Shark skull evolution updated

Short one today,
updating an older graphic with the latest ReptileEvolution images in order (Fig. 1).

Figure 1. Shark skull evoluition updated today, June 2, 2021.

More taxa =
more parsimony, smoother transitions, greater detail, more support.

Loganellia is Early Silurian, so more primitive taxa are earlier, in the Ordovician.

start with Chondrosteus.

Multiple opercula (gill openings)
begin with Rhincodon.

Marginal teeth
start with Ginglymostoma.

Nasals extend beyond the jaws
(convergent with or reversal to sturgeons) with Ginglymostoma and Sphyrna, the hammerhead.

Nasals extend waaay beyond the jaws
in guitarfish (Rhinobatos) + sawfish (Pristis) on one branch and Squaloraja + the goblin shark (Mitsukurina) on another by convergence.

Nasals are much smaller
in Isistius and Ozarcus leading to Hybodus and bony fish, which also return to a single gill opening (as in Prohalecites Fig. 2, Amia, Fig. 1).

Ancestors of Gregorius also evolve to
the moray eel (Gymnothorax, not shown), Harpagofutator (Figs. 1, 2), Edestes, and Helicoprion (Fig. 1).

Ancestors of Gregorius also evolve to
spiny sharks, like Homalacanthus (Early Devonian, Fig. 2) so all more primitive taxa appeared earlier, in the Silurian.

Ratfish and Iniopterygians
arise from horn sharks like Heterodontus. These taxa also reduce the number of gill openings.

Torpedo rays
arise from hammerheads. Other skates, rays and manta rays have separate origins.

Paddlefish (Polyodon)
with a single gill opening arise from basking sharks (Cetorhinus) with multiple gill openings.

Figure 2. Representatives from the Early Devonian radiation that gave us bony fish, including Prohalecites and Homalcanthus.

This appears to be a novel hypothesis
of interrelationships (Fig. 3). If someone proposed this earlier, let me know so I can promote that citation.

Figure 3. Subset of the LRT focusing on elasmobranchs.

Don’t put all your ‘faith’ in one to a dozen traits
(like gill opeinings) in order to exclude taxa. That’s called “Pulling a Larry Martin.” Expand your taxon list and see where evolution takes your cladogram.

According to the LRT,
humans, bats, frogs, dinosaurs and pterosaurs all have shark ancestors, so we are all sharks. Something else to get used to.

15 thoughts on “Shark skull evolution updated

  1. This appears to be a completely random selection of non teleosts drawn from a hat. If I could be bothered I would see if purely randomly selecting these names would give a more or less parsimonious comparison to other independently derived (modern, fossil, genetic) analyses that all give essentially identical results for the included taxa. There is no point going into this in detail, so I’ll pick one example- the relationship of Heterodontus (I have looked at CT scans, preserved, taxidermy and fossil examples) and Chimaera (CT scan etc, as well as dissecting loads of them). I cannot come up with a single character that is demonstrably homologous that links Heterodontus with Chimaera and excludes all, or most, other neoselachians or neoselachian sharks. I have looked at the chondrocranium, jaws and hyoids, branchial skeleton, dentition, paired fins, vertebral column, dermal skeleton (denticles, spines) and soft tissue anatomy. The closest I can come up with is presence of fin spines, but those of Heterodontus share characters of number, micro and macro structure, attachment and insertion, growth mechanism, etc. with Squalus, Protospinax, Palidiplospinax, Belemnobatis and even Hybodus. Please enlighten us with just a character or two in each of those categories, so no need for a full analysis.

    • The same characters are used for reptiles, birds, mammals. Per your request, some traits that relate to sharks and chimaera include: 1) orbit longer than or equal to rostrum or not. 2) nasal shape in dorsal view, ten possibilities. 3) lacrimal deeper than maxilla or not or fused. The light tan parts are lacrimal homologs. The premaxilla and maxilla are absent until marginal teeth appear and those two homologs first appear as no deeper than a ribbon. Hope this helps. I know it’s different than you were taught. I am teaching myself with every new taxon, and have made over 100,000 changes over the last ten years. So nothing is set in stone, but in detail and overall the present results appear to show a gradual accumulation of derived traits with not untenable ‘jumps’ between taxa.

      • Not sure how you define the first one, as rostral cartilages are often unmineralised, and almost certainly not homologous (look at the embryological development). Indeed if you include any rostral structures, those of chimaeroids are really large, but folded back on themselves.
        The others make no sense as they seem to refer to dermal bones that are lacking in both taxa.

        As I say, on the other hand where are the characters based on chondrocranium, jaw cartilages (as opposed to non existent bones) and hyoids, branchial skeleton, dentition, paired fins, vertebral column, dermal skeleton (denticles, spines) and soft tissue anatomy?

      • re: dermal bones lacking… Contra tradition, sturgeons precede sharks. Sharks lose bone. Bony fish regain it from cartilage over cartilage precursors. We can see this in hatchling bowfins. This post may help:

        This one, too:

        re: where are the characters based on…? They are the same characters used for the rest of the LRT, in the .nex file, available on request, and always going through improvements and additions. The character list has not been posted online yet. This is an ongoing study, never quite finished.

      • These are circular arguments- you arbitrarily draw non existent dermal bone on animals which have none and use that as evidence that they had dermal bone. Start with one, very simple set of characters in the dentition (a small and easily observed set of features). The following are shared by Heterodontus and all neoselachians, but lacked by Chimaera and all other post Palaeozoic holocephalans: dental lamina producing separate teeth, teeth grow from outside in, teeth shed by cutting off vital supplies at jaw margin, tooth files in alternate pattern, enameloid present (the complexity of enamelled is a set of characters), dental laminae do not fuse during ontogeny, osteodentine present, ability to produce teeth at symphysis (lost in lamniformes), lack of whitlockite, lack of tritoral tissue enclosed within modified dentine, lack of tubular dentine (tubular structures in myliobatids and Ptychodus is structurally different); do you want more or shall I move on to the vertebral column?

      • You wrote: “you arbitrarily draw non existent dermal bone ” …No. I follow shapes specified by geography and landmarks. Everyone knows there is no dermal bone… but there is a homolog structure.

        re: dentition… I have resisted adding more dental traits (including tooth order, chemistry, etc), opting instead for larger traits more easily seen in the photo data available. So far, this has not been a problem.

        re: lacked by Chimaera… as you know, some traits appear only once. Others appear and remain. Others fade and disappear.

        Clearly you know your category. Like you, many paleontologists are specialized to their favorite subject. Few have decided to generalize, studying both birds and fish and everything in between. That’s my niche. I need to see how chimaeras are related to elephants and everything in between. At present the cladograms presented are the latest results, always subject to change if a more parsimonious arrangement with more gradual accumulations (or losses) of traits can be recovered. Suggestions along those lines would be very helpful.

      • For comparing across large clades there is no choice but to break it down and build back up, because homology becomes almost impossible to prove. It is unlikely that the teeth of chondrochthyans is homologous with that of osteichthyans, but that is uncertain. The gene expression is the same but embryological development of the dental lamina is completely different. Likewise the skull of chondrochthyans is not homologous with that of osteichthhans in entirety. The osteichthyes skull is essential dermal whilst the chondrichthyes skull is mesodermal- fundamentally different. The osteichthyes skull essentially uses the chondrichthyes skull as a template (really easy to see in the lower jaw). The most likely homology is the chondrichthyes skull is homologous to the lateral and ventral parts of the osteichthyes braincase (the dorsal part being fused to dermal bone). It is easily seen embryologically that facial/lateral bones of osteichthyes encase the braincase/chondrocranium that itself ossifies (again there is no ossification in chondrichthyes, other mineralisation is in its place). Trying to compare (non braincase) bones of osteichthhans with a chondrichyan skull is madness – it is like trying to make the dermal armour of an armadillo and the (mesodermal) ribs of a turtle homologous because they enclose the trunk. Put in more characters and be careful of homology and certainly do not make up nonsense homologies, and you will end up with the same trees as everyone else, whether they are looking at genetics or characters. Interesting that in chondrichthyans trees using fossils and skeleton, modern and soft tissues, and 3 totally independent genetic markers all give virtually identical results (the few exceptions are the reason for keeping at this). And at order level this pretty much mirrors pre cladistics too.
        Boring, but true.

      • You wrote: “It is unlikely that the teeth of chondrochthyans is homologous with that of osteichthyans, but that is uncertain.”

        Adding taxa in the LRT shows shark marginal teeth are ancestral to bony fish teeth. The same can be said of the skull elements. By minimizing taxon exclusion the LRT answers many such questions. Even so, I eagerly look forward to other studies that duplicate the LRT taxon list. Everything is at the hypothesis stage until tested by others.

        You wrote: “whether they are looking at genetics or characters.”

        Please send citations or PDFs for such character studies. The LRT falsifies deep time genetic studies in many vertebrate and tetrapod clades.

        You wrote: “Trying to compare (non braincase) bones of osteichthhans with a chondrichyan skull is madness –”

        That’s not a good answer. Taxa need to be compared, whatever the results may be. Please send citations that test similar taxon lists, including hybodontids, sturgeons, paddlefish, moray eels and their extinct relatives like Prohalecites. Do not send studies that list only suprageneric taxa.

        Thank you for your expertise.

  2. Honest question – can you define what you mean, precisely, by “precursor” in the context of a dermal bone on a chondrocranium? I’ve made the same point as the previous poster several times – coding dermatocranial characters for sharks ls like coding phalangeal characters for snakes – but you’re still doing this.

    • Sorry if this is a repeat. I don’t see my just posted reply in the comment list. To repeat:
      Identification is based on homologous shapes surrounding landmark openings like the naris, orbit, jaws, occiput, teeth, etc. Start with Acipenser (sturgeon) and Chondrosteus (basalmost gnathostome) as outgroup taxa. Bowfin (Amia) hatchlings demonstrate how bone develops on chondrocranium precursor structures as islands that spread to touch one another in adults. Chondrocrania appear on sharks in patterns homologous with those appearing on bony fish close to sharks. Both overall and in detail the patterns evolve from one genus to another. That’s what I’m trying to show. Is there a more parsimonious alternative? If so, please advise.

      • The actual alternative, parsimonious or otherwise, is that sharks simply don’t have any sort of “precursors” on their chondrocrania for dermal bones. They’re not there. You cannot score dermatocranial characters for sharks.

      • You wrote: “You cannot score dermatocranial characters for sharks.” I falsified your statement by doing so. And… it works.

        Perhaps what you meant was, “You should not score…” or perhaps, “You will be making a mistake if you score…” Sounds like those who said, you cannot or should not lump and separate 360 to 1870 taxa with only 235 characters. I falsified that statement, too. Chris, what I need from you is an alternative to the LRT phylogeny for sharks. Where do they arise from? What do they evolve into (if anything)?

    • PS you wrote: “coding dermatocranial characters for sharks ls like coding phalangeal characters for snakes” except that snakes don’t have fingers. Sharks do have heads shaped by structures, as in all craniates. Try to keep your metaphors in the realm.

      • I stand by my metaphor. Sharks have heads, but not dermatocrania. Scoring characters reflecting dermatocranial variation in a shark is exactly the same as scoring characters reflecting digit variation in a snake. As I’ve stated, there are no precursors to any dermal bone on the chondrocranium of a shark.

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