The origin of the Holocephali (= chimaeras, ratfish)

Updated December 14, 2020
with the addition of many more taxa (see cladograms below).

Didier 1995 reports,
“There are two hypotheses on the origin of Holocephali (Bonaparte 1832). The first and most generally accepted scenario is that holocephalans have evolved from some lineage of bradyodont sharks. The second hypothesis suggests that holocephalans are most closely related to placoderms.”

FIgure 1. Ratfish (chimaera) and Heterodontus to scale.

FIgure 1. Ratfish (chimaera) and Heterodontus to scale.

By contrast
in the large reptile tree (LRT, 1635+ taxa then, 1776 now, Fig. 3) the few tested holocephalians (Chimaera (Fig. 1) and Belantsea)) arise from the horn shark, Heterodontus, (Fig. 1).

Didier 1995 reports,
“The adductor muscles of Heterodontus also lie anterior to the eye and superficially they resemble chimaeroid fishes in this respect. I interpret this as a convergent feature of heterodontids and chimaeroids.” That is the only mention of Heterodontus (Fig. 1) in the text. Squalus is the outgroup taxon in Didier’s figure 46 cladogram.

Figure y. Basal Gnathostomata with the addition of Rhinochimaera.

Figure y. Basal Gnathostomata with the addition of Rhinochimaera.

References
Bonaparte CL 1832. Iconografia delle fauna italica per le quattro classi degli animali vertebrati. Tomo III. Pesci. Roma. [Issued in puntata (installments), without pagination; total of 556 pp., 78 pls.
Didier DA 1995. Phylogenetic Systematics of Extant Chimaeroid Fishes (Holocephali, Chimaeroidei). American Museum Novitates 3119:86pp.

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