| Protocetidae | |
|---|---|
 | |
| Maiacetus | |
Scientific classification  | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Artiodactyla |
| Infraorder: | Cetacea |
| Parvorder: | † Archaeoceti |
| Family: | † Protocetidae Stromer 1908 |
| Subfamilies | |
3, See text | |
Protocetidae, the protocetids, form a diverse and heterogeneous group of extinct cetaceans known from Asia, Europe, Africa, South America, and North America.
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 – a bone that in land-mammals is a fusion of five vertebrae that connects the pelvis with the rest of the vertebral column – was divided into loose vertebrae. However, the pelvis retain a sacroiliac joint. 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 , [2] in which the fossilised fetus was positioned for a head-first delivery, suggesting that Maiacetus gave birth on land. The ungulate ancestry of these early whales is still underlined by characteristics like the presence of hooves at the ends of toes in Rodhocetus.
The protocetid subfamilies were proposed by Gingerich et al. 2005. 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: [3]
Protocetinae- Protocetines are Lutetian 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), [4] they possess a pelvis similar to those in land-living mammals, a sacrum articulated to the ilia and innominates and large hindlimbs used for foot-powered propulsion. [3] Genera:
Georgiacetinae- Georgiacetines are Bartonian 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 sacroiliac joint, 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. [3] [5] Genera:
Rodhocetus is an extinct genus of protocetid early whale known from the Lutetian of Pakistan. The best-known protocetid, Rodhocetus is known from two partial skeletons that taken together give a complete image of an Eocene whale that had short limbs with long hands and feet that were probably webbed and a sacrum that was immobile with four partially fused sacral vertebrae. It is one of several extinct whale genera that possess land mammal characteristics, thus demonstrating the evolutionary transition from land to sea.
Pakicetidae is an extinct family of Archaeoceti that lived during the Early Eocene in Pakistan.
The evolution of cetaceans is thought to have begun in the Indian subcontinent from even-toed ungulates (Artiodactyla) 50 million years ago (mya) and to have proceeded over a period of at least 15 million years. Cetaceans are fully aquatic marine mammals belonging to the order Artiodactyla and branched off from other artiodactyls around 50 mya. Cetaceans are thought to have evolved during the Eocene, the second epoch of the present-extending Cenozoic Era. Molecular and morphological analyses suggest Cetacea share a relatively recent closest common ancestor with hippopotami and that they are sister groups. Being mammals, they surface to breathe air; they have 5 finger bones (even-toed) in their fins; they nurse their young; and, despite their fully aquatic life style, they retain many skeletal features from their terrestrial ancestors. Research conducted in the late 1970s in Pakistan revealed several stages in the transition of cetaceans from land to sea.
Ambulocetus is a genus of early amphibious cetacean from the Kuldana Formation in Pakistan, roughly 48 or 47 million years ago during the Early Eocene (Lutetian). It contains one species, Ambulocetus natans, known solely from a near-complete skeleton. Ambulocetus is among the best-studied of Eocene cetaceans, and serves as an instrumental find in the study of cetacean evolution and their transition from land to sea, as it was the first cetacean discovered to preserve a suite of adaptations consistent with an amphibious lifestyle. Ambulocetus is classified in the group Archaeoceti—the ancient forerunners of modern cetaceans whose members span the transition from land to sea—and in the family Ambulocetidae, which includes Himalayacetus and Gandakasia.
Basilosauridae is a family of extinct cetaceans. They lived during the middle to the early late Eocene and are known from all continents, including Antarctica. They were probably the first fully aquatic cetaceans. The group is noted to be a paraphyletic assemblage of stem group whales from which the monophyletic Neoceti are derived.
Archaeoceti, or Zeuglodontes in older literature, is a paraphyletic group of primitive cetaceans that lived from the Early Eocene to the late Oligocene. Representing the earliest cetacean radiation, they include the initial amphibious stages in cetacean evolution, thus are the ancestors of both modern cetacean suborders, Mysticeti and Odontoceti. This initial diversification occurred in the shallow waters that separated India and Asia 53 to 45 mya, resulting in some 30 species adapted to a fully oceanic life. Echolocation and filter-feeding evolved during a second radiation 36 to 35 mya.
Protocetus atavus is an extinct species of primitive cetacean from Egypt. It lived during the middle Eocene period 45 million years ago. The first discovered protocetid, Protocetus atavus was described by Fraas 1904 based on a cranium and a number of associated vertebrae and ribs found in middle Lutetian Tethyan marine limestone from Gebel Mokattam near Cairo, Egypt.
Ichthyolestes is an extinct genus of archaic cetacean that was endemic to Indo-Pakistan during the Lutetian stage. To date, this monotypic genus is only represented by Ichthyolestes pinfoldi.
Remingtonocetus is an extinct genus of early cetacean freshwater aquatic mammals of the family Remingtonocetidae endemic to the coastline of the ancient Tethys Ocean during the Eocene. It was named after naturalist Remington Kellogg.
Artiocetus is an extinct genus of early whales belonging to the family Protocetidae. It was a close relative to Rodhocetus and its tarsals indicate it resembled an artiodactyl.
Georgiacetus is an extinct genus of ancient whale known from the Eocene period of the United States. Fossils are known from Georgia, Alabama, and Mississippi and protocetid fossils from the right time frame, but not yet confirmed as Georgiacetus, have been found in Texas and South Carolina.
Maiacetus is a genus of early middle Eocene cetacean from the Habib Rahi Formation of Pakistan.
Remingtonocetidae is a diverse family of early aquatic mammals of the order Cetacea. The family is named after paleocetologist Remington Kellogg.
The Qasr el Sagha Formation is a geological formation located in Egypt. The formation is part of the Wadi El Hitan World Heritage Site. The Qasr el Sagha Formation overlies the Birket Qarun Formation and is overlain by the Gebel Qatrani Formation. The sandstones and shales of the formation were deposited in a deltaic to shallow marine environment. It dates to the Late Eocene.
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.
Qaisracetus is an extinct protocetid early whale known from the Eocene of Baluchistan, Pakistan.
Carolinacetus is an extinct protocetid early whale found in the Bartonian Tupelo Bay Formation in Berkeley County, South Carolina.
Eocetus is an extinct protocetid early whale known from the early late Eocene Giushi Formation in Gebel Mokattam, outside Cairo, Egypt. The specimen was first named by Fraas as Mesocetus schweinfurthi. However, the name Mesocetus was previously used causing a change to the species name to Eocetus schweinfurthi. Since the genus was first described in the early 20th century, several other specimens, mostly isolated vertebrae, have been attributed to Eocetus, but the taxonomic status of these widely distributed specimens remain disputed.
Aegicetus is an extinct genus of protocetid whale based on a partial skull with much of an associated postcranial skeleton discovered in Egypt. It lived around 35 million years ago, making it the youngest known protocetid to date. Aegicetus was discovered in 2007 at Wadi El Hitan as a relatively complete skeleton and a partial second specimen. They were assigned to a new genus and species in 2019 by Philip D. Gingerich et al.
Sobrarbesiren is a genus of extinct sirenian that lived in the Eocene, about 47 million years ago. The type and only species is S. cardieli, known from a multitude of specimens from the Spanish Pyrenees. Sobrarbesiren was a medium-sized animal, 2.7 m long and still retaining both pairs of limbs. Although initially thought to be amphibious, later studies instead suggest that they would have been fully aquatic and been selective sea grass browsers. Unlike modern dugongs and manatees, they likely lacked a tail fluke, although it would have appeared horizontally flattened.
Archaeocete genera by family | |||||||
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| Pakicetidae | |||||||
| Ambulocetidae | |||||||
| Remingtonocetidae | |||||||
| Protocetidae |
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| Basilosauridae |
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