To open the door to the "lost world", the "key" is actually in the poop?

The mass extinction event at the end of the Permian period 252 million years ago caused unprecedented damage to the entire marine ecosystem, and 80% of the species in the ocean disappeared from the earth. So, how did the nearly collapsed ecosystem gradually recover from the Great Depression? This scientific mystery has become the focus of paleontologists and paleoecologists.

Recently, a Chinese and foreign research team led by the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, revealed the truth about the early Triassic biological recovery and the great radiation. This truth was found from coprolites that are not popular with the public. The relevant research results were published in the latest issue of the international geoscience journal "Paleogeography, Paleoclimate, Paleoecology".

Poop fossils are a useful supplement to bone fossil research

Coprolites, also known as poop fossils, are a type of fossil in the fossil family and are classified as trace fossils. Poop fossils are very rare because soft poop is difficult to preserve and the probability of becoming a fossil is even smaller. However, poop fossils often contain some animal and plant debris, which can not only provide information about animal diet, appetite size, and habitat, but also have special importance in paleoecology and paleontology research. It is not only a good supplement to the study of bone fossils, but also regarded as a magical object that reveals the mysteries of the paleontological world.

The preserved feces fossils are quite characteristic, generally in the shape of convexities. Due to the different food and digestive tract characteristics of different vertebrates, their excrement often has different shapes and characteristics. Some fish feces fossils are spiral-shaped, while mammal feces fossils are generally oval to long strips. Among them, carnivorous feces often contain bone fragments, while herbivorous feces are composed entirely of plant fibrous structural materials.

Poop fossils are mostly composed of calcium phosphate and come in different sizes: large ones are mostly between 2 and 5 centimeters in diameter, or even 1 meter long; small ones are also called fecal pellets, and most have a diameter of less than 5 millimeters.

Changes in phytoplankton and zooplankton affected Mesozoic marine ecosystems

The Triassic Luoping Biota is a microcosm of the biological world's return to glory after the mass extinction at the end of the Permian period. However, the scientific community is not very clear about the deep reasons why organisms recovered from the mass extinction and regained their evolutionary brilliance.

Research on ecosystem recovery generally starts from the diversity changes of individual species, and then the restoration of the entire ecosystem structure. Its integrity is the sign of recovery. However, in the process of building an ecosystem, changes in the bottom layer of the ecosystem, such as the transformation of primitive producers and primary consumers, can often produce bottom-up correlation changes in the entire ecosystem and trigger fundamental changes in the ecosystem. In the Mesozoic Era, changes in marine phytoplankton such as dinoflagellates, diatoms and coccoliths had a fundamental impact on the marine ecosystem.

The research team discovered a new type of poop fossils in a thin fossil layer beneath the Luoping Biota of the Guanling Formation in the Middle Triassic in Yunnan, providing important clues for deciphering the scientific issues of biological recovery during this period.

Cyanobacteria and mysid shrimp are the basis for biological recovery

The new poop fossils are ellipsoidal to short columnar, composed of microcrystalline calcium carbonate and a small amount of residual organic matter, and are associated with fossils of warty mysid shrimp. Under the CT scanner, the poop fossils have multiple pairs of crescent-shaped cavity structures inside, accompanied by euhedral pyrite crystals.

The crescent-shaped cavity structure is an important identification mark of the poop fossils produced by crustaceans. In the continuous ion beam cutting imaging, the researchers found a morphologically stable tubular cavity structure inside the poop fossils. After three-dimensional restoration, the tubular structure was stable in shape and had a uniform diameter, ranging from 3 to 4 microns in size and 60 microns in length. This morphological feature is very similar to that of mineralized cyanobacteria.

In addition, in terms of characteristics, the new feces fossils and the mineralized intestinal morphology and composition of the accompanying warty shrimp are highly similar.

The study showed that the new poop fossils were produced by a large number of warty mysid shrimp zooplankton. More importantly, the tubular cyanobacteria preserved in the poop fossils may indicate that the original producers of the marine ecosystem at that time were still primitive groups such as cyanobacteria.

Therefore, the basis for biological recovery is to build an ecological chain from the bottom primitive producer cyanobacteria to the primary consumer Mysid shrimp and even higher-level consumers. By comparing with the Guiyang biota in the early stage of recovery, it is further shown that the revolutionary changes in the Mesozoic marine ecosystem may have begun at a certain period after the Anisian period of the Mesozoic era and gradually dominated.

(The author is a researcher at the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, and the honorary curator of the Nanjing Paleontological Museum)