An ocean floor in the sand
The Western Desert of Egypt was not always a desert. During the Eocene epoch, between 40 and 36 million years ago, this entire area was a shallow, tropical extension of the ancient Tethys Sea. This former sea floor, now a wind-carved landscape of sandstones and mudstones, is Wadi Al-Hitan. Designated a UNESCO World Heritage Site, the valley contains an enormous concentration of high-quality fossils that record a major shift in life on Earth. Hundreds of marine vertebrate skeletons are found here, including over 400 whales. The sheer number and exceptional preservation of these fossils show a detailed picture of this ancient marine ecosystem.
The most common fossils are two types of early whales, or archaeocetes. The first is the massive Basilosaurus isis, a serpentine predator that could reach up to 21 meters in length. A complete skeleton of an 18-meter-long Basilosaurus has been found at the site. The second is the smaller, dolphin-like Dorudon atrox, which measured about 5 meters long. These creatures were fully aquatic, with streamlined bodies and forelimbs evolved into flippers. Yet, they retained distinct skulls and teeth similar to their land-based carnivorous ancestors. Some Basilosaurus fossils have been found with their last meals preserved, which included smaller whales like Dorudon, as well as crabs and sawfish.
The legs that changed everything
The most important feature of the Wadi Al-Hitan whales is the presence of hind legs. A 1989 expedition led by paleontologist Philip Gingerich discovered skeletons with small, but fully formed, hind limbs. These legs, complete with a femur, tibia, fibula, and articulated feet and toes, were previously unknown in this sub-order of whales.
The limbs were far too small to support the animals' weight on land, proving they lived a fully marine existence. Their function is a subject of study, but one leading hypothesis suggests they were used as copulatory guides during mating. The discovery of these vestigial legs showed the evolutionary transition of whales from terrestrial mammals to ocean dwellers. The pelvis was not connected to the vertebral column, another major shift from their land-walking ancestors.
The site also preserves a wide array of other marine life. Fossils of early sea cows (sirenians), crocodiles, sea turtles, and sea snakes have all been recorded. The teeth of numerous shark and ray species are also common finds. Together, these fossils allow for a detailed reconstruction of the coastal environment these legged whales inhabited millions of years ago.