Fossil-rich site in China offers new insights into early mammal evolution in the Jurassic

Fossil-rich sites in northeastern China have been accurately dated for the first time, giving palaeontologists a new window into the evolution of the animals that lived in the region during the Middle to Late Jurassic.

Sites studied are in the Liaoning and Hebei Provinces near the Korean peninsula. Fossilised creatures from the region dating back to the Jurassic are referred to as Yanliao Biota.

Researchers studying the sites have determined that the Yanliao Biota date back to between 164 million and 157 million years ago. Their results are published in the journal Earth and Planetary Science Letters.

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Among the fossils are well-preserved insects, as well as pterosaurs, feathered dinosaurs, amphibians and early mammals. This includes the earliest recorded salamanders, which are now dated to 164 million years.

Volcanic layers of ash, known as tuff, were used to date the outcrops which hold the Yanliao Biota. A Jurassic volcanic eruption caused ash to fall into a lake which existed in the region at the time. This ash was not subsequently moved by geological events, providing an opportunity for scientists to use it to date the fossil-rich layers.

Researchers at the Chinese Institute of Vertebrate Paleontology and Paleoanthropology, and their collaborators, used cathodoluminescence to assess the age of the biota.

This method involves polishing grains of a crystal, exposing the crystal’s centre. Electrons fired at the grain cause particles of light to be emitted. The different wavelengths of light being emitted reveals particular characteristics of the crystal.

Sediment holding Yanliao biota were dated using 129 grains of zircon crystal. Different levels of uranium, thorium and lead were measured. This allowed scientists to use radiometric dating to analyse the levels of different radioisotopes present in the crystal to date the sediment.

For example, the two radioisotopes of uranium have different half lives (the time it takes for half of a sample to radioactively decay). Uranium-238 decays into thorium-234 with a half life of about 4.5 billion years. Uranium-235 has a half life of about 704 million years, decaying to thorium-231.

Using this technique, the scientists were able to accurately date various layers of sediment in which the Yanliao Biota are found. This includes early squirrel-like mammals called euharamiyidans, dated back to 163 million years.

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Today, mammals are unlike any other group of animals. While there are some which continue to straddle the evolutionary line between mammal and reptile (like echidnas and platypuses), true mammals are furry, lactating creatures with big brains, differentiated teeth and keen senses of smell and hearing.

Mammals diverged from reptiles about 325 million years ago. “Mammal-like” reptiles, or synapsids, dominated in the Permian period (299–252 million years ago), just before the age of dinosaurs. But exactly when the first true mammals emerged continues to be subject to debate.

One theory, known as the “explosive model”, suggests that early mammals originated in the Middle Jurassic before diversifying rapidly.

Another theory, called the “long-fuse model”, posits that mammals emerged in the Late Triassic and diversified in the Jurassic.

The researchers who dated the Yanliao Biota took this second model as their starting point. They developed software to work backwards from the Yanliao euharamiyidans and identified the root of mammalian evolution in the Late Triassic, 208 million years ago.

However, they note that, whatever the model, diversification of mammals took place in the Jurassic, and further discoveries may yield “ghost lineages” of mammal ancestry which have yet to be identified in the fossil record.