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.
Perhaps more importantly,
this taxon addition sparked another look at all the sharks and rays, based on phylogenetic bracketing
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 Tinkering remains a good way to learn and not be hamstrung by outdated traditions sometimes found in textbooks.
Here are the phylogenetic results
subject to further improvements.
(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.
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.