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

The traditional ‘placoderm’, Jagorina, enters the LRT with Manta.

Ray-like fish can be confusing
especially if more of the exterior than the interior is preserved (Figs. 2-4). Scoring for the large reptile tree (LRT, 1786+ taxa is based largely on bones, but also on proportions and shapes in detail and overall.

To make matters worse,
ray-like fish like to wrap and fuse their pectoral fins around their face. I mean, who else does that? This one trait is convergent across three clades. Traditional workers suffering from taxon deletion keep mantas, skates and rays in one clade, the invalidated Batoidea, which we looked at earlier here and here.

Jagorina pandora
(Fig. 1) is known from skeletal casts (hollow shapes in stone).

Figure 1. Jagorina in two views from Carroll 1988 and here colored with tetrapod homologs.

Figure 1. Jagorina in two views from Carroll 1988 and here colored with tetrapod homologs. Note the separation of the tan postparietals and red tabulars from the rest of the skull as in Manta (figure 6).

Jagorina pandora
(Jaekel 1921; 6cm snout tip to synacrural length; Mb.f 510.2; Fig. 1; Late Devonian) was originally considered a type of placoderm different from the rest, but here nests basal to the manta ray, Manta (Fig. 6). The purported dorsal nostrils are instead left and right fontanelles that merge in Manta. The toothless mouth parts were largely transverse and faced forward, as in Manta. The tooth-bearing elements: premaxilla, maxilla and dentary were not present. The post parietal and tabulars were detached from the parietal as in Manta. The purported operculum is the supratemporal. Gill openings could be ventral.

By contrast,
a traditionally related taxon, Gemuendina (Figs. 2–4), is known from exquisitely preserved dermal materials with very little of the skeleton visible beneath. Perhaps someday µCT scans will reveal the interior architecture. (PhD candidates, are you listening?)

Figure 1. Based on the scale bar, this a surprisingly small specimen, full scale on a typical computer monitor.

Figure 2. Based on the scale bar, this a surprisingly small specimen, shown full scale on a typical computer monitor of 72 dpi. Other specimens are 30cm to 1 m in length.

Gemuendina stuertzi 
(Traquair 1903; 30cm to 1m; Early Devonian; Figs. 2–4) was originally considered a placoderm close to Jagorina (Fig. 1). Here both are related to Manta (Figs. 5–6). The famous and often copied diagram above only loosely matches the in situ specimen (Figs. 3–4). The purported dorsal eyes are not dorsal, but tiny and lateral. The purported dorsal nostrils are instead left and right fontanelles that merge in Manta. The purported jutting mouth oriented upward now appears to be a pair of curled under cephalic fins, as in Manta. A mosaic of tessellated scales covers the body, obscuring the skeleton.

Figure 2. Gemuendina in situ. So much skin and ornament cover the bone, this taxon has been withdrawn from the LRT. Apparently the skull and pectoral girdle have separated from the pectoral fins during taphonomy.

Figure 3. Gemuendina in situ. So much skin and ornament cover the bone, this taxon has been withdrawn from the LRT. Apparently the skull and pectoral girdle have separated from the pectoral fins during taphonomy.

Gemuendina is an excellent specimen,
unfortunately obscuring too much of the interior architecture upon which the LRT is built.

Figure 4. Gemuendina skull in situ. So much skin makes this taxon too confusing to score, but note the apparent cephalic fins previously interpreted as a jutting mouth (as in figure 4).

Figure 4. Gemuendina skull in situ. So much skin makes this taxon too confusing to score, but note the apparent cephalic fins previously interpreted as a jutting mouth (as in figure 4).

In the above photo
(Fig. 4) I could not find the large eyes and jutting mouth illustrated by Gross 1963 (Fig. 2). But I could find soft remains of cephalic fins and tiny lateral eyes, as in Manta (Figs. 5, 6).

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 5.  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.

These taxa
(Fig. 5) form a clade of early gnathostomes lacking marginal teeth as adults. Since more primitive sturgeons have marginal teeth as hatchlings, data is needed on the embryos and hatchlings of whale sharks and mantas to see if they have marginal teeth that are ultimately lost. (PhD candidates, are you listening?)

Both extant taxa (whale sharks + manta rays)
have carpets of palatal teeth that look like patches of sharp-to-blunt scales.

Figure 6. Three views of the skeleton of Manta, colors added. Note the terminal mouth, distinct from other rays, skates and guitarfish. The cephalic fins are continuous from the large face-wrappiing pectoral fins.

Figure 6. Three views of the skeleton of Manta, colors added. Note the terminal mouth, distinct from other rays, skates and guitarfish. The cephalic fins are continuous from the large face-wrappiing pectoral fins.

This clade of marginally toothless gnathostomes
all feed on free-swimming open-water plankton, rather than the benthic (buried) prey other rays and skates prefer with their ventral mouths full of pavement-like teeth. They filter vast quantities of sea water in a large gill chamber (Fig. 7).

Figure 3. The gill chamber and digestive track of Manta shown in ventral view.

Figure 7. The gill chamber and digestive track of Manta shown in ventral view.

My earlier attempts at understanding
Gemuendina were hampered by not knowing the skin was so thick it obscured the skeleton beneath. Hopefully that mistake is repaired now. If not, further corrections will be made. The addition of Jargorina to the LRT and the deletion of Gemuendina from the LRT brings a more complete understanding of this clade and its ray-like, filter-feeding members.


References
Gross W 1963. Gemuendina stuertzi Traquair. Notizblatt des Hessischen Landesanstalt für Bodenforschung 91, 36–73.
Jaekel O 1921. Die Stellung der Pala¨ontologie zu einigen Problemen der Biologie und Phylogenie. Pal Zeit 3:213–239.
Traquair RH 1896. The extinct vertebrate animals of the Moray Firth area. Pp. 235–285 in Harvie-Brown J.A and Buckley TE (eds.): A Vertebrate Fauna of the Moray Firth Basin, Vol. II. Harvie Brown and Buckley, Edinburgh.

wiki/Turinia
wiki/Manta
wiki/Jagorina
wiki/Gemuendina

 

Traditional batoids (skates + rays): taxon exclusion hampers prior phylogenetic results

McEachran and Aschliman 2004 reported,
“all authors agree that batoids constitute a monophyletic group.”

Underwood et al. 2015 reported, 
“While the monophyly of the Batoidea is not in doubt, phylogenetic relationships within the group are uncertain.”

By including a wider gamut of taxa,
the large reptile tree (LRT, 1785+ taxa, subset Fig. 1) recovers rays apart from skates and mantas apart from both. So the monophyly of the Batoidea is in doubt when more taxa are added. It is also surprising that a character list with no batoid characters is able to lump and split them, indicating the primacy and necessity of adding taxa.

Figure 1. Subset of the LRT focusing on basal gnathostomes. Traditional rays and skats are highlighted.
Figure 1. Subset of the LRT focusing on basal gnathostomes. Traditional rays and skates are highlighted along with Squaloraja, a traditional chimaerid with a sawshark appearance and Tristychius, a flattened nurse shark relative with large fins.

Franklin et al. 2014 wrote:
“A database of 253 specimens, encompassing 60 of the 72 batoid genera, reveals that the majority of morphological variation across Batoidea is attributable to fin aspect-ratio and the chordwise location of fin apexes. Both aspect-ratio and apex location exhibit significant phylogenetic signal.”

Figure 2. Four 'batoid' cladograms published in Underwood et al. 2015 with citations listed.
Figure 2. Four ‘batoid’ cladograms published in Underwood et al. 2015 with citations listed. They don’t agree with each other largely due to taxon exclusion and inappropriate outgroup taxa.

For those who want evidence of evolution
the four cladograms offered in Underwood et al. 2015 (Fig. 2) offer little.

  1. They employ suprageneric taxa for outgroup taxa
  2. They exclude pertinent taxa (see Fig. 2) from both the in-group and out-group.
Figure 3. Batoid cladogram frrom Sasko et al. 2006 with notes on swimming motions.
Figure 3. Batoid cladogram frrom Sasko et al. 2006 with notes on swimming motions. Note the differences compared to those in figure 2. 

Sasko et al. 2006 published a batoid phylogeny
that included notes on swimming styles. Taxon exclusion also mars this study. As a result convergence is ignored. These authors didn’t think they were cherry-picking taxa… but they were doing exactly that. They thought they were covering ‘all the bases’. The editors and referees agreed. That’s why the LRT tests a wider gamut of taxa to minimize the possibility of this sort of taxon exclusion. Outgroups are important. Omit pertinent outgroups and nothing else goes right.

Figure 4. Shark skull evolution according to the LRT. Compare to figure 1. Note the sturgeon-like reversal in the guitarfish, Rhinobatos.

By contrast
in the LRT (subset Fig. 1, diagram Fig. 4) Holocephali (=ratfish) is a derived clade, not a basal bauplan upon which rays and skates evolved. While more rays and skates are listed in the four Underwood et al. cladograms, the LRT includes outgroup taxa back to headless chordates. Long nosed sawfish and guitarfish nest together in the LRT. Marginally toothless and filter-feeding mantas nest with similar whale sharks and kin (not found in Underwood et al. cladograms). Bottom line: prior authors assumed too much. More taxa would have helped, as shown in Fig. 2.

Figure 2. The spotted eagle ray, Aetobatus in vivo.
Figure 5. The spotted eagle ray, Aetobatus in vivo.

A key to understanding evolution
is to understand that most of the time (tunicates, starfish and kin a clear exception), simpler taxa evolve into more complex taxa by the gradual accumulation of derived traits. In vertebrates, jawless chordates appear first. Then pre-jaws appear ventrally in sturgeons. In mantas and whale sharks marginally toothless jaws migrate anteriorly. In the rest, the sensitive rostrum continues to overhang the now tooth-lined jaws. Starting with this scenario, the rest of the chondrichthyes evolves wither a shorter or longer rostrum, pectoral fins might take over propulsion (convergent with mantas), and teeth might turn into pavement stone analogs.

Figure 5. Sturgeon mouth animated from images in Bemis et al. 1997. This similar to ostracoderms, basal to sharks.
Figure 6. Sturgeon mouth animated from images in Bemis et al. 1997. This returns in guitarfish (Fig. 7).
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 7. Rhinobatus jaw mechanism animation. This is how skates and rays eat, distinct from the Thelodus/ whale shark/ manta ray method of ram feeding. Compare to the sturgeon in figure 6.

While we’re at it,
please note the overlooked sturgeon-like reversal displayed by the guitarfish, Rhinobatos (Figs. 4, 7), basal to skates. That tiny-extending mouth morphology (Figs. 6, 7) didn’t appear de novo. It was waiting in the sturgeon-shark-skate gene pool to return.


References
Aschliman NC, Nishida M, Miya M, Inoue JG, Rosana KM and Nayloer GJP 2012. Body plan convergence in the evolution of skates and rays (Chondrichthyes: Batoidea). Mol Phylogenet Evol 63(1):28-42. doi: 10.1016/j.ympev.2011.12.012. Epub 2011 Dec 22.
Franklin O, Palmer C and Dyke G 2014. Pectoral fin morphology of batoid fishes (Chondrichthyes: Batoidea): explaining phylogenetic variation with geometric morphometrics. J Morphol 275(10):1173-86. doi: 10.1002/jmor.20294. Epub 2014 May 5.
Hall KC, Hundt PJ, Swenson JD, Summers AP and Crow KD 2018. The evolution of underwater flight: The redistribution of pectoral fin rays, in manta rays and their relatives (Myliobatidae). J Morphol 279(8):1155-1170. doi: 10.1002/jmor.20837. Epub 2018 Jun 7. PMID: 29878395
Larouche O, Zelditch ML and Cloutier R 2017. Fin modules: an evolutionary perspective on appendage disparity in basal vertebrates. BMC Biol. 2017 Apr 27;15(1):32. doi: 10.1186/s12915-017-0370-x. PMID: 28449681
Martinez CM, Rohlf FJ and Frisk MG 2016. Re-evaluation of batoid pectoral morphology reveals novel patterns of diversity among major lineages. J Morphol. 277(4):482-93. doi: 10.1002/jmor.20513. Epub 2016 Feb 11. PMID: 26869186
McEachran JD, Dunn KA and Miyake T 1996. Interrelationships of the batoid fishes (Chondrichthyes: Batoidei). Pp. 63–84 in Stiassny MLJ, Parenti LR, Johnson G D eds. Interrelationships of fishes. Academic Press, San Diego.
McEachran JD and Aschliman N 2004. Phylogeny of Batoidea. Chapter 3 in: Biology of Sharks and Their Relatives, Second Edition. DOI: 10.1201/9780203491317.ch3
Pavan-Kumar A et al. 2013. Molecular phylogeny of elasmobranchs inferred from mitochondrial and nuclear markers. Mol Biol Rep 41:447–457. doi: 10.1007/s11033-013-2879-6 PMID: 24293104.
Rosenberger LJ 2001. Pectoral fin locomotion in batoid fishes: undulation versus oscillation. J Exp Biol 204(Pt 2):379-94. PMID: 11136623
Undersood CJ et al. (6 co-authors) 2015. Development and Evolution of Dentition Pattern and Tooth Order in the Skates And Rays (Batoidea; Chondrichthyes). PLoS ONE 10(4): e0122553. doi:10.1371/journal.pone.0122553

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Pelvic claspers also appear, by convergence, in placoderms.

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

Steven E Campana Lab webpage:

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

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

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

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

The nurse shark, Ginglymostoma cirratum, enters the LRT

The nurse shark,
Ginglymostoma (Figs. 1, 2) enters the large reptile tree (LRT, 1771+ taxa) as a sister to Tristychius (Figs. 3, 4) from the Early Carboniferous.

Figure 1. The nurse shark, Ginglymostoma, in vivo.

Figure 1. The nurse shark, Ginglymostoma, in vivo.

Figure 2. The skull of Ginglymostoma in two views. The gill basket and pectoral fins are shown in dorsal view only. Tetrapod homologs are added as colors.

Figure 2. The skull of Ginglymostoma in two views. The gill basket and pectoral fins are shown in dorsal view only. Tetrapod homologs are added as colors.

Ginglymostoma cirrum (Bonaterre 1788; Müller and Henle 1837) is the extant nurse shark, a lethargic, bottom dweller. According to Wikipedia, Members of this genus have the ability to suck in water in order to remove snails from their shells in a manner that can be described as ‘vacuum-like’. The anterior displacement of the anterior gill bars (= labial cartilages, Fig. 2, red color) helps provide a circular form for the suction tube.

Note the unique (at present) posterior placement of the orbit surrounded by circumorbital cartilage relative to the ‘lacrimal’ (= palatoquadrate, tan color). This is convergent with a similar shift seen in Hybodus basanus and bony fish. The large lateral process of the hyomandibular (Fig. 2, dark green color) is not seen in other tested sharks. Tabulars are missing as in Tristychius.

Figure 1. Tristychius, a basal shark from the Early Carboniferous,

Figure 1. Tristychius, a basal shark from the Early Carboniferous, Early this skeleton was compared erroneously to Squatina and other rays.

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

Figure 4. CT scans of Tristychius skull from Coates et al. 2019.

Tristychius arcuatus (Agassiz 1837; Early Carboniferous; 60cm est.) was a small relative of Ginglymostoma with a short torso, large pectoral and pelvic fins and large dorsal spines. It is not related to skates or rays.

The nares pointed anteriorly. Teeth are nearly absent with only a few in the anterior dentary. The postorbital is strongly developed here. Tabulars are absent. Note the low position of the gill slits. Note the large anterior gill bars (= labial cartilages in red) that restrict jaw depression and create lateral walls for the open jaws.

Coates et al. 2019 described this specimen with regard to suction feeding 50 million years before the bony fish equivalent. The authors also report, “The labial cartilages are large and comparable to examples known in Mesozoic hybodontids (8) and modern suction feeding elasmobranchs such as nurse sharks (genus Ginglymostoma).”

Developmental note:
A kind reader (CB) commented on a previous post, (in short): “Most of the bones you’re trying to identify on shark chondrocrania are dermal bones. That means they don’t pre-form in cartilage. Which means animals without a bony skull cannot have them.”

I replied, (in short), “It is axiomatic that sharks cannot stand alone. There must be a relationship to other vertebrates. The renaming of skull topologies along the lines of tetrapod homologies has opened a door that had previously been closed due to a separate naming system that I and others have suggested unifying. The LRT shows that vertebrate skulls _alone_ took a detour after paddlefish, then returned to bony skull elements with sutures with hybodontids and their descendants. I don’t know the mechanism for this. The LRT provides the map for future scholars.”

Click here for the complete set of comments and replies.

Seasonal note: 
Another blogpost has finally overtaken ‘the origin of bats‘ as the most popular one over the last few weeks. The new most popular post is the one that deals with ancient astronomy, rather than paleontology. Evidently there is either some interest in ‘A Christmas Story‘, or the web page comes up whenever someone is looking for the movie with the same name.


References
Agassiz L 1837. Recherches Sur Les Poissons Fossiles. Tome III (livr. 8-9). Imprimérie de Petitpierre, Neuchatel viii-72
Bonaterre PJ 1788. Tableau encyclopédique et méthodique des trois règnes de la nature …, Ichthyologie. Panckoucke, Paris 1788.
Coates MI, Tletjen K, Olsen AM and Finarelli JA 2019. High performance suction feeding in an early elasmobranch. Science Advances 2019:5: eaax2742.
Motta PJ and Wilga CD 1999. Anatomy of the Feeding Apparatus of the Nurse Shark, Ginglymostoma cirratum. Journal of Morphology 24:33–60.

wiki/Tristychius
wiki/Ginglymostoma
wiki/nurse shark

Ferromirum, a little sister to the mysterious giant shark, Megachasma

Frey et al. 2020 brought us
a new, small, Late Devonian shark with large eyes. Ferromirum (Fig. 1). In the large reptile tree (LRT, 1770+ taxa, subset Fig. 4) Ferromirum has a long, lost living relative, the megamouth shark, Megachasma (Figs. 2, 3), a taxon overlooked by Frey et al.

Figure 1. Ferromirum a Late Devonian shark in several views from Frey et al. 2020 and colorized here.

Figure 1. Ferromirum a Late Devonian shark in several views from Frey et al. 2020 and colorized here.

While Ferromirum may look toothless
so does Megachasma (Figs. 2, 3) at the same scale. Rows of tiny sharp teeth lined  like cemetery headstones filled the jaws of both taxa.

From the Frey et al. abstract:
“The Palaeozoic record of chondrichthyans (sharks, rays, chimaeras, extinct relatives) and thus our knowledge of their anatomy and functional morphology is poor because of their predominantly cartilaginous skeletons.”

Not that poor in 2020. And we have living specimens to dissect. The taxon exclusion problem experienced by Frey et al. is of their own doing.

“Here, we report a previously undescribed symmoriiform shark, Ferromirum oukherbouchi, from the Late Devonian of the Anti-Atlas. Computed tomography scanning reveals the undeformed shape of the jaws and hyoid arch, which are of a kind often used to represent primitive conditions for jawed vertebrates.

Not true. Taxon exclusion is the problem here. That led the Frey et al. team to a misunderstanding of shark origins (Fig. 4) in which they made the placoderm, Entelognathus, the outgroup and the bony fishes Guiyu, Climatius, Acanthodes and Doliodus nested basal to sharks. In the LRT these taxa are all derived from sharks. Essentially the cladogram in Frey et al. is upside down. See below for details.

Figure 1. Megachasma in vivo. Note the single cusp teeth.

Figure 2. Megachasma in vivo. Note the single cusp teeth.

Except for size, Megachasma provides many clues
to the in vivo appearance and habits of Ferromirum due to its close nesting and phylogenetic bracketing.

Figure 3. Megachasma skull.

Figure 3. Megachasma skull.

The Frey et al. abstract continues:
“Of critical importance, these closely fitting cartilages preclude the repeatedly hypothesized presence of a complete gill between mandibular and hyoid arches. We show that the jaw articulation is specialized and drives mandibular rotation outward when the mouth opens, and inward upon closure.”

As in the megamouth shark, “so unlike any other type of shark that it is usually considered to be the sole extant species in the distinct family.” according to Wikipedia, which also suffers from taxon exclusion.

“The resultant eversion and inversion of the lower dentition presents a greater number of teeth to prey through the bite-cycle. This suggests an increased functional and ecomorphological disparity among chondrichthyans preceding and surviving the end-Devonian extinctions.”

It’s good to know the megamouth shark is no longer alone.

Figure x. Subset of the LRT focusing on sharks.

Figure 4. Subset of the LRT focusing on sharks.

Basically the Frey et al. cladogram is upside down
due to taxon exclusion. In addition to the issues listed above, the authors nest the dogfish shark, Squalus, at a highly derived node. The LRT (Fig. 4) nests it close to the base. The authors nest Ferromirum, Cladoselache, Ozarcus, and Akmonistion within the Holocephali (chimaeras). The LRT nests these taxa with sharks. The last two nest as proximal outgroups to the derived clade Osteichthyes (bony fish), which is a clade basal to sharks in Frey et al. More taxa from a wider gamut resolves all such issues. Don’t rely on the work of others or tradition. Find all this out for yourself.


References
Frey L, Coates MI, Tietjen K, Rücklin M and Klug C 2020. A symmoriiform from the Late Devonian of Morocco demonstrates a derived jaw function in ancient chondrichthyans. Nature Communications Biology 3:681 | https://doi.org/10.1038/s42003-020-01394-2 | http://www.nature.com/commsbio

The LRT confirms a 2015 paper: sharks had a semi-bony past

Long et al. 2015
described a few bits and pieces from a Late Devonian shark, Gogoselachus (Fig 1), with a “highly distinctive type of calcified cartilage forming the endoskeleton.”

I came across this 2015 paper a few days ago, long after sharks nested as taxa derived from cartilaginous sturgeon and semi-bony paddlefish ancestors in the Large Reptile Tree (LRT, 1744+ taxa, subset Fig. 2). So having a basal shark with ‘calcified cartilage’ comes as no surprise to the LRT.

Figure 1. Gogoselache restoration based on the few parts shown here.

Figure 1. Gogoselache restoration based on the few parts shown here from Long et al. 2015.

From the Long et al. ‘Background’
“Living gnathostomes (jawed vertebrates) comprise two divisions, Chondrichthyes (cartilaginous fishes, including euchondrichthyans with prismatic calcified cartilage, and extinct stem chondrichthyans) and Osteichthyes (bony fishes including tetrapods).”

This is the traditional view of fish interrelations, the one told in lectures and textbooks. See figure 2 for the more complete LRT view.

“Most of the early chondrichthyan (‘shark’) record is based upon isolated teeth, spines, and scales, with the oldest articulated sharks that exhibit major diagnostic characters of the group—prismatic calcified cartilage and pelvic claspers in males—being from the latest Devonian, c. 360 Mya. This paucity of information about early chondrichthyan anatomy is mainly due to their lack of endoskeletal bone and consequent low preservation potential.”

Figure x. Subset of the LRT focusing on fish.

Figure 2. Subset of the LRT focusing on fish.

From the Long et al. 2015 Methodology/Principal Findings
“Here we present new data from the first well-preserved chondrichthyan fossil from the early Late Devonian (ca. 380–384 Mya) Gogo Formation Lägerstatte of Western Australia. The specimen is the first Devonian shark body fossil to be acid-prepared, revealing the endoskeletal elements as three-dimensional undistorted units: Meckel’s cartilages, nasal, ceratohyal, basibranchial and possible epibranchial cartilages, plus left and right scapulocoracoids, as well as teeth and scales. This unique specimen is assigned to Gogoselachus lynnbeazleyae n. gen. n. sp.”

From the Long et al. 2015 Conclusions/Significance
“The Meckel’s cartilages show a jaw articulation surface dominated by an expansive cotylus, and a small mandibular knob, an unusual condition for chondrichthyans.”

In the LRT the Meckel’s cartilages are scored as fused mandible bones based on ancestral states in which the mandible bones are not fused.

“The scapulocoracoid of the new specimen shows evidence of two pectoral fin basal articulation facets, differing from the standard condition for early gnathostomes which have either one or three articulations.”

“The tooth structure is intermediate between the ‘primitive’ ctenacanthiform and symmoriiform condition, and more derived forms with a euselachian-type base.”

Of special interest is the highly distinctive type of calcified cartilage forming the endoskeleton, comprising multiple layers of nonprismatic subpolygonal tesserae separated by a cellular matrix, interpreted as a transitional step toward the tessellated prismatic calcified cartilage that is recognized as the main diagnostic character of the chondrichthyans.”

In the LRT phylogenetic analysis, rather than a few interesting and traditionally unexpected traits, elucidates interrelationships. Having bony traces in a basal shark skeleton could have been predicted from the LRT, but the fish portion of the LRT came later, this time providing confirmation rather than prediction.

The Long et al. 2015 paper
earned a fair bit of publicity five years ago.

From Phys.org
“Fossil ancestor shows sharks have a bony past.
This study further supports the idea that sharks must have evolved from bony primitive ancestors and lost their bone early on in the race as they acquired their predatory body shape.”

From: theconversation.com
“No bones about it. Sharks evolved cartilage for a reason.
Most people know that sharks have a distinctive, all-cartilage skeleton, but now a fossil from Western Australia has revealed a surprise ‘missing link’ to an earlier, more bony form of the fish.”

“In testing fossil remains discovered by Professor Long in July 2005 at Gogo in the Kimberley in Western Australia, detailed CT scanning analysis has shown that the three-dimensional remnant skeleton contains a small proportion of bone as well as cartilage.”

“Because sharks and rays have entirely cartilaginous skeletons, Professor Long said it was traditionally thought that they were part of a primitive evolutionary pathway, and that bone in other fish was the more advanced condition. But a series of discoveries in recent years has suggested that sharks are “more evolutionarily derived”, and are likely to be descended from bony ancestors.”


References
Long JA, Burrow CJ, Ginter M, Maisey JG, Trinajstic KM, et al. 2015. First Shark from the Late Devonian (Frasnian) Gogo Formation, Western Australia Sheds New Light on the Development of Tessellated Calcified Cartilage.” PLoS ONE10(5): e0126066. DOI: 10.1371/journal.pone.0126066

Early Silurian Sinacanthus compared to Early Cretaceous Bonnerichthys

Zhu 1998 brought us a peek at Early Silurian vertebrates
represented by distinctive Sinacanthus fin-spines (Fig. 1). These were variously considered acanthodian-like and shark-like.

Zhu argues
“Sinacanths are one of the oldest known chondrichthyans (sharks + ratfish) rather than acanthodians, and their spines are the oldest known shark fin spines.”

Figure 1. Sinacanthus fin spine.

Figure 1. Sinacanthus fin spine. Scale unknown.

No comparable fin spines are currently
documented among the sharks and their kin at ReptileEvolution.com. So I looked at other taxa. After seeing a comparable fin spine that belonged to the Late Cretaceous bony fish, Bonnerichthys (Figs. 2, 3), I wondered if Sinacanthus was similar enough? This time I’ll let you decide because…

Figure 2. Bonnerichthys pectoral fin for comparison.

Figure 2. Bonnerichthys pectoral fin for comparison.

…there is no way
the large reptile tree (LRT, 1720+ taxa) can nest this fin alone given its present set of characters, none of which lump and split fin details.

According to (P’an 1959, 1964)
Sinacanthus wuchangensis (MG.V103a) and its relatives have fins with 15 to 50 ridges per side. Acanthodians have fewer ridges generally, which is why Zhu et al. preferred allying those fins with elasmobranchs rather than acanthodians.

Zhu et al. present a diagnosis
“Sinacanths with long and slender fin spines; spine gradually tapering, recurved posteriorly and dagger-shaped.” 

Zhu documented the histology of sinacanths.
Given today’s observations, perhaps the histology of Bonnerichthys will be worth looking into and comparing it to Sinacanthus. There’s another project for a grad student!

Zhu goes on to say in their Remarks:
“Since the phylogeny of early elasmobranchs remains obscure, the diagnosis given above is more descriptive than phylogenetic.”

Actually the phylogeny of early elasmobranchs is clear in the LRT. Nevertheless, nothing quite like this fin spine has shown up yet in tested elasmobranchs,

Chronology
in the LRT Late Cretaceous Bonnerichthys phylogenetically precedes some derived Late Silurian taxa (e.g. Entelognathus, Qilinyu, Romundina and Guiyu). So maybe this chronology jump is another opportunity to explore for that grad student.

Figure 1. Bonnerichthys parts from Friedman et al. 2010 and colorized here.

Figure 3. Late Cretaceous Bonnerichthys parts from Friedman et al. 2010 and colorized here.

Apparently the earliest radiation of fish clades
occurred during the Ordovician. Unfortunately only a few ‘armored lancelet’ fossils, like Arandaspis, have been found in Ordovician strata so far. The LRT indicates there are more fish and more derived fish waiting to be found in that early stratum.

Figure x. Subset of the LRT focusing on fish.

Figure x. Subset of the LRT focusing on fish. Spiny sharks are related to osteoglossiformes here.

A little housekeeping note…
the skull of Pachycormus (Figs. 4, 5) has been reviewed, overhauled and this taxon is now happier in its new home (Fig. x) with Bonnerichthys, the extant arowana, Osteoglossum (Fig. 6) and two extinct pseudo-swordfish, Protosphyraena and Aspidorhynchus.

Figure 1. Pachycormus fossil. Pelvic fins vestigial near vestigial anal fin. See figure 2 for closeup of the skull.

Figure 4. Pachycormus fossil. Vestigial pelvic fins seem to appear near the vestigial anal fin. See figure 2 for closeup of the skull. Note the extra set of spines medial to the spiny pectoral fins.

Still wondering if
those slender curved spines medial to the pectoral fins are pelvic fins or new structures? Normal pelvic fins are absent, but a long set of dorsal ribs suggests the anus is still just anterior to the anal fin, as in all related taxa. Osteoglossum (Fig. 6) seems to have posterior pelvic fins, but the skeleton does not show them. So the situation is confusing at present.

Figure 8. Pachycormus macropterus has a new skull reconstruction. Originally I did this without template or guidance. Now osteoglossiformes provide a good blueprint.

Figure 5. Pachycormus macropterus has a new skull tracing and reconstruction. Originally I did this without template or guidance. Tinkering. Now osteoglossiformes provide a good blueprint.

These osteoglossiformes/pachcormiformes
arise from spiny sharks, a novel hypothesis of phylogenetic relationships recovered by the LRT earlier.

Figure 1. The arowana, an Amazon River predator, nests with Late Jurassic Dapedium in the LRT.

Figure 6. The arowana (Osteoglossum) an Amazon River predator has posterior pelvic fins and no mid pectoral fins in life, but the skeleton does not show that. Confusing.

This fish phylogeny recovered by the LRT,
including certain taxa not traditionally included with certain other taxa (Fig. x), is a novel hypothesis of interrelationships awaiting confirmation from an independent study with another character list, but a similar taxon list.


References
P’an K 1959. [Devonian fish fossils of China and their stratigraphic and geographic distributions.] Monographic summary of basic data on Chinese geology 1:1–13 [in Chinese].
Zhu M 1998. Early Silurian sinacanths (Chondrichthyes) from China. Palaeontology 41(1):157–171.

The goblin shark (Mitsukurina) enters the LRT

Revised December 7, 2020
with a new nesting of Mitsukurina closer to paddlefish than to guitarfish (Fig. x) despite the presence of gill slits on Mitsukurina and gill covers on paddlefish. That would be “Pulling a Larry Martin” as all other traits support the new relationship.

Figure 4. Subset of the LRT focusing on sharks.

Figure x. Subset of the LRT focusing on sharks.

 

Often hailed as ‘the most bizarre shark’,
the goblin shark, Mitsukurina owstoni (Jordan 1898; Figs. 1, 2) nests with the paddlefish  (Polyodon) in the large reptile tree (LRT, 1710+ taxa then, 1772 taxa now).

Figure 1. The skull of the goblin shark, Mitsukurina.

Figure 1. The skull of the goblin shark, Mitsukurina. Red arrow points to naris. The extended nasal region is full of ampullae that sense electrical activity produced by prey twitching muscles while hiding in the sea floor.

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

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

Mitsukurina owstoni (Jordan 1898; 3-4+m long) is the extant goblin shark, a dead end form close to paddlefish, converging on sharks with regard to the gill slits. This is a sluggish swimmer feeding on sea floor prey, sensing their electrical fields, snatching them with protrusible jaws, (as in paddlefish).

FIgure 3. Scapanorhynchus, and Early Cretacous goblin shark.

FIgure 3. Scapanorhynchus, and Early Cretacous goblin shark.

Scapanorhynchus lewisii (Davis 1887; 65cm to 3m long; Early Cretaceous) is a fossil goblin shark.

Figure 2. Rhinobatos, the guitarfish, and Rhina the bowhead guitarfish, are transitional to skates and rays, but not mantas. Note the ventral mouth and pectoral fins extending anterior to the orbits.

Figure 4. Rhinobatos, the guitarfish, and Rhina the bowhead guitarfish, are transitional to skates and rays, but not mantas. Note the ventral mouth and pectoral fins extending anterior to the orbits.

Rhinobatos rhinobatos (Linneaus 1758; up to 1.47m) is the extant common guitarfish, a transitional taxon between sharks and skates. The mouth and gills are below the pectoral fins that extend forward anterior to the eyes, which rise to the top of the skull.

Rhina anclystoma (Bloch and Schneider 1801) is a type of guitarfish. Still waiting for skull data to score this fish.


References
Bloch MC and Schneider JG 1891. Systema anclystoma.
|Davis JW 1887. The fossil fishes of the chalk of Mount Lebanon, in Syria. Scientific Transactions of the Royal Dublin Society, 2 (3): 457–636, pl. 14–38.
Jordan DS 1898. Description of a species of fish (Mitsukurina owstoni) from Japan, the type of a distinct family of lamnoid sharks. Proceedings of the California Academy of Sciences, Zoology. Series 3. 1 (6):199–204.
Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.

wiki/Common_guitarfish
wiki/Mitsukurina
wiki/Goblin_shark
wiki/Scapanorhynchus