Early Carboniferous, eel-like Chondronchelys enters the LRT, but not with ratfish

Saturday, November 20, 2021 13:57

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From the Finarelli and Coates 2014 abstract:
“Chondrenchelys problematica, from the Viséan (Holkerian) of Scotland, is the earliest holocephalan known from extensive cranial and postcranial material.

When tested
in the large reptile tree (LRT, 2001+ taxa) Chondrenchelys (Fig. 1) nests with Heteropetalus (Fig. 2) and these with Harpragofututor (Fig. 3) and Edestus, far from the clade Holocephali (= ratfish). The Chondrenchelys clade is transitional from the derived shark, Hybodus, and basal bony fish like Homalacanthus and Prohalecites (Fig. 3).

Finarelli and Coates do not mention Heteropetalus.
They note, “Lund (1982) re-described key anatomical features of Chondrenchelys in his comparative description of the slightly younger holocephalan Harpagofututor volsellorhinus from the Bear Gulch fauna, placing both genera in the Family Chondrenchelyidae (Berg 1940), based on their anguilliform (eel-like) body plans, and general similarities in the morphologies
of the pectoral fin and tooth plates.”

The authors do not mention Gregorius (FIg. 3), Edestes, or Gymnothorax, all three related to Chondronchelys in the LRT (subset FIg. 4).

Figure 1. Chondrenchelys in various views.

The abstract continues:
“Here, we provide a comprehensive new description of this taxon using three new specimens, in which we observe many morphological features for the first time. Much of the cranial morphology is closer to that of living chimaeroid holocephalans than was previously appreciated.“

The phylogenetic splitting of chimaeroid holocephalians
from these moray eel (= Gymnothorax) relatives is unique to the LRT due to traditional taxon exclusion elsewhere. Add pertinent taxa to your favorite claodgram to split these clades.

Figure 3. Representatives from the Early Devonian radiation that gave us bony fish, including Prohalecites and Homalcanthus.

The abstract continues:
“The conjunction of numerous distinctly holocephalan features with those that are otherwise general to Chondrichthyes demonstrates a decoupling of several of the structural conditions that characterise the distinctive morphological complex of the extant holocephalan skull.

The superficial convergence
is readily observable.

The abstract continues:
“The anguiliform postcranium is more elongate than previously reconstructed, and it is now clear that the axial skeleton extended beyond the posterior extremity of the elongate dorsal fin. Morphological characters are reviewed with a view to further phylogenetic analyses.

The abstract continues:
“We recommend using the appearance of Chondrenchelys at 336.5 Ma as a hard minimum age for the last common ancestor of elasmobranchs and chimaeroids, because of its secure association with other holocephalans, and current uncertainties concerning elasmobranch stem lineage membership.”

Or not.
Finarelli and Coates report, “published phylogenies of fossil holocephalans show little agreement. Grogan et al.’s (2012) tree of chimaeriforms and their fossil relatives differs profoundly from Stahl (1999), and both conflict with the topology of the holocephalan taxa in Pradel et al. (2011).”

The authors continue:
“Relating the morphological disparity of living chondrichthyans to the increasingly detailed range of Palaeozoic forms presents a growing challenge; hence the significance of the new Mumbie Quarry specimens, which offer a more detailed and effective comparison between ancient and modern holocephalans.”

Figure 4. Subset of the LRT focusing on basal vertebrates. Note the clade Holocephali nests apart from Chondrenchelys and kin, including moray eels.

The authors conclude:
“We do not undertake a phylogenetic reconstruction here; a comprehensive analysis will be undertaken in a related study (currently in preparation) on undescribed specimens from Bearsden, Scotland.”

Figure 5. Akmonistan, a relative of Stethacanthus.

A Google search failed to find
a more recent (2015–2021) paper on Holocephali from Bearsden, Scotland, home of Stethacanthus. However, the authors of Wikipedia consider stethacanthids “prehistoric holocephalians’. Akmonistion (Fig. 5) is considered a stethacanthid, but it does not nest within the clade Holocephali in the LRT (Fig. 4). The LRT indicates it is time to split up the traditional members of this clade.

This appears to be a novel hypothesis of interrelationships.
If not, please provide a citation so I can promote it here.

Thoughtful readers will note
the LRT now has more than 2000 taxa, still splitting and lumping virtually all of them using the same 238 multi-state characters. They said it could not be done, but that’s the current state of this online experiment. The LRT continues to be relegated by professors, students, paleoartists, underemployed PhDs and trolls worldwide, often because it contradicts textbooks. Corrections continue wherever warranted. The early tree topology from 2011 has remained essentially unchanged, but for the addition of 10x more taxa, including several hundred fish. Thank you for your comments and readership as the pool of untested taxa continues to dry up. Hopefully new/old taxa will keep showing up.

References
Finarelli JA and Coates MI 2014. Chondrenchelys problematica (Traquair, 1888) redescribed: a Lower Carboniferous, eel-like holocephalan from Scotland. Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 105 , Issue 1 , March 2014 , pp. 35 – 59 DOI: https://doi.org/10.1017/S1755691014000139

wiki/Chondrenchelys

Source: https://pterosaurheresies.wordpress.com/2021/11/21/early-carboniferous-eel-like-chondronchelys-enters-the-lrt-but-not-with-ratfish/