Protocetidae, the protocetids, form a diverse and heterogeneous group of extinct

cetacea Cetacea (; , ) is an infraorder of aquatic mammals that includes whales, dolphins, and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively carnivorous diet. They propel th ...

ns known from Asia, Europe, Africa, South America, and North America.

Description

There were many genera, and some of these are very well known (e.g., '' Rodhocetus''). Known protocetids had large fore- and hindlimbs that could support the body on land, and it is likely that they lived amphibiously: in the sea and on land. It is unclear at present whether protocetids had flukes (the horizontal tail fin of modern cetaceans). However, what is clear is that they are adapted even further to an aquatic life-style. In '' Rodhocetus'', for example, the

sacrum The sacrum (plural: ''sacra'' or ''sacrums''), in human anatomy, is a large, triangular bone at the base of the spine that forms by the fusing of the sacral vertebrae (S1S5) between ages 18 and 30. The sacrum situates at the upper, back part o ...

– a bone that in land-mammals is a fusion of five vertebrae that connects the

pelvis The pelvis (plural pelves or pelvises) is the lower part of the trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis, or pelvic skeleton). The ...

with the rest of the

vertebral column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordate ...

– was divided into loose vertebrae. However, the pelvis retain a

sacroiliac joint The sacroiliac joint or SI joint (SIJ) is the joint between the sacrum and the ilium bones of the pelvis, which are connected by strong ligaments. In humans, the sacrum supports the spine and is supported in turn by an ilium on each side. T ...

. Furthermore, the nasal openings are now halfway up the snout; a first step towards the telescoped condition in modern whales. Their supposed amphibious nature is supported by the discovery of a pregnant ''

Maiacetus ''Maiacetus'' ("mother whale") is a genus of early middle Eocene (c. 47.5 mya) cetacean from the Habib Rahi Formation of Pakistan. Paleobiology The genus contains a single species, ''Maiacetus inuus'', first described in 2009 on the basis of ...

'', in which the fossilised fetus was positioned for a head-first delivery, suggesting that ''Maiacetus'' gave birth on land. The

ungulate Ungulates ( ) are members of the diverse clade Ungulata which primarily consists of large mammals with hooves. These include odd-toed ungulates such as horses, rhinoceroses, and tapirs; and even-toed ungulates such as cattle, pigs, giraffes, ...

ancestry of these early whales is still underlined by characteristics like the presence of hooves at the ends of toes in ''Rodhocetus''.

Taxonomy

The protocetid subfamilies were proposed by . They placed ''

Makaracetus ''Makaracetus'' is an extinct protocetid early whale the remains of which were found in 2004 in Lutetian layers of the Domanda Formation in the Sulaiman Range of Balochistan, Pakistan (, paleocoordinates ). ''Makaracetus'' is unique among ar ...

'' in its own subfamily (Makaracetinae) based on its unique adaptations for feeding (including only two incisors in each premaxilla). They then erected two subfamilies for the rest of the protocetids based on their degree of aquatic adaptation: Protocetinae- Protocetines are

Lutetian The Lutetian is, in the geologic timescale, a stage or age in the Eocene. It spans the time between . The Lutetian is preceded by the Ypresian and is followed by the Bartonian. Together with the Bartonian it is sometimes referred to as the ...

protocetids with generalized skulls retaining three incisors in the premaxilla and three molars in the maxilla. To the extent postcrania are known (primarily from '' Rodhocetus'', and more recently ''Peregocetus''),Olivier L, Bianucci G, Salas-Gismondi R, Di Celma C, Steurbaut E, Urbina M & de Muizon C (2019). "An amphibious whale from the Middle Eocene of Peru reveals early South Pacific dispersal of quadrupedal cetaceans". ''Current Biology'' 29(8): p. 1352–1359.e3. they possess a pelvis similar to those in land-living mammals, a

articulated to the ilia and innominates and large hindlimbs used for foot-powered propulsion. Genera: Georgiacetinae- Georgiacetines are

Bartonian The Bartonian is, in the ICS's geologic time scale, a stage or age in the middle Eocene Epoch or Series. The Bartonian Age spans the time between . It is preceded by the Lutetian and is followed by the Priabonian Age. Stratigraphic defini ...

protocetids, considered transitional to the basilosaurids. Their skulls and dentition are similar to those of protocetines, but the pelvis in ''Georgiacetus'' and ''Aegicetus'' indicate a reduced

, with no substantial articulation between backbone and innominates. Until recently, no hindlimb material was known for this subfamily, although the pelvic morphology seemed to hint at a tail-based rather than foot-based mode of aquatic locomotion. The discovery of ''Aegicetus'' supports this notion.Gingerich P.D., Antar M.S.M., Zalmout I.S. (2019). "''Aegicetus gehennae'', a new late Eocene protocetid (Cetacea, Archaeoceti) from Wadi Al Hitan, Egypt, and the transition to tail-powered swimming in whales". ''PLoS ONE'' 14(12
e0225391
/ref> Genera:

Notes

References

* * * {{Taxonbar, from=Q2721468 Protocetidae Eocene first appearances Eocene extinctions Prehistoric mammal families Paraphyletic groups