Cretaceous Aquilolamna nests with Devonian Palaeospondylus in the LRT

Summary for those in a hurry
The authors excluded related taxa that would have helped them identify their strange, new 1.6 m shark with elongate pectoral fins. The authors also failed to identify the correct mouth, eyes, nasal capsules and gill slits.

Vullo et al. 2021 bring us a wonderful new 1.6m Turonian elasmobranch
with graceful, really long, pectoral fins, Aquilolamna milarcae (INAH 2544 P.F.17, Figs. 1, 2). The authors tentatively assigned (without a phylogenetic analysis) their fossil shark to lamniformes, like the mako shark, Isurus, which has a standard underslung mouth and overhanging rostrum. Vullo et al. thought Aquilolamna was a filter-feeder by assuming that it had a wide, ‘near-terminal mouth’ without teeth, as in the manta ray (genus: Manta). That morphology is distinct from lamniformes like Isurus.

This is a difficult fossil to interpret.
More than the fins make Aquilolamna different than most other fossil and extant sharks.

Unfortunately
Vullo et al. put little effort (Fig. 2 diagram) into their attempt to understand the many clues Aquilolamna left us. Those clues are documented here (Fig. 2) by using DGS (= color tracings) and tetrapod homologs for skull bones.

Figure 1. Aquilolamna in situ from Vullo et al. 2021. Colors added here.

Figure 1. Aquilolamna in situ from Vullo et al. 2021. Colors added here.

For proper identification, it didn’t help that Vullo et al. 

  1. imagined the mouth wide and in front, instead of small and below the occiput
  2. imagined the eyes on the sides, instead of on top
  3. imagined the gill slits on the sides, instead of ventral
  4. did not perform a phylogenetic analysis with a wide gamut of taxa
  5. did not consider Middle Devonian Palaeospondylus (Figs. 3, 4) as a taxon worthy of their time and consideration
  6. did not consider the torpedo ray, Tetronarce (Fig. 5), or the hammerhead, Sphyrna, taxa worth comparing in analysis (as in Fig. 4).
Figure 2. Skull of Aquilolamna and diagram from Vullo et al. 2021. Colors and new labels applied here. The mouth (magenta) appears under the occiput, overlooked by Vullo et al.

Figure 2. Skull of Aquilolamna and diagram from Vullo et al. 2021. Colors and new labels applied here. The mouth (magenta) appears under the occiput, overlooked by Vullo et al. White lines indicate symmetries. The hyomandibulars are small with fused quadrates at the new jaw corners and link to the intertemporals, as in all other vertebrates.

Despite these issues, Vullo et al. thought there was enough of Aquilolamna
that was strange, new and easy to understand to make it worthy of publication. And it is. And that’s okay. In science it’s okay to leave further details to other workers. Keeps us busy and feeling helpful! It’s okay to make mistakes. Others will fix those. That’s all part of the ongoing process.

From the abstract:
“Aquilolamna, tentatively assigned to Lamniformes, is characterized by hypertrophied, slender pectoral fins. This previously unknown body plan represents an unexpected evolutionary experimentation with underwater flight among sharks, more than 30 million years before the rise of manta and devil rays (Mobulidae), and shows that winglike pectoral fins have evolved independently in two distantly related clades of filter-feeding elasmobranchs.”

By contrast, in the LRT filter-feeding manta rays are more primitive than sharks that bite for a living.

Unfortunately the authors omitted important sister taxa recovered by the LRT from their comparison studies. They looked at other elamobranchs, but not the electric torpedo ray, hammerhead and Palaeospondylus (Figs. 3, 4).

By focusing on just a few traits the authors are trying to “Pull a Larry Martin.” Instead they should have performed a wide-gamut phylogenetic analysis with hundreds of traits.

Figure 1. A specimen of Palaeospondylus in situ with colors added here. This appears to be a ray in the hammerhead shark, Sphyraena family.

Figure 3. A specimen of Palaeospondylus in situ with colors added here. This appears to be a ray in the hammerhead shark, Sphyrna family, that also includes the electric torpedo ray, Tetronarce.

Figure 4. Palaeospondylus diagram from Joss and Johanson 2007 who mistakenly considered Palaeospondylus a hatchling lungfish.

Figure 4. Palaeospondylus diagram from Joss and Johanson 2007 who mistakenly considered Palaeospondylus a hatchling lungfish.

From the taphonomy section of the SuppData:
“No teeth can be observed in INAH 2544 P.F.17, possibly due to rapid post-mortem disarticulation and scattering affecting the dentition.”

Turns out the authors were looking for teeth in the wrong place. The real jaws with tiny teeth were partly hidden below the occiput, as in Middle Devonian Palaeospondylus (Fig. 4), not at the anterior skull rim of Aquilolamna.

Figure 4. Subset of the LRT focusing on the shark clades related to Aquilolamna and Palaeospondylus.

Figure 4. Subset of the LRT focusing on the elasmobranch clades related to Aquilolamna and Palaeospondylus.

The reported lack of pelvic fins in Aquilolamna
is unexpected in sharks, which otherwise always have pelvic fins. This lack of pelvic fins could turn out to be a synapomporphy of taxa descending from Palaeospondylus. We’ll have to have more taxa for that.

From the Vullo et al. 2021 diagnosis of the ‘family, genus and species’:
“Medium-sized neoselachian shark that differs from all other selachimorphs in having hypertrophied, scythe-shaped plesodic pectoral fins whose span exceeds the total length of the animal. High number (~70) of anteriorly directed pectoral radials. Head short and broad, with wide and near-terminal mouth. Caudal fin markedly heterocercal. Caudal fin skeleton showing a high hypochordal ray angle (i.e., ventrally directed hypochordal rays). Caudal tip slender with no (or strongly reduced?) terminal lobe. Squamation strongly reduced (or completely absent?).”

Figure 1. Tetronarce fairchildi (originally Torpedo fairchildi Hutton 1872, 1m)

Figure 5. Tetronarce fairchildi (originally Torpedo fairchildi Hutton 1872, 1m). Note the robust caudal fin. The hyomandibular links the jaw joint to the braincase.

Aquilolamna has more vertebrae than Palaeospondylus,
but the former is much larger, an adult and geologically younger by 280 million years. We looked at Palaeospondylus just three days ago here. Very lucky timing to have Palaeospondylus for comparison prior to studying Aquilolamna.

Figure 6. Ontogenetic growth series of an electric torpedo ray. Pectoral fins in green.

Figure 6. Ontogenetic growth series of an electric torpedo ray from Madl and Yip 2000. Pectoral fins in green. Pectoral fins enlarge with maturity. Eyes migrate dorsally. Perhaps the same occurred with Aquilolamna and Palaeospondylus.

Taxon exclusion
continues to be the number one problem in paleontology. Phylogenetic analysis with a wide gamut of hundreds of taxa continues to be the number one solution to nesting all new and enigma taxa. Contra the assertions of dozens of PhDs, first-hand examination of the fossil is not required, nor is a degree or doctorate. This is the sort of profession where you learn on the job with every new taxon that comes along. This one was not in any textbooks, so everyone started like a September freshman with Aquilolamna.

And finally, if you can’t find the mouth where you think it should be,
look somewhere else.


References
Madl P and Yip M 2000. Essay about the electric organ discharge (EOD) in Colloquial meeting of Chondrichthyes head by Goldschmid A, Salzberg, January 2000. Online here.
Vullo R, Frey E, Ifrim C, Gonzalez Gonzalez MA, Stinnesbeck ES and Stinnesbeck W 2021. Manta-like planktivorous sharks in Late Cretaceous oceans. Science 371(6535): 1253-1256. DOI: 10.1126/science.abc1490
https://science.sciencemag.org/content/371/6535/1253

Online Publicity for Aquilolamna:

  1. sciencemag.org/news/2021/03/eagle-shark-once-soared-through-ancient-seas-near-mexico
  2. phys.org/news/2021-03-discovery-winged-shark-cretaceous-seas.html
  3. nationalgeographic.com/science/article/shark-like-fossil-with-manta-wings-is-unlike-anything-seen-before
  4. livescience.com/ancient-shark-flew-through-dinosaur-age-seas.html

From Tapanila et al. 2020: Edestus, a scissors ‘shark’, enters the LRT

Among the oddest fish in the late Paleozoic seas,
are relatives of Helicoprion, famous for its buzz saw teeth (Fig. 3) that grew in a single spiral set in the mid-line of the jaws. Unfortunately not enough is known of the skull to attempt a nesting in the large reptile tree (LRT, 1812+ taxa) at present.

Figure x. Shark skull evolution diagram.

Earlier
Harpagofututor (Figs. 2, 3), a Carboniferous relative of today’s moray eel (Gymnothorax), was tentatively allied with the buzz-saw clade because it shared a small medial tooth row.

Today
Edestus heinrichi (Leidy 1856; Fig. 1; FMNH PF2204; 25cm skull length; Pennsylvanian) joins this clade with its scissors-like medial jaws (Fig. 1) and enough skull to work with.

Figure 1. Skull of Edestus from Tapanila et al. 2018 and reconstructed here using DGS methods.

From the Tapanila et al. 2020 abstract
“Sharks of Late Paleozoic oceans evolved unique dentitions for catching and eating soft bodied prey. A diverse but poorly preserved clade, edestoids are noted for developing biting teeth at the midline of their jaws. Helicoprion has a continuously growing root to accommodate >100 crowns that spiraled on top of one another to form a symphyseal whorl supported and laterally braced within the lower jaw. Reconstruction of jaw mechanics shows that individual serrated crowns grasped, sliced, and pulled prey items into the esophagus.”

Figure 2. Harpagofututor skull colored and reconstructed here. Compare to Gymnothorax in figure 4.
Figure 2. Harpagofututor skull colored and reconstructed here. Compare to Edestus in figure 1

From the abstract, continued.
“A new description and interpretation of Edestus provides insight into the anatomy and functional morphology of another specialized edestoid. Edestus has opposing curved blades of teeth that are segmented and shed with growth of the animal. Set on a long jaw the lower blade closes with a posterior motion, effectively slicing prey across multiple opposing serrated crowns.

Figure 5. Harpagofututor male and female skulls. Added here is the best partial skull of the buzz tooth shark, Helicoprion.
Figure 3. Harpagofututor male and female skulls. Added here is the best partial skull of the buzz tooth shark, Helicoprion.

Tradtionally
Edestus and Helicoprion are portrayed with a shark-like body, but phylogenetic bracketing gives it a moray eel-type body (Fig. 5), like Harpagofututor.

Figure 4. From Tapanila et al. 2020, animated here to show the biting cycle of Edestes.
Figure 4. From Tapanila et al. 2020, animated here to show the biting cycle of Edestus.

From the abstract, continued:
“The symphyseal dentition in edestoids is associated with a rigid jaw suspension and may have arisen in response to an increase in pelagic cephalopod prey during the Late Paleozoic.”

Note that Gymnothorax, the extant moray eel (Fig. 6), also has large palatal teeth along the symphysis (= midline) by homology.

Figure 1. Harpagofututor female from Lund 1982.
Figure 5. Harpagofututor female from Lund 1982.
Figure 2. The skull of the moray eel, Gymnothorax, in 3 views. Colors added as homologs to tetrapod skull bones. The nares exit is above the eyes.
Figure 6. The skull of the moray eel, Gymnothorax, in 3 views. Colors added as homologs to tetrapod skull bones. The nares exit is above the eyes.

Pulling prey deeper into the jaws
is a trait shared with the moray eel (Gymnothorax, Fig. 6), though done with an extra set of esophageal jaws (Fig. 7), distinct from the buzz-saw and scissors sharks.

Figure 7. GIF animation showing the dual bite of the dual jaws in moray eels. Both are derived from gill bars.
Figure 7. GIF animation showing the dual bite of the dual jaws in moray eels. Both are derived from gill bars.

Special thanks to reader JeholornisPrima
who sent a link to the Tapanila et al. 2020 paper on Edestus this morning to get this ball rolling.


References
Leidy J 1856. Indications of five species, with two new genera, of extinct fishes. Proc Acad Nat Sci Philadelphia 7:414.
Tapanila L, Priuitt J, Wilga CD and Pradel A 2020. Saws, Scissors, and Sharks: Late Paleozoic Experimentation with Symphyseal Dentition. The Anatomical Record 303:363–376. https://doi.org/10.1002/ar.24046

wiki/Edestus

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

The paddlefish (Polyodon) and basking shark (Cetorhinus) are closely related

The ‘key trait’: having one gill cover or several gill covers
(as in sharks, Fig. 1) turns out to be a trivial trait in a matrix of 235 traits in the large reptile tree (= LRT, subset Fig. 2). Only one gene has to change to make one type of gill or the other as recently documented (see below).

Figure 1. The basking shark (Cetorhinus) compared to the paddlefish (Polyodon).
Figure 1. The basking shark (Cetorhinus) compared to the paddlefish (Polyodon). Note the gelatinous rostrum in the paddlefish juvenile. That trait is retained in mako sharks, as we learned earlier.

What does ‘closely related’ actually mean?
No other tested taxon shares as many traits with paddlefish (Polyodon) as the basking shark (Cetorhinus, Fig. 1) in the LRT. Someday a taxon might be added that nests between them. At present such taxa remain unknown and untested. Both taxa are derived from the Polyodon hatchling taxon (Fig. 3), which has a shorter rostrum and a more basking shark-like appearance overall. Back in the Silurian, pre-paddlefish hatchlings were likely much smaller and adults were likely the size of present day hatchlings, but that’s not a requirement. No other analysis that I am aware of has ever included paddlefish hatchlings as taxa, but that morphology is key to understanding various lineages within Chondrichthyes. So, here’s a case where adding a taxon is much more important than adding a character.

Figure 6. Adding Debeerius to the LRT helped revise the shark-subset. Note the shifting of the basking shark, Cetorhnus within the paddlefish clade.
Figure 2. Adding Debeerius to the LRT helped revise the shark-subset. Note the shifting of the basking shark, Cetorhnus within the paddlefish clade.

Note the gelatinous rostrum
in the paddlefish juvenile (Fig. 1). That trait is retained from mako sharks (Figs. 3, 6, as we learned earlier here. The rostrum of the adult basking shark is likewise filled with gelatin supported by a thin frame of cartilage (Fig. 4). The shark-like appearance of paddlefish has been noted previously. Previously the presence of one enormous gill cover in paddlefish has excluded them form prior shark studies. The LRT minimizes such taxon exclusion by simply adding taxa.

We’ve always known
that ratfish (with one gill cover, Fig. 3) nest with sharks (with several gill covers separating slits). No one has complained about that yet.

Then we learned
that sturgeons and Chondrosteus (with one gill cover, Fig. 3) nest basal to whale sharks and mantas (with several gill covers). The pattern of gill covers was presented and revised recently here.

Figure 3. Shark skull evolution according to the LRT. Compare to figure 1.

Now
paddlefish (Polyodon) nests with basking sharks (Cetorhinus, Fig. 1) in the large reptile tree (LRT, 1785+ taxa, subset Fig. 2). Evolution is full of such trivial exceptions.

Paddlefish inhabit rivers. Basking sharks inhabit the sea.
They both feed the same way. Basking sharks reach 30 feet in length. Paddlefish reach 7 feet in length. The two likely went their separate ways in the Silurian (prior to 420mya), so they had plenty of time to evolve on their own since then.

Figure 2. Skull of Cetorhinus adult and juvenile showing differences in the rostrum and fusion of skull elements in the adult.
Figure 4. Skull of Cetorhinus adult and juvenile showing differences in the rostrum and fusion of skull elements in the adult.

A recent study on gill covers by Barske et al. 2020
“identify the first essential gene for gill cover formation in modern vertebrates, Pou3f3, and uncover the genomic element that brought Pou3f3 expression into the pharynx more than 430 Mya. Remarkably, small changes in this deeply conserved sequence account for the single large gill cover in living bony fish versus the five separate covers of sharks and their brethren.”

Figure 4. Skull of Polyodon from a diagram published in Gregory 1938, plus a dorsal view and lateral photo.
Figure 5. Skull of Polyodon from a diagram published in Gregory 1938, plus a dorsal view and lateral photo.

While comparisons to the feeding technique in paddlefish and basking sharks
appear in the literature (Matthews and Parker 1950, Haines and Sanderson 2017), these were presumed to be by convergence based on the single gill cover vs. multiple gill cover difference.

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 6. 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).

Relying on one, two or a dozen traits
to trump the other 234, 233 or 213 is called “Pulling a Larry Martin.” You don’t want to do that. Put aside your traditions, add taxa and let the unbiased software figure out where your taxon nests using the widely accepted hypothesis of maximum parsimony (= fewest changes) over a large set of character traits.

The present hypothesis of interrelationships
(Fig. 2) appears to be novel. If not, please advise so I can promote the earlier citation.


References
Barske L et al. (10 co-authors) 2020. Evolution of vertebrate gill covers via shifts in an ancient POU3f3 enhancer. PNAS 117(40):24876–24884.
Integration of swimming kinematics and ram suspension feeding in a model American paddlefish, Polyodon spatula. The Journal of Experimental Biology, 10.1242/jeb.166835, 220, 23, (4535-4547), (2017).
Matthews LH, Parker HW 1950. Notes on the anatomy and biology of the basking shark (Cetorhinus maximus (Gunner)). Proceedings of the Zoological Society of London 120(3):535–576.

What do larval sturgeons eat (when they have teeth)?

Today more evolutionary gaps are filled
and gaffs are rectified as novel hypotheses of interrelationships between sturgeons and sharks are further cemented with more data. When all the parts keep falling into place like this, a hypothesis is more likely to be correct.

Yesterday I learned
that larval sturgeons (like larval paddlefish) have small sharp teeth. These are lost when hatchlings grow more than 3cm in length. At this point their diet changes from open water microscopic copepods to river bottom macroscopic arthropods. At the same time their mouth parts become extensible vacuum cleaner tubes, usually carried inside, sometimes everted (Fig. 2b).

Figure x. Medial section of Acipenser larva with temporary teeth from Sewertzoff 1928.
Figure 1. Medial section of Acipenser larva with temporary teeth from Sewertzoff 1928. Looks more like a shark than a sturgeon here because this is where sharks come from in the LRT.

Zarri and Palkovacs 2018 described
larval green sturgeon diets. “Fish smaller than 30 mm had teeth on the oral jaws and showed a strong reliance on zooplankton prey. The developmental loss of teeth in fish greater than 30 mm was associated with decreased zooplankton consumption and increased richness of benthic macroinvertebrates in diets.”

Figure 2. Growth stages in Acipenser transmontanus, a species of white sturgeon.
Figure 2. Growth stages in Acipenser transmontanus, a species of white sturgeon. First the yolk sac is absorbed, then external feeding begins. Adult armor is derived from ostracoderm armor.

According to the online The Fish Report
“The study found that the most common larval sturgeon prey included copepods (a kind of tiny zooplankton), and macroinvertebrates such as mayflies, midges, and blackflies. The scientists also noted an interesting diet shift: larval sturgeon consumed zooplankton and macroinvertebrates in roughly equal amounts until they grew to 30 millimeters in total length, at which point their macroinvertebrate consumption increased. This shift coincided with the young sturgeon losing their teeth (fun fact: unlike humans, sturgeon start out life with teeth and lose them as they grow older).”

Zooplankton prey
include copepods (a kind of tiny zooplankton) that floats freely in open waters.

Benthic macroinvertebrates
such as larval mayflies, midges, and blackflies that live in river sands and muds.

Figure 5. Sturgeon mouth animated from images in Bemis et al. 1997. This similar to ostracoderms, basal to sharks.
Figure 2b. Sturgeon mouth animated from images in Bemis et al. 1997. This is similar to ostracoderms and basal to sharks. The barbels are retained buccal cirri.

Muir et al. 2000 report
a burrowing river amphipod about 1 cm long, Corophium spp., is the most important prey for bottom-feeding juvenile and sub-adult white sturgeon. In adult sturgeons,small bottom-dwelling fish, larvae, crayfish, snails, clams and leeches are on their prey list.

So the loss of teeth and the change in diet
reflects a change from open water predation of microscopic forms that other fish would filter to visible worms and larvae living in river bottoms.

This somewhat mirrors more primitive behavior in lancelets
that feed in open waters as juveniles, then burrow tail first in river bottoms and become sessile feeders. One branch of lancelets kept evolving to become crinoids and later, starfish. The other branch, the one that kept active as adults, became vertebrates.

Figure 11.  Manta compared to Thelodus (Loganellia) and Rhincodon. All three have a terminal mouth essentially straight across, between the lateral eyes, distinct from most fish. Note the lack of teeth. 
Figure 3.  Manta compared to Thelodus (Loganellia) and Rhincodon. Note the lack of teeth in this large, open water filter feeders.

This supports the phylogeny
of the large reptile tree (LRT, 1780+ taxa) which recovers the toothless Chondrosteus + Rhinchodon + Manta clade as the proximal descendants of sturgeons. These increasingly larger taxa continue to feed like larval sturgeons on plankton filtered from open water with larger, more anteriorly directed jaws and branchial cavities.

Figure 4. Shark skull evolution according to the LRT. Compare to figure 1.

The second largest and second most basal shark in the LRT,
the basking shark, Cetorhinus, is likewise toothless and feeds on open water zooplankton.

Phylogenetically
it was not until larval teeth were retained in adults, like Isurus and similar sharks, that made the capture of larger and larger prey in open water conditions possible. Contra tradition,  filter-feeding, like a whale shark, is a a primitive trait, as documented in the LRT.

After marginal teeth appeared on shark jaws, and stayed there in adults,
evolution took several courses, including a return to benthic feeding in guitarfish, sawfish, rays, ratfish all with pavement-like teeth. Sharks with sharp teeth kept their open water feeding habits. Some of these gradually lost the long rostrum and evolved into several forms, including 2m Hybodus close to the base of bony fish represented by 4cm Prohalecites (Fig. 5).

Figure 1. Prohalecites porroi in situ from Arratia 2015, colors added. No dorsal spines here.
Figure 1. Prohalecites porroi in situ from Arratia 2015, colors added. No dorsal spines here.

Given the above gathered data points,
now I’m looking for a juvenile osteostracan. Wonder what it looks like? If less bony, as in sturgeons, they might be hard to find.


References
Muir WD, McCabe, Parsley and Hinton 2000. Diet of first-feeding larval and young-of-the-year white sturgeon in the Lower Columbia River. Northwest Science 74(1):25–33.
Sewertzoff AN 1928. The head skeleton and muscles of Acipenser ruthensus. Acta Zoologica 13:193–320.
Zarri LJ and Palkovacs EP 2018. Temperature, discharge and development shape the larval diets of threatened green sturgeon in a highly managed section of the Sacramento River. Ecology of freshwater fish 28(2): https://doi.org/10.1111/eff.12450

Prohalecites: the new tiny ancestor of all bony ray fin fish

Updated February 10 2021
with a some details modified in the morphology resulting in a shifting of this taxon to the base of the ray-fin fish clade (not the spiney/lobe-fin fish clade).

…and it still looks like Hybodus,
(Fig. 2) its 50x larger proximal ancestor.

A nice surprise today
as a phylogenetically miniaturized hybodontid shark with gill covers and ray fins, Prohalecites porroi (Figs. 1, 4, 5, Belloti 1857, Deecke 1889, Tintori 1990, MCSNIO P 348, Middle Triassic; 4cm), enters the large reptile tree (LRT, 1780+ taxa) as THE basalmost bony ray-fin fish. 

Figure 1. Prohalecites porroi in situ from Arratia 2015, colors added. No dorsal spines here.

Figure 1. Prohalecites porroi in situ from Arratia 2015, colors added. No dorsal spines here.

That makes Prohalecites close to a late-surviving
human, mammal, reptile, tetrapod and bony fish ancestor. Prohalecites needs to be in every paleo textbook from here on out.

No trace of scales is preserved
in any specimen. No neurocranial material is preserved. Hemichordacentra are present. The preoperular is so slender it is twig-like.

Figure 2. Hybodus fraasi fossil in situ is 50x larger than an adult Prohalecites, the basalmost bony fish.

Figure 2. Hybodus fraasi fossil in situ is 50x larger than an adult Prohalecites, the basalmost bony fish.

Surprisingly little has been written
about Prohalecites. While Arratia 2015 considered it “the oldest of the Teleosti”, she did not mention Hybodus, its proximal ancestor in the LRT.

Tintori 1990 left Prohalecites as a Neopterygian incertae sedis,
“because its characters do not perfectly fit in any of these cited groups.” 

Arratia and Tintori 1998 wrote, 
“Prohalecites possesses an interesting mosaic of primitive and advanced chalacters, some of which have been previously interpreted as synapomorphies of Teleostei.” 

“The election of the outgloup plays a significant role in the phylogenic position of Prohalecites and other neopterygians. Unquestionably, Prohalecites is not a Teleostei.”

Their cladograms nested Prohalecites between Amia and all higher bony fish. Neither sharks nor Hybodus are mentioned in the text. So taxon exclusion hampers an otherwise highly focused study.

Figure 1. Subset of the LRT focusing on ray-fin fish, their speed, niches and extant.

Figure 1. Subset of the LRT focusing on ray-fin fish, their speed, niches and extant.

Once again,
too much focus, not enough of a wide-angle view hampered prior workers. Whenever taxa are tested together that have never been tested together before, new relationships can be recovered. That’s why the LRT was created 10 years ago. You should have so many taxa in your cladogram that it tells you which taxa to include in your more focused study. Cherry-picking taxa has become outdated. That traditional practice leads to false positives and enigmas.

Figure 4. Prohalecites skull from Arratia 2015, colors added.

Figure 4. Prohalecites skull from Arratia 2015, colors added.

Arratia 2015,
wrote on the history and current status of the fish clade Teleostei (Müller 1845).

Figure 3. Prohalecites diagram from Tintori 1990, colors added.

Figure 5. Prohalecites diagram from Tintori 1990, colors added.

According to Arratia,
Müller defined the clade based on soft tissue traits not visible in fossils. Thus, the taxon content of the clade has changed several times over the last century.

From the Arratia abstract:
“The monophyly of the total group Teleostei, which now includes Triassic pholidophorids, is supported by numerous synapomorphies.” This, of course, would be “Pulling a Larry Martin“, which happens frequently out there. Remember, it is better to define a clade by establishing two taxa that recover a last common ancestor on a wide gamut, comprehensive cladogram. Don’t rely on a few or a few dozen traits. Convergence must be allowed in your hypothetical model, because convergence and reversal did happen.

Arratia also wrote,
“Prohalecites from the Ladinian/Carnian (Triassic; c. 240 Ma) boundary represents the oldest stem teleost.” That piqued my interest in this tiny fish only half as long as a human finger.

According to Arratia, 
“during most of the last 170 years there has been a dichotomy in the treatment of teleosts, where fossil and living groups have been studied separately, including distinct classifications.”

Looking at the simple cladogram in figure 5 of Arratia 2015,
it looks like Teleostei includes Pholidophorus, Leptolepis, their last common ancestor and all descendants. Two other cladograms are shown in Arratia figure 8 based on earlier analyses. A full page cladogram is shown in Arratia figure 9 placing Amia and Lepisosteus as two of five outgroup taxa. Basal ingroup taxa include Pachycormus and Aspidorhynchus. Synapomorphies were listed for each node followed by a report on the pertinent traits. All this was for nought because the phylogenetic context was incomplete and invalid.

Arratia concludes, 
“The results demonstrate the importance of including fossil teleosts in the phylogenetic analysis, especially because some of their characters and combination of characters introduce a new perspective in understanding the origin and early radiation of the group, and indirectly provide a new scenario to interpret homologous characters.”

It goes without saying that Arratia 2015 did not include
placoderms, or lobefins in a teleost clade defined by Pachycormus and Aspidorhynchus in the LRT.

Prohalecites demonstrates, once again,
phylogenetic miniaturization at the genesis of a major clade, despite its late (Middle Triassic) appearance in the fossil record.

Was Prohalecites larger in the Silurian?
Maybe. It’s worth looking for. Or maybe bony fish began by neotony.

Earlier we looked at the origin of bone ‘islands’
on a cartilage substrate in the hatchlings and juveniles of the extant taxon, Amia. Prohalecites documents the origin of bone in the tiny adult bony fish descendants of hybodont sharks.


References
Arratia G and Tintori A 1999. The caudal skeleton of the Triassic actinopterygian †Prohalecites and its phylogenetic position, p. 121–142. In: Mesozoic Fishes 2—Systematics and Fossil Record. G. Arratia and H.-P. Schultze (eds.). Verlag Dr. F. Pfeil, München.
Arratia G 2015. Complexities of early Teleostei and the evolution of particular morphological structures through time. Copeia 103(4):999–1025.
Bellotti C 1857. Descizione di alcune nuove specie di pesci fossili di Perledo e di altre localtta lombarde. 419–432. In Sopani A (ed) Studi geologici sulla Lomabardia. Editore Turati, Milano.
Deecke W 1889. Über Fischea ùs verschiedenen Horizonten der Trias. Palaeontogaphica 45:97–138.
Müller J 1845. Über den Bau und die Grenzen der Ganoiden, und über das natürliche System der Fische. Physikalisch-Mathematische Abhandlungen der ko¨niglichen Akademie der Wissenschaften zu Berlin 1845:117–216.
Tintori A 1990. The actinopterygian fish Prohalecites from the Triassic of northern Italy. Palaeontology 33:155–174.

 

wiki/Prohalecites
wiki/Teleostei

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.

 

Why sharks have no bones: Borrell 2014

Several years ago
Nature published a report by Borrell 2014 titled, “Why sharks have no bones” referencing a report in the same issue by Venkatesh et al. 2014 featuring their work on the ‘elephant shark’, Callorhinchus milii (Fig. 1). This follows recent discussions and comments on cartilage and bone in sharks and bony fish respectively.

Callorhinchus is a chimaera, a ratfish and an elasmobranch,
but note (Fig. 1) the presence of a single gill opening covered by a broad operculum, otherwise found sturgeons, paddlefish and Osteichthys (other bony fish). The upper jaw is fused to the cranium, distinct from basal sharks, paddlefish and sturgeons, convergent with Osteichthys. That’s why they call it a chimaera!

Figure 1. Callorhinchus, the subject of the Venkatesh et al. 2014 study on cartilage in sharks, in vivo.
Figure 1. Callorhinchus, the subject of the Venkatesh et al. 2014 study on cartilage in sharks, in vivo.

According to Borrell,
“The DNA sequence of the elephant shark helps to explain why sharks have a cartilaginous skeleton and how humans and other vertebrates evolved acquired immunity.”

“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.”

From the Venkatesh et al. 2014 text:
“All gene family members involved in bone formation were present, except the secretory calcium-binding phosphoprotein (SCPP) gene family.”

Other fish experts note in the article, “Antarctic icefish (Notothenioidei (Fig. 2), lost the ability to form bone over the course of evolution.”

Notothenia, the namesake for the icefish clade, has not been added to the LRT yet, but is clearly a relative of Coryphaena, the open seas mahi-mahi, a bony fish not related to chimaeras. We’ll look at that taxon soon.

From the Venkatesh et al. 2014 abstract:
“Our functional studies suggest that the lack of genes encoding secreted calcium-binding phosphoproteins in cartilaginous fishes explains the absence of bone in their endoskeleton.”

Figure 2. Cladogram from Venkatesh et al. 2015. Second frame shows repairs based on the LRT.
Figure 2. Cladogram from Venkatesh et al. 2015. Second frame shows repairs based on the LRT.

All this is interesting news,
but Venkatesh et al. was using a traditional and outdated cladogram (repaired in Fig. 2). Parts of the Venkatesh et al. phylogenetic context were invalid due to taxon exclusion. Paddlefish, sturgeons and other pertinent taxa are missing. With regard to the included taxa:

  1. Heterostracan head shields are either made of bone.
  2. Osteostracan head shields are made of bone.
  3. Placoderms had bony armor, but the vertebrae, braincase, fin supports and gill arches were all made of cartilage. In the LRT, catfish are living relatives and they have bony skeletons and some catfish have bony armor.
  4. Acanthodians had jaw bones are preformed in cartilage then ossified with mesoderm-derived bone. Though the parts of the jaws have a similar origin, the teeth are very different from those of modern fish histologically, lacking enamel and apparently were not replaced, so had to last for the life of the fish.

So sharks pretty much stand alone lacking bone in most of their skeleton.

Figure 4. Shark skull evolution according to the LRT. Compare to figure 1.

Among extinct sharks,
Hybodus
 (Fig. 3), the proximal outgroup to bony fish in the LRT, “seems to have had highly ossified cartilage making it more like solid bone. This has meant that impressions have been quite well preserved revealing the morphology of the living animal.” Reference here.

Traditional workers never linked
the short-snouted shark, Hybodus, to bony fish, but considered the high degree of ossification convergent. By adding taxa, Hybodus nests basal to bony fish, demonstrating homology (rather than convergence) in the re-acquisition of bone, likely by the reactivation of that one bone gene, That’s what we’re looking for: a gradual accumulation of traits modeling actual microevolutionary events.


References
Borrell B 2014. Why sharks have no bones. Nature online here
Venkatesh B et al. 2014. Elephant shark genome provides unique insights into gnathostome evolution. Nature 505:174–179.

wiki/Australian_ghostshark
austhrutime.com/acanthodians.htm

The sawfish, Pristis, enters the LRT

Continuing with long-nosed basal chondrichthyans
the large reptile tree (LRT, 1772 taxa; subset Fig. 4) would not be complete without a sawfish.

Figure 1. The sawfish (Pristis pristis) in vivo.

Figure 1. The sawfish (Pristis pristis) in vivo.

Pristis pristis 
(Linck 1790; up to 7.6m) is the extant sawfish, a sister to the guitarfish (Rhinobatos, Fig. 5)) in the LRT. That’s no surprise. All prior workers have this in their hypotheses of interrelationships.

The nasal dermal denticles are larger than the teeth. They become worn with age and are not replaced. As in sister taxa the rostrum is a huge sensory organ for finding buried prey. The saw is also used to swipe and incapitate swimming fish and pin fish against the sea floor. Prey are swallowed whole.

The prefrontals (brown) contribute to the width of the skull. Gills and nares are ventral.

Figure 2. Pristis the sawfish from Digimorph.org, used with permission and colorized here.

Figure 2. Pristis the sawfish from Digimorph.org, used with permission and colorized here.

Both the guitarfish and sawfish
are derived from the dogfish shark, Squalus, which is also basal to another clade of bottom-feeding taxa, the chimaera or ratfish.

Figure 3. Ventral view of Pristis micro don. Note the ventral gills, nares and flat surface.

Figure 3. Ventral view of Pristis micro don. Note the ventral gills, nares and flat surface.

The addition of the Prisitis, the sawfish,
provoked a reexamination of Rhinobatos, the guitarfish (Fig. 5).

Figure 5. Updated image of Rhinobatos showing a lateral expanded prefrontal (brown) on top of the nasal (pink). Apologies for the earlier misunderstanding. I'm learning as I go and no prior studies attempt to color shark skulls with tetrapod homologs.

Figure 5. Updated image of Rhinobatos showing a lateral expanded prefrontal (brown) on top of the nasal (pink).

Figure 4. Subset of the LRT focusing on sharks.

Figure 4. Subset of the LRT focusing on sharks.

Bone identities in the guitarfish, Rhinobatos
(Fig. 5) were modified today. Apologies for the earlier misunderstanding. I’m learning as I go and no prior studies have attempted to color shark skulls with tetrapod homologs.


References
Linck HF 1790. Versuch einer Eintheilung der Fische nach den Zähnen. – Magazin für das Neueste aus der Physik und Naturgeschichte 6 (3): 28-38. Gotha.
Welten M et al. 2015. Evolutionary origins and development of saw-teeth on the sawfish and sawshark rostrum (Elasmobranchii; Chondrichthyes). Royal Society Open Science 2(9):150189.

wiki/Pristis
wiki/Sawfish

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