Isistius: the cookie cutter shark, enters the LRT, as a lamprey mimic

The latest addition to the LRT,
Isistius brasiliensis, the extant cookie-cutter shark (Figs. 1–3), was a puzzle to try to figure out, given that the entire skull turned to cartilage and can only be figured out by looking at the bumps and valleys, along with the overall proportions.

To no one’s surprise,
Isistius
nests in the LRT with other small cigar-shaped sharks with large eyeballs, including Carboniferous Falcatus.

Figure 1. Isistius, the cookie cutter shark animated.

Figure 1. Isistius, the cookie cutter shark animated.

Perhaps more importantly,
this taxon addition sparked another look at all the sharks and rays, based on phylogenetic bracketing

Figure 2. Isistius brasiliensis in several views.

Figure 2. Isistius brasiliensis in several views.

But that can only take place
in a valid phylogenetic context, so the shark subset of the large reptile tree (LRT, 1770 taxa) itself came under scrutiny. You might recall, I knew nothing about fish anatomy a year ago when I started posting sharks to the LRT trying to use tetrapod homologs for skull bones. Tinkering remains a good way to learn and not be hamstrung by outdated traditions sometimes found in textbooks.

Figure 3. Isistius skull. Note all the former bones are now fused together into a precise lump of cartilage.

Figure 3. Isistius skull. Note all the former bones are now fused together into a precise lump of cartilage.

Here are the phylogenetic results
subject to further improvements.

Figure x. Subset of the LRT focusing on sharks.

Figure x. Subset of the LRT focusing on sharks.

Isistius brasiliensis
(Quoy and Gaimard 1824) is the extant cookiecutter shark, a living sister to Falcatus. This deep-water shark has light-emiting photophores covering its underside. It migrates to the surface every day to take a circular bite out of giant whales and sharks. In this way it can be seen as a sort of lamprey-mimic. Isistius also consumes smaller free-swimming prey, like squid. Note the anterior nostrils and larger dentary teeth.


References
Quoy JRC and Gaimard JP 1824–1825. des Poissons. Chapter IX”. In de Freycinet, L (ed.). Voyage autour du Monde…exécuté sur les corvettes de L. M. “L’Uranie” et “La Physicienne, pendant les années 1817, 1818, 1819 et 1820. Paris 192–401.

wiki/Isistius

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.