A research team from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences reported on Apr. 2 that changes in lung ventilation mechanisms played a key role in shaping body size limits among early land vertebrates. The findings were published in Science Advances on April 1.
The study is significant because it addresses how respiratory systems influenced the evolutionary pathways and ecological dominance of two major groups of terrestrial vertebrates: amniotes, which include reptiles and mammals, and lissamphibians, such as modern amphibians.
Researchers analyzed a dataset comprising 344 fossil species from the Middle Devonian to Early Permian periods. They found that lissamphibians are limited to small body sizes due to their reliance on cutaneous gas exchange and buccal pumping for lung ventilation—modes efficient in water but less so on land. This constraint favors smaller bodies with higher surface area-to-volume ratios for effective gas exchange.
In contrast, amniotes evolved costal lung ventilation—a method involving rib movements—which enabled them to ventilate lungs more efficiently and excrete carbon dioxide effectively on land. This adaptation allowed amniotes to surpass the body size limitations seen in lissamphibians, leading to a wide range of sizes from tiny lizards weighing only grams to massive mammalian herbivores weighing several tons.
The study also confirms that both lineages descended from large-bodied ancestors but diverged due to differences in respiratory traits. Amniote-lineage vertebrates experienced fewer constraints on skull shape as well, enabling them to develop deeper skulls suited for herbivory—a key factor behind their ability to occupy new ecological niches during evolutionary history.
Researchers conclude that these biological differences between amniotes and lissamphibians emerged soon after their split hundreds of millions of years ago, laying the groundwork for modern terrestrial ecosystems.
