Insights into Mammal Evolution Since the Triassic: China’s Yanliao Biota Explores a Fresh Outlook

China’s Yanliao Biota, a collection of well-preserved fossils in northeastern China from the Middle to Late Jurassic period (174 to 145 million years ago), provides valuable insights into vertebrate evolution. This Lagerstätte reveals exceptional examples of insects, feathered dinosaurs, transitional pterosaurs, salamanders, and early mammals.

Understanding the timing of evolutionary events during these periods is crucial for uncovering the history of life on Earth. Researchers have utilized volcanic ash layers, known as tuff, to date the fossil-bearing units in the Yanliao Biota, as reported in Earth and Planetary Science Letters.

By creating an artificially exposed cave in sedimentary rocks deposited in a lakustrine environment, located in the Liaoning and Hebei Provinces of China, scientists were able to study the ash that fell into the lake during a volcanic eruption. The ash remained undisturbed, allowing for precise dating of the fossils.

The researchers at the Institute of Vertebrate Paleontology and Paleoanthropology in China, along with their collaborators, employed a technique called cathodoluminescence. This technique involved polishing grains of a crystal and using a specialized microscope to examine its center. This microscope used light to reveal specific characteristics of the crystal, while the cathodoluminescence process utilized electrons being fired at the grain to cause luminescence and emit photons at different wavelengths. As the crystals were nearly pristine and not transported, their original location could be determined.

The sedimentary layers were dated using 129 grains of zircon crystal. The levels of uranium, thorium, and lead in the crystal were measured through U-Pb radiometric dating. This dating method is based on the principle that uranium decays over time into a more stable form, which is lead. By determining the half-life of uranium, which is the time it takes for half the amount of uranium to decay, scientists can calculate the age of the crystal. In this case, the Yanliao Biota was dated to be between 164 and 157 million years old.

Within the sampled sections, six salamanders were discovered and dated to be 164 million years old. This represents one of the earliest diversification events of amphibians belonging to the group Caudata. Sedimentary rocks containing membrane-winged dinosaurs were dated to be between 164 and 160 million years old, aligning with previous studies on the development of flight in theropod dinosaurs. The Yanliao Biota also contains Euharamiyidans, some of the earliest mammal ancestors, which were dated to units aged 163 to 158 million years old.

These findings were incorporated into a technique called Bayesian tip-dated phylogeny, which helped determine the origin date of true mammals prior to the early mammal ancestors. By analyzing the evolutionary relationships and patterns of fossils, the researchers revealed that the root of mammalian evolution occurred in the Late Triassic, approximately 208 million years ago. This supports the long-fuse model, in which the diversification of early mammals did not occur until the Middle Jurassic. However, the researchers emphasize that there may be more undiscovered ghost lineages of hypothesized mammal ancestors as further exploration of Earth’s ancient past continues.

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