Rhinochimaera is added to the LRT as another ratfish

Adding taxa continues to clarify hypothetical interrelationships
among all included taxa in the LRT. especially in the shark grade/clade where skull elements are cartilaginous and tend to fuse together leaving fewer clues/landmarks in the more derived taxa.

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.”

In the LRT placoderms are not basal to sharks, but nest with bony fish in a clade that reverted to a cartilaginous internal skeleton while keeping a bony dermal skeleton. So the second hypothesis is falsified.

According to Wikipedia, “Most Bradyodonti fossils consist of jaws and teeth. These indicate that Bradyodonti ate mollusks and other shelled invertebrates. Their bodies were probably broad and flattened, like modern rays.”Bradyodonti” can also refer to the present-day Chimaera or ratfish of the order Chimaeriformes, which have an upper jaw fused to the braincase and a flap of skin covering the gill slits.”

So, once again suprageneric taxon labels leave us all a little confused since Bradyodonti = Chimaeriformes in some circles.

Figure 4. Shark skull evolution according to the LRT. Compare to figure 1.

By adding taxa
to the large reptile tree (LRT, 1776 taxa, subset Figs. 1, 2) holocephali (= chimaera, ratfish) arise from taxa near Squalus and Heterodontus (Fig. 8) close to the shark/ray split where marginal teeth become like paving stones.

By adding colors to skulls,
fused and obscure elements may be identified with tetrapod homologs. That makes scoring and identifying errors easier.

Earlier we looked at
a dorsal view of the skull of the dogfish shark, Squalus (Fig. 1) here. You’ll note that there is much more fusion in the skull cartilage of ratfsh (Figs. 5–7) including the fusion of the lacrimal complex (= traditional palatoquadrate) with the neurocranium and dermocranium.

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.

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 (Fig. 3).

Figure 1. Cladogram from Didier 1995, colors added to reflect taxon inclusion, exclusion according to the LRT (see figure 2).
Figure 3. Cladogram from Didier 1995, colors added to reflect taxon inclusion, exclusion according to the LRT (see figure 2).

Getting back to Rhinochimaera
(Fig. 4–6). It should come as no surprise that, with its long rubbery snout, Rhinochimaera is among the most derived chimaeras in the LRT (subset Fig. 2). That proboscis is supported by a single slender nasal cartilage articulating on a joint with the rest of the smaller, underlying nasal cartilages (Fig. 6), homologs with similar, smaller elements in Callorhinchus (Fig. 7).

Figure 1. The long-nosed chimaera (Rhinochimaera africana?).
Figure 4. 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 5. 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 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.

Rhinochimaera nests with
Callorhinchus (Fig. 6) in the LRT (subset Fig 2), The latter helps identify elements in the former (Figs. 5, 6). Note the lateral rostral rods (l. rost. rod) arises from the lacrimal. The medial rostral rod (m. rost. rod) arises from the nasal.

Figure 7. Callorhinchus milii skull in several views. All the cartilage is fused here, so color identifies elements. Note the tactile rostral elements are smaller and not associated with the premaxilla (contra Didier 1995).
Figure 7. Callorhinchus milii skull in several views. All the cartilage is fused here, so color identifies elements. Note the tactile rostral elements are smaller and not associated with the premaxilla (contra Didier 1995).

Heterodontus
(Fig. 8) likewise helps identify skull elements in chimaeroids prior to the fusion of the lacrimal complex (= traditional palatoquadrate) with the dermocranium and neurocranium.

Figure 8. Heterodontus skull with colors added to identify elements as tetrapod homologs.
Figure 8. Heterodontus skull with colors added to identify elements as tetrapod homologs.

Every added taxon
helps clarify the position of every nested taxon as every included taxon affects every other. Likewise, all included and tested taxa help identify fused elements whenever they appear in taxa like Rhinochimaera (Figs. 4–6). Earlier guesses have been repaired. Current guesses will be repaired as soon as errors are discovered. Sutures may not be visible, but the jaw joint is still the quadrate. The large strut above it is still the hyomandibula. The nasal still extends over the nares. Etc. etc.

Some say I need to look at specimens firsthand.
There’s an answer to that. Although I have examined many specimens firsthand, that’s not my job at present. Precision observations will come, but first a wide-angle view of hundreds of taxa is what is required, because 1) specialists, by definition, are not going to look outside their speciality, and 2) no one has done this before. This one time, just let one guy on the planet do this and the rest of the specialists can benefit from whatever insights are recovered here in this wide-angle view of many taxa at once, the LRT.


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.
Lund R 1977. New information on the evolution of the bradyodont chondrichthyes. Fieldiana 33(28)521–538.
Venkatesh B et al. 2014. Elephant shark genome provides unique insights into gnathostome evolution. Nature 505:174–179.

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