Why sharks have no bones: Borrell 2014

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

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

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

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

“Although scientists knew what genes were involved in bone formation, it wasn’t clear whether sharks had lost their bone-forming ability or just never had it in the first place. After all, sharks do make bone in their teeth and fin spines.”

“The sequence reveals that members of this group are missing a single gene family that regulates the process of turning cartilage into bone, and that a gene duplication event gave rise to the transformation in bony vertebrates.”

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

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

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

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

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

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

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

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

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

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

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


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

wiki/Australian_ghostshark
austhrutime.com/acanthodians.htm

The sawfish, Pristis, enters the LRT

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Figure 4. Subset of the LRT focusing on sharks.

Figure 4. Subset of the LRT focusing on sharks.

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


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

wiki/Pristis
wiki/Sawfish

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

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

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

Backstory
Several recent reader comments disputed and/or cast doubt on the identity of shark skull bones (Fig. 2) and the shark-to-bony fish transition recovered by the large reptile tree (LRT, 1771+ taxa, see Fig. 1 diagram). Objections were  based on developmental grounds. One reader (CB) wrote: Most of the bones you’re trying to identify on shark chondrocrania are dermal bones. That means they don’t pre-form in cartilage. Which means animals without a bony skull cannot have them.”

That is the traditional view found in current textbooks.

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

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

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

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

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

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

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

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

Keys to understanding this issue include:

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

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

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

Pehrson 1940 examined
a series of embryonic stages of the extant bowfin, Amia calva (Fig. 3), one of the most primitive bony fish in the LRT. Pehrson 1940 reports: “Three different stages of the formation of the premaxillary are shown. The anterior, dental part of the bone is clearly distinguishable from the posterior and dorsal part, situated above the cartilage.”

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

Figure x. Embryo development in the bowfin, Amia. The facial bones develop as buds surrounding dermal sensory organs 'floating' on top of a cartilage base.

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

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

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

Surprisingly,
Pehrson was keen on naming fish bones in accord with those of pre-tetrapods. He reports, “There seems to be no doubt that the intertemporal and supratemporal parts of the developing composite bone correspond to the similarly named bones in Osteolepidae and Rhizodontidae.” Not sure if Pehrson was the first to do this, but it should be standard.

Supporting evidence that sturgeons are shark ancestors:
According to Wikipedia, notable characteristics of Acipenseriformes include:

  1. Cartilaginous endoskeleton – as in sharks and fish more primitive than sharks
  2. Lack of vertebral centrum – as in fish more primitive than sharks
  3. Spiral valve intestine – as in sharks, bichirs, gars and lungfish, the last two by reversals.
  4. Conus arteriosus = infundibulum, a conical pouch found in the heart from which the pulmonary trunk artery arises (not sure how this relates, but there it is).

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

Indeed. The LRT comes to the same conclusion.

“Acipenseriforms also are noteworthy because of their unusual mixture of characters, which caused early debate about their classification. Two aspects of living Acipenseriformes were especially problematic for early ichthyologists: (1) reduced ossification of the endoskeleton combined with presence of an extensive dermal skeleton; and (2) the presence of a hyostylic jaw suspension and protrusible palatoquadrate recalling the jaws of sharks.”

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

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

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

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

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

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


References
Bemis WE, Findeis EK and Grande L 1997. An overview of Acipenseriformes. Environmental Biology of Fishes 48: 25–71, 1997.
Gillis JA 2019. ‘Secondary’ cartilage and the vertebrate dermal skeleton in Reference Module in Life Sciences.
Hall BK 2005. Bones and Cartilage. Academic Press, London. ISBN: 978-0-12-319060-4
Maisey JG 1983. Cranial anatomy of Hybodus basanus Egerton from the Lower Cretaceous of England. American Museum Novitates 2758:1–64.
Maisey JG 1987. Cranial Anatomy of the Lower Jurassic Shark Hybodus reticulatus
(Chondrichthyes: Elasmobranchii), with Comments on Hybodontid Systematics. American Museum Novitates 2878: 1–39.
Pehrson GT 1940. The development of dermal bones in the skull of Amia calva. Acta Zoologica 21:1–50.
Wagner DO and Aspenberg P 2011. Where did bone come from? An overview of its evolution. Acta Orthopaedica. 82(4):393–398.
The Skull, Volume 1. Eds. Hanken J and Hall BK University of Chicago Press Books, 1993.

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

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