Chlamydoselachus and the loss of gill slit sets

The frilled shark
(genus: Chlamydoselachus anguineus, Garman 1884; 2m in length; Figs. 1–3) enters the large reptile tree at the base of sturgeons + sharks AND at the base of all bony fish and stem tetrapods, like Osteolepis (Fig. 3).

Also noteworthy…
Chlamydoselachus is derived from the placoderm Entelognathus with no taxa between them. And that makes the frill neck yet another taxon close to our own lineage of descent. Apparently this is a novel hypothesis of relationships. I can’t find it online. If anyone can find something similar in the literature, please let me know and we can call this a confirmation, rather than a discovery.

Fossil and living frill sharks
are known from the Late Cretaceous to recent times, but had their genesis no later than the Late Silurian, following Entelognathus and prior to Guiyu and Psarolepis.

So, once again,
we get to see, feel, interact with and dissect a living ancestor from deep time.

That carpet of teeth on the palate of Chlamydoselachus
(Fig. 1) is probably a more recent acquisition not present on the original frilled shark.

A variety of gill openings
Anterior to the standard shark-like five pairs of gill slits, the frill neck has a sixth pair that meet one another at the base of the throat (Fig. 2), similar to those in sturgeons and other bony fish (by convergence in the LRT). The front pair disappear in later traditional sharks. The back five pair disappear, or get covered by the operculum, in sturgeons and other bony fish.

Distinct from other sharks,
but like bony fish other than sturgeons, the bony mandible of Chlamydoselachus is not underslung, but extends as long as the bony rostrum (Fig. 1).

According to Wikipedia. “This species is aplacental viviparous: the embryos emerge from their egg capsules inside the mother’s uterus, where they survive primarily on yolk. The gestation period may be as long as three and a half years, the longest of any vertebrate. Litter sizes vary from two to fifteen, and there is no distinct breeding season.”

Taxonomy according to Wikipedia, “Several early authors believed the frilled shark to be a living representative of otherwise long-extinct groups of elasmobranchs (sharks, rays, and their ancestors), based on its multiple-pointed teeth, the articulation of its upper jaw directly to the cranium behind the eyes (called “amphistyly”), and its “notochord-like” spinal column with indistinct vertebrae. Garman proposed that it was allied with the cladodonts, a now-obsolete taxonomic grouping containing forms that thrived during the Palaeozoic era, such as Cladoselache from the Devonian period (416–359 Mya). His contemporaries Theodore Gill and Edward Drinker Cope suggested it was instead related to the hybodonts, which were the dominant sharks during the Mesozoic era. Cope went as far as to assign this species to the fossil genus Didymodus.

“More recent investigations have found the similarities between the frilled shark and extinct groups may have been overstated or misinterpreted, and this shark exhibits a number of skeletal and muscular traits that firmly place it with the neoselachians (modern sharks and rays), and more specifically with the cow sharks in the order Hexanchiformes.”

“The head is broad and flattened with a short, rounded snout. The nostrils are vertical slits, separated into incurrent and excurrent openings by a leading flap of skin. The moderately large eyes are horizontally oval and lack nictitating membranes (protective third eyelids).”

“The pectoral fins are short and rounded. The single, small dorsal fin is positioned far back on the body, about opposite the anal fin, and has a rounded margin. The pelvic and anal fins are large, broad, and rounded, and also positioned well back on the body. The caudal fin is very long and roughly triangular, without a lower lobe or a ventral notch on the upper lobe.”

“The dermal denticles are small and shaped like the tip of a chisel, becoming enlarged and sharp on the dorsal margin of the caudal fin. It is one of the few sharks with an “open” lateral line, in which the mechanoreceptive hair cells are positioned in grooves that are directly exposed to the surrounding seawater.”

References
Ebert DA and Compagno LJV 2009. Chlamydoselachus africana, a new species of frilled shark from southern Africa (Chondrichthyes, Hexanchiformes, Chlamydoselachidae). Zootaxa. 2173: 1–18.
Garman S 1884. An Extraordinary Shark. Bulletin of the Essex Institute. 16: 47–55.

wiki/Frilled_shark
wiki/Southern_African_frilled_shark