Debeerius: enters the LRT basal to Rhinochimaera

Figure 1. Debeerius reconstruction from Grogan and Lund 2000. Compare the skull to Figure 2.

Figure 1. Debeerius reconstruction from Grogan and Lund 2000. Compare the skull to Figure 2.

Figure 2. Debeerius skull, colors added here.

Figure 2. Debeerius skull, colors added here.

Debeerius ellefseni 
(Grogan and Lund 2000; Mississipian, 320 mya) was originally considered intermediate betweeen sharks and ratfish, but here nests between Chimaera and Rhinochimaera (Figs 3–5).

Figure 1. The long-nosed chimaera (Rhinochimaera africana?).

Figure 3. The long-nosed chimaera (Rhinochimaera africana?).

Figure 5. Fused cartilage skull of Rhinochimaera lacking the tactile/sensory probe supports. Compare to diagram in figure 6.

Figure 4. Fused cartilage skull of Rhinochimaera lacking the tactile/sensory probe supports. Compare to diagram in figure 6.

Figure 6. Diagram of Rhinochimaera pacifica from Didier 1995. Inverted area and colors added to show interpretations of element boundaries based on Callorhinchus (Fig. 7) and other related taxa.

Figure 5. Diagram of Rhinochimaera pacifica from Didier 1995. Inverted area and colors added to show interpretations of element boundaries based on Callorhinchus (Fig. 7) and other related taxa.

Rhinochimaera pacifica
(Mitsukuri 1895, 130cm in length) is the extant Pacific spookfish. A long nasal bone greatly extends the rostrum, creating a larger sensory surface, as in sawfish, hammerheds and paddlefish by convergence.

Figure 6. Adding Debeerius to the LRT helped revise the shark-subset. Note the shifting of the basking shark, Cetorhnus within the paddlefish clade.

Figure 6. Adding Debeerius to the LRT helped revise the shark-subset. Note the shifting of the basking shark, Cetorhnus within the paddlefish clade.

Revisions to the LRT
(Fig. 6) shift the second largest living fish, the basking shark (Cetorhinus) with paddlefish (Polyodon). Both are filter feeders with huge gapes. Among the few differences is the presence of gill slits vs. gill covers. Turns out to be a trivial trait in the scheme of things. The basking shark is a giant hatchling paddlefish.

Figure 7. Polyodon hatchling. Compare to the Polyodon adult (figure 8) and Cetorhinus (figure 9).

Figure 7. Polyodon hatchling. Compare to the Polyodon adult (figure 8) and Cetorhinus (figure 9).

Figure 4. Skull of Polyodon from a diagram published in Gregory 1938, plus a dorsal view and lateral photo.

Figure 8. Skull of Polyodon from a diagram published in Gregory 1938, plus a dorsal view and lateral photo. compare to figures 7 and 9.

FiIgure 1. The basking shark, Cetorhinus maximus, in lateral and ventral views.

Figure 9. The basking shark, Cetorhinus maximus, in lateral and ventral views. Compare to figures 7 and 9.

References
Grogan ED and Lund R 2000. Debeerius ellefseni (Fam. Nov.,Gen. Nov., Spec Nov.), an autodiastylic chondrichthyan from the Misssissippian Bear Gulch limestone of Montana (USA), the relationships of the Chondrichthyes, and comments on gnathostome evolution. Journal of Morphology 243:219–245.

wiki/Debeerius

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