Mass Extinctions Hitting the Larger Vertebrates the Hardest

There are numerous biggest and most magnificent species on earth, from sharks to giraffes, who are threatened with extinction.

A new study has taken an interesting look at why animals, especially sea creatures, became significantly smaller for millions of years after a mass extinction event that took place about 360 million years ago.

As reported in NY Times on November 12, 2015.

According to this rule the size of a particular group of animals has a tendency to improve eventually over time owing to the evolutionary benefits of larger body size, which include the ability to catch prey and avoid being hunted.

Sallan says that as many global fish populations are now endangered and ecologists fear that Earth may now be on the brink of a sixth major extinction event caused by human activities.

Instead, the researchers point to an advantage of small size: Among today’s creatures, diminutive species tend to grow and reproduce faster, so that they can produce a new generation before they are killed by predators. This is an example of the so-called Lilliput Effect, or a trend towards small body size following a mass die-off.

According to scientists, this mass extinction was the result of a global chill that brought glaciers to the tropics.

For at least 40 million years following the mass extinction, known as the Hangenberg event, oceans were teeming with smaller fish. Aside from fish being generally smaller animals, many fish populations are suffering the dire aftereffects of overfishing. The event in question took place 359 million years ago, at the tail-end of the Devonian Period. “But, in the aftermath of the extinction, that ends up being a bad strategy in the long term”.

To sort out the body-size trends around the Hangenberg Event, Sallan and her coauthor, Andrew K. Galimberti, now a graduate student at the University of ME, amassed a dataset of 1,120 fish fossils spanning the period from 419 to 323 million years ago.

Apart from this, the researchers also examined museum specimens and photographs along with other published papers to gain more information about fish body sizes where they concluded that the vertebrate size increased during the Devonian period a few 419 to 359 million years ago. “There were fish called arthrodire placoderms with large slashing jaws that were the size of school buses, and there were relatives of living tetrapods, or land-dwelling vertebrates, that were nearly as large”.

The study finds that, similar to the mass extinction that’s underway now, the end-Devonian extinction resulted in the loss of most large-bodied vertebrates. “These disturbances are shifting natural selection so that smaller, faster-reproducing fish are more likely to keep going, and it could take a really long time to get those bigger fish back in any sizable way”. The ecosystem, suddenly interrupted by fire, is first dominated by smaller, faster-growing grasses and shrubs. However, they claimed that the cause is less important than understanding how long it would take for these species to recover.