Cetorhinus maximus, the giant, filter-feeding, basking shark, enters the LRT

Revised December 27, 2020
with new light shed by the relationship of Cetorhinus with paddlefish, particularly the short-nosed hatchlings of of paddlefish (Polyodon).

Cetorhinus maximus
(originally Squalus maximus, Gunnerus 1765; Figs. 1, 2) is the extant basking shark, the second largest fish in today’s oceans.

FiIgure 1. The basking shark, Cetorhinus maximus, in lateral and ventral views.

Figure 1. The basking shark, Cetorhinus maximus, in lateral and ventral views.

The basking shark is distinct from the largest fish in the sea,
the unrelated whale shark (Rhincodon typus). In the large reptile tree (LRT, 1768+ taxa), Cetorhinus nests with Polyodonthe paddlefish (Figs. 4, 5).Figure 2. Skull of Cetorhinus adult and juvenile showing differences in the rostrum and fusion of skull elements in the adult.

Figure 2. Skull of Cetorhinus adult and juvenile showing differences in the rostrum and fusion of skull elements in the adult.

Figure 7. Polyodon hatchling. Compare to the Polyodon adult (figure 8) and Cetorhinus (figure 9).

Figure 3. Polyodon hatchling. Compare to the Polyodon adult (figure 8) and Cetorhinus (figure 9).

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.

Slow-moving, filter feeding
basking sharks regularly reach 7–8m in length. Marginal teeth are tiny and essentially useless. So is the basking shark’s brain. Gill slits nearly encircle the head, doubling as plankton collectors. They balloon out when the mouth is open, straining great quantities of water for tiny prey items.

The pineal foramen is anterior to the raised frontal (light blue), as in related taxa.

Former skull bones are here fused together and turned to cartilage, more so in the adult where the deteriorated naris capsule is confluent with the orbit.

Developing eggs remain in the mother for over a year. Up to six young may be carried at a time. Sexual maturity and lifespan are similar to humans.

As a juvenile the protruding nasals curl down in front of the mouth and rise with maturity. The rest of the skull also changes shape with maturity.

As an adult the basking shark has few enemies other than skin parasites, including lampreys and cookie-cutter sharks (which we’ll learn about soon).

Figure 6. Adding Debeerius to the LRT helped revise the shark-subset.

Figure x. Adding Debeerius to the LRT helped revise the shark-subset.

Sharp-eyed readers will note
changes in the shark subset of the LRT (Fig. x). Hybodus remains the proximal outgroup taxon to bony fish. The mako shark, Isurus, moves closer to the origin of sharks and closer to the shark-like toothless taxon, Chondrosteus.


References
Gunnerus JE 1765. Brugden (Squalus maximus), Beskrvenen ved J. E. Gunnerus. Det Trondhiemske Selskabs Skrifter, 3: 33–49, pl. 2
Izawa K and Shibata T 1993. A young basking shark, Cetorhinus maximus, from Japan. Japanese Journal of Ichthyology, 40 (2): 237-245, figs 1-4).
Matthews. L. H. and H. W. Parker 1950. Notes on the anatomy and biology of the basking shark (Cetorhinus maximus (Gunner)). Proc. Zool. Soc. Lond., 120: 535- 576. pls. 1-8.
Pavesi, P. 1874. Contribusiine alia storia naturale del genere Selache. Ann. Mus. Stor. Nat. Genova, 6: 5-72, pls. 1-3.

shark-references.com/species/view/Cetorhinus-maximus
shark-references.com/post/526
wiki/Basking_shark

3 thoughts on “Cetorhinus maximus, the giant, filter-feeding, basking shark, enters the LRT

  1. The skeleton figured by Pavesi (1874) was not an adult, but a 2.95 m juvenile. He also illustrated the same individual in vivo, which has a quite different head shape from your silhouette.
    https://www.biodiversitylibrary.org/item/40858#page/10/mode/1up

    The form of the rostrum in juveniles seems to vary by individual, evidenced by the other specimen which had a very curved rostrum yet was similar in length to Pavesi’s (2.6 m).
    https://www.jstage.jst.go.jp/article/jji1950/40/2/40_2_237/_article

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