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

cetacea Cetacea (; , ) is an infraorder of aquatic mammals belonging to the order Artiodactyla that includes whales, dolphins and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively c ...

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 ''Rodhocetus'' (from ''Rodho'', the geological anticline at the type locality, and ''cetus'', Latin for whale) is an extinct genus of protocetid early whale known from the Lutetian of Pakistan. The best-known protocetid, ''Rodhocetus'' is known ...

''). 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 ''

'', for example, the

sacrum The sacrum (: sacra or sacrums), in human anatomy, is a 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 of the pelvic cavity, ...

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

pelvis The pelvis (: pelves or pelvises) is the lower part of an Anatomy, anatomical Trunk (anatomy), trunk, between the human abdomen, abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also c ...

with the rest of the

vertebral column The spinal column, also known as the vertebral column, spine or backbone, is the core part of the axial skeleton in vertebrates. The vertebral column is the defining and eponymous characteristic of the vertebrate. The spinal column is a segmente ...

– 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. The ...

. 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 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 two specimens, including a sp ...

'', 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 Euungulata ("true ungulates"), which primarily consists of large mammals with Hoof, hooves. Once part of the clade "Ungulata" along with the clade Paenungulata, "Ungulata" has since been determined ...

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'' 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 (stratigraphy), stage or age (geology), 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 ...

protocetids with generalized skulls retaining three incisors in the premaxilla and three molars in the maxilla. To the extent postcrania are known (primarily from ''

'', 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 International Commission on Stratigraphy's (ICS) geologic time scale, a stage or age in the middle of the Eocene Epoch or Series. The Bartonian Age spans the time between . It is preceded by the Lutetian and is follow ...

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 Protocetidae, the protocetids, form a paraphyletic, diverse and heterogeneous group of extinct cetaceans known from Asia, Europe, Africa, South America, and North America. Description There were many genera, and some of these are very well kno ...

Eocene first appearances Eocene extinctions Prehistoric mammal families Paraphyletic groups