Mesonychians were a diverse group of carnivorous mammals that lived during the Eocene epoch, around 50 to 34 million years ago. These animals inhabited forests and were somewhat similar in appearance to modern-day wolves or bears, with long bodies and relatively short limbs.
Over time, some mesonychians began to adapt to a semi-aquatic lifestyle, possibly due to the abundance of food resources in aquatic environments. As they spent more time in the water, their limbs gradually modified to become more paddle-like, and their bodies became streamlined. Their teeth also transformed, becoming more suitable for catching and consuming fish and other aquatic prey.
As these adaptations continued, some mesonychians transitioned into fully aquatic creatures, ultimately giving rise to the diverse groups of whales, dolphins, and porpoises that we see today. This evolutionary process is known as cetaceanization.
It's important to note that the exact evolutionary relationships between different cetacean groups are still being studied, and some aspects of whale origins remain subjects of debate and further investigation. However, the general understanding is that whales evolved from land-dwelling mesonychians through a series of gradual adaptations and transitions over millions of years.