What challenges do reptiles face when moving from land to sea?

On August 27, 2024, Jiang Dayong, professor at the School of Earth and Space Sciences of Peking University, gave a speech entitled "Living towards the Sea after the Disaster" at the Science Popularization China Guizhou Special Event.

As intelligent human beings, we often think about our own existence, asking ourselves questions about our identity, origin, and future, and considering what kind of changes we will experience. These questions actually contain our exploration of the essence of life.

To understand the changes and nature of life, we need to study life on Earth in depth. Among them, the study of paleontology plays a vital role. Because paleontology research provides us with an understanding of the changes in life and the relationship between life and the environment, as well as an important time frame. Compared with modern biology, which focuses more on all life phenomena and life laws in the present period, paleontology has an additional time scale. Through them, we can understand the course of life more comprehensively: the evolutionary process of organisms is not a simple linear change, nor a single spiral line. It includes both slow accumulation processes and major mutations.

From the birth of the earth

The Earth was born 4.6 billion years ago, and the earliest life appeared 3.8 billion years ago, including primitive single-cell prokaryotes such as bacteria and cyanobacteria.

After more than a billion years of evolution, we gradually evolved from prokaryotes to eukaryotes, and then evolved into multicellular organisms. After a snowball event on Earth, the entire Earth was covered in ice and snow, and then multicellular life began to flourish.

About 530 to 520 million years ago, the Cambrian explosion occurred, a major evolutionary event in which all of the advanced taxa today appeared on Earth.

The period before the Cambrian period was called the Cryptozoic Eon because life forms were relatively simple and fossil records were relatively scarce; the period before and after the Cambrian period was called the Phanerozoic Eon because after the Cambrian explosion, life forms became more diverse, including the emergence of organisms with hard skeletons, and the fossil records of the Cambrian period and subsequent periods were relatively complete.

Starting with the Cambrian period, the Paleozoic Era is considered the age of marine invertebrates, including trilobites, brachiopods, bivalves, and tetracorals, which flourished during the Paleozoic Era (520 million to 252 million years ago).

It was also from the Cambrian period that life gradually became more complex, leaving behind a large number of fossils, proving that the rate of life evolution was gradually accelerating. During the Cambrian period, the most primitive chordates appeared in the Chengjiang fauna, although the Paleozoic was mainly dominated by marine invertebrates.

As time went by, vertebrates gradually developed. In the Devonian period, fish became dominant, and in the late Devonian period, some fish evolved limbs and went ashore, evolving into amphibians. Plants also evolved, and they came onto land in the earlier Silurian period, gradually developing from the earliest marine algae to land plants, turning the earth into a green environment.

By the Carboniferous period, amphibians had evolved into reptiles, which were more advanced animals with stronger bones and smarter brains.

At the end of the Permian period, the most significant mass extinction event in the history of the Earth occurred. However, reptiles ushered in a period of great development in the Triassic period after the mass extinction event and eventually became the main rulers of the Mesozoic Era.

In the Mesozoic Era, dinosaurs ruled the land, various marine reptiles such as ichthyosaurs and plesiosaurs occupied the oceans, and pterosaurs and birds ruled the sky. At the same time, the ancestors of mammals also originated in the Mesozoic Era. A mass extinction event occurred at the end of the Cretaceous Period. After the mass extinction event, mammals had the opportunity to develop in the Cenozoic Era and eventually evolved into modern humans.

In general, biological evolution is not a linear change. Its complexity can be compared to a tree, where each branch represents a type of life form. The complexity of the fossil record means that our understanding of the evolution of life is still limited. At the same time, the movement of the earth's plates and changes in the environment are also affecting the process of biological evolution. To fully understand the evolution of organisms, we must comprehensively consider multiple factors such as the environment, climate and geology, so that we can truly understand the essence of life.

Mass extinction: disaster and opportunity

At the end of the Permian period 252 million years ago, there was a mass extinction event that was speculated to have been caused by the eruption of a large volcano in Siberia.

This mass extinction event was the most significant in Earth's history. In this mass extinction event, about 95% of marine life became extinct. For example, trilobites that flourished in the Cambrian period and tetracorals in the Paleozoic era were completely extinct. At the same time, the Earth's environment also experienced drastic changes, including high temperatures, lack of oxygen, and acidification of seawater, which led to a catastrophic collapse of the Earth's ecosystem.

This marked the beginning of a new era on Earth, the Mesozoic Era.

The Mesozoic Era was the era of reptiles, which flourished unprecedentedly, forming the famous "Dinosaur Era". Dinosaurs were reptiles that could walk upright on two legs and successfully dominated the land. The reason they were able to adapt to life on land was that reptiles lay amniotic eggs, which made the development of embryos and individuals not restricted by the external water environment, and achieved autonomous hatching of eggs. This characteristic was the foundation and material basis for reptiles to become the masters of the earth in the Mesozoic Era.

It is important to emphasize that disasters and opportunities coexist. During this mass extinction, some organisms also gained opportunities for development. For example, reptiles successfully withstood the extreme high temperatures and ushered in a period of great development.

The first period of the Mesozoic Era was the Triassic, followed by the Jurassic and Cretaceous periods.

Starting from the Triassic period, organisms experienced recovery after the mass extinction. During this period, reptiles entered a period of radiation development, and some branches began to evolve in the sea, land, and air. The appearance of dinosaurs on land marked the beginning of reptiles' development inland. In the early Triassic period, about 3 million to 4 million years after the mass extinction event at the end of the Permian period, the earliest ichthyosaurs appeared in the ocean. In the late Triassic period, some reptiles evolved into pterosaurs and evolved into the sky. Under the extreme environmental pressures of high temperature and lack of oxygen on the earth at that time, some reptiles developed into the ocean and evolved into marine reptiles. In the Jurassic period, some feathered dinosaurs also grew wings and turned into birds and flew into the sky - the reptiles that flew into the sky were not only pterosaurs, but also today's birds.

In other words, this mass extinction event gave rise to a diverse evolutionary path for reptiles, including reptiles in the sea, dinosaurs on land, and pterosaurs in the sky. In this catastrophic environmental change, reptiles successfully adapted to different ecosystems.

What challenges do land animals face in adapting to life in the ocean?

Reptiles are true land animals, and even though some crocodiles and turtles can live in water, they are still land animals.

However, as land animals, how do reptiles adapt to marine life? For example, reptiles in the ocean today include sea turtles, while mammals include dolphins and whales. Both dragons and mammals are amniotes, which are typical land creatures. Although humans also dream of swimming in the blue ocean, in reality, without equipment, the time a person can stay in the water is very limited.

The marine environment is quite challenging for any land animal compared to land animals.

First, in the ocean, the light gradually becomes dim or even dark, far less bright than on land. This change in light will have an impact on the lives and emotions of creatures, because we usually feel happier in bright sunlight. Moreover, underwater, the way of breathing is also facing changes. Now we have to breathe air on the surface of the water, and the same is true for dragons and mammals in the ocean, which also need to return to the surface of the water regularly to breathe. In addition, the way sound propagates in water is very different from that in air, which is also an aspect that needs adaptation.

Secondly, the way of movement will also change with the transition from land to sea, because walking in water is a completely different experience than walking on land. At the same time, eating in the ocean also faces challenges, such as eating in water may cause choking.

In addition, changes in reproduction methods are also an important challenge. Organisms in the ocean must adapt to different reproduction methods. For example, ichthyosaurs are typical Mesozoic marine reptiles, but they can no longer come ashore to lay eggs; for another example, reptiles such as ichthyosaurs and plesiosaurs are likely to adopt a method that is more adapted to the aquatic environment, such as viviparity, rather than the oviparity on land. Although these species are extinct, by observing modern life phenomena, we can infer that their lifestyle in the ocean may be more adapted to viviparity.

In general, the process from land to sea is a challenge for any organism, which requires adapting to various environmental changes.

Author: Jiang Dayong, Professor of School of Earth and Space Sciences, Peking University