Elephants and related forms are collectively known as Proboscideans.
The origin of proboscideans happened around 60 million years ago. At the end of Pleistocene period i.e, around 10,000 years ago they were witnessed with spectacular radiation and the evolutionary change was marked by the overall increase in body size. During the early eocene period (~55 million years ago) they occupied almost every continental habitat type.
The oldest proboscidean was called Moeritherium whose fossil aging 50 million years was found in the 20th century. Moeritherium are 3 feet tall and just have a broad upper lip without any trunk to eat aquatic vegetation. The warm shallow waters along the fringes of tethys sea led to the evolutionary divergence of proboscidea, which are the largest of land animals and sirenia, which are strictly aquatic.
Numidotherium belongs to the same species that lived in shallow muddy and warm water environments with fluctuating shorelines. During this period the climate grew warmer and drier and many other herbivorous animals evolved to digest coarse plants. Numidotherium evolved in size, the larger body size is a result of nutritional needs for processing greater quantities of low quality vegetation.
Later, they grew small tusks because of the need to be able to have the shearing action to crop land vegetation and are known as Barytherium, which resemble mostly with the modern day hippopotamus with eight small tusks with the upper pairs vertical and lower pairs horizontally projecting forwards from the mouth.
Around 35 million years ago, the temperature started to grow cooler and seasonal during this transition to the oligocene . Tropical vegetation and polar deciduous forests also started to shrink. Then palaeomastodon started to originate and dwell in forest and open woodland. These had shorter and flattened tusks possibly aiding them in feeding.
Around 25 million years ago, the climate started to warm up again being even drier than earlier.Due to the shift in continental plates, uplifting of mountains causing change in oceanic and atmospheric circulations set up new climatic conditions. Then Deinotherium started to originate for which a movable trunk appeared with tusks attached to the lower jaw curving downwards without any tusks attached to the upper jaw. This form has been very stable and perfectly adapted to the conditions that it lasted around 20 million years with remarkable increase in body size( a height of 4m is reached by Deinotherium Giganteum). There was a hypothesis stating that the downward curving tusks help the animals to anchor themselves to the river bank while sleeping or possibly used for digging up plant roots and vegetation.weathering which removed carbon dioxide from the atmosphere and transported it to the oceanic sink. This reverse greenhouse effect accompanied a cooler trend in climate, this increasing aridity led to the expansion of grasslands and drier vegetation plants and all the animals started to eat grass as main part of their food which led to dental adaptations like complex surface patterns and stronger reinforcements. Finally a form named as stegodon ganesa, which was 3.5 m tall, with very large tusks that touch the ground emerging from lower jaws sideways and upward curved at the end was originated.
Dwarfing of mammoths and Elephas which used to be around 4-6 feet tall started and over 5000 years period of time the species height is reduced by 40%. There are several explanation for dwarfing of mammoths and Elephas. When ice age was completed and climate started to warm up the water resources increased rapidly. Often an island is colonized by a large predator or too small to hold a viable predator population. But, once the island is free from the predators this large body size is not useful for a herbivore. Also, an island has limited resources for which a smaller body size is favoured and thus finally we get our present day elephants with 4000-6000kg and 2.5-3m height.
There are several evolution stages in between with small changes that I didn’t include in this article, but we can see that present day elephants that we see have gained different characteristics from different families of proboscideans
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