Mass Extinction Brought About Dominance of Small Fish

Mass Extinction Brought About Dominance of Small FishA paleontologist from the University of Pennsylvania released a study this week that suggests a mass extinction event millions of years ago had resulted in smaller fish dominating the seas, instead of larger ones, and doing so for longer than once believed.

Larger fish such as sharks are considered by many to be the kings of the seas, but by and large, it is smaller sea creatures that populate the Earth’s waters. But it was much more than just that in the first several million years after a mass extinction that took place after the Devonian Period. According to the University of Pennsylvania’s Lauren Sallan, the so-called Hangenberg Event, a mass extinction dating almost 360 million years ago, had resulted in a major change in the size of vertebrates that are most prevalent in the world’s seas and oceans. Prior to this event, large creatures were far more common than smaller ones, but for 40 million years or more following the extinction, much smaller fish were more dominant than the bigger ones.

“Rather than having this thriving ecosystem of large things, you may have one gigantic relict, but otherwise everything is the size of a sardine,” said Sallan in a statement. She believes that while the dominance of smaller fish technically lasted at least 40 million years, it may have had an impact on the trends in our ecosystem nowadays. Aside from fish being generally smaller animals, many fish populations are suffering the dire aftereffects of overfishing.

Sallan and co-researcher Andrew Galimberti, from the University of Maine, analyzed data of 1,120 fish fossils that existed 419 to 323 million years ago. Their findings revealed that vertebrates had gradually become larger during the Devonian Period, which took place 419 to 359 million years ago. By the end of that period, some fish were as big as the average school bus. “You had some vertebrates that are small, but the majority of residents in ecosystems, from bottom dweller to apex predator, were a meter or more long,” added Sallan.

However, the mass extinction event did happen, resulting in die-offs of several species on water and land alike. Sallan and Galimberti’s study shows that over 97 percent of vertebrate species became extinct during the event. In the years that followed, animals became smaller and their size trended downward for much longer than they normally should. Sallan observed that the decrease in animal size in the 40 million years after the mass extinction had nothing to do with climate or oxygen, thus challenging a widely-held convention; instead, she believes it was ecological factors that drove the size decrease.

But what happened to larger animals? What share did they take up of the 3 percent of vertebrates that remained post-extinction event? It turns out that even animals we consider to be large nowadays had become much smaller than what one would think. “Some large species hung on, but most eventually died out,” Sallan explained. “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.” This is an example of the so-called Lilliput Effect, or a trend towards small body size following a mass die-off.

Sallan also explained the logic behind a Lilliput Effect that favors smaller creatures. She said that organisms growing to large sizes before reproduction becomes a “bad strategy in the long term” following an extinction, which allows tiny, fast-reproducing fish, for instance, to “take over the entire world.” Further, she noted that the cause behind the large fish’s extinction or the instability of an ecosystem isn’t important at the end of the day. Instead, it may be more important to take stock of how long it could take large species to recover from a mass extinction, due to natural selection being shifted to favor the smaller, faster-reproducing animals.