Around 50 million years ago, whales began inching toward planet-wide colonization. At the time, reveals research published Thursday in Current Biology, they were small, hooved, legged, and land-locked animals living in South Asia — quite unlike the giant, streamlined humpbacks and bowheads we know today.
While scientists know that whales’ ancestors came from the sea onto land, then evolved to once again live in the sea, the exact details of that journey have been sparse. The new paper reveals an important piece of the puzzle: evidence of a four-legged whale that lived along the coast of Peru 42.6 million years ago. The international team named this newly identified species Peregocetus pacificus or “the traveling whale that reached the Pacific.”
This skeleton, dug out from the coastal desert Playa Media Luna, is the first indisputable record of a quadrupedal whale skeleton for the whole Pacific Ocean. Additionally, it’s the most complete skeleton found outside of India and Pakistan, and it is the oldest found yet in the Americas.
Appearance-wise, this whale did not look like the whales we have come to know. Like its ancestors in South Asia, it still had small hooves, which indicate that it was still capable of standing and even walking on land. Bones in its tail are reminiscent of beavers’ and otters’ tails, suggesting that the body part was essential to its swimming capabilities. Overall, the ancient animal was four meters long, and the physical evidence suggests it possessed locomotion abilities that enabled it to travel great distances.
This specimen’s existence also demonstrates that four-legged whales were able to cross the South Atlantic Ocean and disperse as far as the Pacific Ocean — all while retaining functional, weight-bearing limbs — less than 20 million years after their origin. The scientists believe that its descendants later and gradually migrated both farther north and south, until whales reached a truly global distribution.
While the journey is still impressive — from South Asia, to the western coast of Africa, to South America — researchers say that was in part possible because the distance between the latter two continents was half what it is today. This ancient whale also would have been assisted by westward surface currents, which pushed it onward as it swam.
Today, all cetaceans — whales, dolphins, and porpoises — are descendants of these four-legged colonizers. Even the earliest fully aquatic whales still wore visual reminders of their past, little external hindlimbs that streamed behind them uselessly. In modern times, scientists still occasionally discover a living whale with the vestiges of small hindlimbs hiding inside its body wall.
Cetaceans originated in south Asia more than 50 million years ago (mya), from a small quadrupedal artiodactyl ancestor. Amphibious whales gradually dispersed westward along North Africa and arrived in North America before 41.2 mya. However, fossil evidence on when, through which pathway, and under which locomotion abilities these early whales reached the New World is fragmentary and contentious. Peregocetus pacificus gen. et sp. nov. is a new protocetid cetacean discovered in middle Eocene (42.6 mya) marine deposits of coastal Peru, which constitutes the first indisputable quadrupedal whale record from the Pacific Ocean and the Southern Hemisphere. Preserving the mandibles and most of the postcranial skeleton, this unique four-limbed whale bore caudal vertebrae with bifurcated and anteroposteriorly expanded transverse processes, like those of beavers and otters, suggesting a significant contribution of the tail during swimming. The fore- and hind-limb proportions roughly similar to geologically older quadrupedal whales from India and Pakistan, the pelvis being firmly attached to the sacrum, an insertion fossa for the round ligament on the femur, and the retention of small hooves with a flat anteroventral tip at fingers and toes indicate that Peregocetus was still capable of standing and even walking on land. This new record from the southeastern Pacific demonstrates that early quadrupedal whales crossed the South Atlantic and nearly attained a circum-equatorial distribution with a combination of terrestrial and aquatic locomotion abilities less than 10 million years after their origin and probably before a northward dispersal toward higher North American latitudes.
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