Acipenser enters the LRT as a basal sturgeon

This one could have been predicted
based on the short rostrum of Acipenser brevisrostrum (Figs. 1, 2), the long rostrum of the other two tested sturgeons (e.g. Pseudoscaphorhynchus Fig. 2) and the short rostrum of ancestral and descendant relatives of sturgeons like Hemcyclaspis and Chondrosteus (Fig. 2). The LRT tests a long list of 234 other traits, too, of course.

Figure 1. Acipenser skulls from Hilton 2005 and colored here.

Figure 1. Acipenser skulls from Hilton 2005 and colored here.

Acipenser brevistrostrum
(Lesueur 1818 non Heckel 1836, Hilton 2005) is the extant short-nosed sturgeon, with fossils going back to the Late Cretaceous (70mya). In the LRT it is the most primitive tested sturgeon.

The short rostrum inherited from Hemicyclaspis (Fig. 2) is retained in Chondrosteus and Isurus. The orbit is low and the oral cavity is set back from the rostral tip, as in Thelodus (Fig. 2), not Hemicyclaspis. Barbels are present, as in Thelodus, not Hemicyclaspis. The dorsal scutes are low and scattered, as in Thelodus, not Hemicyclaspis.

Younger shortnose sturgeons tend to have longer snouts compared to their older counterparts, but that is not reflected in the skull. After losing hatchling teeth, adult sturgeons have bony plates along their esophagus that help them crush hard items.

Figure 2. Top to bottom: Thelodus a soft jawless fish with a ventral oral opening and gill slits, perhaps a hint of diamond-shaped armor laterally. Hemicyclaspis, adds extensive armor. Acipenser brevirostrum, a short-listed sturgeon with a protrusible tube for a mouth and reduced armor. Pseudoscaphorhynchus, a derived sturgeon. Chondrosteus, a fish with jaws, but no marginal teeth.

Figure 2. Top to bottom: Thelodus a soft jawless fish with a ventral oral opening and gill slits, perhaps a hint of diamond-shaped armor laterally. Hemicyclaspis, adds extensive armor. Acipenser brevirostrum, a short-listed sturgeon with a protrusible tube for a mouth and reduced armor. Pseudoscaphorhynchus, a derived sturgeon. Chondrosteus, a fish with jaws, but no marginal teeth.

Shortnose sturgeons
are cartilaginous overall with bones in the skull, jaw and pectoral girdle. Bone is further reduced in descendant sharks, then reversed to more bone in basal bony fish following Hybodus. The sturgeon swim bladder is connected to the intestinal tract by a narrow duct, representing the origin of lungs. This is lost in sharks, then a reversal returns this to certain bony fish, including lungfish and crossopterygians.

Acipenser gives us our clearest clue into the soft tissue
of ancestral thelodonts and osteostracans. At present, Acipenser is the oldest surviving vertebrate, a living fossil, with phylogenetic origins deep into the Silurian based on Thelodus (Fig. 2) from the Early Silurian.


References
Hilton EJ 2005. Observations on the skulls of sturgeons (Acipenseridae): shared similarities of Pseudoscaphirhynchus kaufmanni and juvenile specimens of Acipenser stellatus. Environmental Biology of Fishes 72:135–144.
Le Sueur CA 1818. Description of several species of chondropterygious fishes of North America, with their varieties. Trans. Amer. Phil. Soc. 1: 383–394.

wiki/Shortnose_sturgeon_Acipenser

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

Updating and inverting Gregory 1933: Pre-shark skulls and the ontogenetic disappearance of teeth

From Gregory 1933:
“The typical fish skull, or syncranium (Fig. 1), notwithstanding the intricacy of its details, is generally recognized to be composed of two sharply contrasting divisions, which may be called the neurocranium, or braincase, and the branchiocranium.”

The neurocranium nests the brain, eyes, pineal and balancing organs.

The branchiocranium includes the gill arches and the mouth parts, which are derived from gill arches.

Some workers include a dermocrarnium, derived from the dermis. That would include the nasals and circumorbitals, not shown in Gregory’s figure (Fig. 1).

Figure 1. Syncranium of a bony fish from Gregory 1933, here with colors added.
Figure 1. Syncranium of a bony fish from Gregory 1933, here with colors added.

From Gregory 1933:
“The subdivision of the skull into separate bones has been conditioned chiefly by the necessities of growth and nutrition and that originally the endocranium was a continuum and the dermocranium consisted of a shell of ectosteal tissue, covering the chief functional regions or organs. Even now after the separate bones have enjoyed many millions of years of individuality, they are primarily regional subdivisions of functionally organic groups or tracts as well as organs in themselves.”

“In nearly all the hosts of typical fishes the syncranium is concerned with the pursuit and capture of living prey, the exceptions being few and peculiar forms such as the parrotfishes and the like, which have given up this freely competitive roving life and become highly specialized for living either on aquatic vegetation or on sessile animals.”

The LRT recovers a different pattern. The earliest ‘fish’ (like Arandaspis) were actually armored lancelets, filtering food in large branchial chambers, rather than pursuing prey. Transitional lancelet-fish, like Birkenia, retained a ventrally open oral cavity, still ventral in osteostracans and sturgeons.

In sturgeons the nasal bone or cartilage becomes an electrosensory organ to detect buried prey. When discovered prey is sucked in with an extensible tube. This is the first step toward feeding on larger prey. That arrangement reappears ventrally in later skates and rays and anteriorly in perch, frogfish, etc.

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

This tube evolves by neotony to become toothless jaws in Chondrosteus, (Fig. 2) basalmost sharks and manta rays that continue filter-feeding in open waters.

When tiny teeth appear in the paddlefish, Polyodon, larger prey is still not pursued. perhaps because only Polyodon larvae (Fig. 3) have teeth. Adults (Fig. 4) loose teeth. I just learned (from Sewertzoff 1928) that Acipenser (a sturgeon, Fig. 6) larvae also have tiny teeth (Fig. 5). Just like growing paddlefish, these tiny teeth also reduce and disappear as this sturgeon matures.

Figure 2. Polyodon hatchling prior to the development of the long rostrum with maturity.
Figure 3. Polyodon hatchling prior to the development of the long rostrum with maturity.
Figure 4. Skull of Polyodon from a diagram published in Gregory 1938, plus a dorsal view and lateral photo.
Figure 4. Skull of Polyodon from a diagram published in Gregory 1938, plus a dorsal view and lateral photo.
Figure 5. Medial section of Acipenser larva with temporary teeth from Sewertzoff 1928.
Figure 5. Medial section of Acipenser larva with temporary teeth from Sewertzoff 1928. Not sure if the yolk sac is absorbed before of after teeth appear.
Figure 1. Acipenser, a sturgeon.
Figure 6. Acipenser, a sturgeon.

From Gregory 1933:
“The profound researches of Stensio (1927) and Kiser (1924) have left no reasonable doubt however, that one or another of the ostracoderms gave rise to the modern class of cyclostomes, including the lampreys and hags, thus confirming the earlier views of Cope and others.”

Just the opposite, according to the LRT.

“The ancient ostracoderms, or pre-fishes, are first known from a single plate found in rocks of Middle Ordovician (Harding) age.”

This gives time for poorly ossified sturgeons, paddlefish, sharks and basal bony fish to appear and evolve during the fossil-poor Silurian making way for derived placoderms, like Entelognathus to appear in the Late Silurian.

“The true or gnathostome fishes are not known until the Devonian period and even up to the present time there are no known forms which definitely connect them with the ostracoderms.” 

That was in 1933. Now we have bony fish, like a mislabeled catfish, an osteoglossimorph Sinacanthus , a few lobefins like Guiyu and Psarolepis, and the derived placoderm Entelognathus, in the Silurian. Poorly ossified sturgeons are proximal descendants of ostracoderms in the LRT.

From Gregory 1933:
“As a class the ostracoderms are so inferior to the gnathostomes in their locomotor apparatus that they have even been assumed to be a specialized bottom-living group with no claim to be considered in the line of ascent to the gnathostomes. That was partly because it was further assumed that the continuous “headshield” must always be the result of the fusion of small polygonal plates. But Stensio’s intensive researches have revealed that the primitive ostracoderm shield was supported by a continuous endoskeleton without sutures, which was covered by a bony membrane.”

Sturgeons still have that head shield supported by a continuous endoskeleton without sutures. When sturgeons appeared, the splanchocranium began to separate once again from the neurocranium, as in Birkenia and the thelodonts. This was yet another reversal.

“According to the evidence adduced by Stensio (1925, pp. 160-164; 187-189) it appears that the cartilaginous condition of the skull in modern elasmobranchs is not improbably a result of degeneration, as in the better known cases of the cartilaginous skulls of sturgeons, spoonbills, Ceratodus [a lungfish], salmon, etc. Thus even the exoskeleton of modern sharks is retrogressive and now represented only by the skin and shagreen armor.”

Just the opposite, according to the LRT.

From Gregory 1933:
“Neither the Catopteridae
[no longer used, but refers to releatives of certain paleoniscid bony fish] nor any other known family of Chondrostei [= polyphyletic in the LRT, but traditionally includes sturgeons, paddlefish, bichirs and several extinct clades] however, appear to be directly ancestral to the typical holostean or protospondylous ganoids and later teleosts.”

Just the opposite, according to the LRT, which nests sturgeons, basal to paddlefish, basal to sharks, basal to all bony fish and tetrapods.

Figure x. Shark skull evolution.

From Gregory 1933:
“Stensio also concludes that the saurichthyids, like the sturgeons, palseoniscids, coelancanthids, dipnoans and arthrodires, form a degenerative series. By this he means especially that in such series the adult endocranium is better ossified, less cartilaginous, in the earlier than in the later members of the series.”

The LRT does not score for “better ossified” but relies more on shapes and proportions of scored elements.

“The sturgeon has specialized in the opposite direction from that of the primitive chondrosteans, as it has acquired an excessively small suctorial mouth which is withdrawn far behind the projecting rostrum.”

Just the opposite. The sturgeon mouth is primitive in the LRT.

[In sturgeons] “The whole snou tand fore part of the braincase is warped downward above the capacious orobranchial cavity in order to bring the snout down parallel to the ground.”

Just the opposite. This is the primitive condition, as seen in osteostracoderms.

From Gregory 1933:
“The rostral barbels are specialized tactile organs,”

Not specialized, but primitive, homologous with the buccal cirri of lancelets in which barbels/cirri serve both a chemoreceptive and mechanorerceptive role.

Figure 8. Extant lancelet (genus: Amphioxus) in cross section and lateral view. The gill basket nearly fills an atrium, which intakes water + food, sends the food into the intestine and expels the rest of the water.

From Gregory 1933:
“The neurocranium of the sturgeon and spoonbill are largely cartilaginous but with more or less extensive centers of ossification. It has been assumed by Watson and Stensio that this partly cartilaginous condition is due to retrogressive development (perhaps to the retention of early larval conditions in the adult). Sewertzoff, however, as a result of his embryological investigations (1928) challenges this view and concludes that the recent chondrosteans are much more nearly related to the elasmobranchs than was formerly suspected and that in many respects they are more primitive than the Palaeozoic palasoniscids. He holds among other things that the numerous ossicles in the snout of the sturgeons are more primitive than the few rostral elements of the palaeoniscids.”

“After a careful consideration of these opposing evidences and interpretations, I can only record my impression that the older view is by far the more probable, and that for many reasons, only a few of which may here be noticed.”

The LRT agrees with Sewertzoff 1928, not with Gregory 1933.

“Whatever may be said as to the sturgeon, it can hardly be doubted that the exoskeleton of the spoonbill {Polyodon) is in a highly retrogressive condition. In place of the fully formed ganoid scales of its palaeozoic relatives it has a practically naked body with a few vestigial horny scales in the upper lobe of its heterocercal fin.”

Just the oppositive. The spoonbill (= paddlefish) is primitive and basal to sharks.

Figure 2. Subset of the LRT focusing on one clade of bony fish that includes lobefins, but not exclusively.
Figure 9. Subset of the LRT focusing on one clade of bony fish that includes lobefins, but not exclusively.

From Gregory 1933:
“Moreover, many of the peculiar characters of the sturgeons are foreshadowed by theJurassic Chondrosteus (Fig. 195), which on the other hand retains features that are clearly inherited from a palseoniscoid stock, as well noted by A. S. Woodward (1895, p. viii). Watson (1925, p. 831) has already shown the annectant character of the Chondrosteidse between the palaeoniscids and the sturgeons.”

Just the opposite. In the LRT Chondrosteus is neotonously derived from sturgeons, basal to sharks. Compare the sturgeon larva (Fig.5) to the adult Chondrosteus, (Fig. 1). On the other hand, palaeoniscids, are no longer considered a natural group.


References
Gregory WK 1933. Fish skulls. A study of the evolution of natural mechanisms. American Philosophical Society 23(2) 1–481.
Sewertzoff AN 1928. The head skeleton and muscles of Acipenser ruthenus. Acta Zool., 9:193–319, 9 pis.

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

Squaloraja: more of a paddlefish than a ratfish

One of the strangest looking of all vertebrates,
Squaloraja polyspondyla, nests alone in the large reptile tree (LRT, 1772+ taxa) between the clade of gnathostomes without marginal teeth and those with marginal teeth.

Figure 1. Squaloraja is not the chimaerid everyone thinks it is, but nests with Scapanorhynchus and Mitsukurina in the paddlefish clade.

Figure 1. Squaloraja is not the chimaerid everyone thinks it is, but nests with Scapanorhynchus and Mitsukurina in the paddlefish clade.

Squaloraja polyspondyla (Agassiz 1843, Woodward 1866, Early Jurassic) is traditionally considered a relative of Chimaera, but here nests as a late-survivor of the basalmost taxon with marignal teeth, here named “Marginodonta“, between the basalmost, toothless, gnathostome clade with Chondrosteus at its base, and all other vertebrates with marginal teeth in the LRT with the paddlefish, Polyodon, at its base.

In a pre-cladistic era,
Squaloraja would have been considered a member of the Chondrostei, since it nests between sturgeons and paddlefish. But now in the LRT, so do whale sharks and mantas.

Grogan, Lund and Greenfest-Allen 2012
nested Squaloraja with chimaerids, but that cladogram excluded Squaloraja pre-shark sister taxa recovered in the wider gamut LRT.

Rather than a continuous notochord,
a series of cartilaginous segments is present, convergent with the situation in bony fish. As an Early Jurassic taxon, Squaloraja had plenty of time to develop this one trait.

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

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

These taxa were bottom feeders with a large wide rostrum full of sensors for detecting buried prey. Distinct from rays and sturgeons, but like Chondrosteus and paddlefish (Polyodon), the mouth was wide. Several excellent specimens preserve soft parts. It is worth comparing Squaloraja to a hatchling Polyodon (Fig. 2).


References
Agassiz L 1843. Recherches sur les Poissons Fossiles, III (IV), Imprimerie de Petitpierre, Neuchatel, pp. 157-390.
Grogan ED, Lund R and Greenfest-Allen E 2012. The origin and relationships of early chondrichthyans. In: Carrier JC, Musick JA and Heithaus MR (eds) Biology of Sharks and their Relatives, Edition 2. CRC Press, Boca Raton, Florida: 3–30.
Woodward AS 1886. On the anatomy and systematic position of the Liassic selachian Squaloraja polyspondyla Agassiz. Proceedings of the Zoological Society of London, 1886: 527–538.

Other references online here and here.

wiki/Squaloraja

Euphanerops: basal to sturgeons with tiny new pelvic fins

Janvier and Arsenault 2007 took another look at
Euphanerops longaevus (Woodward 1900; Late Devonian, Figs. 1, 2) comparing it uncertainly to living lampreys and extinct jawless, finless fish. They report, “The anatomy of Euphanerops longaevus is reconstructed here on the basis of 17 specimens, 14 of which were hitherto undescribed. Practically all the mineralized elements that can be observed in the largest individuals of E. longevous display the same structure, which strikingly recalls that of lamprey cartilage, despite the uncertainty as to the origin of its mineralization.”

Elongated and confluent paired fins
“The new material of E. longaevus described here provides strong support for the presence of ventrolateral, ribbon-shaped, paired fins armed with numerous parallel radials. These fins extend from the anus to the anterior part of the branchial apparatus anteriorly, and are the first instance of paired fins with radials, whose anteroposterior extension largely overlaps that of the branchial apparatus in a vertebrate.”

Mostly true, but let’s not forget in manta rays and guitarfish, skates and rays, paired pectoral fins indeed do overlap the branchial apparatus (= gill basket), IF that is happening in Euphanerops (see below).

From the abstract
“Owing to the uncertainty as to the biogenic or diagenetic nature of the anatomical features described in E. longevous, no character analysis is proposed. Only a few possible homologies are uniquely shared by euphaneropids and either lampreys or anaspids, or both.”

Phylogenetically, the authors note:
“Euphanerops longaevus has been referred to as an anaspid, chiefly because of its distinctive hypocercal tail and anal fin. However, since it apparently has no mineralized dermal skeleton, E. longaevus lacks evidence for the tri-radiate postbranchial spine, which Forey (1984) proposed as the defining character of the Anaspida. Consequently, it is now often treated in recent phylogenetic analyses as a separate terminal taxon, alongside other scale-less (or “naked”) jawless vertebrate taxa also once regarded as anaspids, namely Endeiolepis and Jamoytius.”

Figure 1. Several basal chordates: Branchiostoma, Euphanerops, Jamoytius and Birkenia. The middle image of Euphanerops is the tracing. The others are freehand interpretations not supported here.

Figure 1. Several basal chordates: Branchiostoma, Euphanerops, Jamoytius and Birkenia. The middle image of Euphanerops is the tracing. The others are freehand interpretations from Janvier and Arsenault 2007.

Here 
(Fig. 2) individual skull bones and tiny overlooked pectoral and pelvic fins are identified. Adding a missing (unossified?) rostrum (= nasal) restores the original profile. In the large reptile tree (LRT, 1717+ taxa) Euphanerops nests basal to sturgeons, like Pseudoscaphirhynchus (FIg. 3), a clade not mentioned by Janvier and Arsenault 2007. A previously enigmatic element in front of the mouth is here identified as a pair of barbels, as in sturgeons. The tiny dorsal spines of Euphanerops are also found as larger dorsal armor in Birkenia, osteostracans and sturgeons.

Figure 7. Transitional Euphanerops is more sturgeon-like, but the location of the operculum and/or gill openings are not apparent here. The nasal is missing from both the plate and counter plate.

Figure 7. Transitional Euphanerops is more sturgeon-like, but the location of the operculum and/or gill openings are not apparent here. The nasal is missing from both the plate and counter plate.

According to Wikipedia
Euphaneropidae have, “greatly elongated branchial apparatus which covers most of the length of the body.”

Here that area is identified as a typical subdivided and flattened ventral surface, as in Birkenia, sturgeons and osteostracans.

Figure 2. Skull of Pseudoscaphorhynchus. Note the mouth is created by the lacrimal and angular, not the maxilla and dentary, which are tooth-bearing bones in more derived fish.

Figure 2. Skull of Pseudoscaphorhynchus. Note the mouth is created by the lacrimal and angular, not the maxilla and dentary, which are tooth-bearing bones in more derived fish.

The hypocercal tail of Euphanerops
has heterocercal elements and this taxon nests between taxa with a heterocercal tail. With an Ordovician genesis, Late Devonian Euphanerops likely developed a dipping tail and larger propulsive dorsal fin secondarily, as a reversal. An ancestor, Birkenia, has a similar dipping tail.

Figure 4. Euphanerops caudal fin with elements re-identified.

Figure 4. Euphanerops caudal fin with elements re-identified.

Small enigmatic squares of rod-like elements near the cloaca
are here identified as primitive pelvic fins or vestiges of the same. More primitive taxa do not have pelvic fins. More derived taxa do.

Figure 3. Euphanerops with elements here identified as tiny pectoral fins just anterior to the cloaca.

Figure 5. Euphanerops with elements here identified as tiny pectoral fins just anterior to the cloaca and posterior to the ventral armor. Images from Janvier and Arsenault 2007.

Primitive pectoral fins
are known in ancestral and descendant taxa, so Euphanerops should have them, too. Here (Fig. 6) they are identified as vestiges.

Figure x. Euphanerops plate and counter plate with colors added identifying elements.

Figure 6. Euphanerops plate and counter plate with colors added identifying elements.

Traditionally sturgeons have not been tested with osteostracans
(Fig. 7) and other jawless fish. The LRT tests a wide gamut of competing candidates and nests sturgeons prior to the advent of jaws and teeth in vertebrates, close to osteostracans and Euphanerops. Do not let one or two traits, like a dipping (hypocercal) tail, steer you off course in your wide-gamut analysis.

Figure 7. Top to bottom: Thelodus a soft jawless fish with a ventral oral opening and gill slits, perhaps a hint of diamond-shaped armor laterally. Hemicyclaspis, adds extensive armor. Euphaneropsps, a late survivor of an Ordovician radiation basal to sturgeons. Acipenser, a sturgeon with a protrusible tube for a mouth and reduced armor. Chondrosteus, a fish with jaws, but no marginal teeth.

Figure 7. Top to bottom: Thelodus a soft jawless fish with a ventral oral opening and gill slits, perhaps a hint of diamond-shaped armor laterally. Hemicyclaspis, adds extensive armor. Euphaneropsps, a late survivor of an Ordovician radiation basal to sturgeons. Acipenser, a sturgeon with a protrusible tube for a mouth and reduced armor. Chondrosteus, a fish with jaws, but no marginal teeth.

The ‘paired fin ridges’ observed by Janvier and Arsenault
may be ray-like ossifications that gathered to produce the ventrolateral armor on sturgeons (Fig. 7) or were vestiges thereof. Additionally, that’s where basal chordate gonads are located.

A set of lamprey-like gill openings appear near the skull
of Euphanerops. This appears to be a retention of or reversal back to similar multiple openings seen in Birkenia (Fig. 1). Again, don’t judge a taxon by one or two traits. Test them all against a wide gamut of taxa, like the LRT. We may be seeing what happens a the transition from multiple gill openings to a sturgeon-like operculum here.


References
Janvier P, Desbiens S, Willett JA and Arsenault 2006. Lamprey-like gills in a gnathostome related Devonian jawless vertebrate. Nature 440:1183–1185.
Janvier P and Arsenault M 2007. The anatomy of Euphanerops longaevus Woodward, 1900, an anaspid-like jawless vertebrate from the Upper Devonian of Miguasha, Quebec, Canada. Geodiversitas 29 (1) : 143-216.
Woodward AS 1900. On a new ostracoderm fish (Euphanerops longaevus) from the Upper Devonian of Scaumenac Bay, Quebec, Canada. Magazine of Natural History ser. 7, 5: 416-419.

wiki/Euphaneropidae

Tanyrhinichthys is an armored goblin shark

Revised December 24, 2020
with additional data moving Tanyrhyichthys closer to Mitsukurina, the goblin shark, a taxon omitted by prior authors.

Sallan et al. 2020 return again to
Tanyrhinichthys mcallisteri (Gottfried, 1987; Stack, Hodnett, Lucas and Sallan 2018; Figs. 1, 2; 15 cm length) and focus on its long rostrum, imagining a bottom-dweller, sturgeon-like lifestyle. Oddly they fail to phylogenetically connect it with the extant goblin shark, Mitsukurina (Figs. 3, 4) due to taxon exclusion and some misinterpretation of the skull and gill bars. In the large reptile tree (LRT, 1498+ taxa then, 1781 taxa now; subset Fig. 5).

Figure 1. Tanyrhinichthys in situ and traced.

Figure 1. Tanyrhinichthys in situ and traced.

Based on that phylogeny,
Tanyrhinichthys had a Silurian genesis. Consider it an armored goblin shark.

Figure 2. Skull of Tanyrhinichthys (above) with two bones relabeled. The other fish, Saurichthys, is clearly unrelated.

Figure 2. Skull of Tanyrhinichthys (above) with two bones relabeled. The premaxilla carries teeth. The nasal does not. The other fish, Saurichthys, is unrelated. Compare to figure 2b.

Figure 2. Tanyrhinichthys face after color tracing.

Figure 3. Tanyrhinichthys face after color tracing. Operculum in lavender. Crowded gill bars in several colors follow.

Figure 4. Mitsukurina, the goblin shark. Note the deep premaxilla, and gracile jugal + fused postorbital as in Tanyrhinichthys in figure 3.

Figure 4. Mitsukurina, the goblin shark. Note the deep premaxilla, and gracile jugal + fused postorbital as in Tanyrhinichthys in figure 3.

Tanyrhinichthys had deep jaws and sharp, narrow, marginal teeth,
so it nests crownward of Chondrosteus (which had jaws, but no marginal teeth), apart from sturgeons (which lack jaws and marginal teeth). Both have underslung, shark-like jaws..

Stack et al. followed tradition
in assuming sturgeons are aberrant actinopterygian (ray-fin) fish. The LRT does not make assumptions, but tests all possibilities and included taxa. In the LRT (fish subset in Fig. 5) sturgeons have an extendible toothless oral cavity, the first step toward the jaws seen in Chondrosteus and all later taxa. They are not aberrant derived taxa.

From the abstract
“The earliest ray-finned fishes (Actinopterygii) with elongate rostra are poorly known, obscuring the earliest appearances of a now widespread feature in actinopterygians.”

This may be an exaggeration. The LRT tests several long rostra taxa. Several nest close to one another. Note the authors are already assuming Tanyrhinichthys is an actinopterygian just because it has ray-fins. Only a comprehensive cladogram can determine whether or not ray fin fish are monophyletic. They are not, according to the LRT, unless sharks are included, which is not the intent of the clade name or definition.

“We redescribe Tanyrhinichthys mcallisteri, a long-rostrumed actinopterygian from the Upper Pennsylvanian (Missourian) of the Kinney Brick Quarry, New Mexico. Tanyrhinichthys has a lengthened rostrum bearing a sensory canal, ventrally inserted paired fins, posteriorly placed median fins unequal in size and shape, and a heterocercal caudal fin. Tanyrhinichthys shares these features with sturgeons, but lacks chondrostean synapomorphies, indicating convergence on a bottom-feeding lifestyle.”

Note: The authors mention sturgeons, but oddly fail to mention goblins sharks.

“Elongate rostra evolved independently in two lineages of bottom-dwelling, freshwater actinopterygians in the Late Pennsylvanian of Euramerica, as well as in at least one North American chondrichthyan (Bandringa rayi).”

These taxa are not ‘evolved independently.’ Actually all are related to goblin sharks in the LRT. This is what tinkering does… it solves problems no one else ever thought was a problem.

“The near-simultaneous appearance of novel ecomorphologies among multiple, distantly related lineages of actinopterygians and chondrichthyans was common during the Carboniferous radiation of fishes.”

This statement assumes Tanyrhinichthys is an actinopterygian. In the LRT Tanyrhinichthys precedes sharks and bony fish, despite having ray fins. (Don’t make the mistake of ‘Pulling a Larry Martin.’)

Quotes from ScienceDaily.com, focusing on the new paper:
“Sturgeon are considered a ‘primitive’ species, but what we’re showing is that the sturgeon lifestyle is something that’s been selected for in certain conditions and has evolved over and over again,” says Sallan, senior author on the work.

Not so, according to the LRT where sturgeons and paddlefish are related with Chondrosteus and goblin sharks.

The Sallen et al. 2020 abstract,
continues to press the resemblance of Tanyrhinichys to sturgeons, while avoiding goblin sharks.

“Tanyrhinichthys mcallisteri, a member of the diverse and well-preserved fish fauna within the Upper Pennsylvanian (Missourian) Atrasado Formation of the Kinney Brick Quarry (KBQ), is a small (standard length ~15 cm), elongated actinopterygian with a lengthened rostrum. New material suggests that Tanyrhinichthys was a bottom feeder morphologically similar to the modern sturgeon (Acipenser). Like sturgeon, Tanyrhinichthys had a rostrum that extended past its lower jaw and a resultant small, subterminal mouth, as well as a number of other convergent features, including a long anal fin set forward of the dorsal, large lateral line scales, and an anteriorly-deepened body with ventral insertion of the paired fins. Two other long-rostrumed actinopterygians, an unnamed taxon from Indiana and Phanerorhynchus from the U.K., are known from similarly-aged, Pennsylvanian freshwater coal deposits. Various skeletal features indicate that these long-rostrumed fishes were not closely related.

In the LRT these ‘long-rostrumed fishes’ are all related. So where are comparisons to paddlefish and goblin sharks? They immediately follow:

“As supported by the existence of the paddlefish-like shark Bandringa in similarly aged deposits from Illinois, there was widespread convergence on a bottom-feeding freshwater morphotype amongst Pennsylvanian fishes.”

“Tanyrhinichthys falls into a group of fishes with short electro-sensory rostra with less skeletal support anteriorly, likely facilitating a bottom-roving feeding strategy. This group of fishes includes living taxa (sturgeon, paddlefish, and armored catfishes), along with fossil taxa such as Phanerorhynchus. Although there are some exceptions, it appears that long-rostrumed fishes are driven to evolve grossly similar structures in pursuit of distinctive life modes.”

Thus the authors make only the vaguest of connections between Tanyrhinichthys and paddlefish and no connection to goblin sharks.

re: Gottfried 1987 (below): the clade ‘Aeduelliform’ seems to have been used only by him and him alone. A Google search revealed no other attributions or usages.

Tanyrhinichthys seems to have had both
crowded gill slits and an operculum (Fig. 3).


References
Gottfried MD 1987. A Pennsylvanian aeduelliform (Osteichthyes, Actinopterygii) from North America with comments on aeduelliform interrelationships.
7Paläontologische Zeitschrift 61(1):141-148.
Stack J, Hodnett JM, Lucas S and Sallan L 2018. Tanyrhinichthys, a long-rostrumed Carboniferous ray-finned fish (Actinopterygii), and the evolution of elogate snouts in fishes. Journal of Vertebrate Paleontology abstracts 2018.
Sallan L, Lucas SG, Hodnett J-P and Stack J 2020. Tanyrhinichthys mcallisteri, a long-rostrumed Pennsylvanian ray-finned fish (Actinopterygii) and the simultaneous appearance of novel ecomorphologies in Late Palaeozoic fishes. Zoological Journal of the Linnean Society, 2020; DOI: 10.1093/zoolinnean/zlaa044

Ancient “sturgeon” was not a sturgeon

https://www.sciencedaily.com/releases/2020/06/200622133022.htm

http://reptileevolution.com/polyodon.htm

Chondrosteus animated

A few bones, no teeth, and all in ventral view
made Chondrosteus (Figs. 1a,b) a difficult taxon to visualize a year ago, especially when I knew less about fish. This taxon has been in, then out, and now back in the large reptile tree again (LRT, 1687+ taxa, subset Fig. 2).

Figure 1a. Chondrosteus in situ drawing from Egerton. This was difficult to understand a year ago.

Figure 1a. Chondrosteus in situ drawing from Egerton. This was difficult to understand a year ago.

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

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

Color + eyeballs + animation helps
one understand this taxon (Fig. 1). So do more taxa in the LRT, giving me an education, and understanding of evolutionary patterns. Like primitive fish, Chondrosteus has an unrestricted notochord, no nasal bones, no prefrontals, no premaxillae, maxillae or dentaries. These last three bones don’t appear until marginal teeth start to grow in more derived taxa, like Falcatus, Hybodus and Ozarcus.

Figure x. Newly revised fish subset of the LRT

Figure 2. Newly revised fish subset of the LRT. This is a novel hypothesis of interrelationships. Typically sturgeons are considered aberrant bony fish nesting with paddlefish, like Polyodon.

Chondrosteus acipenseroides (Agassiz 1843; Egerton 1858; Early Jurassic 190mya; 1.2m; BMNH P3361) is transitional between sturgeons and sharks + bony fish. The jaws are just begning to form into clamping mechanisms here, but marginal teeth are not yet present.

The above YouTube video shows a feeding sturgeon.
Click to play. Advance to 2:25 to see the ventral tubular feeding activity. This is the transitional morphology from the inactive oral cavity in jawless lancelets, thelodonts and osteostracans to biting jaws in Chondrosteus, sharks + bony fish. Notably, some fish and sharks + rays retain and/or redevelop tubular ventrally-oriented jaws (Fig. 3).

Figure 3. Rhinobatus jaw mechanism animation. This is how skates and rays eat, distinct from the Thelodus/ whale shark/ manta ray method of ram feeding.

Figure 3. Rhinobatus jaw mechanism animation. This is how skates and rays eat, distinct from the Thelodus/ whale shark/ manta ray method of ram feeding.

Part of the learning process in science
is ‘taking a step back’ to review and falsify difficult subjects. Figure 1 (above) is  a good example of scientific ‘tinkering‘ to figure out how all the parts are arranged and what they do. I didn’t get it right the first time.

The traditional method
is to shovel a year’s salary at a university only to be taught what you could have read in three seconds at Wikipedia. Many young paleo PhDs have gone this route.


References
Agassiz L 1843. Recherches Sur Les Poissons Fossiles. Tome I (livr. 18). Imprimerie de Petitpierre, Neuchatel xxxii-188
Egerton PDMG 1858. On Chondrosteus, an extinct genus of the Sturionidae, found in the Lias formation at Lyme Regis. Philosophical Transactions of the Royal Society of London 148:871-885.

wiki/Chondrosteus

An overview of sturgeons: Bemis et al. 1997

Bemis et al. 1997 looked at sturgeons,
those strange semi-armored, lake-dwelling fish without a proper set of jaws (Fig. 1).

From the abstract:
“Acipenseriformes [= sturgeons and kin] has existed at least since the Lower Jurassic (approximately 200 MYBP), and all fossil and recent taxa are from the Holarctic. Phylogenetic relationships among Paleozoic and Early Mesozoic actinopterygians are problematic, but most workers agree that Acipenseriformes is monophyletic and derived from some component of ‘paleonisciform’ fishes.”

In the large reptile tree (LRT, 1684+ taxa; subset Fig. 2) sturgeons, like Pseudoscaphirhynchus, do not nest with bony fish, or sharks, but prior to the shark-bony fish split. Sturgeons can be thought of as less-armored osteostracans and more-armored thelodonts.

Figure 1. Top to bottom: Thelodus a soft jawless fish with a ventral oral opening and gill slits, perhaps a hint of diamond-shaped armor laterally. Hemicyclaspis, adds extensive armor. Acipenser, a sturgeon with a protrusible tube for a mouth and reduced armor.

Figure 1. Top to bottom: Thelodus a soft jawless fish with a ventral oral opening and gill slits, perhaps a hint of diamond-shaped armor laterally. Hemicyclaspis, adds extensive armor. Acipenser, a sturgeon with a protrusible tube for a mouth and reduced armor.

Again from the abstract:
“We discuss five features fundamental to the biology of acipenseriforms that benefit from the availability of our new phylogenetic hypothesis:

  1. specializations of jaws and operculum relevant to jaw protrusion, feeding, and ram ventilation;
  2. anadromy or potamodromy and demersal spawning;
  3. paedomorphosis and evolution of the group;
  4. the biogeography of Asian and North American polyodontids and scaphirhynchines; 
  5. and the great abundance of electroreceptive organs in the rostral and opercular regions.”

The LRT (Fig. 2) recovers sturgeons prior to the invention of terminal jaws and teeth, so what sturgeons have is not a specialization, but a primitive state.

Figure x. Newly revised fish subset of the LRT

Figure 2. Newly revised fish subset of the LRT. Sturgeons appear in the small gray box above.

Bemis et al. do not discuss
possible relationships with osteostracans and their thelodont ancestors. They do remark on the interest in sturgeon systematics, “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.”

In the LRT sturgeons are less armored osteostracans, arising from even less armored thelodonts. The ossification is not ‘reduced’. Rather the ossification is at a precursor stage.

From Bemis et al.
“Linneaus 1758, Lacépede 1797 and Heckel 1836 the classical idea was sturgeons must be closely related to sharks based on a similar endoskeleton and jaw suspension.”

Polyodon, the paddlefish (Fig. 3), has been lumped with sturgeons, but that is not supported by the LRT. Polyodon has real jaws.

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

Figure 3. Skull of Polyodon from a diagram published in Gregory 1938, plus a dorsal view and lateral photo. Traditionally lumped with sturgeons, the LRT does not support that interrelationship.

The fossil taxon, Chrondrosteus, has been lumped with sturgeons. In the LRT this taxon was added, then deleted due to a lack of good reconstruction due to a lack of skull bones and my lack of experience with fish. Putative relatives were studied yesterday, and do not nest with sturgeons. I will discuss this toothless taxon soon.

From Bemis et al.
“Müller 1846 defined the three grades of bony fish: Chondrostei, Holostei and Teleostei base on increasing degrees of ossification. Müller accepted sturgeons as osteichthyans.”

Those divisions are not supported by the LRT.

The Bemis et al. cladogram
nests sturgeons with paddlefish, both derived from Polypterus, the bichir (a lungfish in the LRT, and a sister to lungfish in Bemis et al). More taxa split three apart from one another in the LRT (Fig. 2). Taxon exclusion is the problem with Bemis et al. as no osteostracans, and dozens of other pertinent taxa, are included in their cladogram.

As mentioned earlier,
this is a novel hypothesis of interrelationships. If there is an earlier version of this hypothesis, send the citation 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.

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

Sturgeons are living osteostracans. So are we.

According to Wikipedia,h
“The class Osteostraci (“bony shields”) is an extinct taxon of bony-armored jawless fish, termed “ostracoderms“, that lived in what is now North America, Europe and Russia from the Middle Silurian to Late Devonian.

“Anatomically speaking, the osteostracans, especially the Devonian species, were among the most advanced of all known agnathans. This is due to the development of paired fins, and their complicated cranial anatomy. The osteostracans were more similar to lampreys than to jawed vertebrates in possessing two pairs of semicircular canals in the inner ear, as opposed to the three pairs found in the inner ears of jawed vertebrates.”

Lampreys have no fins, nor do they have disc-shaped skulls and armor, like sturgeons do.

Figure 1. The osteostracan Cephalaspis (above) compared to the sturgeon Pseudoscaphorhynchus (below). The similarities of these armored morphologies have been overlooked previously. In both cases the jawless or tubular mouth is below the skull, the former towards the front, the latter below the eyes.

Figure 1. The osteostracan Cephalaspis (above) compared to the sturgeon Pseudoscaphorhynchus (below). The similarities of these armored morphologies have been overlooked previously. In both cases the jawless or tubular mouth is below the skull, the former towards the front, the latter below the eyes.

The similarities of these armored morphologies
have been overlooked previously. In both cases the jawless or tubular mouth is below the disc-like skull. In sturgeons the lacrimal forms the top of the tube.

A premaxilla and maxilla arrive
with the genesis of teeth.

Here’s what Wikipedia has to say,
“These jawless vertebrates are the sister-group of gnathostomes. Several synapomorphies support this hypothesis, such as the presence of: sclerotic ossicles, paired pectoral fins, a dermal skeleton with three layers (a basal layer of isopedin, a middle layer of spongy bone, and a superficial layer of dentin), and perichondral bone.

“Most osteostracans had a massive cephalothorac shield, but all Middle and Late Devonian species appear to have had a reduced, thinner, and often micromeric dermal skeleton. This reduction may have occurred at least three times independently because the pattern of reduction is different in each taxon.”

Sturgeons retained that osteostracan cephalic shield
for the last 400 million years. And a bit of the armor,  too. The primitive nature of sturgeons has been understood since the beginning of paleoichthyology, but just how primitive has eluded workers until now. If there is an earlier citation that links sturgeons to osteostracans, let me know, so I can give proper credit.

Based on the node
on which the osteostracan Hemicyclaspis, all later taxa, including Tetrapoda and Homo, can also be considered part of the clade Osteostraci.

Figure x. Newly revised fish subset of the LRT

Figure x. Newly revised fish subset of the LRT

Pseudoscaphirhychus kaufmanni (Nikolskii 1900) is the extant amu darya sturgeon. Distinct from traditional cladograms, the LRT nests this sturgeon between the osteostracan, Hemicyclaspis and the last common ancestor of sharks and bony fish, Chondrosteus. All have weak ventral jaws, no teeth and a shark-like heterocercal tail.

The lacrimal and mandible support a bottom-feeding extendable tube disconnected from the quadrate. Breathing is hampered whenever the mouth is in the mud, so water enters the top of the operculum, then exits laterally.


References
Nikolskii AM 1900. Pseudoscaphirhynchus rossikowi, n. gen, et spec. Ann. Mus. Imp. Sci. St. Petersburg 4, 257–260 (text in Russian).

wiki/Pseudoscaphirhychus
wiki/Sturgeon 
wiki/Chondrosteus
wiki/Osteostraci