Are whales artiodactyls? No.

Prothero and 15 co-authors 2021
report the clade name Cetioartiodactylia “is a junior synonym for Artiodactylia.” That confirms Spauling, O’Leary and Gatesy (2009), but that’s not the point. In Prothero et al. 16 PhDs think whales are monophyletic and evolved from either small, deer-like taxa… or, according to Graur and Higgins 1994, Irwin and Arnason (1994) and Gatesy et al. (1996): hippos, universally and mistakenly considered to be artiodactyls (see Fig. 3). All prior workers overlooked, omitted, ignored and excluded mysticete and ondontocete precursor taxa with legs recovered in the LRT (Figs. 1–3).

From the abstract:
“The name “Cetartiodactyla” was proposed in 1997 to reflect the molecular data that suggested that Cetacea is closely related to Artiodactyla. Since then, that taxon has spread in popularity, even outside the scientific literature. However, the implications of the name are confusing, because Cetacea and Artiodactyla are not sister-taxa.

Cetacea and Artiodactyla are also not sister-taxa in the large reptile tree (LRT, 1936+ taxa; subset Fig. 3) because Cetacea is an invalid clade. Members of the Odontoceti arise completely separate from Mysticeti (Figs. 1, 2) and apart from Artiodactyla.

Are hippos artiodactyls? No. In the LRT (subset Fig. 3) hippos nest outside the Artiodactyla, between oreodonts + mesonychids and anthracobunids + desmostylians + mysticetes.


“Instead, the evidence clearly shows that cetaceans are a group embedded within Artiodactyla, not a sister-taxon of equal rank. It has long been accepted practice that systematists do not modify the names of higher groups when new subgroups are added to them. For example, Owen’s original concept of Artiodactyla did not change its name when more and more disparate taxa were added to it. Dinosauria did not become “Avedinosauria” when it became clear that birds are a subgroup of dinosaurs, nor did Reptilia become “Avereptilia”. In the interests of taxonomic priority and stability, and especially because the name is inherently misleading, we recommend that the name “Cetartiodactyla” be abandoned. If one wishes to make a reference to the fact that whales are now considered to be a subgroup of artiodactyls, they could be referred to informally as “whales and other artiodactyls” or “whales and terrestrial artiodactyls” without using a formal taxonomic name that is confusing and misleading.”

Curious why 16 authors signed their names to this nomenclature paper? It seems to be an opportunity for 15 PhDs to stamp their approval on this junior synonym effort led by Dr. Prothero. Unfortunately this turns out to be too small a bandage for such a large wound.

Figure 8. Odontoceti (toothed whale) origin and evolution. Here Anagale, Andrewsarchus, Sinonyx, Hemicentetes, Tenrec Indohyus and Leptictidium precede Pakicetus. Maiacetus and Orcinus are aquatic odontocetes.
Figure 8. Odontoceti (toothed whale) origin and evolution. Here Anagale, Andrewsarchus, Sinonyx, Hemicentetes, Tenrec Indohyus and Leptictidium precede Pakicetus. Maiacetus and Orcinus are aquatic odontocetes.

Vislbokova 2013
wrote on the origin of Cetartiodactyla. “The data on phylogeny and early evolution of Cetartiodactyla are analyzed and a model for the initial stage of their history is proposed. It is shown that the roots of Cetartiodactyla go back to generalized Cretaceous terrestrial Eutheria, and a hypothetical basal group of Cetartiodactyla was probably ancestral to the orders Artiodactyla and Cetacea. The Artiodactyla-Cetacea divergence and adaptive radiation of Artiodactyla, which gave rise to the suborders Ruminantia, Tylopoda, and Suiformes, apparently occurred in the pre-Eocene time, earlier than 55 Ma. Molecular similarity between Hippopotamidae and Cetacea is evidence of common origin of Artiodactyla and Cetacea and adaptation to aquatic environment.”

This 2013 paper also suffers from taxon exclusion and dependence on deep time molecular studies. Note the lack of specific taxa and the use of “generalized Cretaceous terrestrial Eutheria’. Just add taxa to find out where whales really came from (Figs. 1–3), all the way back to Ediacaran worms.

Figure 1. Taxa in the lineage of right whales include Desmostylus, Caperea and Eubalaena. The tiny bit of jugal posterior to the orbit (in cyan) is found in all baleen whales tested so far. The frontals over the eyes are just roofing the eyeballs in Desmostylus, much wider in Caperea and much, much longer in Eubalaena.
Figure 1. Taxa in the lineage of right whales include Desmostylus, Caperea and Eubalaena. The tiny bit of jugal posterior to the orbit (in cyan) is found in all baleen whales tested so far. The frontals over the eyes are just roofing the eyeballs in Desmostylus, much wider in Caperea and much, much longer in Eubalaena.

Prothero et al. 2021 report:
“The relationship of Cetacea and Hippopotamidae has since been confirmed by all the studies already cited and is now well established by paleontological studies that demonstrate the origin of hippos from a common ancestor with whales within the paraphyletic group Anthracotheria,
which most systematists regard as the sister-group of whales and hippos.”

This is incorrect when you add even a few pertinent taxa (Figs. 1–3).

Figure 3. Subset of the LRT focusing on placentals. Note the separation of Odontoceti from Mysticeti. Note the splitting of the two Vulpavis species. Note the placement of Hippopotamus close to Mesonychus, not within Artiodactyla. Simply adding taxa gives you this family tree topology. Taxon exclusion spoils prior efforts at understanding whale origins.

Spauliding, O’Leary and Gatesy 2009
ran their own analysis and likewise omitted desmostylians, anthracobunids, tenrecs and anagalids. They recovered a monophyletic Cetacea with toothless Mysticeti (= baleen whales) nesting by default between Basilosaurus and Physeter, two toothed whales. This is an untenable association given the competing hypothesis that includes more taxa that demonstrate a gradual accumulation of derived traits, leading to convergence between Mysticeti and Odontoceti. The LRT was able to lump and separate these taxa with the present character list.

As demonstrated here, you can learn more
by simply doing the work yourself rather than sitting in university lecture halls and reading university textbooks on vertebrate paleontology. Professors continue to promote taxon exclusion even though it spoils efforts at understanding whale origins, turtle origins, snake origins, pterosaur origins, shark origins, etc.

Prothero et al. 2021 relies on several molecule studies.
Colleagues in paleontology: Let’s get back to bones, lots and lots of bones from lots and lots of taxa. Molecules and taxon exclusion are providing false positives and leading to very sketchy conclusions. You were inspired to get into paleontology because you thought bones were fascinating. Get back to that original inspiration.

References
Gatesy J, Hayashi C, Cronin MA, and Arctander P 1996. Evidence from milk casein genes that cetaceans are close relatives of hippopotamid artiodactyls. Mol Biol Evol 13(7):954-963.
Graur D and Higgins DG 1994. Molecular evidence for the inclusion of cetaceans within the order Artiodactyla. Mol Biol Evol 11(3):357-364.
Irwin DM and Árnason U 1994. Cytochrome b gene of marine mammals: Phylogeny and evolution. J Mamm Evol 2(1):37-55.
Prothero et al. (15 co-authors) 2021. On the Unnecessary and Misleading Taxon “Cetartiodactyla”. Journal of Mammalian Evolution. https://doi.org/10.1007/s10914-021-09572-7
Spaulding M, O’Leary MA and Gatesy J 2009. Relationships of Cetacea (Artiodactyla) among mammals: increased taxon sampling alters interpretations of key fossils and character evolution. PLoS One 4(9): e7062. https://doi.org/10.1371/journal.pone.0007062
Vislobokova IA 2013. On the Origin of Cetartiodactyla: Comparison of Data on Evolutionary Morphology and Molecular Biology. Palaeontological Journal 47(3):321–334.

Rejected by referees including P. Gingerich:
Peters D 2013. The triple origin of whales. https://www.researchgate.net/publication/328388746_The_triple_origin_of_whales

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