Shubin NH and Jenkins FA, Jr 2006. A Devonian tetrapod-like fish and the evolution of the tetrapod body plan. Nature. 440 (7085): 757–763.
Watson DMS 1929. Croonian Lecture. The evolution of the Amphibia. Philosophical Transactions of the Royal Society, London B 214:189-257.

No wikipedia page yet for Koilops.

6 thoughts on “Tiktaalik and Spathicephalus now united

  1. Off topic: you asked me in e-mail for the definition of Temnospondyli, and my response bounced. Here’s the definition:

    we propose that Temnospondyli be defined as a stem-based taxon including all choanates more closely related to Eryops than to Pantylus (Microsauria)

    Yates & Warren 2000
    doi: 10.1006/zjls.1998.0184

    This definition is simple, practical, and still in use.

    On topic: Postcrania are unknown for Spathicephalus, which is a pretty obvious baphetoid and comes out as such in my analyses; what it shares with Tiktaalik is really just the flatness of the head. Koilops is so poorly known – at least judging from what’s published; as with the other new taxa in Clack et al. (2016), it’s possible that a more detailed description is forthcoming – that I didn’t even bother adding it to my matrix; I don’t have much of an opinion on what it could be.

    • Thanks, David. In the meantime I found out that Schoch 2013 defined the Temnospondyli as the least inclusive clade of Edops and Mastodonsaurus. That seems to agree with your definition, which I wish would include two taxa, their LCA and all descendants.

      • Oh no, Schoch’s (2013) definition does not agree with that of Yates & Warren (2000) at all. First, it’s node-based instead of branch-based, so it applies to a smaller clade on any particular phylogeny. Second, it might exclude all of Dissorophoidea, for example – Schoch’s (2013) phylogeny of Temnospondyli is much less robust than he thinks!

      • Some postcranial material is known of Baphetes: the open-access paper is here.

        Then there’s similar material that may belong to Doragnathus, which is likely a baphetoid for other reasons: closed-access paper by Smithson & Clack (2013), DOI: 10.1016/j.crpv.2013.06.006

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