Scientists have offered a new perspective on evolutionary transitions

Ancient oceans could become especially diverse not only after mass extinctions, but also during periods when endangered groups of animals still coexisted with those that had yet to spread. This is the conclusion reached by researchers who studied the fossil remains from Colombia. They proposed a new concept to describe such evolutionary transitions. See the Izvestia article about what happened in the ancient ocean and why it can change ideas about evolution.

An ancient ocean with giant predators

The study is published in the journal Frontiers in Ecology and Evolution (FEE). Scientists from McGill University in Canada and the Columbia Paleontological Research Center studied fossil data from the Paha Formation, a geological formation in the central part of modern Colombia. It was formed in the Early Cretaceous period, about 145-100 million years ago, and has preserved traces of a rich marine ecosystem.

The ancient ocean was home to large marine reptiles, including long-necked plesiosaurs and their relatives, saltwater crocodylomorphs, ichthyosaurs, and turtles. Other inhabitants included sharks, bony fish, crustaceans, ammonites, and a variety of invertebrates. Some marine predators reached more than 10 m in length. This diversity allowed animals to occupy different ecological niches and formed a complex food network.

One of the features of the Groin formation is the large number of large predators that occupied the upper levels of the food chain. In previous work, the researchers reconstructed the food connections between the inhabitants of the ancient ocean and discovered a complex structure of interactions. According to the authors, this ecosystem provides a rare opportunity to study how marine communities have changed during a period of large-scale evolutionary transformations.

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How endangered species coexisted with new ones

The fossils studied have preserved representatives of different stages of the evolution of marine animals. Some groups descended from ancient lineages that were widespread as early as the Jurassic period and were gradually approaching extinction. Others belonged to evolutionary groups that were just beginning to actively develop and diversify in the Cretaceous period. At the same time, their representatives lived in the same region and could coexist in the same ecosystem.

This combination is especially noticeable among marine reptiles. For example, representatives of the late lineages of pliosaurs, large predators, some of which reached impressive sizes, were found in the Paha formation. At the same time, plesiosaurs and ichthyosaurs were found in the region, including animals with different eating habits. The researchers also pay attention to the diversity of fish, sea turtles and invertebrates that formed the basis of the food web.

The authors believe that such transitions did not always occur through the abrupt disappearance of some organisms and the subsequent spread of others. In some cases, evolutionary changes could unfold gradually: the old lines were still preserved, while the new ones already occupied accessible ecological niches. The Groin formation allows us to trace such an intersection of different stages of the development of marine fauna.

What helped the ancient ocean become so diverse

Researchers attribute the richness of the ancient ecosystem to a combination of climatic, geographical, and biological factors. In the Early Cretaceous period, the Earth had a warm climate, the level of the world's oceans was significantly higher than the modern one, and the concentration of carbon dioxide in the atmosphere was higher than the pre—industrial one. Such conditions affected marine habitats and could contribute to the development of diverse communities.

The geographical location of the region also played an important role. In the area of modern Colombia, sea routes were formed that connected the eastern Pacific Ocean with the ancient Atlantic through the so-called Spanish Corridor. These water spaces could facilitate the settlement of marine animals and the exchange of species between different regions. Scientists consider an additional factor to be the supply of nutrients from land to coastal waters, which could support high biological productivity.

The interactions between the organisms themselves also influenced the diversity. The appearance of new species of fish and other animals expanded the food possibilities of predators, and competition for resources contributed to the division of ecological niches. As a result, different groups of large predators could have existed simultaneously in the ancient ocean, hunting different prey. The authors emphasize that the significance of each of these factors has yet to be studied in more detail.

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What is a "biotic fracture"?

To describe the discovered combination of disappearing and developing evolutionary lines, the researchers proposed the term "biotic fracture." It marks a period when ancient groups of animals approaching extinction continue to exist simultaneously with new lineages that are beginning to actively spread and diversify. According to the authors, the Groin formation provides a particularly vivid example of this process.

This concept complements the understanding of major changes in biodiversity. When studying evolutionary transitions, attention is usually paid to mass extinctions, after which other organisms occupy the vacated ecological niches. However, the history of the ancient ocean in Colombia shows that important changes could occur without the simultaneous extinction of a large number of species — through the long-term coexistence of different evolutionary groups.

At the same time, scientists do not call the Groin formation an officially recognized "hotspot" of biodiversity. Although it is distinguished by the richness of fossils and the complex structure of the ancient ecosystem, the available data is not yet sufficient for such a classification according to modern criteria. To find out how widespread the "biotic fracture" was in the history of the Earth, new findings are needed, more accurate dating of geological layers and comparison with other ancient ecosystems.