Crassigyrinus nests with Whatcheeria

Crassigyrinus scoticus (Watson 1926, Clack 1998; 2m in length; Early Carboniferous, Viséan, 340 mya; Fig. 1) has been described as taxonomically enigmatic (see below). The large reptile tree (LRT) nests it clearly and robustly with Whatcheeria (Fig. 1), more or less confirming Clack (1998).

Figure 1. Crassigyrinus compared to Whaatcheeria.

Figure 1. Crassigyrinus compared to Whaatcheeria. It appears that the quadratojugal on Whatcheeria has been rotated dorsally. A ghosted drawing on an unrotated QJ is shown.

This aquatic tetrapod
had tiny limbs and likely a long deep tail. The palate has been described as ‘very fish-like’. The vertebrae were not well ossified with no sign of posterior facets to unite them. The postfrontals contacted each other medially, separating the frontals from the parietals. The skull was relatively tall on this active predator with large teeth. The basioccipital is not developed into a formed occipital condyle, but then the neck is so short that the pectoral girdle starts beneath the lateral skull bones.

Panchen reported
the tabular contacts the parietal, as in the Seymouriamorpha, but that is not the case in the Clack data (Fig. 1). Some workers report a preopercular bone as in Whatcheeria, but that is likewise not shown in the Clack data, even though its sister, Whatcheeria, evidently retains a preopercular. Panchen 1990 suggested, “The homology [of the ischium] wth the pelvic fin basal scute of osteolepiform fishes is proposed.” Unfortunately Crassigyrinus comes too late both phylogenetically and chronologically and the basal scute is way too far back beneath the tail on Osteolepis (Fig. 2).

Figure 2. Ostelepis has a large bone basal to the pelvic fin. IMHO it is too far back to be a possible ischium, contra Panchen.

Figure 2. Osteolepis has a large bone basal to the pelvic fin. IMHO it is too far back below the causals to be a possible ischium homolog, contra Panchen.

Neotony
Crassigyrinus retains many juvenile (tadpole) and/or primitive traits. Ahlberg & Milner (1994) reported: “Instead of being the first tetrapod to ‘return to the water’, it may be the last survivor of the primitive tetrapods that never left the water.'” That hypothesis is not confirmed by the LRT.

Figure 3. Pederpes is a basal taxon in the Whatcheeria + Crassigyrinus clade.

Figure 3. Pederpes is a basal taxon in the Whatcheeria + Crassigyrinus clade.

Taxonomy
Panchen 1991 considered Crassigyrinus the sister-group of the Anthracosauroideae (Proterogyrinus and kin), but in the LRT Crassigyrinus nests as derived within that clade without known descendants. A basal taxon is Pederpes (Fig. 3). Clack 1998, 2002 considered Crassigyrinus close to Whaatcheeria and anthracosaurs, which the LRT supports. Wikipedia has more on its long history of discovery and taxonomy.

Old news
Earlier the LRT nested the basal tetrapod Parioxys with temnospondyls and I was warned that all known specimens are difficult. I was working from a simple illustration provided by Carroll 1964. With the addition of big-headed Acheloma (=TrematopsParioxys nested as a sister to it within the Dissorophoidea and within the Lepospondyli. That being said, as illustrated by Carroll, Parioxys has so many autapomorphies that I’m going to exclude it, for now, from the LRT. I understand better data is coming soon.

References
Ahlberg PE and Milner AR 1994. The origin and early diversification of tetrapods. Nature 368, 507-514.
Clack JA 1998. The Scottish Carboniferous tetrapod Crassigyrinus scoticus (Lydekker) – cranial anatomy and relationships. Transactions of the Royal Society of Edinburgh: Earth Sciences 88, 127-142.
Clack JA 2002. Gaining Ground: The origin and evolution of tetrapods. Indiana University Press.
Lydekker R 1890. On two new species of labyrinthodonts. Quarterly Journal of the Geological Society, London 46, 289-294.
Panchen AL 1985. On the amphibian Crassigyrinus scoticus Watson from the Carboniferous of Scotland. Philosophical Transactions of the Royal Society of London B 309: 505-568.
Panchen AL 1990. The pelvic girdle and hind limb of Crassigyrinus scoticus (Lydekker) from the Scottish Carboniferous and the origin of the tetrapod pelvic skeleton. Earth and Environmental Science Transactions of the Royal Society of Edinburgh 81(1):
Panchen AL 1991. The early tetrapods: classification and the shapes of cladograms in: Origins of the Higher Groups of Tetrapods: Controversy and Consensus. Eds. Schultze HP and Trueb L. Comstock Publishing Associates, Cornell University Press, Ithaca and London.
Watson DMS 1926. Croonian Lecture – The evolution and origin of the Amphibia. Philosophical Transactions of the Royal Society B 214:189–257.

wiki/Crassigyrinus
Tetrapod Zoology/Crassigyrinus/2007

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3 thoughts on “Crassigyrinus nests with Whatcheeria

  1. That’s almost orthodox. :-) I’m curious to know what character states unite Crassigyrinus with the whatcheeriids; they’re fairly close together in published trees (mine included), but not sister-groups.

    The question of whether Crassigyrinus retained the preopercular shouldn’t be regarded as a disagreement between different people who have looked at the same (difficult) material. Clack prepared the specimen further than before and looked at it for years; I’m confident Crassigyrinus lacks preoperculars.

    Spelling mistakes in your post: “Osteleopis”, “Ostelepis” should be Osteolepis (‘bone scale’); “Panioxys” (twice) should be Parioxys.

    That hypothesis is not confirmed by the LRT.

    IMNSHO, the terrestriality of such animals as Pederpes, Ossinodus or Eoherpeton has been greatly exaggerated. I think they were no more terrestrial than Silurus.

    I understand better data is coming soon.

    Oh, that might well take another few years. Marcello seems to be working on lots of different things at the same time, or to be working on some of them while keeping the others on the backburner for years. I saw the loan slips for Parioxys in 2013.

    but in the LRT Crassigyrinus nests as derived within that clade without known descendants.

    You’ll never learn it, will you?

    All available software for phylogenetic analysis assumes that none of the OTUs are ancestors of any others. If you put a literal father and a literal son in your data matrix, they’ll come out as sister-groups. They cannot do otherwise! If you’re lucky, the father’s branch will have a length of zero, but you won’t know that until you plot the apomorphies.

    Phylogenetic analysis does not show ancestors and descendants. That’s not even its purpose.

    • Sorry I overlooked this for so long! Yes, you can infer ancestral character states from a phylogenetic tree.

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