Fish grew larger as the oceans cooled over millennia

Two hypotheses describe changes in animal size. Cope’s rule says that body size tends to increase in a given lineage throughout time. Bergmann’s rule states that animals in cold climates tend to have larger bodies while those in warm climates tend to have smaller bodies. 

These rules aren’t universally true among all groups of animals but have been shown to be largely true among “warm blooded” endotherms. A team of researchers recently tested these ideas among tetraodontiform fishes. This group of mainly tropical fishes includes pufferfish, boxfish, and filefish. 

“Cope’s and Bergmann’s rules are fairly well-supported for endotherms, or warm-blooded species, such as birds and mammals. However, among ectothermic species, especially vertebrates, these rules tend to have mixed findings,” explained Emily Troyer, a graduate student at the University of Oklahoma’s Fish Evolution Lab.

To study the trends in the size of ectothermic fish as the oceans cooled over time, the researchers used a mixture of paleontology and genomics to understand the lineage’s evolution.

“When you look across different groups in the tree of life, then you will notice that there are a limited number of groups that actually have a good fossil record, but the larger marine fish group (known as Tetraodontiformes) that includes the popular pufferfish, ocean sunfish and boxfish, is remarkable in that it has a spectacular paleontological record,” said Dahiana Arcila, professor and assistant curator at Sam Noble Museum of Natural History, 

“So, by integrating those two fields, genomics and paleontology, then we’re actually able to bring into the picture new results that you won’t be able to obtain using just one data type.”

What the researchers found is that the ectothermic fishes seemed to follow the two rules much like their endothermic counterparts among mammals and birds. The changes over time followed Cope’s rule, but the changes also coincided with cooling ocean temperatures, following Bermann’s rule as well.

“Based on fossil data, we’re showing that these fish started very small, but you can see that living species are much larger, and those changes are associated with the cooling temperature of the ocean over this very long period of time,” said Arcila. 

The research is published in The Proceedings of the National Academy of Sciences.

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By Erin Moody , Earth.com Staff Writer