Chondrosteus animated

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

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

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

Figure 3. Chondrosteus animation (2 frames) in situ and reconstructed in lateral view. This is the transitional taxon linking sturgeons to bony fish + sharks.

Figure 3. Chondrosteus animation (2 frames) in situ and reconstructed in lateral view. This is the transitional taxon linking sturgeons to bony fish + sharks.

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

Figure x. Newly revised fish subset of the LRT

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

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

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

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

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

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

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


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

wiki/Chondrosteus

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