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.
Keys to understanding this issue include:
- The elements of the dermocranium in shark outgroup taxa (sturgeon and paddlefish) = bone sheath over cartilage.
- The elements of the dermocranium in sharks = prismatic cartilage, more ossified in hybodonts
- 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).
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.
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.”
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.
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:
- “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)
- “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)
- “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).
- “the biogeography of Asian and North American polyodontids and scaphirhynchines;
- “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:
- Cartilaginous endoskeleton – as in sharks and fish more primitive than sharks
- Lack of vertebral centrum – as in fish more primitive than sharks
- Spiral valve intestine – as in sharks, bichirs, gars and lungfish, the last two by reversals.
- 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.
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.
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.
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.