Gaviacetus

Gaviacetus Temporal range: Middle Eocene, 45–43.5 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Artiodactyla
Infraorder:	Cetacea
Family:	†Protocetidae
Genus:	†Gaviacetus Gingerich, Arif, and Clyde, 1995
Species:	†G. razai
Binomial name
†Gaviacetus razai Gingerich, Arif, and Clyde, 1995[1]

Gaviacetus (from Latin Gavia, "loon" and cetus, "whale")^[2] is an extinct archaeocete whale that lived approximately 45 million years ago. Gaviacetus was named for its characteristic narrow rostrum and the fast pursuit predation suggested by its unfused sacral vertebrae.^[2]

The only known Gaviacetus razai specimen was found in Punjab, Pakistan. (30°06′N 70°24′E / 30.1°N 70.4°E / 30.1; 70.4, paleocoordinates 11°36′N 65°30′E / 11.6°N 65.5°E / 11.6; 65.5)^[3]

The skull of Gaviacetus is characteristic of protocetids, but the rostrum is extremely narrow anterior to P³, suggesting some kind of trophic specialization.^[4]

The well-preserved auditory bulla in Gingerich, Arif & Clyde 1995's specimen is dense like in other archaeocetes, is equipped with a prominent sigmoid process, but has 3-5 contacts with the rest of the cranium.^[5]

Preserved alveoli (tooth sockets) show that Gaviacetus had double- and triple-rooted cheek teeth,^[6] but some controversy remains regarding the number of molars. Based on other cranial characters, Gingerich, Arif & Clyde 1995 concluded that Gaviacetus is a protocetid (more primitive archeocetes with a third upper molar) and therefore assumed the presence of M³ though no traces thereof are preserved in their specimen. In opposition to this, Bajpai & Thewissen 1998, whose specimen is also lacking the essential maxillar part, thought the cranium above the very small M² to be to narrow for the presence of M³, hence indicating Gaviacetus is a basilosaurid (a more derived archaeocetes lacking a third upper molar).^[7] Uhen 2009 argued against this assignment until more solid evidence has been found.^[8]

The preserved sacral vertebra was not fused with its posterior neighbour, indicating that Gaviacetus was a tail-powered swimmer like Protocetus, better adapted to pursuit predation than Rodhocetus. The preserved transverse process of the sacral vertebra is distally expanded, suggesting a synchondrosal joint between the vertebral column and pelvis.^[4]

Although often represented as having small hind limbs,^[9] this is an inference from the general progression of other fossil species towards limb loss; the only postcranial remains found for Gaviacetus are a rib and several vertebrae. At least three popular science books misspell this genus as Gaviocetus.^[10][11][12]

The genus Gaviacetus was established in 1995 with the description of G. razai. A second species, G. sahnii, was described in 1998, but transferred to its own genus, Kharodacetus in 2014 based on newly discovered remains.^[13]