Decline of Early Crocodile Helped the Early Marine Turtle

Marine turtles experienced an evolutionary windfall thanks to a mass extinction of crcodyliforms around 145 million years ago. 

Crocodyliforms comprise modern crocodiles and alligators and their ancient ancestors, which were major predators that thrived on Earth millions of years ago. They evolved into a variety of species including smaller  ones that lived on land through to mega-sized sea-swimming species that were up to 12 metres long. However, around 145 million years ago crocodyliforms, along with many other species, experienced a severe decline -- an extinction event during a period between two epochs known as the Jurassic/Cretaceous boundary. 

During this boundary up to 80 percent of species on land and in marine environments were wiped out . This decline was primarily due to a drop in sea level, which led to a closing off of shallow marine environments such as lagoons and coastal swamps. These were the homes and primary hunting grounds for may crocodyliforms. The decimation of many marine crocodyliforms may also have laid the way for their ecological replacement by other large predatory groups such as modern shark species  and new types of plesiosaurs. Plesiosaurs were long-necked, fat-bodied and small headed ocean-going creature with fins, which later went extinct around 66 million years ago.

Other factors that contributed to the decline of marine crocodyliforms included a change in the chemistry of ocean water with increased sulphur toxicity and a depletion of oxygen. While primitive crocodilyform species on land also suffered major declines, the remaining species diversified into new groups such as  the new extinct notosuchians, which were much smaller in size at around 1.5 metres in length. Eusuchians also came to prominence after the extinction which led to today's crocodiles.

For this research work the researchers analyzed almost 1,200 crocodyliforms fossil records.

Source: Biological Science