While it is commonly believed that Hippopotamus are related to horse or pigs, Hippopotamus is actually more closely related to Whales.
The name of the animal “hippopotamus” itself have rooted from ancient Greek words “hippos” which means horses and “potamos” which means river. From the name alone, we can deduce that ancient morphologist tend to think about Hippopotamus as horses that live in the river. However, a combination of palaeontological research and genetic research have proved that hippopotamus are actually more related to whales than horses.
image taken from www.physorg.com
The genetic evidence was derived from genetic sequence evolutionary history tracking method developed by scientists at Tokyo Institute of Technology. Genetic-wise, hippopotamus DNA have a lot of similarity with those of whales. It is theorized that both whales and hippopotamus was descended from a common ancestor who happen to love water about 50-60 million years ago. This common ancestor lineage later split into two groups the cetaceans who like water so much that they abandon land and anthracotheres who still live in fresh water area inside continental mass.
The anthracotheres later branched into many species that later went extinct during the Pliocene epoch, leaving hippopotamus as their only extant descendant.
image taken from www.palaeo.gly.bris.ac.uk
In the sea, the cetacean family split into many species such as whale, dolphins and porpoise. They have abandoned their land live entirely and their legs specialize into fins. Even now, the skeletal structure of their bone still resemble the skeletal structure of hands in land mammal.
image taken from www.nationalgeographic.com
The common ancestor
image taken from www.dinozaury.com
At the moment, no one is really sure about which animal is actually the common ancestor of both whales and hippopotamus. The first candidate is an animal living during early Eocene known as pakicetus. Pakicetus fossils were found in Pakistan in 1983 by Philip Gingerich, Neil Wells, Donald Russell, and S. M. Ibrahim Shah.
Pakicetus are carnivorous land animals with unusual ear anatomy. The auditory bulla was formed only by ectotympanic bones, which is the feature that suggest that this animal was related to whales. The other features suggesting the relation between Pakicetus and whales are the folding in a bone of the middle ear and the cusps arrangement on the molar teeth.
image taken from www.adm.es
The second possible common ancestor was an cat-sized Eocene animal from India known as Indohyus. This animal was considered habitually aquatic because it have osteosclerotic bones. This means that Indohyus have thick and heavy outer coating to bones which is useful to reduce buoyancy and make it possible for the animal to stay submerged. This is similar to the bone composition of Hippopotamus.
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