The nurse shark, Ginglymostoma cirratum, enters the LRT

The nurse shark,
Ginglymostoma (Figs. 1, 2) enters the large reptile tree (LRT, 1771+ taxa) as a sister to Tristychius (Figs. 3, 4) from the Early Carboniferous.

Figure 1. The nurse shark, Ginglymostoma, in vivo.

Figure 1. The nurse shark, Ginglymostoma, in vivo.

Figure 2. The skull of Ginglymostoma in two views. The gill basket and pectoral fins are shown in dorsal view only. Tetrapod homologs are added as colors.

Figure 2. The skull of Ginglymostoma in two views. The gill basket and pectoral fins are shown in dorsal view only. Tetrapod homologs are added as colors.

Ginglymostoma cirrum (Bonaterre 1788; Müller and Henle 1837) is the extant nurse shark, a lethargic, bottom dweller. According to Wikipedia, Members of this genus have the ability to suck in water in order to remove snails from their shells in a manner that can be described as ‘vacuum-like’. The anterior displacement of the anterior gill bars (= labial cartilages, Fig. 2, red color) helps provide a circular form for the suction tube.

Note the unique (at present) posterior placement of the orbit surrounded by circumorbital cartilage relative to the ‘lacrimal’ (= palatoquadrate, tan color). This is convergent with a similar shift seen in Hybodus basanus and bony fish. The large lateral process of the hyomandibular (Fig. 2, dark green color) is not seen in other tested sharks. Tabulars are missing as in Tristychius.

Figure 1. Tristychius, a basal shark from the Early Carboniferous,

Figure 1. Tristychius, a basal shark from the Early Carboniferous, Early this skeleton was compared erroneously to Squatina and other rays.

Figure 1. CT scans of Tristychius skull from Coates et al. 2019.

Figure 4. CT scans of Tristychius skull from Coates et al. 2019.

Tristychius arcuatus (Agassiz 1837; Early Carboniferous; 60cm est.) was a small relative of Ginglymostoma with a short torso, large pectoral and pelvic fins and large dorsal spines. It is not related to skates or rays.

The nares pointed anteriorly. Teeth are nearly absent with only a few in the anterior dentary. The postorbital is strongly developed here. Tabulars are absent. Note the low position of the gill slits. Note the large anterior gill bars (= labial cartilages in red) that restrict jaw depression and create lateral walls for the open jaws.

Coates et al. 2019 described this specimen with regard to suction feeding 50 million years before the bony fish equivalent. The authors also report, “The labial cartilages are large and comparable to examples known in Mesozoic hybodontids (8) and modern suction feeding elasmobranchs such as nurse sharks (genus Ginglymostoma).”

Developmental note:
A kind reader (CB) commented on a previous post, (in short): “Most of the bones you’re trying to identify on shark chondrocrania are dermal bones. That means they don’t pre-form in cartilage. Which means animals without a bony skull cannot have them.”

I replied, (in short), “It is axiomatic that sharks cannot stand alone. There must be a relationship to other vertebrates. The renaming of skull topologies along the lines of tetrapod homologies has opened a door that had previously been closed due to a separate naming system that I and others have suggested unifying. The LRT shows that vertebrate skulls _alone_ took a detour after paddlefish, then returned to bony skull elements with sutures with hybodontids and their descendants. I don’t know the mechanism for this. The LRT provides the map for future scholars.”

Click here for the complete set of comments and replies.

Seasonal note: 
Another blogpost has finally overtaken ‘the origin of bats‘ as the most popular one over the last few weeks. The new most popular post is the one that deals with ancient astronomy, rather than paleontology. Evidently there is either some interest in ‘A Christmas Story‘, or the web page comes up whenever someone is looking for the movie with the same name.


References
Agassiz L 1837. Recherches Sur Les Poissons Fossiles. Tome III (livr. 8-9). Imprimérie de Petitpierre, Neuchatel viii-72
Bonaterre PJ 1788. Tableau encyclopédique et méthodique des trois règnes de la nature …, Ichthyologie. Panckoucke, Paris 1788.
Coates MI, Tletjen K, Olsen AM and Finarelli JA 2019. High performance suction feeding in an early elasmobranch. Science Advances 2019:5: eaax2742.
Motta PJ and Wilga CD 1999. Anatomy of the Feeding Apparatus of the Nurse Shark, Ginglymostoma cirratum. Journal of Morphology 24:33–60.

wiki/Tristychius
wiki/Ginglymostoma
wiki/nurse shark

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