Earth is home to almost nine million plant and animal species, and three quarters of them are on land. Faced with this staggering diversity, the man on the street may wonder how it all came to be. But scientists say that this question has yet to be answered with any clarity.
To address this knowledge gap, the EU-funded TERRA research project, led by Professor Richard Butler of the University of Birmingham, is taking a fresh look at the diversification of land animals with a focus on early four-limbed vertebrates, or tetrapods.
Tetrapods first appeared on land about 360 million years ago and established their first communities over the next 70 million years or so, during the Carboniferous and early Permian periods. These two latter periods of the Palaeozoic Era played a critical role in the evolution of these animals. During that time, Europe and North America was a single, large land mass called Euramerica, which lay at the equator and was covered by tropical rainforests. The forests’ warm and humid climate provided the ideal conditions for early tetrapods to evolve and diversify, leading to the gradual emergence of amniotes
as well as a variety of synapsids
, reptiles and amphibians.
However, towards the end of the Carboniferous, at around 303 to 307 million years ago, these rainforests began to disappear from large parts of the globe. By the early Permian, the climate had become arid and many regions were covered with dryland vegetation. This major environmental change, referred to as the carboniferous rainforest collapse (CRC), resulted in the mass extinction of many plant species and was a critical period in tetrapod evolution. Nevertheless, few studies have focused on how it affected these early vertebrates’ diversity at the time.
Previous efforts to gauge the impact of the CRC failed to take into account spatial and temporal sampling biases in existing fossil data. Furthermore, they analysed data at the family level (two taxonomic ranks higher than species) rather than focusing on species. To address this deficiency, the TERRA project team members have compiled new sets of data and developed improved methods for estimating changes in diversity. They describe their findings in the journal ‘Proceedings of the Royal Society B’
The researchers found that when the environment changed with the onset of the CRC, the number of tetrapods dropped substantially, before recovering gradually during the Permian. However, unlike earlier studies, their data showed that habitat loss didn’t result in the species being restricted to specific geographic areas. Instead, while the CRC may have led to lower levels of global species diversity, the surviving tetrapod communities didn’t diversify in isolation, but in fact became better connected.
The TERRA (375 Million Years of the Diversification of Life on Land: Shifting the Paradigm?) project aims to establish a new and accurate model of terrestrial diversification in order to bridge the knowledge gap that exists in this field.
For more information, please see: CORDIS project website