Squaloraja: more of a paddlefish than a ratfish

One of the strangest looking of all vertebrates,
Squaloraja polyspondyla, nests alone in the large reptile tree (LRT, 1772+ taxa) between the clade of gnathostomes without marginal teeth and those with marginal teeth.

Figure 1. Squaloraja is not the chimaerid everyone thinks it is, but nests with Scapanorhynchus and Mitsukurina in the paddlefish clade.

Figure 1. Squaloraja is not the chimaerid everyone thinks it is, but nests with Scapanorhynchus and Mitsukurina in the paddlefish clade.

Squaloraja polyspondyla (Agassiz 1843, Woodward 1866, Early Jurassic) is traditionally considered a relative of Chimaera, but here nests as a late-survivor of the basalmost taxon with marignal teeth, here named “Marginodonta“, between the basalmost, toothless, gnathostome clade with Chondrosteus at its base, and all other vertebrates with marginal teeth in the LRT with the paddlefish, Polyodon, at its base.

In a pre-cladistic era,
Squaloraja would have been considered a member of the Chondrostei, since it nests between sturgeons and paddlefish. But now in the LRT, so do whale sharks and mantas.

Grogan, Lund and Greenfest-Allen 2012
nested Squaloraja with chimaerids, but that cladogram excluded Squaloraja pre-shark sister taxa recovered in the wider gamut LRT.

Rather than a continuous notochord,
a series of cartilaginous segments is present, convergent with the situation in bony fish. As an Early Jurassic taxon, Squaloraja had plenty of time to develop this one trait.

Figure 2. Polyodon hatchling prior to the development of the long rostrum with maturity.

Figure 2. Polyodon hatchling prior to the development of the long rostrum with maturity.

These taxa were bottom feeders with a large wide rostrum full of sensors for detecting buried prey. Distinct from rays and sturgeons, but like Chondrosteus and paddlefish (Polyodon), the mouth was wide. Several excellent specimens preserve soft parts. It is worth comparing Squaloraja to a hatchling Polyodon (Fig. 2).


References
Agassiz L 1843. Recherches sur les Poissons Fossiles, III (IV), Imprimerie de Petitpierre, Neuchatel, pp. 157-390.
Grogan ED, Lund R and Greenfest-Allen E 2012. The origin and relationships of early chondrichthyans. In: Carrier JC, Musick JA and Heithaus MR (eds) Biology of Sharks and their Relatives, Edition 2. CRC Press, Boca Raton, Florida: 3–30.
Woodward AS 1886. On the anatomy and systematic position of the Liassic selachian Squaloraja polyspondyla Agassiz. Proceedings of the Zoological Society of London, 1886: 527–538.

Other references online here and here.

wiki/Squaloraja

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