Deep-sea Daliatias enters the LRT with rows of big, sharp teeth and giant labial cartilages

Daliatias licha
(Bonnaterre 1788, Rafinesque 1810; 1 to 1.8ml Fig. 1) is the extant kite fin shark. This slow-moving sea floor skimmer is the largest luminous vertebrate.

Here,
in the large reptile tree (1839+ taxa, subset Fig. 3), this solitary predator nests with the cookie cutter shark, Isistius (Fig. 2), another deep sea luminous vertebrate.

The jaws of the kitefin shark
do not extend to produce suction as in the cookie-cutter shark. In the kitefin shark the gill openings extend both above and below the pectoral fin (Fig. 1). The large labial cartilages restrict jaw depression and create lateral walls for the open jaws.

Figure 1. Daliatias overall and in detail.

Given the phylogenetic nesting of
Isistius and Daliatias, (Fig. 4) these taxa are surprisingly close to the lineages of sharks that gave rise to bony fish and thereafter to tetrapods and ultimately humans. Since those actual bony fish ancestors would have lived during the Silurian, we can safely assume that Silurian ancestors of Isistius and Daliatias were more plesiomorphic in appearance with smaller teeth and less specialized jaws.

Figure 2. Isistius brasiliensis in several views.
Figure 2. Isistius brasiliensis in several views.

Isistius brasiliensis
(Quoy and Gaimard 1824; Fig. 2) is the extant cookiecutter shark, a living sister to Daliastias (Fig. 1). 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 larger vertebrates, like 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.

Figure x. The mako shark, Isurus, had little to no labial cartilages.
Figure 3. The mako shark, Isurus, had little to no labial cartilages. It nests basal to the kitefin and cookie cutter sharks in the LRT, skipping the long rostrum taxa of sawfish, guitarfish, etc.

The mako shark,
Isurus (Fig. 3), is ancestral to the kitefin and cookie cutter sharks. The teeth are smaller, the rostrum is longer, the fins are all larger in this open sea fast predator, distinct from its more plesiomorphic and currently unknown Silurian ancestors resembling the nurse shark, Ginglymostoma, which is among the most basal vertebrates with marginal teeth.

Figure 4. Subset of the LRT focusing on cartilaginous fish. There are several changes here from prior tree topologies, but sharks still arise from sturgeons and give rise to bony fish. So this is a grade, not a monophyletic clade.

The present subset of the LRT
(Fig. 4) represents changes made over the weekend after a continuing review of sharks and other basal chordates. Readers are watching this experiment in real time, with changes appearing whenever new light is shed on hypothetical interrelationships. Thank you for your patience and understanding. I’m learning as I go.

References
Bonaterre PJ 1788. Tableau encyclopédique et méthodique des trois regnes de la nature.
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.
Rafinesque CS 1810. Caratteri di alcuni nuovi generi e nuove specie di animali e piante della sicilia, con varie osservazioni sopra i medisimi. Per le stampe di Sanfilippo: Palermo, Italy. pp. 105, 20 fold. Pl., online

wiki/Isistius
wiki/Kitefin_shark

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