paddlefish (Polyodon spathula Walbaum 1792; Grande and Bemis 1991), sturgeons and bichirs (Fig. 1) are considered members of the clade Chondrostei. According to Wikipedia, “Chondrostei are primarily cartilaginous fish showing some degree of ossification. The cartilaginous condition is thought to be derived, and the ancestors of this group were bony fish with fully ossified skeletons. Members of this group share with the Elasmobranchii certain features such as the possession of spiracles, a heterocercal tail, and the absence of scales.”
in the large reptile tree (LRT, 1632 taxa; Fig. F) these taxa are primitive, not derived, and not related to one another.
- The sturgeon (Pseudoscaphirhynchus) nests at the base of Gnathostomata (jawed fish)
- The paddlefish (Polyodon) nests at the base of Chondrichthyes (ratfish and sharks)
- The bichir (Polypterus) nests close to lobefin Dipnomorpha (lungfish).
- Chondrosteus, nests with lizardfish.
So that means
another traditional clade, Acipenseriformes, (sturgeons + paddlefish) is also polyphyletic, invalidated by the LRT.
Polyodon spathula (Walbaum 1792, up to 2.5m) is the extant American paddlefish, native to tributaries of the Misssissippi River. Traditionally considered closely related to sturgeons, paddlefish are closer to chimaeras and sharks. Paddlefish young have teeth and a shorter rostrum. Adults lose their teeth and consume vast quantities of water filtering out the zooplankton on gill rakers, as in related whale sharks and mantas (Rhincodon and Manta). The skeleton is made of cartilage. An unrestricted and continuous notochord persists, not individual vertebrae. The heterocercal tail resembles that of shark. The rostrum is highly sensitive. They eyes are tiny.
“Fossil records of paddlefish date back over 300 million years. American paddlefish are a highly derived fish because they evolved with adaptations such as filter feeding.”
Paddlefish in the LRT nest close to other primitive filter feeders, like the whale shark and manta. So they are NOT highly derived.
This hypothesis of interrelationships is novel.
If you know of earlier hypotheses along these lines, let me know and I will promote their citation.
Grande L and Bemis WE 1991. Osteology and phylogenetic relationships of fossil and Recent paddlefishes (Polyodontidae) with comments on the interrelationships of Acipenseriformes. Society of Vertebrate Paleontology Memoir 1. Journal of Vertebrate Paleontology 11, Supplement to Number 1. 121pp.
Walbaum J 1792. Petri Artedi renovati. Part 3. Petri Artedi sueci genera Piscium in quibus systema totum ichthyologiae proponitur cum classibus, ordinibus, generum characteribus, specierum diffentiis, observationibus plumiris. Redactis Speciebus 2. Ichthyologiae, III: 723.