Shocking news! The torpedo is a hammerhead!

This one came as a surprise
as I scored Tetronarce (= the New Zealand torpedo, an electric ray, Fig. 1), I thought:

  1. this taxon is breaking some rules, and
  2. I’ve seen that bizarre nasal before… but where?
Figure 1. Tetronarce fairchildi (originally Torpedo fairchildi Hutton 1872, 1m)

Figure 1. Tetronarce fairchildi (originally Torpedo fairchildi Hutton 1872, 1m). The long red elements are tabular homologs, separated from the rest of the skull.

Tetronarce fairchildi 
(originally Torpedo fairchildi Hutton 1872, 1m) is the extant New Zealand torpedo, an electric ‘ray’ on the outside. Here it nests with Sphyrna, the hammerhead shark, based on its skeleton. So this ‘ray’ is convergent with other rays. Note the broad nasals with open medial architecture, underslung jaws with tiny, single-cusp teeth and shark-like tail. Here the eyeball stalks are preserved, distinct from most other tetrapods tested in the LRT, probably due to careful dissection to get at its cartilaginous skeleton. Two dorsal fins are preserved.

Figure 1. The small hammerhead shark, Sphyrna tutus, is best appreciated in dorsal or ventral view.

Figure 1. The small hammerhead shark, Sphyrna tutus, is best appreciated in dorsal or ventral view.

Skates,
like the guitarfish, Rhinobatos, and the sawfish, Pristis, have an elongate narrow rostrum and nasal. Angel sharks and eagle rays have other distinguishing traits that nest them with each other and not with the aforementioned. So do manta rays. When more rays and skates are added to the LRT that may change. Or not.

Every possibility must always be left open,
as Torpedo gently, but firmly reminds us. Do not be tempted into “Pulling a Larry Martin” here. A short list of traits don’t make a taxon. Only a nesting in a wide gamut phylogenetic analysis can do that.

Yes, outward appearances are very different.
But when you look at the skeletons and test them in phylogenetic analysis no other taxon shares as many traits with hammerheads as torpedoes. Evolutiion leaves clues. It’s up to us to find them. You won’t find a similar laterally extended nasal with a perforated medial architecture in any other tested sharks or rays. Though many skeletal traits are indeed different, taken as a suite of characters no other tested taxon comes closer.

Figure 2. Skull of Sphyrna tutus in three views from Digimorph. org and used with permission. Colors added.

Figure 2. Skull of Sphyrna tutus in three views from Digimorph. org and used with permission. Colors added.

Sphyrna tudes 
(orignally Zygaena tudes Valenciennes 1822; 1.3m in length) is the extant smalleye hammerhead shark. It prefers muddy habitats with poor visibility. Sphyrna has a tendency to inhabit coastal waters along the intertidal zone rather than the open ocean, as their prey item, invertebrates, fish, rays, small crustaceans and other benthic organisms hide in the sands and sediment along these zones. Gestation is 10 months. Females produce 19 pups each year. The eyes and nares are further separated by the lateral expansion of nasals, prefrontals and postfrontals creating the cephalofoil. Compare to Torpedo (above).

The largely overlooked value of the LRT 
comes from testing together taxa that have never been tested together before with a generic character list not designed specifically for sharks and rays, other fish, birds and mammals. Convergence runs rampant in the Vertebrata. Scientists need a wide gamut cladogram that minimizes taxon exclusion and character selection bias.

By the present evidence
the former clade Batoidea has now been divided into quarters. This appears to be a novel hypothesis of interrelationships. If there is a prior publication, let me know so I can promote it.


References
Hutton FW 1872. Catalogue with diagnoses of the species. Ed. Hutton, FW and Hector J (eds), Fishes of New Zealand, pp. 1-88 pls 1-12, Colonial Museum and Geological Survey Department, Wellington.

wiki/Electric_ray
wiki/Sphyrna
wiki/Torpedo_fairchildi

Overlooked convergence: sharks and whales have a gelatinous snout

Short one today.
The pictures tell the story.

Everyone knows
the snout of the sperm whale is shaped by large sacs of spongy gelatinous material, the spermaceti organ and the melon (Fig. 1).

Figure 1. Sperm whale head diagram showing  the spermaceti organ and the junk (melon) sitting atop the elongate rostrum, as in sharks, more or less.  See figure 2.

Figure 1. Sperm whale head diagram showing the spermaceti organ and the junk (melon) sitting atop the elongate rostrum, as in sharks, more or less. See figure 2.

Shark skulls are not shaped like hydrodynamic bullets.
like the skulls of sturgeons, paddlefish and bony fish. Rather, shark skulls (Fig. 2), like sperm whale skulls, have gelatins that fill the voids and support their bullet-shaped snouts.  Since I didn’t see anything like this when I ‘googled’ it, I thought to add it to mix.

Figure 2. Skull of the dogfish shark, Squalus, superimposed on a graphic of the invivo shark. Yellow areas added to show the extent of the gelatinous material that fills the empty spaces above and below the cartilaginous rostrum (nasal homolog).

Figure 2. Skull of the dogfish shark, Squalus, superimposed on a graphic of the invivo shark. Yellow areas added to show the extent of the gelatinous material that fills the empty spaces above and below the cartilaginous rostrum (nasal homolog).

Yesterday’s post on shark skull cartilage
and the bony homologs one can clearly see by coloring the elements (the now common DGS method) invited a reader’s comments that what I’m doing ‘is the death of science.’ As longtime readers know, I follow the evidence and point out flaws in traditional hypotheses, including instances of taxon omission. That this is necessary points not to the death of science, but to the willingness of someone to test untested hypotheses and taxon lists.

I welcome evidence to the contrary.
I make changes constantly. I follow the evidence, not the textbooks and not the professors, unless the evidence supports them.

Thank you
for your interest in this ongoing online experiment of a life-long learner and heretic.

 

Identification of shark skull elements: a closer look at the evidence

In recent months
I’ve been applying tetrapod skull bone homologies to cartilaginous shark skulls (Fig. 1). This has never been done before because paleontologists and ichthyologists do not consider cartilage homologous with bone. Only a few fish skull names have tetrapod homologs. That number increases with lungfish and crossopterygians, because these taxa approach the tetrapod grade. Even so, wouldn’t it be better if all craniate skull bones and cartilage had tetrapod names. Is it even possible?

Phylogenetically,
at least in the large reptile tree (LRT, 1775+ taxa), sharks follow sturgeons and paddlefish. Sharks precede bony fish based on the application of tetrapod skull bone homologies to all fish. But is this possible? Some say no.

Sharks, lacking bone, provide a controversy without possible resolution
according to some workers. So, how can we keep sharks in the LRT and score them with the present set of characters? Maybe the divide is not so divisive after all, contra tradition.

Let’s look at
two available lines of evidence (Figs. 1, 2).

Figure 1. Squalus skull in dorsal view. Changing the contrast enables seeing the cartilage sutures that had bone precursors.

Figure 1. Squalus skull in dorsal view. Changing the contrast enables seeing the cartilage sutures that had bone precursors. The pineal opening homolog is between the nasals and frontals here.

The first line of evidence
is a dorsal view photo of the dogfish (Squalus) skull (Fig. 1). It is made of cartilage, but you can’t tell that by looking at this photo. Normally bright white, the skull image above has been multiplied in several layers of Photoshop to bring up the contrast. One more multiplied layer provides colors and labels. This process is called Digital Graphic Segregation or DGS and is being used more and more often in paleontology, especially in µCT scans. I’ve been using DGS since 2003.

In the old days of black and white plus halftone publication in journals
outline tracings were used because color incurred an extra charge. With online publishing, color is not an extra charge. So, why not use it?

Here in the dorsal view of the skull of the dogfish, Squalus,
(Fig. 1) the tetrapod-homolog nasal (pink) is still out front, over the nares. The circumorbital cartilage has sutures that match the prefrontal (brown) and postfrontal (orange). Sutures also mark the intertemporal (yellow-green), supratemporal (green) and tabular (red) rimming the lateral cranium. The parietal (lavender) and post parietal (tan) appear to have switched places here, but that is due to a previous complete splitting and re-melding of the parietal in more primitive taxa.

In sturgeons, paddlefish and sharks the jaws
are often separate from the cranium. The upper jaw (= traditional palatoquadrate) here (Fig. 2 color overlay) consists of the large lacrimal + jugal + preopercular + quadrate all fused together. The tooth-bearing premaxilla and maxilla are thin sheets on the jaw rims. Shark teeth have no roots, so the premaxilla and maxilla need not be deeper. In fish and tetrapods with tooth roots the premaxilla and maxilla are deeper and the lacrimal shrinks.

Figure 3. Online diagrams of a shark skull with all sutures obliterated with an airbrush. Compare to figure 1, a real shark (Squalus) skull.

Figure 2. Online diagrams of a shark skull with all sutures obliterated with an airbrush. Compare to figure 1, a real shark (Squalus) skull.

The alternative view
(Fig. 2, gray layer) comes in the form of an airbrushed diagram of a shark skull that does not show any sutures. Even so, DGS colors can still be added based on the bumps and valleys of skull topography. Figure 2 is a generalized shark skull done freehand. It has labels. That’s good for translating traditional shark nomenclature to tetrapod nomenclature, but such diagrams do not provide the overlooked details present in photography.

If this method and attitude toward sharks skulls is adopted
Squalus (Fig. 1) will no longer have to disqualify itself from tetrapod homologies based on skull sutures and architecture. Actually, Squalus is a great example of the homologies found in shark cartilage and tetrapod bone. Graphically the two cannot be distinguished from one another. Present day diagrams lacking necessary details (Fig. 2) need to be updated to reflect tetrapod homologies. If cartilage or bone sutures are obliterated on certain taxa, then we can use skull topography and phylogenetic bracketing to estimate where the fusion took place, or score the suture for fusion. Some birds likewise fuse skull elements. That doesn’t seem to be a problem for ornithologists.

Some notes from the literature follow.
While describing the origin of the fish skull, Richter and Underwood 2019 report: “The evolutionary origin of the brain and braincase of fishes remains largely elusive.”

Adding taxa to the LRT has improves that situation, revealing a tree topology featuring the gradual accumulation of derived traits among all included taxa that all cladograms are supposed to have, but too often don’t.

“The development of the vertebrate skull is dependent on the presence of an embryonic neural crest whose cells migrate to induce the formation of various elements of the cranial skeleton, dentitions and certain soft tissues. Much progress has been made in the understanding of the vertebrate skull since pioneering anatomical descriptions made last century.”

So, which is it?remains elusive‘ or ‘much progress‘? This is no reason to build up drama. This is science, not Shakespeare. Just start with ‘much progress’ if that is so.

“In the last few decades, studies involving micro-anatomy, ontogenetic development, molecular biology and gene expression have shed light on key developmental processes that seem to be widely shared among vertebrates. However, molecular biology and ontogenetic studies have been restricted to a small number of fish species.”

As readers know, molecular studies (= genomics) recover false positives way to often. Toss out the gene studies. Add fossils. Score traits. See what the software recovers. Phenomics works better than genomic ichthyologists ever imagined.

Figure 3. Pineal body in a primitive jawless fish, like the lamprey.

Figure 3. Pineal body in a primitive jawless fish, like the lamprey.

Richter and Underwood continue:
“There is still much uncertainty about precise homologies between parts of the skull of distinct groups of fishes, due to the fact that the vertebrate skull shows a remarkable morphological and anatomical plasticity.”

As readers know, precise homologies have been offered here between parts of the skull of distinct groups of fishes while maintaining a standard gradual accumulation of derived traits. The Early Carboniferous nurse shark, Tristychius (Fig. 4), is a good  example of how DGS can work on a µCT scan.

Figure 1. CT scans of Tristychius skull from Coates et al. 2019.

Figure 1. CT scans of Tristychius skull from Coates et al. 2019.

ScienceDirect.com presented some traditional thinking
on the topic of shark chondrocrania. Several shark experts helped produce this online summary.

Iuliis and Pulera 2011 provide a definition:
“The chondrocranium is the large single element of the head skeleton.. It surrounds and provides support for the brain and sense organs.” 

You’ll notice the skull (= chondocranium, Fig. 1) can have several openings and medial fenestra in dorsal view in some sharks. Most of these correspond to narial, optical, spinal and pineal openings (close to the nasals), plus space for various jaw muscles, as in all craniates. On top of the nasal in sharks is a large pre-cerebral cavity typically not found in tetrapods, sturgeons or bony fish. The cavity communicates posteriorly with the cranial cavity by way of the pre-cerebral fenestra. In life this area is filled with gelatinous material. Rostrum cartilage in sharks is spongy and flexible, allowing the shark to absorb considerable impact with its nose.

“This chapter provides the anatomy of the shark. [which] belongs to Chondrichthyes, which first appeared in the Silurian Period and is among the earliest to branch off from the rest of the gnathostomes (jawed vertebrates).

By contrast the LRT recovers sharks derived from paddlefish and bony fish derived from hybodontid sharks. Sharks are not a separate clade. Tetrapods, including mammals are highly derived hybodontid sharks.

“Among the specialized features that unite these groups [Chondrichthyes] are unique perichondral and endochondral mineralization, distinctive placoid scales, an inner ear that opens exter­nally through the endolymphatic duct, pelvic claspers in males, and a cartilaginous skeleton.”

“The perichondrium is a dense layer of fibrous connective tissue that covers cartilage in various parts of the body.”

“Endochondral ossification takes place at the base of the skull, vertebrae, hips, and limbs through the replacement of a cartilaginous rudiment with bone.”

Pelvic claspers also appear, by convergence, in placoderms.

“The tail and caudal fin are generally reduced and often whip–like. Locomo­tion is accomplished through wave-like flapping of the fins rather than lateral undulations of the trunk and tail.”

This is false. Generally we see rays, skates and chimaera swimming by flapping their pectoral fins. Sharks swim with rhythmic undulations of the torso tipped by a large V-shaped tail.

Chondrocranium: according to Wikipedia
“In cartilaginous fishes (e.g. sharks and rays) and agnathans (e.g. lampreys and hagfish), the chondrocranium persists throughout life. Embryologically, the chondrocranium represents the basal cranial structure, and lays the base for the formation of the endocranium in higher vertebrates.”

Dermal bone: according to Wikipedia
“In contrast to endochondral bone, dermal bone does not form from cartilage that then calcifies, and it is often ornamented. Dermal bone is formed within the dermis and grows by accretion only – the outer portion of the bone is deposited by osteoblasts.

Endochondral ossification: according to Wikipedia
“Unlike intramembranous ossification, which is the other process by which bone tissue is created, cartilage is present during endochondral ossification. Endochondral ossification is also an essential process during the rudimentary formation of long bones, the growth of the length of long bones, and the natural healing of bone fractures.”

Here’s an invalidated shark skull story:
According to Guardian.com
“Fossil upends theory of how shark skeletons evolved, say scientists. The partial skull of an armoured fish that swam in the oceans over 400m years ago could turn the evolutionary history of sharks on its head, researchers have said.”

“The fossil, about 410m years old and reported in the journal Nature Ecology & Evolution, was unearthed in western Mongolia in 2012, and belongs to a placoderm that has been dubbed Minjinia turgenensis and would have been about 20-40cm in length. “This fossil is probably the most surprising thing I have ever worked on in my career. I never expected to find this,” Dr Martin Brazeau of Imperial College London, first author of the research, said.”

Figure 1. Minjina in 4 views, mirror-image and colors added.

Figure 2. Minjina in 4 views, mirror-image and colors added.

We looked at Minjinia earlier
here and here. It is indeed a bottom-dwelling placoderm with reduced jaws and eyes. So it is no surprise that bone was present because placoderms nest with bony fish in the LRT. Contra traditional views, placoderms do not precede sharks in the LRT.

The root word chondro,
as in ‘Chondrichthyes’ indicates cartilage. Immature tetrapods and bony fish have a chondrocranium that gets replaced by a skull made of bone. Sturgeons, paddlefish and sharks had not yet gained the ability to replace cartilage with bone. According to the LRT, hybodontids, with their highly ossified skulls and the bony fish that succeed them regain that ability to replace cartilage with bone while losing the flexibility in the rostrum of sharks.

Kaucka and Adameyko 2019
review the evolution of cartilage in the cranial region and discuss shaping of the chondrocranium in different groups of vertebrates.

Compagnucci et al. 2013
review several then current, now out-of-date hypotheses of jaw development, all without a cladogram.

Finally, let’s not forget
Borrell 2014 found it only took one gene turned off to stop bone production in shark-relative chimaeras. Whenever that gene was turned on it restarted bone production.


References
Borrell B 2014. Why sharks have no bones. Nature online here
Compagnucci C et al. (11 co-authors) 2013.
Pattern and polarity in the development and evolution of the gnathostome jaw: both conservation and heterotopy in the branchial archesof the shark, Scyliorhinus canicula, Dev. Biol. 377(2): 428–448.
De Iuliis G Pulerà D 2011. Chapter 3. The Shark in The Dissection of Vertebrates (Second Edition), Science Direct online
Kaucka M and Adameyko I 2019. Evolution and development of the cartilaginous skull: From a lancelet towards a human face. Seminars in Cell & Developmental Biology 91:2–12. https://doi.org/10.1016/j.semcdb.2017.12.007
Richter M and Underwood C 2019. Chapter 8 – Origin, development and evolution of the fish skull. Pages144–159 in Evolution and development of fishes Eds. Johanson Z, Underwood C and Richter M. Cambridge University Press DOI: https://doi.org/10.1017/9781316832172.009
Venkatesh B et al. 2014. Elephant shark genome provides unique insights into gnathostome evolution. Nature 505:174–179.

Steven E Campana Lab webpage:

https://www.sciencedirect.com/topics/veterinary-science-and-veterinary-medicine/chondrocranium

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

https://uni.hi.is/scampana/sharks/shark-anatomy/skeleton/

https://www.theguardian.com/environment/2020/sep/07/fossil-upends-theory-of-how-shark-skeletons-evolved-say-scientists

The bone 2 cartilage 2 bone transition from sturgeons 2 sharks 2 bony fish

Short summary for those in hurry:
There is support in Pehrson 1940 for the origin of facial (dermal) bones on a cartilaginous template (contra Hall 2005) in a proximal shark descendant.

  1. Sturgeons (shark ancestors in the LRT) have facial bones sheathed to a cartilage template.
  2. Sharks lose all trace of bone, but keep the cartilage.
  3. Bony fish (shark descendants in the LRT) reacquire facial bones on a cartilage template

Backstory
Several recent reader comments disputed and/or cast doubt on the identity of shark skull bones (Fig. 2) and the shark-to-bony fish transition recovered by the large reptile tree (LRT, 1771+ taxa, see Fig. 1 diagram). Objections were  based on developmental grounds. One reader (CB) 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.”

That is the traditional view found in current textbooks.

First:
my guess is this comment resulted after reading any of several authors all citing Hall 2005, who wrote, “The vertebrate dermal skeleton includes the plate-like bones of the skull, and, in reptiles and fishes, also includes various scales, scutes, denticles and fin rays. Dermal bone forms via a process known as intramembranous ossification, with mesenchymal condensations differentiating directly into bone without a cartilaginous template.”

Second:
As everyone knows, no part of shark skulls is bone. It’s all cartilage. Nevertheless and despite obliteration and/or fusion of most skull sutures, shark ‘nasal’ templates still cover the snout and nares. Shark ‘frontal’ templates are still located between the eyes. I have retained tetrapod skull nomenclature for shark skull template elements in order to include shark taxa in the LRT.

Third:
A valid phylogenetic context, like the LRT (diagram in Figs 1, 4), is vital in matters like this. Taxon exclusion leading to an improper cladogram is the root cause of most prior misunderstandings, as readers well know.

Wagner and Aspenberg 2011 wrote:
“Bone is specific to vertebrates, and originated as mineralization around the basal membrane of the throat or skin, giving rise to tooth-like structures and protective shields in animals with a soft cartilage-like endoskeleton.”

That’s not correct. In sharks dentine and enamel from the skin and teeth are not bone. Instead, bone first appears in sturgeons and kin. Then it disappears in sharks only to reappear in bony fish + tetrapods, according to the LRT. Traditionally and mistakenly sturgeons were considered relatives of derived bony fish, which is part of the problem.

In sturgeons and paddlefish, Bemis et al. 1997 report, 
“the bones more or less closely ensheath the underlying endochondral rostrum”. Sharks lack this sheath of bone on the rostrum. Instead, remaining more flexible cartilage supports the skull and skeleton.

Figure 2. Acipenser brevirostrum, 1m typical length. Records up to 1.47m.

Figure 2. Acipenser brevirostrum, 1m typical length. Records up to 1.47m.

Keys to understanding this issue include:

  1. Elements of the dermocranium in shark outgroup taxa: sturgeons (Fig. 1) and paddlefish = bone sheath over cartilage.
  2. Elements of the dermocranium in sharks (Fig. 2) = prismatic cartilage
  3. Elements of the dermocranium in proximal shark descendants: the bowfin, Amia (Figs. 2, 3) = bone patches develop around sensory cells over a cartilage template, according to Pehrson 1940.
Figure 2. Fish evolution from Hybodus to Amia documenting the shark to bony fish transition.

Figure 2. Fish evolution from Hybodus to Amia documenting the shark to bony fish transition.

Pehrson 1940 examined
a series of embryonic stages of the extant bowfin, Amia calva (Fig. 3), one of the most primitive bony fish in the LRT. 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.”

The ontogenetic origin of bone in Amia (Fig. 3) first appears in embryos as tiny islands on the skull surface over a cartilage or pre-cartilage template. This proximal descendant of hybodontid sharks (Fig. 2) documents many skull homologies.

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.

It is noteworthy
that the appearance of bone surrounding sensory cells all over the skull in bony fish followed the reduction of the long, sensory-cell-filled rostrum in bony fish. Taking the other evolutionary route, other shark descendants (e.g. hammerheads, skates, rays, goblin sharks, elephant-nosed chimaera, sawfish), further elongated the rostrum for increased acuity in finding bottom-dwelling prey.

Pehrson also described
the appearance of ossification where prior cartilage dissolved, convergent with the process of fossilization. Thereafter some embryos began to develop ossified skull bones without a cartilaginous template, in accord with Hall 2005, who did not cite Pehrson 1940.

Surprisingly,
Pehrson was keen on naming fish bones in accord with those of pre-tetrapods. 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.” Not sure if Pehrson was the first to do this, but it should be standard.

Supporting evidence that sturgeons are shark ancestors:
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 sure how this relates, but there it is).

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. 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.”

These aspects are not problematic of sturgeons and paddlefish are basal to sharks.

The palatoquadrate is neither a palatine nor a quadrate. It is largely homologous to the lacrimal with fusion of the tiny quadrate and tall, curved, preopercular in most taxa, fusion of the premaxilla and maxilla (tooth-bearing elements) on taxa with teeth. The former and future jugal is also typically fused.

Figure 5. Sturgeon mouth animated from images in Bemis et al. 1997. This similar to ostracoderms, basal to sharks.

Figure 5. Sturgeon mouth animated from images in Bemis et al. 1997. This similar to ostracoderms, basal to sharks.

“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.”

That is incorrect according to the LRT, which tests a wider gamut of fish and nests traditional acipenseriformes basal to unarmored sharks and derived from armored osteostracoderms (Fig. 4). There was no paedomorphic reduction of the skeleton at the origin of sturgeons. The sturgeon feeding system is not ‘specialized’. It is primitive.


References
Bemis WE, Findeis EK and Grande L 1997. An overview of Acipenseriformes. Environmental Biology of Fishes 48: 25–71, 1997.
Gillis JA 2019. ‘Secondary’ cartilage and the vertebrate dermal skeleton in Reference Module in Life Sciences.
Hall BK 2005. Bones and Cartilage. Academic Press, London. ISBN: 978-0-12-319060-4
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 GT 1940. The development of dermal bones in the skull of Amia calva. Acta Zoologica 21:1–50.
Wagner DO and Aspenberg P 2011. Where did bone come from? An overview of its evolution. Acta Orthopaedica. 82(4):393–398.
The Skull, Volume 1. Eds. Hanken J and Hall BK University of Chicago Press Books, 1993.

https://en.wikipedia.org/wiki/Acipenseriformes
https://www.zoology.ubc.ca/~millen/vertebrate/Bio204_Labs/Lab_3__Skull.html
G Torsten Pehrson bio

Evolution of the shark skull illustrated

In comparative anatomy,
macroevolutionary events can be diagrammed (Figs. 1, 2). Hopefully this can act as a visual shorthand that models real evolutionary events. It’s a way of checking that things look right after the software tells you whatever it recovers after ‘crunching the numbers’.

A diagram documenting the evolution of shark skulls
was posted online by ScienceDirect (Berkowitz and Shellis 2017; Fig. 1). This represents the traditional view and includes a hypothetical ancestor. That means the authors did not know the proximal outgroup for sharks.

Figure 1. Traditional diagram of shark jaw evolution.

Figure 1. Traditional diagram of shark jaw evolution from ScienceDirect website. Compare to figure 2.

By contrast
in the large reptile tree (LRT, 1770+ taxa) shark ancestors are known back to Cambrian chordates. Sturgeons and paddlefish are shark ancestors (Fig. 2). If one proves to be not suitable, there are a dozen more. Shark descendants include bony fish.

Traditionally
(Fig. 1) chimaeras and sharks form a basal dichotomy from their hypothetical ancestor.

By contrast
in the LRT (Figs. 2, 7) horn sharks (Heterodontus) and chimaeras (Chimaera) arise from dogfish sharks (Squalus) far from the base of sharks. The goblin shark (Mitsukurina) nests closer to the paddlefish (Polyodon).

Traditionally and in the LRT
dogfish (Squalus) gave rise to guitardfish (Rhinobatos) and skates. Traditionally skates are ray sisters, but In the LRT most rays arise from angel sharks (Squatina) and earlier sharks with giant pectoral fins (Cladoselache). The manta ray (Manta) has a separate ancestry close to whale sharks (Rhincodon, not shown in figure 2).

Figure 2. Shark skull evolution according to the LRT. Compare to figure 1.

Figure 2. Shark skull evolution according to the LRT. Compare to figure 1.

Pre sharks in the LRT have an operculum.
Post sharks in the LRT have an operculum. Transitional sharks do not have an operculum. Instead they have five to seven, tiny to enormous gill slits. As an added twist, chimaeras have four gill slits covered by an operculum… and goblin sharks lack an operculum, but nest with paddlefish with an giant operculum.

Pre sharks in the LRT have individual skull ‘bones’,
whether highly ossified or not. Post sharks in the LRT likewise have individual skull bones. Sharks do not. They tend to fuse several jaw bones together apart from the skull bones, which also tend to fuse together.

Lacking an operculum and individual skull bones
traditionally sets sharks apart from other fish with these traits. As readers know by now, using a few to a dozen traits to describe a clade is called ‘Pulling a Larry Martin‘. Instead, run a phlogenetic analysis with enough traits to lump and separate all included taxa. In that way convergence and reversals will sort themselves out without being hampered by traditional bias, outdated orthodoxy and cherry-picking a short list of preferred taxa with preferred traits (Fig. 1).

Gillis et al. 2011 tested genes
in a shark and a chimaera. They concluded, “the common ancestor of Elasmobranchs and Holocephalans had the gills of a shark, rather than the gills of a chimaera.”

That’s not what the LRT found
after testing traits and expanding the taxon list.

Vertebrate embryos (including humans)
have several gill pouches/clefts that either become gill slits (sharks) gill slits covered by an operculum (chimaeras and many bony fish) or throat elements (tetrapods). Since shark gill pouches are similar to those in higher vertebrates, their origin can be traced by to more primitive ancestors, like lampreys (before they evolved to become blood-suckers, Fig. 3).

Figure 3. Pineal body in a primitive jawless fish, like the lamprey.

Figure 3. Pineal body in a primitive jawless fish, like the lamprey.

The loss and later reacquisition of opercula
in sharks and bony fish respectively appears to be a novel hypothesis of traits and interrelations recovered by the LRT. If there was a prior paper on this subject, let me know so I can promote it here.

The fusion and reduced ossification of shark skull bones
has traditionally marked them as ‘primitive’, but recent studies indicate sharks arose from bony ancestors, agreeing with the LRT (Fig. 2).

Figure 8. The KUVP 83503 specimen of Tanyrhinichthys is reconstructed here differently than originally proposed, largely based on a different specimen. Here a preopercular is present for the first time.

Figure 4. The KUVP 83503 specimen of Tanyrhinichthys is reconstructed here differently than originally proposed, largely based on a different specimen. Here a preopercular is present for the first time.

Tanyrhinichthys
(Fig. 4) nests basal to sharks along with paddlefish (Polyodon) and hatchling paddlefish (Fig. 5), which don’t have a long rostrum and look even more shark-like. Tanyrhinichys has a preoperculum (light yellow, Fig. 4), a trait traditionally lacking in sharks, but look again. In many sharks the traditional ‘palatoquadrate’ has a curved and robust lateral rim extending over the jaw joint. According to phylogenetic bracketing, that is the preoperculum now fused to the ‘palatoquadrate’ (light tan, actually the lacrimal using tetrapod homologs), which forms before the advent of the premaxilla and maxilla). Fusion of former jaw elements in sharks is a novel identification.

Figure 2. Polyodon hatchling prior to the development of the long rostrum with maturity.

Figure 5. Polyodon hatchling prior to the development of the long rostrum with maturity.

The long straight tail of the paddlefish hatchling 
is similar to the long straight tail of the goblin shark (Mitsukurina, Figs. 2, 6), which is one more reason why the goblin shark nests at the base of sharks, along with its protrusible jaws. The similarity of paddlefish and goblin sharks has not gone unnoticed. Several authors have written about the similarity, but all have attributed the similarity to convergence.

Figure 2. Classic diagram of the goblin shark, Mitsukurina.

Figure 6. Classic diagram of the goblin shark, Mitsukurina.

Figure x. Subset of the LRT focusing on sharks.

Figure 7. Subset of the LRT focusing on sharks.

Adding taxa
resolves all sorts of phylogenetic issues.


References
Berkovitz B and Shellis P 2017. Chondrichthyes 1 Elasmobranchs in The Teeth of Non-Mammalian Vertebrates. online at ScienceDirect.com
Gillis JA, Rawlinson KA, Bell J, Lyon WS, Baker CVH and Shubin NH 2011. Holocephalan embryos provide evidence for gill arch appendage reduction and opercular evolution in cartilaginous fishes. Proceedings of the National Academy of Sciences USA, 2011, 108:1507-1512

https://www.sharks.org/blog/blogs/science-blog/sharks-and-chimaeras-and-a-hedgehog

 

Caupedactylus ybaka (Kellner 2012) enters the LPT

Kellner (2012, 2013) described
the skull of an Early Cretaceous sinopterid pterosaur, Caupedactylus ybaka (MN 4726-V, Fig. 1). The skull is about forty-six centimetres long. (Hope I got this right this time).

Earlier the same skull was posted online.

Figure x. Caupedactylus in situ and restored by sculptors.

Figure x. Caupedactylus in situ and restored by sculptors. Or a different specimen.

New Tapejarid-Tupuxuarid skull.

Figure 1. New Tapejarid-Tupuxuarid skull now named Caupedactylus.

Bones colorized in this tapejarid / tupuxuarid.

Figure 2. Bones colorized in this tapejarid / tupuxuarid, named Caupedactylus.

Abstract
“A new unusual tapejarid pterosaur from the Early Cretaceous Romualdo Formation (Araripe Basin, Brazil) is described, based on a skull, lower jaw and some postcranial elements. Caupedactylus ybaka gen. et sp. shows the typical high nasoantorbital fenestra of the Thalassodrominae but lacks a palatal ridge, and shares with the Tapejarinae several features, including a downturned rostral end, allowing its allocation to that clade.”

The new skull compared to other tapejarids. Click to enlarge.

Figure 2. Click to enlarge. The rising size of the tapejaridae.

Abstract continues
“Furthermore, the new species differs in having an anteriorly and posteriorly expanded premaxillary sagittal crest, the lacrimal process of the jugal strongly inclined, and a slit-like postpalatine fenestra, among other characters. The region of the left jugal-quadratojugal-quadrate shows a pathology that is likely the result of an infection. The lateral surface of the premaxillary crest presents grooves that were interpreted in other pterosaurs as impressions of blood vessels, corroborating growing evidence that cranial crests could have been involved in thermoregulation.”

“Also, the new species has a well-preserved palate with a large palatine forming the anterior region of the choanae and the postpalatine fenestra and a secondary subtemporal fenestra. Since the latter has been regarded as unique to non-pterodactyloids, its occurrence in Caupedactylus demonstrates that the evolution of palatal region in pterosaurs is more complex than previously thought.”

Perhaps to no one’s surprise, this specimen nested in 2013 in the large pterosaur tree (LPT) between Sinopterus dongi and Tupandactylus.


References
Campos HBN and Headden JA 2013. A review of Tupuxuara deliradamus (Pterosauria, Azhdarchoidea, Thalassodromidae) from the Early Cretaceous Romualdo Formation of Brazil. International Symposium on Pterosaurs – Rio Ptero 2013.
Elgin RA 2015. Paleobiology, Morphology and Flight Characteristics of Pterodactyloid Pterosaurs. Dissertation, University of Heidelberg.
Kellner AWA 2012. A new unusual tapejarid (Pterosauria, Pterodactyloidea) from the Early Cretaceous Romualdo Formation, Araripe Basin, Brazil. Cambridge University Press 103 (3-4) The Full Profession: A Celebration of the Life and Career of Wann Langston, Jr., Quintessential Vertebrate Palaeontologist September 2012 , pp. 409-421.
Kellner AWA 2013. A new unusual tapejarid (Pterosauria, Pterodactyloidea) from the Early Cretaceous Romualdo Formation, Araripe Basin, Brazil. Earth and Environmental Science Transactions of the Royal Society of Edinburgh 103(3-4): 409-421.
Manzig PC et al. (10 co-authors) 2014. Discovery of a Rare Pterosaur Bone Bed in a Cretaceous Desert with Insights on Ontogeny and Behavior of Flying Reptiles. PLoS ONE 9(8): e100005.
Martill DM and Naish D 2006. Cranial Crests Development in the Azhdarchoid Pterosaur Tupuxuara, With Review of the Genus and Tapejarid Monophyly. Palaeontology 49(4): 925-941.

wiki/Caupedactylus
pterosaurheresies.wordpress.com/2013/06/06/tapejarid-or-tupuxuarid/

Marjanović 2018 suggested homologizing fish and tetrapod skull bones

As longtime readers know,
tetrapod bone colors were used here on fish skull photos and diagrams as they were added to ReptileEvolution.com (Figs. 1a, b) over the past two years. That was necessary in order to score several dozen new fish taxa for a growing online phylogenetic analysis that, until then, included only tetrapods.

Figure 1. Amia juvenile with DGS colors added. Image from Digimorph.org and used with permission.

Figure 1a. Amia juvenile with DGS colors added. Image from Digimorph.org and used with permission.

Somehow it all worked out.
I was pleasantly surprised from the start at how readily tetrapod bone colors could be applied to fish skulls (Figs. 1a, b).

Figure 4. Skull of the extant bowfin (Amia). Compare to figure 3.

Figure 1b. Skull of the extant bowfin (Amia).

A few days ago, I learned that back in 2018,
Dr. David Marjanović (researcher, Museum für Naturkunde, Berlin) suggested fish workers do the same in an SVP abstract: “It is difficult to tease apart the homologies of bones across Osteichthyes, often even within Actinopterygii. For a long time, it seems, anatomists gave up the attempt; numerous separate—sometimes contradictory—nomenclatures were used in different decades for different taxa or by different authors. However, a flood of recent discoveries provides grounds for optimism.”

The time to do this is now. It is a great idea, a necessary idea.

“The tetrapod stem is much more densely sampled than 25 years ago, confirming
unambiguously that the large bones of the actinopterygian skull table—which lie in roughly
the same places as the frontal and parietal of crown-group tetrapods—are homologous to
the parietal (the “preorbital” of “placoderms”) and the postparietal. This affects the next
more lateral series as well: as recently proposed, the “dermosphenotic”/“infraorbital 5” is
the intertemporal (which participates in the orbit margin in a few early tetrapods), the
“dermopterotic”/“intertemporal” is the supratemporal and the “supratemporal” is the
tabular.”

Fish nomenclature can get confusing if you’re a tetrapod fan. Here (Figs. 1a, 1b, 2) I don’t identify fish nomenclature at all. I let colors tell the tale: pink for nasals, cyan for jugals, orange for postfrontals, yellow green for intertemporals (= prootics), etc. (Fig. 1). That way if bones split or appear as a result of a split, they can be identified in several views. Unfortunately many earlier tetrapods were colored in a more slipshod manner, not in accord with these standards. Over time these will be repaired.

“Further, the base of the tetrapod stem clarifies the original spatial relationships of other
bones: the bone dorsal of the (anterior) naris is plesiomorphically not the nasal, but the so called anterior tectal, and the one ventral to it is the so-called lateral rostral (apparently
homologous to the septomaxilla of crown-group tetrapods), making it likely that these are
the homologs of the actinopterygian “nasal” and “antorbital” respectively. Unlike in
tetrapods, the squamosal of many other sarcopterygians has a long contact with the maxilla and could be homologous to the (second) “supramaxilla”.

Beyond actinopterygians, tetrapod homologies must be extended back to all craniates and gnathostomes.

Figure 2. Eurynotus is another platysomid, basal to the placoderms Coccosteus and Entelognathus.

Figure 2. Eurynotus is another platysomid, basal to the placoderms Coccosteus and Entelognathus, the most derived of these taxa, not the least derived.

“Outside the tetrapod stem, the placoderm-grade animal Entelognathus has shown that some homologies can be traced beyond Osteichthyes.”

By contrast, the LRT nests Entelognathus (Fig. 2) deep within the Osteichthys, close to extant catfish, a traditionally excluded set of taxa.

Nothing else can proceed unless a valid phylogenetic cladogram, like the LRT, has been established.

“I further propose that the unpaired “vomer” of various actinopterygians is the “prerostral plate” seen in “placoderms” and the Silurian osteichthyan Guiyu, the actual paired vomers being represented by the “vomerine toothplates”. 

The present cladogram (subset Fig. 3) and the colors traced on taxa in ReptileEvolution.com documents the splitting and fusion of the tetrapod homologs of bones in various fish. Sometimes two or three bones represent the lacrimal. The squamosal and the quadratojual result from such bone splits. The maxilla and premaxilla appear on the lower rim of the lacrimal as new bones holding the marginal teeth. In the LRT (subset Fig. 3) Guiyu is a derived taxon close to coelacanths.

Figure x. Subset of the LRT focusing on fish.

Figure 3. Subset of the LRT focusing on fish.

“The braincase remains underresearched even within crown-group tetrapods, and
neomorphic bones seem more common there than in the dermal skeleton; still, it seems
clear that the best candidates for homologs of the opisthotic are the “autopterotic” and/or
perhaps the “epiotic”/“epioccipital” of actinopterygians, not the “intercalary” sesamoid.”

“I propose further homologies throughout the skeleton based on ontogenetic data and the
rich fossil record, and hope to start a discussion on this promising field. Confidently
identified homologs would give a boost to phylogenetics and evolutionary biology.”

I agree!


References
Marjanović D 2018.  Yes, we can homologize skull (and other) bones of actinopterygians and tetrapods. Abstracts Society of Vertebrate Paleontology 2018.

Lobalopex: Finally another therapsid enters the TST!

It’s been awhile
since the last therapsid was traced and scored. Today two taxa enter the TST.

Figure 1. Cladogram of ten taxa employed by Sidor et al. 2004.

Figure 1. Cladogram of ten taxa employed by Sidor et al. 2004.

Sidor, Hopson and Keyser 2004
introduced a new ‘biarmosuchian’ and ‘burnetiamorph” therapsid from the Permian of South Africa. They named their new find Lobalopex mordax (CGP/1/61, Fig. 2, skull length 15cm). They reported, “a cladistic analysis including ten biarmosuchian taxa indicates that Lobalopex is the sister taxon to Burnetiidae and that Lemurosaurus is the most primitive burnetiamorph.” (Fig. 1).

After testing
in the Therapsid Skull Tree (= TST, 73 taxa, Fig. 3) Lobalopex nested  as a member of the Burnetia clade, matching the nesting of Sidor, Hopsona and Keyser 2004.

Figure 1. Lobalopex added to previous nested burnetiidae.

Figure 2. Lobalopex added to previous nested burnetiidae. It has a longer skull than others in this clade. Look to Herpetoskylax to imagine an uncrushed Lobalopex skull.

Sidor et al.
note the dorsal skull is crushed (it looks melted!) dorsoventrally. The rest of the skull is not crushed. Photos of the original material have not been published. Look to Herpetoskylax (Fig. 2) to imagine an uncrushed Lobalopex skull (Fig. 2).

A tiny horn boss
is present on the rostrum of Lobalopex. That horn boss gets bigger in Proburnetia and Burnetia (Fig. 2), but skips Lemurosaurus. The ventral portion of the Lobalopex retroarticular process wa lost during collection. The supratemporals (bright green) are fused to underlying bones. Lobalpex has a longer rostrum, relative to orbit length, than others in its clade, convergent with Eotitanosuchus.

Strangely,
little to no post-cranial material is known from this clade. Was it an herbivore or carnivore?

Figure 3. TST with the addition of Lobalopex nesting in the Burnetia clade.

Figure 3. TST with the addition of Lobalopex nesting in the Burnetia clade.

Lobalopex is derived from Lemurosaurus
in Sidor, Hopson and Keyser (Fig. 1), but the other way around in the TST (Fig. 3). Ictidorhinus (Fig. 2) is the outgroup in both cladograms. Herpetoskylax was not mentioned by Sidor, Hopson and Keyser 2004 because it was published later, not until 2006.

Figure 4. More recent cladograms that include Herpetoskylax and Lobalopex.

Figure 4. More recent cladograms that include Herpetoskylax and Lobalopex.

More recent cladograms 
that include Herpetoskylax and Lobalopex (Kruger et al. 2015, Kammerer 2016; Fig. 4) presume Biarmosuchus is the outgroup taxon. By contrast in the TST (Fig. 3) Biarmosuchus is the outgroup to more derived therapsid taxa, as is Rubidgina (Fig.5), which also enters the TST (Fig. 3) today, nesting basal to gorgonopsids and therocephalians + cynodonts + mammals.

Figure 5. Rubidgina skull in 4 views. Note the wide cheeks rotating the orbits anteriorly.

Figure 5. Rubidgina skull in 4 views. Note the wide cheeks rotating the orbits anteriorly. This taxon is not basal to the Burnetia clade in the TST.

With Rubidgina added to the deep time lineage of humans
it would not be too far off the mark to say, the deep time ancestors of Little Red Riding Hood once looked quite a bit like the big bad wolf. How ironic. ‘Grandma’ really did have such big teeth!


References
Sidor CA, Hopson JA and Keyser AW 2004. A new burnetiamorph therapsid from the Teekloof Formation, Permian, of South Africa. Journal of Vertebrate Paleontology 24(4):938–950.

wiki/Lobalopex

Lepidosaurian epipterygoids in basal pterosaurs

In 1998 lepidosaurian epipterygoids
were found in the basal lepidosaur tritosaur, Huehuecuetzpalli (Fig. 1, Reynoso 1998; slender magenta bones inside the cheek area).

Figure 2. Huehuecuetzpalli has a tall, narrow epipterygoid, as in other lepidosaurs, and just a pore of an antorbital fenestra in the maxilla.

Figure 1. Huehuecuetzpalli has a tall, narrow epipterygoid, as in other lepidosaurs, and just a pore of an antorbital fenestra in the maxilla.

About two years ago
previously overlooked lepidosaurian epipterygoids were identified here in a more derived lepiodaur tritosaur, Macrocnmeus (Fig. 2, slender green bones in the orbit area) for the first time.

Figure 1. Macrocnemus fuyuanensis (GMPKU-P-3001) in situ and as traced by the original authors, (middle) flipped with colors applied to bones, and (above) bone colors moved about to form a reconstruction. Darker yellow and darker green are medial views of premaxilla and maxilla. Note the long ascending process of the premaxilla and the palatal elements seen through the various openings all overlooked by those with firsthand access to the fossil. Epipterygoids are lepidosaur synapomorphies not present in protorosaurs.

Figure 2. Macrocnemus fuyuanensis (GMPKU-P-3001) in situ and as traced by the original authors, (middle) flipped with colors applied to bones, and (above) bone colors moved about to form a reconstruction. Darker yellow and darker green are medial views of premaxilla and maxilla. Note the long ascending process of the premaxilla and the palatal elements seen through the various openings all overlooked by those with firsthand access to the fossil. Epipterygoids are lepidosaur synapomorphies not present in protorosaurs.

Until now,
no one has ever positively identified lepidosaurian (slender strut-like) epipterygoids in a pterosaur. In the large reptile tree (LRT, 1737+ taxa) and the large pterosaur tree (LPT, 251 taxa) Bergamodactylus (MPUM 6009) nests as the basalmost pterosaur. Here is the skull in situ with DGS colors applied, as traced by Wild 1978 (above), and reconstructed in lateral and palatal views (below) based on the DGS tracings.

Figure 3. Bergamodactylus skull in situ and reconstructed. Wild 1978 tracing above.

Figure 3. Bergamodactylus skull in situ and reconstructed. Wild 1978 tracing above. Note the break-up of the jugal. Note the fusion of the ectopterygoids with the palatines producing ectopalaatines.

The lepidosaurian epipterygoids of Bergamodactylus
(slender bright green struts in the cheek/orbit area in figure 3), or any pterosaur over the last 200 years, are identified here for the first time, further confirming the lepidosaurian status of pterosaurs (Peters 2007, the LRT). Sorry I missed these little struts earlier. When you don’t think to look for them, you can overlook them.

Figure 5. Eudimorphodon epipterygoids (slender green struts).

Figure 4. Eudimorphodon epipterygoids (slender green struts).

Now you may wonder how many other pterosaurs
have overlooked epipterygoids? A quick look at Eudimorphodon reveals epipterygoids (Fig. 4, bright green struts). Other Triassic pterosaurs include:

  1. Austriadactylus SMNS 56342: slender strut present
  2. Austriadactuylus SC 332466: slender strut present
  3. Raeticodactylus : slender strut is present (identified on link as a stapes)
  4. Preondactylus: slender strut present
  5. Dimorphodon: amber strut over squamosal (Fig. 5 in situ image), 
  6. Seazzadactylus MFSN 21545: slender struts present, tentatively identified by Dalla Vecchia 2019, but as more than the slender struts they are) (Fig. 6).
The skull of Dimorphodon macronyx BMNH 41212.

Figure 5. The skull of Dimorphodon macronyx BMNH 41212. Above: in situ. Middle: Restored. Below: Palatal view. The slender yellow strut on top of the red squamosal in situ is a likely epipterygoid.

Figure 6. Seazzadactylus from Dalla Vecchia 2019. Here the epipterygoid struts are more correctly and less tentatively identified.

Figure 6. Seazzadactylus from Dalla Vecchia 2019. Here the epipterygoid struts are more correctly and less tentatively identified.

Hard to tell in anurognathids
where everything is crushed and strut-like. Hard to tell in other pterosaurs because the hyoids look just like epipterygoids. Given more time perhaps more examples will be documented that are obvious and irrefutable.

Added a few days later:

Added Figure. Here's the Triebold specimen of Pteranodon (NMC41-358) with epipterygoid splinters in bright green.

Added Figure. Here’s the Triebold specimen of Pteranodon (NMC41-358) with epipterygoid splinters in bright green.

Here’s the Triebold specimen of Pteranodon
(NMC41-358, added figure) with epipterygoid splinters in bright green. So start looking for the epipterygoid in every pterosaur. We’ll see if it is universal when more pterosaur specimens of all sorts are presented.


References
Dalla Vecchia FM 2019. Seazzadactylus venieri gen. et sp. nov., a new pterosaur (Diapsida: Pterosauria) from the Upper Triassic (Norian) of northeastern Italy. PeerJ 7:e7363 DOI 10.7717/peerj.7363
Peters D 2007. The origin and radiation of the Pterosauria. In D. Hone ed. Flugsaurier. The Wellnhofer pterosaur meeting, 2007, Munich, Germany. p. 27.
Reynoso V-H 1998. Huehuecuetzpalli mixtecus gen. et sp. nov: a basal squamate (Reptilia) from the Early Cretaceous of Tepexi de Rodríguez, Central México. Philosophical Transactions of the Royal Society, London B 353:477-500.
Wild R 1978. Die Flugsaurier (Reptilia, Pterosauria) aus der Oberen Trias von Cene bei Bergamo, Italien. Bolletino della Societa Paleontologica Italiana 17(2): 176–256.

wiki/Bergamodactylus
wiki/Huehuecuetzpalli
wiki/Homoeosaurus
wiki/Bavarisaurus

Evolution and synonyms of the hyomandibular and intertemporal

A major issue still facing paleontology and comparative anatomy
is the different names given to homologous bones in fish, reptiles and mammals. For example:

  1. the hyomandibular of fish is the stapes in tetrapods;
  2. the sphenotic in fish is the intertemporal in basal tetrapods, the prootic + opisthotic in reptiles and mammals;
  3. in fish the supraoccipital is the postparietal in stem tetrapods. That bone splits transversely to produce a postparietal and a supraoccipital in reptiles (Fig. 9);
  4. sometimes the jugal, lacrimal, nasal, maxilla and other bones also split into two or more bones. Other times they fuse together;
  5. some bones do not appear until later, de novo or by the product of a split;
  6. likewise, marginal teeth appear, disappear, fuse, unfuse, become more complex and simpler during evolution.
  7. … and that’s not counting the bones that have been traditionally mislabeled (Fig. 10).

From the genesis of the vertebrate skeleton
in Middle Silurian Birkenia (Fig. 1), a tiny hyomandibular articulates with the intertemporal dorsally and the tiny quadrate ventrally. The hyomandibular, a former dorsal gill arch segment, would ultimately evolve to become the most robust bone in the architecture of certain basal bony fish (Fig. 2) before shrinking in stem tetrapods (Fig. 6), ultimately becoming the stapes in basal reptiles (Fig. 9), and a tiny middle ear bone in mammals and humans.

Figure 2. Birkenia in situ with precursor facial bones labeled. This Middle Silurian taxon is basal to Furcacaudiformes and all other vertebrates.

Figure 1. Birkenia in situ with precursor facial bones labeled. This Middle Silurian taxon is basal to Furcacaudiformes and all other vertebrates.

In the first fish with jaws,
Chondrosteus (Fig. 2) the hyomandibular pivots to thrust the jaws forward during a bite, an action originated in tube-mouth osteostracans and sturgeons.

Figure 1. Chondrosteus animation (2 frames) in situ and reconstructed in lateral view. This is the transitional taxon linking sturgeons to bony fish + sharks.

Figure 1. Chondrosteus animation (2 frames) in situ and reconstructed in lateral view. This is the transitional taxon linking sturgeons to bony fish + sharks.

In the paddlefish ancestor,
Tanyrhinichthys (Fig. 3), the hyomandibular (deep green again) is no longer as mobile.

Figure 2. Tanyrhinichthys face after color tracing.

Figure 2. Tanyrhinichthys face after color tracing.

The hyomandibular becomes a massive immobile element
in the Early Devonian bony fish and spiny shark  Homalacanthus (Fig. 4). It continues to link the intertemporal with the quadrate.

Figure 4. Homalacanthus in situ and reconstructed.

Figure 4. Homalacanthus in situ and reconstructed. The massive hyomandibular is dark green.

In the fish portion
of the large reptile tree (LRT, 1710+ taxa; Fig. x) we’ve just crossed the major dichotomy separating stem lobefins (many of which are still ray fins) from stem frog fish + mudskippers, sea robins and tripod fish, which also use their pectoral fins to walk along the sea floor. (Let’s save that bit of interest for another blogpost).

Figure x. Subset of the LRT, focusing on fish for July 2020.

Figure x. Subset of the LRT, focusing on fish for July 2020.

Just across the dichotomy,
the tiny (3cm) spiny shark, Mesacanthus (Fig. 5) has a slender hyomandibular with forked tips. Thereafter the hyomandibular is largely covered up by cheek bones.

Figure 1. Early Devoniann Mesacanthus in situ. This 3 cm fish is a typical acanthodian here traced using DGS methods and reconstructed. Distinct from other spiny sharks, this one lacks large cheek plates, as in the extant Notopterus (Fig. 3).

Figure 5. Early Devoniann Mesacanthus in situ. This 3 cm fish is a typical acanthodian here traced using DGS methods and reconstructed. The hyomandibular is dark green.

In the stem tetrapod and large osteolepid,
Eusthenopteron (Fig. 6) the hyomandibular (dark green) attaches to a largely submerged intertemporal (yellow-green) with little dorsal exposure. The quadrate (red) contact with the hyomandibular is only tentative.

Figure 5. Eusthenopteron hyomandibular (dark green) still linking a largely submerged intertemporal (yellow-green) and a small quadrate (red).

Figure 6. Eusthenopteron hyomandibular (dark green) still linking a largely submerged intertemporal (yellow-green) and a small quadrate (red). Here the pterygoid (dark red) is essentially vertical, distinct from most tetrapods (e.g. Figs. 7-9).

In the flattend skull of a basal tetrapod, like
Laidleria (Fig. 7), the hyomandibular / stapes is horizontal and the intertemporal does not have a dorsal exposure. The quadrate connection is broken as the stapes contacts the small posterior tympanic membrane.

Figure 6. Early tetrapod Laidleria. The intertemporal disappears from the dorsal skull and the hyomandibular / stapes dark green)  is oriented horizontally here without a quadrate connection.

Figure 7. Early tetrapod Laidleria. The intertemporal disappears from the dorsal skull and the hyomandibular / stapes dark green)  is oriented horizontally here, perhaps without a quadrate connection, but note the extent of the stapes in palate view vs. occiput view.

In the aquatic reptilomorph,
Kotlassia (Fig. 8), the hyomandibular / stapes is tiny and oriented dorsolaterally in contact with a large tympanic membrane filling a posterior notch. The intertemporal reappears on the dorsal surface of the skull and expands internally to form the paraoccipital process (opisthotic).

Figure 7. The reptilomorph, Kotlassia, skull. Note the reappearance of the intertemporal here called the prootic. The hyomandibular / stapes is tiny and dark green.

Figure 8. The reptilomorph, Kotlassia, skull. Note the reappearance of the intertemporal here called the opisthotic in occipital view. The hyomandibular / stapes is tiny and dark green. The stapes contacts the tympanic membrane laterally.

In the basal and fully terrestrial archosauromorph,
Paleothyris (Fig. 9), the intertemporal is no longer exposed on the dorsal surface, but is exposed in occipital view, where it is called the opisthotic. The otic notch is now absent as the eardrum is reduced and relocated posterior to the jaw hinge. The former robust hyomandibular continues thereafter to shrink, becoming more sensitive to eardrum vibrations enabling a greater range of sound frequencies to be transmitted to the inner ear and brain.

Figure 8. The early archosauromorph, Paleothyris. Here the hyomandibular / stapes is oriented ventrolaterally. The intertemporal is not exposed dorsally.

Figure 9. The early archosauromorph, Paleothyris. Here the hyomandibular / stapes is oriented ventrolaterally. The intertemporal is not exposed dorsally, only occipitally where it is called the opisthotic.

On a slightly different subject:
bone misidentification by Thomson 1966

has been something of a problem ever since that publication. Here (Fig. 10) are the original bone IDs along with revised IDs on separate frames. Principally the relabeled intertemporal and parietal move behind the dorsal braincase division (Fig. 11).

Figure 2. Ectosteorhachis skull from Thomson 1966 with layers to show the brain case and palatoquadrate. Some bones are relabeled in the revised view.

Figure 10. Ectosteorhachis skull from Thomson 1966 with layers to show the brain case and palatoquadrate. Some bones are relabeled in the revised view. Note the intertemporal becomes the prootic + opisthotic at this point.

Thomson 1966 erred
when he put these elements anterior to the split, probably in order to locate the pineal opening between the parietals, which is typical of tetrapods. In osteolepids and their kin the pineal opening is between the relabeled frontals anterior to the transverse cranial split (Fig. 11).

Figure 11. Eusthenopteron and Osteolepis with skull bones relabeled.

Figure 11. Eusthenopteron and Osteolepis with skull bones relabeled.

Why is this so?
Under this new labeling system the contact between the intertemporal and hyomandibular is maintained (Figs. 6, 10). Outgroups to these taxa, like Cheirolepis (Fig. 12) likewise run a portion of the postorbital over the orbit, separating the postfrontal from the orbit margin. Now the ostelepids follow that trait despite the two-part postorbital.

Figure 11. Cheirolepis is an outgroup taxon to the ostelepids that includes a postorbital that extends over the orbit, separating the postfrontal from the orbit margin.

Figure 12. Cheirolepis is an outgroup taxon to the ostelepids that includes a postorbital that extends over the orbit, separating the postfrontal from the orbit margin.

In earlier posts
on hyomandibular evolution. and juvenile Eusthenopteron (Fig. 13; Schultze 1984) corrections have now been made. This bit of relabeling is a new hypothesis awaiting confirmation from others. At present phylogenetic bracketing (Fig. 12) supports this interpretation.

Figure 1. Eusthenopteron juvenile in situ from Schultze 1984. Large plate ventral to the mandible overlaps a convex ventral margin. The quadratojugal is not labeled here. Several bones are re-labeled here.

Figure 1. Eusthenopteron juvenile in situ from Schultze 1984. Large plate ventral to the mandible overlaps a convex ventral margin. The quadratojugal is not labeled here. Several bones are re-labeled here.

For those interested,
these changes affected only 4 character traits out of 238. These scoring changes did not affect the tree topology.


References

Schultze H-P 1984. Juvenile specimens of Eusthenopteron foordi Whiteaves, 1881 (Osteolepiform Rhipidistian, Pisces) from the Late Devonian of Miguasha, Quebec, Canada. Journal of Vertebrate Paleontology 4(1):1–16.
Thomson KS 1966. The evolution of the tetrapod middle ear in the rhipidistian-amphibian transition. American Zoologist 6:379–397.
Westoll TS 1943. The hyomandibular of Eusthenopteron and the tetrapod middle ear. Transactions of the Royal Society B 131:393–414.