D. TAB RASMUSSEN (top)/TRENT SCHINDLER/NSF (bottom)

Fossils recovered from the Ethiopian highlands are helping scientists fill in long-lost branches on the family tree of modern-day elephants. According to a report published today in the journal Nature, five kinds of proboscidean (the group that includes elephants and their extinct relatives) were recovered, as well as three other types of prehistoric creatures. The discoveries should help explain why certain mammal species survived and thrived once a land bridge granted access to Eurasia some 24 million years ago.

Between 32 and 24 million years ago, Africa and Arabia formed a single continent that was isolated from other landmasses. The fossils--including jaws, molars and partial skeletons--were found in the Chilga region of what is now Ethiopia and date to 27 million years ago. "These are the 'missing years' for Afro-Arabia, and what, exactly, happened to mammals during this eight-million-year period has long remained a mystery to science," says project leader John Kappelman of the University of Texas. The remains of the proboscideans (such as the teeth shown above) included both extremely primitive varieties as well as advanced species that are more closely related to extant elephants, suggesting that new species continued to evolve throughout the missing years on the isolated continent. "These ancestral elephants were much smaller than today's African elephants, but at nearly 1,000 kilograms--about that of a medium-sized Texan longhorn--they were still a bit too big to keep in your backyard," notes study co-author William J. Sanders of the University of Michigan.

The remains also shed light on animals that lived in the region but left no living descendants. For example, the arsinoithere (reconstructed in the bottom image above), did not survive past 24 million years ago. Scientists posit that it was most likely outcompeted by novel species originating from Eurasia. Although the new finds help to fill in some blanks of the story of Afro-Arabian mammalian evolution, the tale is far from complete. "We have unveiled only a few of the secrets of the mammal evolution on the Afro-Arabian continent," notes Jean-Jacques Jaeger of the University of Montpellier in France. "Many more surprising discoveries are to be expected."