Artiodactyla and Cetacea

Cetacea no longer applies to an ordinal level taxon: all members of Cetacea currently are included within the order Artiodactyla. Montgelard et al. (1997) proposed the name “Cetartiodactyla” to reflect the growing body of data showing Cetacea nested within Artiodactyla. However, use of the name Cetartiodactyla has been controversial because Cetacea and Artiodactyla are not sister-taxa: molecular data distinctly show cetaceans embedded within Artiodactyla (Prothero et al. 2021). Exceptionally rapid and disparate evolution of the cetacean skull has obscured an accurate assessment of their phylogenetic relationships with other groups of mammals (Goswami et al. 2022). As a result, the initial—and apparently incongruous—assignment of Cetacea to Artiodactyla generally is ascribed to molecular data from amino acid and nucleotide sequence data (Goodman et al. 1985; Irwin et al. 1990; Graur & Higgins 1994), pinpointing Hippopotamidae as the sister taxon of Cetacea (Gatesy et al. 1996). Paleontological evidence subsequently corroborated this relationship (Gingerich et al. 1990, 2001; Thewissen & Hussain 1993; Thewissen 1994; Thewissen & Madar; 1999; Thewissen et al. 2001). Molecular data have provided increasing support and definition for these relationships (Upham et al. 2019; McGowen et al. 2020). However, the name and taxonomic rank of the group remains controversial.

A variety of propositions have been put forward to address this controversy. We noted Cetartiodactyla above, a name that has been recommended for disuse by Asher & Helgen (2010) and Prothero et al. (2022) for the ordinal group. An intraordinal alternative was proposed by Waddell et al. (1999): Whippomorpha (“ wh ales” plus “ hippo s”), as the clade within Artiodactyla that includes Hippopotamidae and Cetacea. The same grouping subsequently was given the name Cetancodonta by Arnason et al. (2000, 2002, 2008). As pointed out by Asher & Helgen (2010) based on the principle of priority espoused by Simpson (1945; also see Art. 23 of the International Code of Zoological Nomenclature), and regardless of the awkward construction of the name, Whippomorpha has temporal priority over Cetancodonta. However, as a “clade”, it is a descriptive appellation for a monophyletic subordinate group, and does not resolve the taxonomic level at which subordinate or superordinate groups may lie; in other words: the taxonomic level of “clade” is nebulous in this instance, besides defining a common ancestry, or circumscribing “delimitable monophyletic units” (Huxley 1957); in the present instance: Hippopotamidae and Cetacea. There are any number of such units in any region of the tree of life one may wish to examine, and a proliferation of names for such clades would serve little useful purpose; Prothero et al. (2022:96) correctly pointed out that “If one wishes to convey the fact that whales are artiodactyls, one can say informally “whales and other artiodactyls” or “whales and terrestrial artiodactyls””. More recently, Whippomorpha has been adopted as a subordinal level group (Lewison 2011).

Linnaeus described whales, dolphins, and their ilk, as the order Cete (Linnaeus 1758:75; also used by Gray 1843; Bonaparte 1851; nec Cete sensu Thewissen 1994), but the currently accepted name (for the same group defined by Linnaeus) is Cetacea Brisson 1762:3 [first summary mention], 215 [unnumbered title page], 217 [diagnosis]. This name became accepted and since has come into widespread use (e.g., Gray 1821 [as a “Class”: “Cetaceae”, containing the order Herbivoraae (including Manatidae and Dugongidae, and Order Carnivorae, with families Monodontidae, Physeteridae, and Balanadae]; Lesson 1827 [as “Cétacées”]; Gray 1846 [as Cetacea, but with the same familial arrangement as in Gray 1821]; Brandt 1873; Lydekker 1887; Trouessart 1898; etc.): all used Cetacea as an ordinal level taxon. The Committee on Taxonomy of The Society for Marine Mammalogy maintains a list of marine mammals and subspecies (https://marinemammalscience.org/science-and-publications/list-marinemammal-species-subspecies/; accessed 20 December 2022) listing Cetacea as an infraorder within Artiodactyla, with Mysticeti and Odontoceti (no rank) and their currently accepted familial level taxa contained therein. Cetacea also has been used at the family level: Doherty (1864:138) used “Cetacidae” [sic] for “whales, etc.”. Doherty (1864) even went so far as to link Cetacidae, in the “Pachydermal Order” with “Pachydermidae” (hippopotamus), albeit containing as well Tapiridae and Proboscidae (tapirs and elephants). While Doherty’s philosophical taxonomic framework was somewhat heterodox, it was not unique and may have had its origins in similar philosophical propositions of Swainson (1835).