Contributed by Ziqi Wu, Tian Mi, Lei Huang, Zhuo Li.

Imagine myriad fishes, including creatures the size of school buses, swimming in the Earth’s seas. Yup. That’s how things used to be 360 million years ago. It was not until the appearance of Devonian-Mississippian vertebrates that the modern range of body sizes became readily visible in the marine system. What happened in between these times? What made the creatures’ sizes shrink and spoiled the fun of scuba diving nowadays?

The answer is that a massive extinction happened about 359 million years ago, at the end of the Devonian Period. The scientists named it the Lilliput Effect: a temporary size reduction following mass extinction, which is usually temporary, but sometimes becomes persistent in specialized groups, such as birds, plankton, or island faunas. But how can a smaller size increase an organism’s fitness? One popular misconception is that the fittest organisms in a population are those that are strongest and largest. However, the Lilliput Effect has proved this wrong.

A giant fish has the length of a school bus before the die-off. However, after the asteroid hit earth and a mass extinction took place, the surviving fishes, and their descendants, are smaller since smaller size gives them an advantage.

The mechanism underlying the effect is controversial. One hypothesized model points to the effect of weather. Researchers (Dahl et al.) tracked down redox history of the atmosphere and oceans and found out that the radiation of large predatory fish (animals with high oxygen demand) correlates with atmosphere oxygenation. Therefore, a lower level of oxygen in the atmosphere might have played a role in the overall shrinkage. A temperature model based on Bergmann’s rule proposes that size is negatively influenced by temperature (Bergmann 1847). Previous studies suggested that warm-blooded vertebrate species are larger in colder climates than their congeners inhabiting warmer climates (Harries and Knorr 2009).

However, a more recent study points to another advantage of smaller size: smaller individuals grow and reproduce faster, and they adapt to their environments more quickly because of their relatively short generation times (Sallan and Galimberti 2015). Using holocephalans and ray-finned fishes as examples, they demonstrated that smaller vertebrates tend to have high reproductive rates, short generation times, and large populations. These traits may increase survival while promoting diversification via higher variation and population fragmentation.

Altogether, although there is empirical evidence suggesting that Lilliput effect could be a general response to environmental stress following various mass extinction events, there remain many puzzles to be solved.

For further reading, see:

Adam K. Huttenlocker and Jennifer Botha-Brink. 2013. Body size and growth patterns in the therocephalian Moschorhinus kitchingi (Therapsida: Eutheriodontia) before and after the end-Permian extinction in South Africa. Paleobiology 39(2), 353-277.

Bing Huang, David A.T. Harper, Renbin Zhan, Jiayu Rong. 2010. Can the Lilliput Effect be detected in the brachiopod faunas of South China following the terminal Ordovician mass extinction? Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 285, Issues 3–4, Pages 277-286.

Keller, G., Abramovich,S. 2009. Lilliput effect in late Maastrichtian planktic foraminifera: Response to environmental stress. Palaeogeography, Palaeoclimatology, Palaeoecology 284: 47-62

Peter J. Harries, Paul O. Knorr. 2009. What does the ‘Lilliput Effect’ mean? Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 284, Issues 1–2, Pages 4-10.

Richard J. Twitchett. 2007. The Lilliput effect in the aftermath of the end-Permian extinction event, Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 252, Issues 1–2, Pages 132-144, ISSN 0031-0182.

Sallan L, Galimberti AK. 2015. Body-size reduction in vertebrates following the end-Devonian mass extinction.Science 350(6262):812-815.