Revised December 27, 2020
with new light shed by the relationship of Cetorhinus with paddlefish, particularly the short-nosed hatchlings of of paddlefish (Polyodon).
(originally Squalus maximus, Gunnerus 1765; Figs. 1, 2) is the extant basking shark, the second largest fish in today’s oceans.
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 Polyodon, the 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.
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).
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.
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.