| Yamatocetus Temporal range: Late Oligocene | |
|---|---|
Scientific classification  | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Artiodactyla |
| Infraorder: | Cetacea |
| Family: | † Eomysticetidae |
| Genus: | † Yamatocetus Okazaki, 2012 |
| Species: | †Y. canaliculatus |
| Binomial name | |
| †Yamatocetus canaliculatus Okazaki, 2012 | |
Yamatocetus canaliculatus is an extinct species of eomysticetid baleen whale from the Late Oligocene of Japan.
The holotype specimen, a partial skeleton, was discovered by curator Toshiyuki Kamei in October 1981 in the Ashiya Group of the Jinnobaru Formation on the island of Kyushu, and stored in the Kitakyushu Museum of Natural History and Human History. The genus name honors the old name of Japan, Yamato, and Latin cetus, "whale." The species name translates to "with groove" in reference to the grooves in its mouth which indicate functional baleen. It was placed into the family Eomysticetidae, a primitive group of baleen whales. [1]
As in other baleen whales, the skull is wide and flat. The skull, along the sides, has several narrow, straight grooves–eight in total–and there are several foramina in the skull used as passage for blood vesselsthough not as many as modern baleen whales–which indicate it had baleen in its mouth. The skull contains several tooth sockets, but no teeth were discovered; it is possible these sockets were vestigal and held no teeth, or the teeth naturally fell out over the course of the animal's life. The blowhole was located midway along the snout, near the position of eyes. Unlike the closely related Eomysticetus , the head is relatively flat, and the snout is blunt. The auditory bulla in the ear is more similar to primitive baleen whales and archaeocetes. [1]
Yamatocetus had seven neck vertebrae, and the complete series was preserved. The neural spine projecting up from the vertebra increases along the series. Thoracic vertebrae one through seven and nine were preserved. The ribs had a double-headed joint with the vertebrae. The shoulder blade was concave on the side facing the ribs, a primitive feature for baleen whales. [1]
The left forearm and fragments of the right were discovered. As in modern whales, the elbow joint probably could not rotate. The humerus is proportionally longer than that of modern baleen whales, a characteristic of eomysticetid whales, suggesting eomysticetids had less swimming capabilities. [1]
Yamatocetus was an early filter feeder, and its teeth, if it had any, were likely vestigial and served no purpose. Since the whale had several double-headed vertebral joints, and modern baleen whales have a reduced number of these to increase the flexibility of the ribcage to aid in deep-sea diving, Yamatocetus likely stayed near the surface. [1]
Several marine vertebrates were found in the Ashiya Group: the baleen whales " Metasqualodon ," Chonecetus , and an unidentified aetiocetid; and an unidentified squalodontid dolphin; a Trionyx softshell turtle; the tortoise Indotestudo takasago ; an unidentified sea turtle; an unidentified crocodile; the seabirds Copepteryx and an unknown pelagornithid; and unknown dugong. An unknown amynodontid, a terrestrial hippo-like animal, was also found. [1]
Baleen is a filter-feeding system inside the mouths of baleen whales. To use baleen, the whale first opens its mouth underwater to take in water. The whale then pushes the water out, and animals such as krill are filtered by the baleen and remain as a food source for the whale. Baleen is similar to bristles and consists of keratin, the same substance found in human fingernails, skin and hair. Baleen is a skin derivative. Some whales, such as the bowhead whale, have longer baleen than others. Other whales, such as the gray whale, only use one side of their baleen. These baleen bristles are arranged in plates across the upper jaw of whales.
The evolution of cetaceans is thought to have begun in the Indian subcontinent from even-toed ungulates 50 million years ago 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 or earlier and to share a relatively recent closest common ancestor with hippopotamuses. 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.
Janjucetus is an extinct genus of cetacean, and a basal baleen whale (Mysticeti), from the Late Oligocene around 25 million years ago (mya) off southeast Australia, containing one species J. hunderi. Unlike modern mysticetes, it possessed large teeth for gripping and shredding prey, and lacked baleen, and so was likely to have been a predator that captured large single prey animals rather than filter feeding. However, its teeth may have interlocked, much like those of the modern-day filter-feeding crabeater seal, which would have allowed some filter-feeding behavior. Its hunting behaviour was probably similar to the modern-day leopard seal, probably eating large fish. Like baleen whales, Janjucetus could not echolocate; however, it did have unusually large eyes, and so probably had an acute sense of vision. The only specimen was found on the Jan Juc beach, where the remains of the extinct whales Mammalodon, Prosqualodon and Waipatia have also been discovered.
Mammalodon is an extinct genus of archaic baleen whale belonging to the family Mammalodontidae.
Mammalodontidae is a family of extinct whales known from the Oligocene of Australia and New Zealand.
Aetiocetus is a genus of extinct basal mysticete, or baleen whale that lived 33.9 to 23.03 million years ago, in the Oligocene in the North Pacific ocean, around Japan, Mexico, and Oregon, U.S. It was first described by Douglas Emlong in 1966 and currently contains known four species, A. cotylalveus, A. polydentatus, A. tomitai, and A. weltoni. These whales are remarkable for their retention of teeth and presence of nutrient foramina, indicating that they possessed baleen. Thus, Aetiocetus represents the transition from teeth to baleen in Oligocene mysticetes. Baleen is a highly derived character, or synapomorphy, of mysticetes, and is a keratinous structure that grows from the palate, or roof of the mouth, of the whale. The presence of baleen is inferred from the fossil record in the skull of Aetiocetus. Aetiocetus is known from both sides of the Pacific Ocean: it was first documented in Oregon, United States, but it is also known from Japan and Mexico. The genus is currently constrained to the Northern hemisphere and has little value in biostratigraphic studies of the Oligocene due to its limited occurrences across the Pacific.
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.
Livyatan is an extinct genus of macroraptorial sperm whale containing one species: L. melvillei. The genus name was inspired by the biblical sea monster Leviathan, and the species name by Herman Melville, the author of the famous novel Moby-Dick about a white bull sperm whale. It is mainly known from the Pisco Formation of Peru during the Tortonian stage of the Miocene epoch, about 9.9–8.9 million years ago (mya); however, isolated teeth from other locations such as Chile, Argentina, South Africa, and Australia implies that either it or a close relative survived into the Pliocene, around 5 mya, and was present throughout the Southern Hemisphere. It was a member of a group of hyper-predatory macroraptorial sperm whales and was likely an apex predator, preying on whales, seals, and so forth. Characteristic of raptorial sperm whales, Livyatan had functional, enamel-coated teeth on the upper and lower jaws, as well as several adaptations for hunting large prey.
Acrophyseter is a genus of extinct sperm whales that lived in the Late Miocene off the coast of Peru comprising two species: A. deinodon and A. robustus. It is part of a group of macroraptorial sperm whales which all shared several features for the purpose of hunting large prey, such as deeply-rooted and thick teeth. Acrophyseter measured 3.9–4.3 metres (13–14 ft), making it the smallest raptorial sperm whale. Because of its short pointed snout, and its strong curved front teeth, it probably fed on the large marine vertebrates of its time, such as seals and other whales.
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Tohoraata is a genus of eomysticetid baleen whale from the Late Oligocene (Chattian) of New Zealand. There are two recognized species, T. raekohao and T. waitakiensis.
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Waharoa is a genus of eomysticetid baleen whale from the Late Oligocene (Chattian) of New Zealand. It was identified with the discovery of Waharoa ruwhenua by Boessenecker and Fordyce (2015), which added a new genus and species to a monophyletic family Eomysticetidae.
Horopeta is a genus of baleen whale from the Late Oligocene (Chattian) Kokoamu Greensand of New Zealand.
Mystacodon is a genus of toothed baleen whale from the Late Eocene Yumaque Formation of the Pisco Basin in southwestern Peru. It is the oldest known baleen whale, and was probably a suction feeder of small prey on the seafloor.
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Ankylorhiza is an extinct genus of toothed whale that lived in what is now the United States during the Oligocene epoch, between 29 and 23.5 million years ago. The type and only known species is A. tiedemani, though two fossil skeletons may represent an additional, second species within the genus. Ankylorhiza was about 4.8 meters (16 ft) long, with a long, robust skull bearing conical teeth that were angled forwards at the tip of the snout.
