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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Just the opposite, according to the LRT.

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

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

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

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

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

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

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

Just the opposite, according to the LRT.

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

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

Figure x. Shark skull evolution.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

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