Tristychius, a Carboniferous suction-feeder

Updated December 4, 2020
with a review of shark morphology after adding taxa. Earlier the LRT (subset Fig. 3) nested Tristychius (Figs. 1, 2) with Squatina and other rays. Now Tristychius nests with the similar, bottom-feeding nurse shark, Ginglymostoma. The clade that includes Squatina and the rays is currently only one node away. In addition, some skull bones were reidentified.

Updated again, May 7, 2021
with new data, Tristychius nests closer to the whale shark, Rhincodon. Click here to view.

Figure 1 updated with new cartilage identities. There is no postotbital here and the jugal is also lacking. As in the whale shark, the postfrontal extends laterally and the eyeball is on a long optic nerve. Compare to earlier misinterpretation below.
Figure 1. CT scans of Tristychius skull from Coates et al. 2019.
Figure 1. CT scans of Tristychius skull from Coates et al. 2019.

Tristychius arcuatus (Agassiz 1837; Early Carboniferous; 60cm est.; Figs. 1, 2) was a small ancestor to angel sharks (Fig. 2) and a sister to dogfish with a short torso, large pectoral and pelvic fins and large dorsal spines. The nares pointed anteriorly, as in Megachasma. Teeth are nearly absent with only a few in the anterior dentary. The postorbital is absent in other taxa, but strongly developed here. Tabulars are absent. Note the low position of the gill slits. Note the large anterior gill bars (= labial cartilages) that restrict jaw depression and create lateral walls for the open jaws.

Figure 1. Tristychius, a basal shark from the Early Carboniferous,
Figure 2. Tristychius, a basal shark from the Early Carboniferous,

Coates et al. 2019 re-described this taxon
with regard to the evolution of suction feeding 50 million years before the bony fish equivalent. They considered Tristychius a hybodont shark, like Hybodus, but the large reptile tree (LRT; 1653+ taxa then, 1771+ taxa now) nests it basal to nurse sharks, like Ginglymostoma (not listed below).

References
Agassiz L 1837. Recherches Sur Les Poissons Fossiles. Tome III (livr. 8-9). Imprimérie de Petitpierre, Neuchatel viii-72
Coates MI, Tletjen K, Olsen AM and Finarelli JA 2019. High performance suction feeding in an early elasmobranch. Science Advances 2019:5: eaax2742.

wiki/Squatina
wiki/Smoothback Angelshark
wiki/Tristychius

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