According to new study led by the University of Pennsylvania, a mass extinction 359 million years ago known as the Hangenberg event was followed by a transformation of the vertebrate community, UPI reports.

The study reports that before the mass extinction occurred, large creatures dominated the earth. However, after the die off that wiped out 97 percent of all vertebrate species, the oceans were filled with smaller sized fish.

"Rather than having this thriving ecosystem of large things, you may have one gigantic relict, but otherwise everything is the size of a sardine," Lauren Sallan, an assistant professor at the University of Pennsylvania, said in a press release.

Laren Sallan, who led the study, and her colleagues decided to find a connection between body size and climatic conditions but failed to find a correlation.

"There was no association with either temperature or oxygen, which overturns everything that has been assumed in vertebrates both today and in the past," Sallan said.

"Instead it tells us that these trends must be based entirely on ecological factors."

The report suggests that it is the catastrophic event itself and the instability that follows that is the deciding factor. Ecological stability puts the Cope rule into effect, while the instability gives way to the Lilliput Effect.

Cope's rule states that the body size of a particular group of species tends to increase over time because of the evolutionary advantages while the Lilliput Effect holds that after mass extinctions, there is a temporary trend toward small body size.

For the study, Sallan and her coauthor, Andrew K. Galimberti, now a graduate student at the University of Maine, collected a data of 1,120 fish fossils spanning the period from 419 to 323 million years ago, Upenn reports.

"Some large species hung on, but most eventually died out," Sallan said.

"So the end result is an ocean in which most sharks are less than a meter and most fishes and tetrapods are less than 10 centimeters, which is extremely tiny. Yet these are the ancestors of everything that dominates from then on, including humans."

The new research was published online this week in the journal Science.