Basal bony fish descendants of hybodontid sharks

Moving on from sharks in general,
hybodontid sharks (Fig. 1)  have the most heavily ossified skulls… without a rostrum… with jaws extending to the anterior margin, as in bony fish.

For those following reader comments
on the latest heresy, reader comments do not refer to ALL the skull bones only the dermatocranium. Keep this in mind when reading the following from the U. West Vancouver labs online study of skulls accessible here.

The neurocranium (= chondrocranium) surrounds the brain and certain sense organs (parietal, postparietal, intertemporal, supratemporal, tabular and all occipital bones). In sharks the neurocranium is composed of cartilage, but in most other vertebrates the cartilage is replaced by bone.

The splanchnocranium consists of the gill arches and their derivatives… part cartilage, part endochondral bone. The splanchnocranium evolved to become the bones of the human face (below the frontals, sans nasals = maxilla + premaxilla + lacrimal + jugal + quadrate + dentary + ear bones (= former hyomandibular + jaw bones)) and the face of Amia the bowfin (Figs. 1, 2). The preopercular disappears in basal tetrapods no longer breathing with gills.

The dermatocranium consists of the original dermal scales (= armor) of ostracoderms and sturgeons. The authors say “The dermatocranium forms most of the skull,” but really all that is left over from the above lists are the nasals, frontals and circumorbitals (= prefrontals, postfrontals, postorbitals). The squamosal and quadratojugal appear later as cheek bones split in two, then split again. And also do so by convergence in unrelated taxa. So what are we arguing about with regard to shark-bony fish homologies? Not many bones after all.

Figure 1. Fish evolution from Hybodus to Amia documenting the shark to bony fish transition.
Figure 1. Fish evolution from Hybodus to Amia documenting the shark to bony fish transition.

Keys to understanding this issue include:

  1. The elements of the dermocranium in shark outgroup taxa (sturgeon and paddlefish)  = bone sheath over cartilage.
  2. The elements of the dermocranium in sharks  = prismatic cartilage, more ossified in hybodonts
  3. The elements of the dermocranium in proximal shark descendants (Amia and the moray eel, Gymnothorax, Fig. 1) = bone redevelops surrounding sensory cells over a cartilage bauplan (Fig. 3).
Figure 4. Skull of the extant bowfin (Amia). Compare to figure 3.
Figure 2. Skull of the extant bowfin (Amia). Compare to figure 3.

As a quick review, Bemis et al. 1997 report, 
“the bones more or less closely ensheath the underlying endochondral rostrum” of sturgeons and paddlefish. Sharks lack this sheath of bone.

As reported earlier, Pehrson 1940 examined
a series of embryonic stages of Amia calva (Fig. 3). Pehrson was a fan of naming fish bones in accord with those of pre-tetrapods, as he reports, “There seems to be no doubt that the intertemporal and supratemporal parts of the developing composite bone correspond to the similarly named bones in Osteolepidae and Rhizodontidae.” Thus Pehrson labels the intertemporal and supratemporal. Perhaps he was the first. I repeated the experiment and came to the same conclusions in sharks. Note the reduction of the long nasals in bony fish precursors, the hybodontid sharks.

Figure x. Embryo development in the bowfin, Amia. The facial bones develop as buds surrounding dermal sensory organs 'floating' on top of a cartilage base.
Figure 3. Embryo development in the bowfin, Amia. The facial bones develop as buds surrounding dermal sensory organs ‘floating’ on top of a cartilage (chondral) and prechondral base.

Some anterior Hybodus teeth start to look like Amia teeth (Fig. 4).
Blazejowski 2004  reported, “Gradual height reduction of the principal cusp is observed in successive tooth rows: the lateral teeth have low, long crowns with characteristic large lingual process, sometimes less pronounced as a buttress. Root is strongly ad−
joined to the crown in every tooth.”

Figure 4. Teeth of Hybodus species from Blazejowski B 2004, colors added. Note the wide variety and how two specimens approach the narrow cone morphology found in the basal bony fish, Amia and Gymnothorax (Fig. 1).
Figure 4. Teeth of Hybodus species from Blazejowski B 2004, colors added. Note the wide variety and how two specimens approach the narrow cone morphology found in the basal bony fish, Amia and Gymnothorax (Fig. 1). Blazejowski reported, “Gradual height reduction of the principal cusp is observed in successive tooth rows: the lateral teeth have low, long crowns with characteristic large lingual process, sometimes less pronounced as a buttress. Root is strongly ad− joined to the crown in every tooth.”

Pehrson 1940 reports:
“Three different stages of the formation of the premaxillary are shown. The anterior, dental part of the bone is clearly distinguishable from the posterior and dorsal part, situated above the cartilage.” Pehrson also describes the appearance of ossification where prior cartilage has dissolved, convergent with the process of fossilization.

Figure x. Shark skull evolution.

On the other hand… What taxa came before sharks?
Phylogenetically, that is (Fig. 5). Answer: Paddliefish. Chondrosteus. Sturgeons. Osteostraci. Birkenia (Fig. 5) in that order. All are bottom feeders with a ventral mouth, like the ventral mouth of basal sharks, like the goblin ‘shark’, now nesting with paddlefish in the LRT.

According to Bemis et al.
“We discuss five features fundamental to the biology of acipenseriforms [= sturgeons + paddlefish] that benefit from the availability of our new phylogenetic hypothesis:

  1. “specializations of jaws and operculum relevant to jaw protrusion, feeding, and ram ventilation;” (Chondrosteus, the goblin shark (Mitsukurina, and other basal sharks also protrude the jaws)
  2. “anadromy or potamodromy and demersal spawning;” (anadromy = migration of fish, from salt water to fresh water, as adults; potamodromy = freshwater fish; demersal spawning = mouth brooding)
  3. “paedomorphosis and evolution of the group;” (= retention of juvenile or larval traits in adulthood. Note the resemblance of larval paddlefish to basal sharks, Fig. 5).
  4. “the biogeography of Asian and North American polyodontids and scaphirhynchines;
  5. “the great abundance of electroreceptive organs in the rostral and opercular regions.” (e.g. sturgeons + paddlefish vs. sawfish, goblin sharks, hammerheads, etc).

According to Wikipedia,
Notable characteristics of Acipenseriformes include:

  1. Cartilaginous endoskeleton – as in sharks and fish more primitive than sharks
  2. Lack of vertebral centrum – as in fish more primitive than sharks
  3. Spiral valve intestine – as in sharks, bichirs, gars and lungfish, the last two by reversals.
  4. Conus arteriosus = infundibulum, a conical pouch found in the heart from which the pulmonary trunk artery arises. (not scored in the LRT, which looks at bones and their homologs).

Bemis et al. report,
“Acipenseriforms are central to historical ideas about the classification and evolution of fishes.”

Indeed. The LRT comes to the same conclusion.

“Acipenseriforms also are noteworthy because of their unusual mixture of characters, which caused early debate about their classification.”

Ray fins + armor + cartilage skeleton + ventral oral cavity + lack of jaws are some of these mixed characters. Actually, these are just primitive, something that has been overlooked until the LRT added taxa to recover a new family tree topology.

“Two aspects of living Acipenseriformes were especially problematic for early ichthyologists: (1) reduced ossification of the endoskeleton combined with presence of an extensive dermal skeleton; and (2) the presence of a hyostylic jaw suspension and protrusible palatoquadrate recalling the jaws of sharks.”

This is going to piss off ichthyologists: The palatoquadrate is not a palatine and only a small portion is a quadrate. The palatoquadrate is largely homologous to the lacrimal with fusion of the preopercular in some taxa. On taxa with teeth we find the fusion of the premaxilla and maxilla (tooth-bearing elements) to the much larger lacrimal. The former and future jugal are also involved.

“The current conventional view (developed and refined by many authors… holds that Acipenseriformes evolved from a ‘paleonisciform’ ancestor via paedomorphic reduction of the skeleton and specialization of the feeding system, but there is much more to the history of ideas about the systematics of this group.”

The current conventional view is incorrect according to the LRT, which tests a wider gamut of fish and nests traditional asipenseriformes basal to unarmored sharks, derived from armored osteostracoderms (Fig. 5). There was no paedomorphic reduction of the skeleton. Instead, sturgeons were basal to the origin of the jaws and skeleton.

Bemis et al. reviewed the history of sturgeon taxonomy, 
reporting: “Throughout this period [Linneaus 1788 through Heckel 1836]. most workers adhered to the classical idea that sturgeons must be closely related to sharks because they appeared to share a largely cartilaginous endoskeleton and similar jaw suspension. Chondrosteus, was named by Agassiz (1844) and described by Egerton (1858). Müller (1846) defined three grades of bony fishes — Chondrostei, Holostei and Teleostei — on the basis of increasing degrees of ossification. In doing this, Müller rejected the classical idea that sturgeons are closely related to sharks and accepted them as osteichthyans. Sewertzoff (1925, 1926b, 1928) was the only 20th century ichthyologist to seriously consider a closer link between sturgeons and chondrichthyans. Sewertzoff (1925) presented his conclusions as a phylogenetic tree, in which chondrosteans are shown as the sister group of all other bony fishes, and emphasized the presence of a protrusible palatoquadrate in both elasmobranchs and sturgeons. We now regard palatoquadrate protrusion as derived independently within chondrosteans (see additional discussion in the final section of this paper). Norris (1925) and others noted neuroanatomical similarities between sturgeons and sharks, but these are almost certainly plesiomorphic features (see Northcutt & Bemis 1993), and few workers ever accepted Sewertzoff’s view (see Berg 1948b and Yakovlev 1977 for additional history and critique).”

“It was not until later, when Gardiner (1984b) published the first generic level cladogram including fossil and recent Acipenseriforms, that interest in their phylogenetic interrelationships began to grow. Gardiner’s (1984b) analysis was controversial because he suggested that paddlefishes were diphyletic,

“From this brief history [much abbreviated above], it is clear that phylogenetic studies of Acipenseriformes are still in their infancy.”

This is only due to taxon exclusion and traditional bias (= textbooks). Including more taxa without bias (Fig. 5) as in the LRT, clarifies phylogenetic studies.

Figure 4. Paddlefish (Polyodon) hatchling in 2 views. This taxon marks the origin of marginal teeth. Barbels go back to whale sharks (Fig. 5). From the caption: Scanning electron micrographs of Polyodon spatula larva: The olfactory pit has not yet completely subdivided into anterior and posterior nares. Many clusters of ampullary electroreceptors are visible on the cheek region dorsal to the upper jaw. The teeth of the upper jaw are erupting in two series. Additional erupting teeth can be seen at the leading edge of infrapharyngobranchial.
Figure 6. Paddlefish (Polyodon) hatchling in 2 views. This taxon marks the origin of marginal teeth. Barbels go back to whale sharks (Fig. 5). From the caption: Scanning electron micrographs of Polyodon spatula larva: The olfactory pit has not yet completely subdivided into anterior and posterior nares. Many clusters of ampullary electroreceptors are visible on the cheek region dorsal to the upper jaw. The teeth of the upper jaw are erupting in two series. Additional erupting teeth can be seen at the leading edge of infrapharyngobranchial.

Sturgeon-like barbels (not those of catfish, carp, hagfish or zebrafish)
originate with sturgeons and continue in paddlefish (Fig. 6). Whale sharks retain barbels (Fig. 7), but they tuck them away into the corners of their mouth. Manta rays (Fig. 8) lose their barbels. Sawsharks keep theirs. Not sure yet about the Mandarin dogfish.

Figure 7. Whale shark (Rhincodon) mouth. Note the lack of marginal teeth, presence of barbels and single nares.
Figure 7. Whale shark (Rhincodon) mouth. Note the lack of marginal teeth, presence of barbels extending the mouth corners  and single nares.
Figure 8. Manta ray mouth lacking a barbel. Compare to its living sister, Rhynchodon, the whale shark.
Figure 8. Manta ray mouth lacking a barbel. Compare to its living sister, Rhynchodon, the whale shark. Cephalic lobes are anterior extensions of the pectoral fins.

The nesting of sturgeons and paddlefish 
primitiive to sharks appears to be a novel hypothesis of interrelationships recovered by the LRT simply by adding taxa. In like fashion, the nesting of moray eels and bowfins arising early from sharks also appears to be a novel hypothesis of interrelationships. If there is a prior citation to either, please let me know so I can promote it.


References
Bemis WE, Findeis EK and Grande L 1997. An overview of Acipenseriformes. Environmental Biology of Fishes 48: 25–71, 1997.
Blazejowski B 2004. Shark teeth from the Lower Triassic of Spitsbergen and their histology. Polish Polar Research 25(2)153–167.
Maisey JG 1983. Cranial anatomy of Hybodus basanus Egerton from the Lower Cretaceous of England. American Museum Novitates 2758:1–64.
Maisey JG 1987. Cranial Anatomy of the Lower Jurassic Shark Hybodus reticulatus
(Chondrichthyes: Elasmobranchii), with Comments on Hybodontid Systematics. American Museum Novitates 2878: 1–39.
Pehrson T 1940. The development of dermal bones in the skull of Amia calva. Acta Zoologica 21:1–50.

Splanchnocranium

https://en.wikipedia.org/wiki/Acipenseriformes

https://www.zoology.ubc.ca/~millen/vertebrate/Bio204_Labs/Lab_3__Skull.html

15 thoughts on “Basal bony fish descendants of hybodontid sharks

  1. Once more unto the breach, I suppose. I really don’t know why I keep trying. You may not listen, but maybe some of your readers will.

    You list several dermal bones as part of the splanchnocranium, except none of them is part of that system. They are part of the dermatocranium. Very little of the mandibular arch ever ossifies. The quadrate and articular are the only consistent sites of ossification. The orbital process of the PQ may ossify in some groups, forming the epipterygoid of many sauropsids and the alisphenoid of mammals, and derived bony fishes sometimes have some additional ossifications (e.g. the metapterygoid, which is more or less adjacent to the quadrate, and an autopalatine further forward; neither of these has a homologue in tetrapods). That’s it. The dentary, premaxilla, maxilla, and so forth are all part of the dermatocranium. That they are functionally linked with the mandibular arch does not, in any way, indicate homology or common derivation.

    And you do know Hybodus doesn’t have a bony skull – right?

    (If you think it does, you really need to go look at one. Not a photograph of one. It’s actually possible to tell bone from cartilage up close.)

    • Chris, apparently you are getting your splanchnocranium homologies from a different source than https://bonebroke.org/2015/06/25/splanchnocranium/ which matches my independent search for the origin of facial bones. You will need to back up your statements with citations and a valid phylogenetic context. Where are your cladograms?

      I never said Hybodus had a bony skull, so don’t try that old debate club trick. But among sharks it is said to be the most highly ossified. I will leave it to the shark experts to tell me the chemistry of their fossils.

      You do know the premaxilla and maxilla don’t appear before the appearance of teeth – right? And they don’t get deep before the appearance of tooth roots – right?

      I was willing to listen to you, to double check my hypotheses, to add taxa, to dive deeper into the literature — but none of that, so far, has supported your assertions. IF true, I need to see some evidence. I need an alternate cladogram. Once more into the breach, if willing and able.

      • Actually, you DID say Hybodus had a bony skull, albeit indirectly. The word “ossified” means “developed into bone.” if the skull is ossified, it’s bony. That’s that. That’s literally what ossification means. A structure cannot be ossified but simultaneously not bony.

        I did a quick search for sources claiming that the chondrocranium of Hybodus is ossified. I found a few web sites, but nothing in the peer-reviewed literature, even though the chondrocranium of Hybodus is fairly well known. No one who has studied Hybodus has suggested ossification of the chondrocranium.

        Several sources, though, say parts of the endoskeleton of Hybodus is calcified. To ossify is to develop actual bone. To calcify is to develop calcium carbonate deposits. This is entirely different from ossification.

        A few textbooks (e.g. Kardong, if I remember correctly) have implied that calcification and ossification are synonyms or near-synonyms. This is presumably because both apatite (calcium phosphate, CaPO4) and calcium carbonate (CaCO3) contain calcium. But in fact, they’re not the same thing at all.

        Calcification frequently happens in cartilage that never ossifies. The vertebral centra of neoselachian sharks are an example. The tracheal rings of some animals sometimes calcify. So does the sternal cartilage of a crocodylian, especially in very old animals. This can also happen in the articular cartilages of our joints, especially in those dealing with osteoarthritis.

        I think this is where the confusion comes from.

        The braincase of Hybodus is not ossified.

        And of course teeth sometimes arise before the dermal bones that bear them. Teeth don’t form the same way – they’re the result of an inductive relationship between multiple tissues. If you think this means the premaxilla is part of the mandibular arch, you are mistaken.

        I looked up the link you indicated. It doesn’t quite say the facial bones can be part of the splanchnocranium – it says they can be treated as such “for practical purposes,” which I take to mean “they occur in the same part of the head.” This isn’t the same thing as saying the dentary and maxilla form in the splanchnocranium, or that they’re developmentally part of that structure. The author would be wrong if that’s what she thinks, though I suspect she’s relaxing the precision of the terms she uses for some reason. I might contact the author of that web site and ask her to rephrase it, because it appears to have misled at least one reader (you) into thinking some dermal bones are actually part of the splanchnocranium, when they are not.

        As for needing a cladogram – I don’t. Neither do you. I don’t know how many ways I can say this. Dermal bones do not have homologues of any kind in cartilaginous elements. I’m going to continue ignoring your trees for purposes of this discussion, because they are irrelevant. The issue isn’t what a tree shows – it’s whether the information used to build it is biologically meaningful.

      • Chris, Tell me about this relevant study:
        Venkatesh B et al. 2014. Elephant shark genome provides unique insights into gnathostome evolution. Nature 505:174–179.

        “Although scientists knew what genes were involved in bone formation, it wasn’t clear whether sharks had lost their bone-forming ability or just never had it in the first place. After all, sharks do make bone in their teeth and fin spines.”

        “The sequence reveals that members of this group are missing a single gene family that regulates the process of turning cartilage into bone, and that a gene duplication event gave rise to the transformation in bony vertebrates.”

        “All gene family members involved in bone formation were present, except the secretory calcium-binding phosphoprotein (SCPP) gene family.”

        Posted earlier here: https://pterosaurheresies.wordpress.com/2020/12/08/why-sharks-have-no-bones-borrell-2014/

        With regard to facial bones, like I said, the palatoquadrate starts off as a rather large lacrimal when more taxa are added. the maxilla and premaxilla appear when teeth appear. Given this hypothesis, everything makes sense with no outliers.

  2. Your use of a dorsal fin spine in Figure 4 as evidence that hybodontid teeth “are slender, long and deeply rooted, like those of the bony fish” suggests you are failing to read pertinent sections of the references you list = both the Maisey monographs you provide have comprehensive descriptions of actual hybodontid teeth and should have prevented such a fundamental error.

    The image itself is taken from Bermúdez-Rochas (2009) Fig. 4, which you do not cite. I strongly suggest you reread the relevant figure caption.

  3. The article you quote is irrelevant to the discussion. Whether shark ancestors had cranial bones is an interesting question, as is whether modern chondrichthyans have the genetic toolbox to make bone, but these have no bearing on whether one can find homologous between the chondrocranium (or splanchnocranium) and dermal bones.

    There is no connection between the lacrimal (which is dermal) and any cartilaginous component of a shark’s head. This is an objective fact.

    • Chris, since you say shark skulls can never enter a phylogenetic analysis with bony fish and sturgeons + paddlefish + shark-like Chondrosteus… and you say a genetic study of shark cartilage vs bone is irrelevant… and you say the lacrimal, a bone intimately involved with respiration, has no connection to any cartilaginous component of a shark’s head, then we’re just going to have to part ways. I’ve stood alone on questions of pterosaur, whale, snake, turtle, reptile and dinosaur ancestry. I can handle this the same way. Thank you for ‘stirring the pot’ because I never would have dug deeper into this subject without your kind remarks.

      • Where, exactly, did I say sharks and bony fishes cannot be analyzed simultaneously?

        Of course they can. Lots of people have done this. There are lots of structures that give us a source of character data. It’s just that the dermatocranium isn’t one of them. Sharks don’t have one, and they have no homologues in their cartilaginous skeleton.

        You can run them in the same matrix. Just don’t code sharks for dermatocranial characters.

        And yes, the paper on the genetics of bone and cartilage really was irrelevant to the alleged presence of dermatocranial homologues in a shark chondrocranium. Whether the genetic blueprints for certain tissues can be found in taxa not expressing them doesn’t mean some sort of homologue for structures comprised of those tissues can be found somewhere in the non-expressor.

        That you think sharks arose from ancestors with bony skulls is one thing. In fact, a lot of people think that, though we would point to a different set of bony ancestors. In that sense, demonstrating bone-producing potential is useful information. But that’s got nothing to do with actually finding existing morphological correlates of those now-lost bones in the chondrocranium.

        The Bipes example I used a while back wasn’t just a wave-away – it was a direct example of what I’m talking about. Bipes has forelimbs, and other amphisbaenians do not. But at least some of the other amphisbaenians have the genetic toolbox to make forelimbs. We know this because those genes got turned back on in the lineage that gave rise to Bipes, thereby allowing it to grow the forelimbs its ancestors had lost. But even if other amphisbaenians have these genes, there is absolutely nothing in their skeletons to suggest their presence. There is simply no morphological homologue to the forelimb.

        This is exactly the situation we’re dealing with in sharks. Having the ability to produce bone might mean their ancestors made bone, but it doesn’t mean indications of certain now-lost bones will still be evident.

      • Hi Chris, I appreciate your efforts. Your point about bone vs cartilage has never been argued. It’s all about shapes and sutures.
        You wrote: “There are lots of structures that give us a source of character data. It’s just that the dermatocranium isn’t one of them. Sharks don’t have one, and they have no homologues in their cartilaginous skeleton.” If there are no homologues in the skull or skeleton, what is there left to score? I’m taking you at your word. Please send citations or clarify. Pectoral fin okay? It’s made of cartilage. Skull okay? just not the nasals, frontals, circumorbitals. Nares, okay? just not the nasals?

        Your point about Bipes remaking forelimbs is not as clear. Citation? In the LRT phylogenetic ancestors of Bipes include Dibamus, which has no limbs (by convergence with Amphisbaena?) and several Cretaceous taxa which have no post-crania (score= ?) until we come to Slavoia and Sineoamphisbaena, which have forelimbs, if not completely preserved in the latter.

  4. At this point, I would recommend looking at morphological phylogenetic analyses that have included sharks and bony fishes for character suggestions. I don’t normally work on fish. Qiao et al. 2016 (PLoS One) might be a good place to start, as would any of the excellent analyses by Brazeau, Friedman, and Coates. All I can tell you is that dermatocranial characters cannot be scored for sharks, because they don’t have dermatocrania.

    I suspect, in part, you’re running into the problem many of us have highlighted for a long time – you can’t keep adding taxa without also adding characters. You’ve hit a point at which your matrix doesn’t express morphology held in common among early-branching gnathostome groups.

    There are also quite a few phylogenetic analyses out there that have included multiple amphisbaenian taxa, including Bipes. A scholar.google search should locate some of them.

    • Actually, just the opposite, Chris. You’re asking me to remove characters from sharks because you said they cannot be scored. The present list of characters was working just fine until you said, in essence, ‘you can’t go down those roads’. So check your logic.

      With regard to shark phylogeny, no one includes Loganiella, Polyodon, Chondrosteus, sturgeons, Amia, Gymnothorax AND Gregorius, not even Qiao et al. 2016. Many workers want to include placoderms, which are not related. Others want to include spiny sharks and Doliodus, which are also in the clade of bony fish (more derived than the moray eel + deep sea gulper clade, but lack bone, except in the spines).

      Qiao et al. 2016 (PlosOne) stated, “Osteichthyans diverged along two major lineages, namely actinopterygians (bichirs, sturgeons, gars, bowfins and teleosts) and sarcopterygians (coelacanths, lungfishes and tetrapods).” That’s traditional, but that’s wrong according to the LRT, which tests a much wider gamut of taxa.

      With regard to Bipes phylogenetic analyses, I will look, but the ones I’ve seen have been genomic, which means no fossils and false positives, like nesting Dibamus basal to squamates. Yuck!

      You’re not up-to-date, Chris. Many traditions and paradigms go by the wayside when you simply add taxa.

      • No, you are not being asked to remove character scores from your shark additions. You are being confronted about your assumption that deramatocranial characters in bony fish can also be found in sharks. That is an extraordinary claim that goes against all we currently know about evolutionary development of the skull in vertebrates. Thus, it requires extraordinary evidence. You have yet to provide said evidence (as a reminder: cladograms are not evidence. They are hypotheses / interpretations of evidence).

        I’m surprised that you don’t understand why a low character: taxon ratio is a detriment to phylogenetic analyses. This is every bit the problem with early molecular analyses, in which one or a handful of genes were used to assess relationships among hundreds of taxa. The seemingly endless amount of nucleotide combinations were an illusion, as nucleotide substitution saturation quickly eliminates the useful information from the data, producing misleading results (e.g., resurrection of the infamous haematothermia hypothesis based on 18s rRNA data).

        Your LRT is most certainly suffering from the same saturation problem, leading to that single “well resolved” tree every time, and a ever growing list of “non-traditional” taxon placements. I get that adding even one new character takes a large amount of time, especially as taxon lists grow, but just because it is hard does not mean it shouldn’t be done. If anything, it’s a sign that your probably on the right track.

        If you can’t use cranial characters to score your sharks (aside from giving everything a 0), then it’s time to look for more characters. There’s just no way around it.

      • Thanks, Jura. See today’s post on eels and four-eyed fish. You are welcome to report any two taxa that should not nest together because they are not related to one another. Unfortunately, you’re a day late for Anguilla and Anableps, which benefitted from the addition of taxa, not characters. With regard to evidence for cartilage acting as a template or a ‘waiting room’ for bone, please review this post: https://pterosaurheresies.wordpress.com/2020/12/14/basal-bony-fish-descendants-of-hybodontid-sharks/ which documents the ontogenic development of bony islands on a cartilage substrate in a proximal shark descendant.

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