A 27cm shark from the Early Carboniferous, Hamiltonichthys, enters the LRT

Updated February 11, 2023
Rescoring several sharks moves extant Chlamydoselachus (Fig 2) and eel-like Thrinacoselache to the Hamiltonichthys clade.

Figure 1. The crushed skull of Hamiltonichthys from Maisey 1989. DGS colors and reconstruction added here.

Maisey 1989 described a small new hybodontid shark
from the Early Cretaceous of Kansas and named it Hamiltonichthys mapesi (Fig 1).

Figure 1. Hamiltonichthys in situ and roughly reconstructed.

When added
to the large reptile tree (LRT, 2219 taxa) Hamiltonichtys nested between the more primitive Palaeobates clade and the more derived Xenacanthus clade basal to the extant seven-gill shark, Chlamydoselachus (Fig 2).

These taxa are all supposed to be more basal than Neoselachii
(= al living sharks and rays), but here Hamiltonichthys nests as a basal member of Neoselachi due to its relationship to the extant seven-gill shark, Chlamydoselachus.

Maisey 1989 wrote,
“The previously popular notion that some or all Recent elasmobranchs are descended
from a Hybodus-like ancestor does not withstand critical investigation and cannot
be substantiated on anatomical grounds, although it is still maintained in some noncladistic studies.”

With this update, the LRT supports the the 1989 hypothesis.

References
Garman S 1884. An Extraordinary Shark. Bulletin of the Essex Institute: 47–55.
Grogan ED and Lund R 2008. A basal elasmobranch, Thrinacoselache gracia n. gen and sp., (Thrinacodontidae, new family) from the Bear Gulch Limestone, Serpukhovian of Montana, USA. Journal of Vertebrate Paleontology. 28 (4): 970–988.
Maisey J 1989. Hamiltonichthys mapesi, g. & sp. nov. (Chondrichthyes; Elasmobranchii), from the Upper Pennsylvanian of Kansas. American Museum Novitates 2931:42pp.

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