The water-to-land transition is one of the most important events in vertebrate evolution, giving rise to two major groups of modern terrestrial vertebrates, amniotes and amphibians, which occupy markedly different ecological niches.
Currently, aquatic amphibians are diversified, but their body sizes are limited to small ones, ranging from 0.03 to 10,800 grams. This limitation may be due to constraints imposed by the respiratory system, which is characterized by cutaneous gas exchange and buccal pumping (i.e., use of the oral cavity) for pulmonary ventilation. While these modes of breathing are efficient in water, they are less efficient in air due to the slow excretion of waste carbon dioxide (CO2). This reduces the volume and maximizes the surface area to volume ratio, which is important for effective skin gas exchange.
In contrast, amniotes exhibit different body sizes between extant species ranging from 0.2 to 180,000,000 grams and a wide dietary range. Because of this, they have been a major group in terrestrial ecosystems since the early Permian period, about 299 million years ago. Amniotes ventilate their lungs by moving their ribs. This process is known as costopulmonary ventilation. This mechanism allows for large amounts of pulmonary ventilation and efficient CO2 excretion on land, allowing amniotes to overcome evolutionary constraints that limit the maximum body size that terrestrial vertebrates can achieve.
An important hypothesis for the terrestrialization of vertebrates proposes a plausible link between the evolution of body size and the emergence of costopulmonary ventilation during the deep divergence of amniote and amphibian ancestors. However, this long-standing hypothesis remained incompletely tested until recently.
To address this gap, a research team from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP), Chinese Academy of Sciences, used a new dataset of 344 fossil species to assess the evolution of body size, skull shape, and respiratory traits. This study focused on the early divergence of terrestrial vertebrates from the Middle Devonian to the Early Permian.
Their discovery is scientific progress April 1st.
The research team’s analysis confirmed that different lineages of early land vertebrates showed different patterns of body size evolution. The ancestors of both amniotes and amphibians moved independently from a large-bodied common ancestor to a small-bodied adaptive region. In particular, amphibian progenitors showed stronger constraints on body size evolution. In contrast, terrestrial vertebrates of the amniote lineage have experienced a relaxation of these constraints and have been able to expand their maximum body size limits over the course of their evolutionary history.
Furthermore, this study demonstrated that buccal lung ventilation was the ancestral breathing mode of all terrestrial vertebrates, which was inherited by our terrestrial amphibian pioneers. Later, traits related to costopulmonary ventilation evolved in the lineage leading to modern amniotes. For example, curved ribs along a mesiodistal axis and an elongated neck region appeared early in pedunculated amniotes, suggesting that the ancestors of modern reptiles and mammals were already using costopulmonary ventilation.
The ancestors of amphibians retained buccal-pulmonary ventilation, increasing their reliance on cutaneous CO2 excretion. This is consistent with strong constraints favoring small body size in this group. The small body maximizes the surface area to volume ratio necessary for efficient skin gas exchange. This trait has also been inherited by modern amphibians.
The evolution of costopulmonary ventilation in terrestrial vertebrates of the amniote lineage not only relaxed body size constraints but also freed skull shape from the functional limitations imposed by buccal pulmonary ventilation. Amniotes evolved deeper skulls, allowing functional division of the adductor muscles. This adaptation enhanced the ability of teeth to apply static pressure during occlusion, an important prerequisite for the evolution of herbivores. This innovation allowed amniotes to occupy new ecological niches by digesting plant resources, and multiple lineages of both herbivores and their predators overcame the maximum body size constraints of the Early Permian.
Today, amniotes exhibit even greater body size differences among terrestrial-adapted species, ranging from gigantic herbivorous mammals weighing several tons to dwarf lizards weighing only a few grams. In contrast, amphibians rely on CO2 excretion through their skin and are therefore still limited to small size.
This study provides convincing evidence that these markedly different biological traits in modern members of both groups diverged soon after their origins and established the basis of modern terrestrial ecosystems hundreds of millions of years before the widespread radiation of modern species.
sauce:
Chinese Academy of Sciences
Reference magazines:
Yu, Y, and others. (2026). Separation of phenotypic constraints framed by respiratory adaptations in the rise of terrestrial vertebrates. Science progresses. DOI: 10.1126/sciadv.aeb0801. https://www.science.org/doi/10.1126/sciadv.aeb0801
