Aetiocetus spp.

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Aetiocetus spp.


Life restoration of A. weltoni displaying both teeth and baleen. © @ Carl Buell

Temporal range: Oligocene (Rupelian-Chattian: ~28-24Ma)

Scientific classification:

Life
Domain: Eukaryota
(unranked): Unikonta
(unranked): Opisthokonta
(unranked): Holozoa
(unranked): Filozoa
Kingdom: Animalia
Subkingdom: Eumetazoa
(unranked): Bilateria
Clade: Nephrozoa
Superphylum: Deuterostomia
Phylum: Chordata
Clade: Olfactores
Clade: Craniata
Subphylum: Vertebrata
Infraphylum: Gnathostomata
Clade: Eugnathostomata
Clade: Teleostomi
Superclass: Tetrapoda
Clade: Reptiliomorpha
Clade: Amniota
Clade: Synapsida
Clade: Eupelycosauria
Clade: Sphenacodontia
Clade: Sphenacodontoidea
Order: Therapsida
Suborder: Cynodontia
Clade: Epicynodontia
Infraorder: Eucynodontia
Parvorder: Probainognathia
Superfamily: Chiniquodontoidea
Clade: Prozostrodontia
Clade: Mammaliaformes
Class: Mammalia
Clade: Holotheria
Superlegion: Trechnotheria
Legion: Cladotheria
Sublegion: Zatheria
Infralegion: Tribosphenida
Subclass: Theria
Clade: Eutheria
Infraclass: Placentalia
Subcohort: Exafroplacentalia
Magnorder: Boreoeutheria
Superorder: Laurasiatheria
Clade: Ungulata
(unranked): Cetartiodactyla
Suborder: Mysticeti
Family: †Aetiocetidae
Genus: †Aetiocetus
Species: †A. cotylalveus
†A. polydentatus
†A. tomitai
†A. weltoni

Aetiocetus was a genus of mysticete that lived in the Oligocene of Japan, Mexico, and the United States (particularly in the state of Oregon)[1] from ~28-24 million years ago.[2]

Description:
Aetiocetus clearly had teeth, but upon closer inspection, the lateral areas of the palate were shown to have had nutrient foramina (groove-like structures) as well. Modern baleen whales possess nutrient foramina that provide blood vessels and nerves to form baleen. Therefore, Aetiocetus was a baleen whale that possessed both teeth and baleen and represents a transitional form between toothed baleen whales and baleen whales with baleen.[2][3]


From Deméré et al. (2008). Ventral view of the upper jaw of A. weltoni. The left image clearly shows teeth while the right image has arrows pointing to the baleen-forming structures.

Feeding:
The possession of teeth allowed for the consumption of large prey while the possession of baleen permitted the sieving of smaller quarry.[2]

References:

[1] fossilworks.org/bridge.pl?action=taxonInfo&taxon_no=36654
[2] whyevolutionistrue.wordpress.com/2010/07/22/baleen-whales-a-lovely-transitional-form/
[3] "Morphological and molecular evidence for a stepwise evolutionary transition from teeth to baleen in mysticete whales" (Deméré et al., 2008).

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This new study provides critical evidence for the nutrient foramina having once supported baleen.

academic.oup.com/zoolinnean/advance-article/doi/10.1093/zoolinnean/zlab017/6278618

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New study finds that lateral palatal foramina are no indication of baleen. Even terrestrial artiodactyls have them.

Lateral palatal foramina do not indicate baleen in fossil whales

Today’s mysticetes filter-feed using baleen, a novel integumentary structure with no apparent homolog in any living mammal. The origins of filter-feeding and baleen can be informed by the fossil record, including rare instances of soft tissue preservation of baleen and also by potential osteological correlates of baleen. Lateral palatal foramina on the roof of the mouth have been proposed as potential osteological correlates of baleen and their presence in some tooth-bearing stem mysticetes has led to the hypothesis that these early mysticetes possessed both teeth and incipient baleen. Here, we test this hypothesis by examining lateral palatal foramina in both filter-feeding and non-filter-feeding cetaceans, including crown and stem odontocetes and in stem cetaceans (or archaeocetes). We also confirm the presence of lateral palatal foramina in 61 species of terrestrial artiodactyls. CT scanning demonstrates consistent internal morphology across all observed taxa, suggesting that the lateral palatal foramina observed in extant mysticetes are homologous to those of terrestrial artiodactyls. The presence of lateral palatal foramina in terrestrial artiodactyls and non-filter-feeding whales (odontocetes and archaeocetes) suggests that these structures are not unique predictors for the presence of baleen in fossil whales; instead, these structures are more probably associated with gingiva or other oral tissue.

www.nature.com/articles/s41598-022-15684-8

This suggests there's no evidence Aetiocetus had baleen, only teeth.