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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. [2] Cetaceans are fully aquatic mammals belonging to the order Artiodactyla and branched off from other artiodactyls around 50 mya.
The order Artiodactyla consists of 349 extant species belonging to 132 genera. This does not include hybrid species or extinct prehistoric species. Modern molecular studies indicate that the 132 genera can be grouped into 23 families; these families are grouped into named suborders and many are further grouped into named clades, and some of these families are subdivided into named subfamilies.
Size varies considerably; the smallest member, the mouse deer, often reaches a body length of only 45 centimeters (18 in) and a weight of 1.5 kilograms (3.3 lb). The largest member, the hippopotamus, can grow up to 5 meters (16 ft) in length and weigh 4.5 metric tons (5 short tons), and the giraffe can grow to be 5.5 meters (18 ft) tall and 4.7 ...
The largest known basilosaurid, Perucetus colossus, is believed to be even bigger, with a length of about 17–20 metres (56–66 ft) and possibly comparable to, if not larger than, the modern blue whale in terms of weight, [15] though other researchers argue that it was much lighter. [16] [17]
Protocetus atavus ("first whale") 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.
Basilosaurus (meaning "king lizard") is a genus of large, predatory, prehistoric archaeocete whale from the late Eocene, approximately 41.3 to 33.9 million years ago (mya). First described in 1834, it was the first archaeocete and prehistoric whale known to science. [2] Fossils attributed to the type species B. cetoides were discovered in the ...
Pakicetus (meaning "whale from Pakistan") is an extinct genus of amphibious cetacean of the family Pakicetidae, which was endemic to Indian Subcontinent during the Ypresian (early Eocene) period, about 50 million years ago. [2]
The interior of a dinosaur egg can be studied using CAT scans or by gradually dissolving away the shell with acid. Sometimes the egg preserves the remains of the developing embryo inside. The oldest known dinosaur eggs and embryos are from Massospondylus, which lived during the Early Jurassic, about 190 million years ago. [2] [3]