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Protocetidae
The protocetids form a diverse and heterogeneous group of cetaceans known from Asia, Europe, Africa, and North America.
==Description==
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 was still connected to one of the sacral vertebrae. 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'', 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''.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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