14 DAY TRIAL // Fossil lower jaw bones recently found in Dogali fossil site (Eritrea) and dated 27 million years old have proved to belong to a missing link between ancient elephant ancestors and the lineage of the modern elephants.
Elephants and their extinct relatives form the Order Proboscidea. The only living species of this order are the African and Asian elephants.
The new species, named Eritreum melakeghebrekristosi, is estimated to have been 1,067 pounds (400 kg) heavy and about 4.2 feet (1,4 m) tall, about the size of a large hog. Proboscidea originated in Africa, in contrast to other groups like rhinos and zebras, which evolved in North America or giraffes and antelopes, which have their origins in Asia and later migrated to Africa.
Proboscidea appeared 50 million years ago. Till 25 million years ago, proboscideans were found only in Africa and close areas in Asia and most of them were palaeomastodonts. These were smaller than today's elephants, with short trunks and tusks and plain dentition.
About 24 million years ago, the group split into two branches: Elephantida, which includes elephants and mammoths, with a very specialized horizontal tooth displacement, and Mammutida, or the mastodons, preserving the initial dentition pattern found in paleomastodonts.
The first Elephantida were the gomphotheres, archaic elephants. But, before this evolutionary split, there was a fossil gap. No proboscidean fossils were known for the period 34 to 24 million years ago. "Throughout Africa, there is a gap in the proboscidean record in the late Oligocene" said Jeheskel Shoshani, professor of biology at the University of Asmara in Eritrea.
"These specimens date from exactly the time where there is a gap."
"The dating of the new fossil pushes the origins of elephants and mastodons five million years farther into the past than previous records," said William J. Sanders, an assistant research scientist in the University of Michigan Museum of Paleontology.
The jawbone pieces were discovered with teeth in place. The scientists were able to visualize the oldest evidence of a proboscidean with horizontal tooth displacement. Thus, Eritreum would be the earliest known right elephant. The size of the teeth is about the same size as the teeth of paleomastodons, and smaller than all known gomphotheres, that lived between 12 and 2 million years ago, and were the earliest previously known direct elephant ancestors.
Eritreum looked very much like a modern elephant, but with a proportionally shorter trunk, based on jaw characteristics and four tusks instead of two (this trait is common in all primitive proboscideans, gomphoteres included).
As Elephantida species evolved, they grew larger and their dentition developed more complex. What is - in fact - the horizontal tooth displacement? Humans and most other mammals have, on the adult stage, a permanent set of teeth. But elephants rotate five teeth in a lifetime on a half jaw.
Their teeth don't emerge vertically from the gums, but are pushed towards the front of the mouth horizontally, one by one, like a conveyer belt. A new tooth grows in the back of the jaw and pushes forward the already existing tooth, which eventually falls out and the next will push this one and so on. The turnover makes an elephant get by with only four teeth, each located in a half jaw.
So, at a given moment, an elephant will present on a half jaw a full tooth or a falling one and an emerging one. "This is one adaptation for extending life and being large," Sanders told.
When an elephant grinds its fifth tooth, it will just starve to death. The only molar tooth at a time has complex textures, providing large surface areas for grinding and extracting precious nutrients from grassy food. "The new fossil from Eritrea is important because it shows aspects of dental anatomy in common with the advanced group, including molars with more cusps and complex crowns and the delayed maturation and emergence of molars. In age and anatomy it is exactly the sort of intermediate evolutionists would expect to bridge the gap between archaic and advanced proboscideans," Sanders said.
"The first proboscideans were the size of dogs, and they've become larger and larger" as they have evolved, he continued. It is more efficient to be bigger. You have fewer enemies, and you can eat more efficiently", said Shoshoni.
Photo credit: PNAS Image credit: Gary H. Marchant