Editing Cetacea (section)

==== Transition from land to sea ====

[[File:Adult female and fetal Maiacetus.jpg|thumb|Fossil of a ''Maiacetus'' (red, beige skull) with fetus (blue, red teeth) shortly before the end of gestation <ref name="Gingerich PD">{{cite journal|author1=Gingerich PD|author2=ul-Haq M|author3=von Koenigswald W|author4=WJ Sanders|author5=Smith BH|year=2009|title=New Protocetid Whale from the Middle Eocene of Pakistan: Birth on Land, Precocial Development, and sexual dimorphism|journal=PLOS ONE|doi= 10.1371/journal.pone.0004366|bibcode = 2009PLoSO...4.4366G|volume=4|issue=2|pages=e4366|pmid=19194487|pmc=2629576|doi-access=free}}</ref>]]

The fossil record traces the gradual transition from terrestrial to aquatic life. The regression of the hind limbs allowed greater flexibility of the spine. This made it possible for whales to move around with the vertical tail hitting the water. The front legs transformed into flippers, costing them their mobility on land.<ref>{{cite web |url=https://evolution.berkeley.edu/what-are-evograms/the-evolution-of-whales/ |title=The Evolution of Whales |publisher=Understanding Evolution, University of California Museum of Paleontology |accessdate=21 January 2025}}</ref>

One of the oldest members of ancient cetaceans ([[Archaeoceti]]) is ''[[Pakicetus]]'' from the Middle Eocene of [[Pakistan]]. This is an animal the size of a wolf, whose skeleton is known only partially. It had functioning legs and lived near the shore. This suggests the animal could still move on land. The long snout had [[Carnivore|carnivorous]] dentition.<ref name="Thewissen"/>

The transition from land to sea dates to about 49 million years ago, with the ''[[Ambulocetus]]'' ("running whale"), also discovered in Pakistan. It was up to {{convert|3|m|ft|abbr=on}} long. The limbs of this archaeocete were leg-like, but it was already fully aquatic, indicating that a switch to a lifestyle independent from land happened extraordinarily quickly.<ref>{{Cite journal |last1=Ando |first1=Konami |last2=Fujiwara |first2=Shin-ichi |date=2016-07-10 |title=Farewell to life on land - thoracic strength as a new indicator to determine paleoecology in secondary aquatic mammals |journal=Journal of Anatomy |volume=229 |issue=6 |pages=768–777 |doi=10.1111/joa.12518 |issn=0021-8782 |pmc=5108153 |pmid=27396988}}</ref> The snout was elongated with overhead nostrils and eyes. The tail was strong and supported movement through water. ''Ambulocetus'' probably lived in mangroves in [[brackish water]] and fed in the [[riparian zone]] as a predator of fish and other vertebrates.<ref>{{cite journal |last=Thewissen |first=Hans |author-link=Hans Thewissen |year=1994 |title=Phylogenetic aspects of Cetacean origins: A morphological perspective |journal=Journal of Mammalian Evolution |pages=157–184 |doi=10.1007/bf01473527 |volume=2 |issue=3 |s2cid=27675176}}</ref>

Dating from about 45 million years ago are species such as ''[[Indocetus]]'', ''[[Kutchicetus]]'', ''[[Rodhocetus]]'' and ''[[Andrewsiphius]]'', all of which were adapted to life in water. The hind limbs of these species were regressed and their body shapes resemble modern whales. [[Protocetidae]] family member ''Rodhocetus'' is considered the first to be fully aquatic. The body was streamlined and delicate with extended hand and foot bones. The merged pelvic [[lumbar spine]] was present, making it possible to support the floating movement of the tail. It was likely a good swimmer, but could probably move only clumsily on land, much like a modern [[Pinniped|seal]].<ref name="Thewissen" />