Ferromirum, a little sister to the mysterious giant shark, Megachasma

Frey et al. 2020 brought us
a new, small, Late Devonian shark with large eyes. Ferromirum (Fig. 1). In the large reptile tree (LRT, 1770+ taxa, subset Fig. 4) Ferromirum has a long, lost living relative, the megamouth shark, Megachasma (Figs. 2, 3), a taxon overlooked by Frey et al.

Figure 1. Ferromirum a Late Devonian shark in several views from Frey et al. 2020 and colorized here.

Figure 1. Ferromirum a Late Devonian shark in several views from Frey et al. 2020 and colorized here.

While Ferromirum may look toothless
so does Megachasma (Figs. 2, 3) at the same scale. Rows of tiny sharp teeth lined  like cemetery headstones filled the jaws of both taxa.

From the Frey et al. abstract:
“The Palaeozoic record of chondrichthyans (sharks, rays, chimaeras, extinct relatives) and thus our knowledge of their anatomy and functional morphology is poor because of their predominantly cartilaginous skeletons.”

Not that poor in 2020. And we have living specimens to dissect. The taxon exclusion problem experienced by Frey et al. is of their own doing.

“Here, we report a previously undescribed symmoriiform shark, Ferromirum oukherbouchi, from the Late Devonian of the Anti-Atlas. Computed tomography scanning reveals the undeformed shape of the jaws and hyoid arch, which are of a kind often used to represent primitive conditions for jawed vertebrates.

Not true. Taxon exclusion is the problem here. That led the Frey et al. team to a misunderstanding of shark origins (Fig. 4) in which they made the placoderm, Entelognathus, the outgroup and the bony fishes Guiyu, Climatius, Acanthodes and Doliodus nested basal to sharks. In the LRT these taxa are all derived from sharks. Essentially the cladogram in Frey et al. is upside down. See below for details.

Figure 1. Megachasma in vivo. Note the single cusp teeth.

Figure 2. Megachasma in vivo. Note the single cusp teeth.

Except for size, Megachasma provides many clues
to the in vivo appearance and habits of Ferromirum due to its close nesting and phylogenetic bracketing.

Figure 3. Megachasma skull.

Figure 3. Megachasma skull.

The Frey et al. abstract continues:
“Of critical importance, these closely fitting cartilages preclude the repeatedly hypothesized presence of a complete gill between mandibular and hyoid arches. We show that the jaw articulation is specialized and drives mandibular rotation outward when the mouth opens, and inward upon closure.”

As in the megamouth shark, “so unlike any other type of shark that it is usually considered to be the sole extant species in the distinct family.” according to Wikipedia, which also suffers from taxon exclusion.

“The resultant eversion and inversion of the lower dentition presents a greater number of teeth to prey through the bite-cycle. This suggests an increased functional and ecomorphological disparity among chondrichthyans preceding and surviving the end-Devonian extinctions.”

It’s good to know the megamouth shark is no longer alone.

Figure x. Subset of the LRT focusing on sharks.

Figure 4. Subset of the LRT focusing on sharks.

Basically the Frey et al. cladogram is upside down
due to taxon exclusion. In addition to the issues listed above, the authors nest the dogfish shark, Squalus, at a highly derived node. The LRT (Fig. 4) nests it close to the base. The authors nest Ferromirum, Cladoselache, Ozarcus, and Akmonistion within the Holocephali (chimaeras). The LRT nests these taxa with sharks. The last two nest as proximal outgroups to the derived clade Osteichthyes (bony fish), which is a clade basal to sharks in Frey et al. More taxa from a wider gamut resolves all such issues. Don’t rely on the work of others or tradition. Find all this out for yourself.


References
Frey L, Coates MI, Tietjen K, Rücklin M and Klug C 2020. A symmoriiform from the Late Devonian of Morocco demonstrates a derived jaw function in ancient chondrichthyans. Nature Communications Biology 3:681 | https://doi.org/10.1038/s42003-020-01394-2 | http://www.nature.com/commsbio

11 thoughts on “Ferromirum, a little sister to the mysterious giant shark, Megachasma

  1. I’ve taken the liberty of looking over your previous discussions of shark morphology and phylogeny.

    Please don’t take this as a personal attack, but all of them can be very easily falsified.

    You’re mapping the homologues of bones found in bony fishes – structures like the nasal, postorbital, and so on – on shark chondrocrania. It’s not just that these homology assessments are wrong on phylogenetic grounds – they’re fundamentally wrong on developmental grounds.

    Most of the bones you’re trying to identify on shark chondrocrania are dermal bones. That means they don’t pre-form in cartilage. Which means animals without a bony skull cannot have them. That’s pretty much by definition, too – if all you have is cartilage, you won’t have any of the bones, or even equivalents of the bones, that never form in cartilage in the first place.

    Every one of these reconstructions should be set aside. The homologies you’ve identified are simply not biologically possible.

    This isn’t the grousing of someone beholden to classical ideas that clash with your ideas, either. For what it’s worth, I don’t care that much about the phylogenetic relationships of fishes. But I do care that someone is posting pictures of shark chondrocrania with supposed homologues to bones that are absolutely not there.

    It’s not a matter of looking at an actual specimen versus looking at a picture. Nor s it the grousing of someone beholden to classical ideas that clash with yours. In fact, the problem is independent of phylogeny; your phylogeny could be completely correct, and your reconstructions would still be wrong. Everything we know about skull development tells us this.

    I’m really not trying to be a jerk here, but you’ve taken yourself on a very long tangent underpinned by a fundamental misunderstanding of what would appear on a chondrocranium. Please – do some research on skull development.

    • I understand your concerns, Chris. I think what you and I are seeing here is a traditional paradigm being broken. The evidence shows that shark chondocrania are homologous to bones found in both predecessors (sturgeons, paddlefish) and successors (the rest of the bony fish clade) based on shape and placement, because sutures are largely obliterated in the cartilage of sharks. Note that this obliteration transitions from sutures in sharks that are closer to paddlefish on one end and sutures return to hybondontids and the rest of the bony fish on the other end. It is axiomatic that sharks cannot stand alone. There must be a relationship to other vertebrates. The LRT has shed new light on that relationship with microevolutionary steps between taxa. The renaming of skull topologies along the lines of tetrapod homologies has opened a door that had previously been closed due to a separate naming system that I and others have suggested unifying.

      At the core of your argument you wrote: “Most of the bones you’re trying to identify on shark chondrocrania are dermal bones. That means they don’t pre-form in cartilage. Which means animals without a bony skull cannot have them.” You are correct in that assessment. So what is the unifying solution? I have not addressed developmental issues. To your point: Darwin and others before him have noted the overall resemblance of vertebrate embryos to one another, including shark embryos. Those 19th century illustrations did not get into the developmental issues you raised, either, but still the point of those early illustrations was clear. Vertebrates share traits from snout to tail. The LRT shows that vertebrate skulls _alone_ took a detour after paddlefish, then returned to bony skull elements with sutures with hybodontids and their descendants. I don’t know the mechanism for this. The LRT provides the map for future scholars.

      Likewise, I don’t know why genomics in deep time do not provide identical tree topologies to phenomics. But clearly they don’t, by a long shot, as everyone already knows. Again, future scholars should dive into this problem.

      Let’s keep talking about this if the above explanation is wrong, insufficient or not clear.

      • There’s no unifying solution here. There are no homologues between dermal cranial bones and anything in the shark chondrocranium. There’s no argument. If you’re seeing what you believe to be correlates of any sort of dermal bone on a shark chondrocranium, you are wrong. I’m sorry to be blunt about it, but that’s how it is.

        Your discussion of similarities in embryos known in the 19th century is a non sequitur. However similar a shark and a human embryo are in early developmental stages, those similarities do not imply homology between the developing chondrocranium in any vertebrate and the dermatocranium in any vertebrate that has one. There is no homology to be found.

        The history of homology assessment in the 18th and 19th centuries is fascinating – there are whole books about it – but developmental pathways and similarities were assessed very differently. It’s worth looking into.

        There’s no “paradigm” to be broken here. It’s like trying to link a lava flow with a coal seam at a different locality. They may look kinda similar – black, tabular, maybe sharply demarcated by underlying and overlying deposits – but one is comprised of various silicate minerals crystallized from molten rock that erupted from a volcano and the other is compressed plant material that developed in a swamp. That they might be relatively close to each other, and they may have even formed at approximately the same time, but one is not an extension of the other.

        This is exactly what you’re trying to do. Dermatocranial bones form in close proximity to the chondrocranium. That both have the suffix “-cranium” certainly reflects this. It would thus be reasonable to expect that, in many cases, dermal ossifications might conform to the surface of the underlying chondrocranium (though in many cases, the cartilage found in a part of a shark chondrocranium never forms in a bony vertebrate). The skin of my hand approximates the overall arrangement of the underlying skeleton, but that doesn’t mean one can find homologues to the hair on my knuckles in the limb skeleton of a lungfish.

        I can only repeat my request that you do some research on how vertebrate skulls develop.

      • Thank you, Chris. I understand your concerns, but homologous shapes are present. Transitional phases leading to and from the shark clades are part of the current data. I report present results. I agree more scientific investigation is needed. I will be glad to retract and/or change reported data if and when that becomes necessary. ReptileEvolution.com is an ongoing experiment, the only one of its kind on the planet at present. Over 100,000 corrections have been administered so far. I will report additional results after further study.

    • This last post and your replies are the ultimate demonstration that you are completely unaware of the groups you talk about.

      Dear Prof. Brochu, that’s one of the problems with Peters’ methods… You can discuss, suggest and prove anything you want… Peters will simply ignore you and years of morphological and molecular studies (that he calls “tradition”). He will simply continue to ignore the suggestions of several experts and the literature only to find and force absurd explanations to his absurd tree topology he gets with 235 characters and more than 1700 taxa.

      • Dear Chris and Giuseppe. Nothing is absurd here. All sister taxa look alike. That cannot be said of taxa within the genomic clades Afrotheria and Laurasiatheria. Golden moles and elephants nest together in genomic studies. Flamingoes and grebes nest together in genomic studies. That is what is absurd. So, moving forward…

        Let’s say there are no dermal homologues between dermal cranial bones and shark chondocrania. Okay, I still need a tree topology that includes sharks and bony fish and a way to score their skulls in the same study. What do you have?

  2. That you find similarly shaped features in the bony fish dermatocranium and the shark chondrocranium means nothing more than that. You have not found homologous similarities at all. That’s because the structures cannot be homologous – they have entirely different developmental origins. Although they are functionally linked, they are otherwise completely independent of each other. Phylogeny is irrelevant to the question.

    Have you followed my advice to do some research on skull development? You really should do that BEFORE responding to anything we’re telling you. If you do, you’ll understand how the similar shapes you’ve seen in photos are misleading you into thinking you’ve found nonexistent homology.

    I, personally, don’t have the characters you need. I don’t do fish. But there are plenty of studies out there that consider the phylogenetic relationships of sharks and bony fishes simultaneously. I understand you think they suffer from some sort of “taxon exclusion” problem, but I would (strongly) urge you to look at the characters they’ve used. Perhaps some of those could be incorporated into your matrix. At the very least, you could reach out to some professional ichthyologists for advice on the matter.

    • Thank you, Chris. This is an ongoing experiment. I have reached out to Long, Friedman and others (who don’t come to mind at the moment). All are professional ichthyologists. None show any interest other than sending requested PDFs at present. I wonder if this is all going to end up like quantum mechanics. No one can fully explain it, but it works when put to practical applications. I do know, through twenty years of experience, that great resistance comes with everything not already in the textbooks (e.g pterosaur ancestry, whale dual origins, turtle dual origins, etc.). Again, I appreciate your professional assistance and attitude.

  3. Dr. Brochu’s request is more than reasonable. I would strongly urge you to obtain and read The Skull series by Brian K. Hall (https://press.uchicago.edu/ucp/books/book/chicago/S/bo3639606.html). It covers many of the important foundational knowledge of skull development. Similarly, I would recommend reading Patterson 1982 (Morphological Characters and Homology) as it provides a means of testing for homology that goes beyond “they look the same”.

    • Thank you, Jura, for the links. I will attend to them. There was never any resistance. Only lack of direction, which you have provided. Meanwhile other random/convenient online studies are yielding little because they are based on traditional phylogenies that exclude pertinent taxa.

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