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

Orodus: another overlooked taxon at the shark-bony fish transition

Another overlooked human ancestor
enters the large reptile tree (LRT, 1793+ taxa) and with it, new light is shed on the history of how we came to be.

Figure 1. Orodus greggi in situ, FMNH specimen. See figure 2 for reconstruction.

Figure 1. Orodus greggi in situ, FMNH specimen. See figure 2 for reconstruction. This black triangle results after Photoshop removal of the original distortion due to perspective visible in the original photo.

Orodus greggi 
(Agassiz 1838, Late Pennsylvanian to Early Permian 300mya, 2m long) is a later surviving descendant at the shark-bony fish split, descending from Hybodus and basal to tiny Prohalecites.

Note: these are both late survivors of a Middle Silurian radiation based on phylogenetic and chronological bracketing. That gives both taxa plenty of time to evolve individual traits that appear, but do not remove both taxa from their phylogenetic order in the LRT.

The mandible of Orodus is massive
(probably a newly evolved trait). The cranium is narrow. The fins are larger than those illustrated by Zangrel 1981. The FMNH specimen preserves skin and gill slits.

Note: the distance between the pectoral fins and skull shrinks in Prohalecites, one way to make five gill opercula shrink to just one.

The FMNH (Field Museum) specimen of Orodus
would make a wonderful project for a PhD candidate. Not much has been written about it. It might be a good idea to run it through an x-ray machine to see the now covered coronoid process.

Figure 2. Orodus reconstructed using DGS from figure 1 alongside Prohalecites x10 and to scale.

Figure 2. Orodus reconstructed using DGS from figure 1 alongside Prohalecites x10 and to scale.

Earlier we looked at
Prohalecites (Fig. 2) a tiny descendent of Orodus also nesting in the LRT between sharks and bony fish and discussed the increasingly common instances of phylogenetic miniaturization at the genesis of major clades.


References
Agassiz L 1838.
 Recherches Sur Les Poissons Fossiles. Tome III (livr. 11). Imprimérie de Petitpierre, Neuchatel 73-140.

wiki/Prohalecites
wiki/Orodus

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

Basal bony fish descendants of hybodontid sharks

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

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

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

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

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

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

Keys to understanding this issue include:

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

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

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

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

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

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

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

Figure x. Shark skull evolution.

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

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

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

According to Wikipedia,
Notable characteristics of Acipenseriformes include:

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

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

Indeed. The LRT comes to the same conclusion.

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

Splanchnocranium

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

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

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