The Aïstopods may be splitting apart

Those long, limbless amphibians,
the Aïstopoda, were once (in the 1920s) the oldest known tetrapods, known from Westphalian (310 mya) strata. Of course, since the publication of Ichthyostega (1932) and the rest of the Devonian tetrapods, that’s old news. Baird 1964 wrote: “The remarkable specialization [in aîstopods] already achieved by the early Mississippian implies an origin well back in Devonian time; a tetrapod ancestry rather than direct derivation from the crossopterygians fishes is indicated. Relationships of the order are obscure.” 

Interesting, that 1964 comment,
as some of the aïstopods continue to nest traditionally with lepospondyls, but others now nest with paratetrapods, closer to crossopterygian fishes.

Aistopods are traditionally considered lepospondyls
because the three parts of each vertebrae are fused to become one. Today three aîstopods were added to the large reptile tree (LRT, 967 taxa). Of those three, two did not nest within the Lepospondyli, or within the Tetrapoda.

Ophiderpeton and Oestocephalus
(Fig. 1) nests with Acherontiscus, within the Lepospondyli and within the Tetrapoda in the LRT. The orbits were far forward and the temples were fenestrated, narrowing the parietal. The supratemporal, tabular, jugal and squamosal and quadratojugal are all reduced, but still present.

Figure 1. Ophiderpeton (dorsal view) and two specimens of Oestocephalus (tiny immature and larger mature).

Figure 1. Ophiderpeton (dorsal view) and two specimens of Oestocephalus (tiny immature and larger mature).

Phlegethontia
Phlegethontia longissima (above; CGH 129)  Phlegethontia linnearis (below; Cope 1871, Anderson 2002, Fritsch 1875; Huxley and Wright 1867) was considered a aîstopod, but it does not nest with Ophiderpeton, despite the complete fusion of each vertebrae by convergence. Here Phlegethontia nests as a basal pro-tetrapod with Pholidogaster and Colosteus (Fig. 5). P. longissima CGH 129 (below) has not yet developed a temporal fenestra.

Unlike most other paratetrapods,
the premaxilla of P. longissima was drawn out to a very long tip and the premaxillary teeth were the largest. The large lateral naris became elongate over the maxilla and prefrontal, perhaps contacting the postorbital and indicating this was a full time air-breather. Not sure what is happening with the supratemporals, which appear to extend laterally. The naris is elongate atop the maxilla.

The vertebrae
of outgroup taxa, like Pholidogaster, are still tripartite. The vertebrae in the transitional taxon, Colosteus, are largely hidden by osteoderms. In Colosteus the forelimbs are vestiges compared to those in Pholidogaster. They disappear in Phlegethontia. Evidently snake-like taxa fuse the neural spine, intercentrum and pleurocentrum as they switch from limb locomotion to vertebral undulation.

Figure 2. Phlegethontia longissima skull (CGH 129) has relatively large temporal plates, a wide flat cranium and a long pointed rostrum.

Figure 2. Phlegethontia longissima skull (CGH 129) has relatively large temporal plates, a wide flat cranium and a long pointed rostrum.

Like Ophiderpeton
AMNH 6966
 (below) has a large temporal fenestra and the parietal is reduced to the portion anterior to the pineal foramen and fused to the fused frontals. The postorbital, suprateomporal and tabular are replaced by a larger occiput (braincase). The rostrum is also shorter.

Figure 2. Phlegethontia linearis and other congeneric taxa with bones identified. Here the traditional lacrimal and prefrontal identities are switched here.

Figure 3. Phlegethontia linearis and other congeneric taxa with bones identified. Here the traditional lacrimal and prefrontal identities are switched here. The squamosal and quadratojugal are fused. The parietal and other cranial bones are absent or vestiges.

Distinct from other paratetrapods,
the dorsal ostederms were absent. The ventral osteoderms had become elongate gastralia, convergent with tetrapods. Having just acquired limbs and girdles, this clade promptly got rid of them and emphasized cerebral undulation. The tiny’ gill bones’ illustrated by Fritsch 1875 (erased here, Fig. 4) are actually displaced gastralia, according to Baird 1964. The naris is larger in Phlegethontia compared to outgroup taxa with legs. So it was likely breathing air, rather than using gills.

Figure 4. Phlegethontia overall with neck and sacral bones colored red. The 'gill bones' are removed. They are gastralia.

Figure 4. Phlegethontia overall with neck and sacral bones colored red. The ‘gill bones’ are removed. They are gastralia.

Despite the many convergent traits
the LRT is able to separate Ophiderpeton from Phlegethontia and from all other long bodied, limbless tetrapods.

Figure 5. Colosteus is covered with dermal skull bones and osteoderms. Those vestigial forelimbs are  transitional to the limbless condition in Phlegethontia.

Figure 5. Colosteus is covered with dermal skull bones and osteoderms. Those vestigial forelimbs are transitional to the limbless condition in Phlegethontia.

The traditional basal taxon
for the Aïstopoda is Lethiscus (Viséan. 340 mya). Hopefully data will come in soon on that taxon so it can be added to the LRT. Wikipedia reports, “The skull is specialised and light, very like that of Ophiderpeton, with the orbits, far forward, and the cheek region unossified (lacking bone). There are approximately 30 closely spaced teeth on the maxilla and dentary, and a sutural pattern of the skull closely resembles that of the Late Carboniferous aïstopod Oestocephalus.”

References
Anderson JS 2002. Revision of the aïstopod genus Phlegethontia (Tetrapoda: Lepospondyli). Journal of Paleontology. 76 (6):1029–1046. Online here.
Baird D 1964. The aïstopod amphibians surveyed. Breviora 206:1-17.
Cope ED 1871. Stated Meeting, Nov. 3d, 1871. Proceedings of the American Philosophical Society 12:176-177
Fritsch A 1875. Über die Fauna der Gaskohle des Pilsner und Rakonitzer Beckens. Sitzungsberichtde er Böhemischen Gesellschaft der Wissenschaften. Prague: 70–79.
Huxley TH 1862. On new labyrinthodonts from the Edinburgh Coal-field. Quarterly Journal of the Geological Society London18:291-296.
Panchen AL 1975. A New Genus and Species of Anthracosaur Amphibian from the Lower Carboniferous of Scotland and the Status of Pholidogaster pisciformis Huxley. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 269(900):581-637.

wiki/Pholidogaster
/wiki/Phlegethontia
wiki/Oestocephalus
wiki/Ophiderpeton

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3 thoughts on “The Aïstopods may be splitting apart

  1. 1) Outdated literature is outdated, including the illustrations in it. Even the specimen drawings to some extent.

    a) Anderson’s 2002 paper explicitly contradicts all the earlier literature on Phlegethontia in several points. You cite it; you should read it. For instance, it presents a good argument that the “frontoparietal” is just the frontal, and the parietal is genuinely lost.

    b) Get your hands on Anderson’s 2003 papers on Pseudophlegethontia and Coloraderpeton + Oestocephalus (March and September issues of JVP) and on his et aliorum 2003 paper on Lethiscus and Oestocephalus (Canadian Journal of Earth Sciences). Also, Anderson’s 2007 paper (Journal of Paleontology) for the true shape of the premaxilla of Phlegethontia.

    c) In several points, all existing literature on aïstopods is outdated; a manuscript currently in the 3rd round of review is going to blow a lot of minds.

    2) Jason Pardo and I gave you a long list of characters where Colosteus has the state found in tetrapods. I see you still haven’t added these characters to your matrix. Why? I really don’t understand.

    The ‘gill bones’ are removed. They are gastralia.

    Nope! They’re cleithra, or perhaps fused cleithra + clavicles. See Anderson (2002), which you’re already citing but evidently haven’t read. Gastralia are present and look quite different.

    (Why do you cite papers you haven’t read? That’s very bad practice.)

  2. Oh, also, even your contribution to fig. 4 is in error: Phlegethontia has lots of caudal vertebrae with ribs – 20 or so. The first caudal vertebra can be recognized as the first that has two ventral keels (between which the caudal artery lay) instead of one. This, too, is in the paper.

  3. The CGH 129 specimen is pretty intricate for being inaccurate, as you suggest it is. So I’m going to take your blackwash of all old literature comment with a grain of salt. So far I did not find a photo of the same specimen. Send it if you have it. It certainly is a distinct genus, as presented, from the AMNH specimens.

    I do cite papers I haven’t read, often because they are not freely available online. I do this not to ‘cover BINGO squares’, as you suggest, but to offer interested readers further reading options. That’s not a ‘bad practice.’ Withholding relevant literature would be bad practice. I appreciate your indignity, but it’s unwarranted here. When I get the Anderson 2002 paper, I’ll read it and make changes if necessary.

    With regard to the sacral vertebrae in figure 4, I based that on the overall similarity to basal tetrapods and osteolepids that don’t have caudal transverse processes (ribs). At present I don’t see caudal ribs until the advent of Trimerorhachis and Ichthyostega which currently nest in other clades distinct from Phlegethontia and Pholidogaster which appear to more fish-like. You may be right about the double vernal ridges in Phlegethontia. Unfortunately figure 4 is a dorsal view. Send data if you want to see corrections made.

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