Chengdu University of Technology team dates China's oldest dinosaur tracks

Scientists have dated dinosaur tracks in China's Sichuan Basin to 218.4 million years ago, making them the oldest known in China. 

The footprint fossils, found in the Upper Triassic Xujiahe Formation of the Sichuan Basin in 1989, were identified by their shape as those of small theropod dinosaurs.

A group of scientists from Chengdu University and British paleontologist Michael Benton, from the University of Bristol, dated the footprints and hypothesized that the dinosaurs migrated eastward from the western Tethys region. The Tethys Sea was a prehistoric ocean that existed before the Indian and Atlantic oceans formed.

Their research appears in the journal Gondwana Research, Feb. 3. Gondwana is the name of the ancient supercontinent that included today's South America, Africa, Australia, the Arabian Peninsula, India and Antarctica.

The authors report that the "theropod tracks ... provide additional evidence of the rise of dinosaurs to ecological dominance at a time of fluctuating paleoclimatic conditions corresponding to the mid-Norian warming event." 

The Norian is part of the Triassic period, from about 227 million to 208.5 million years ago. Paleoclimatic conditions at that time and the thermal physiology of these dinosaurs determined their distribution patterns, the researchers say.

Dating fossil footprints

Dinosaur footprints were first collected from the Late Triassic Xujiahe Formation in 1989 during a regional geological survey. The fossils are on 16 sandstone slabs, stored in the Museum of Chengdu University of Technology. Previously, the fossil tracks had not been definitively dated. Now the advanced technology of extracting information from zircon crystals enables more specific dating.

The research group used the radioactive isotopes of uranium and lead present in zircon crystals in the surrounding sediment, and made use of the known decay rates of uranium to date the fossils.

The team went on to describe the analysis process.

"Zircon grains were separated by standard heavy-liquid and magnetic methods, then elongated grains with better crystal shapes were hand-picked under the stereo microscope," they wrote. "After that these zircon grains were mounted in epoxy and polished to show internal structures for target selection, combined with cathodoluminescence (CL) images, transmitted and reflected light micrographs, and further for laser ablation analyses."

The researchers isolated more than 200 zircon grains, and selected 74 of them for analysis.

Identifying the track makers

To date the only dinosaur fossils in China have been footprints, and no dinosaur skeletal fossils have been found. Previous ichnology, the study and dating of fossilized footprints, in China has not been reliable, the authors report.

The research group measured and analyzed 22 fossil tracks. The key questions they asked about the tracks are "whether they all belong to a single ichnotaxon or represent different icnotaxa," and what the tracks reveal about dinosaurs in the Late Triassic period.

The researchers note that it can be tricky to identify the exact type of dinosaur from footprints, but they conclude these tracks are "small, functionally tridactyl [three-digit] theropod tracks."

"The abundance of small to moderately sized tridactyl pes [foot] imprints from the Tianquan area lets us catch a glimpse of a stable theropod population that lived in South China," they write. "Further, these theropod tracks from eastern Tethys match the timing of the early rise and rapid radiation of dinosaurs during the Late Triassic [mid- Norian] associated with fluctuating paleoclimatic extremes."

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Shenyuan Peng et al. "The first dinosaurs in China: Dating Late Triassic footprint fossils from the Sichuan Basin." Gondwana Research, Feb. 3, 2023.

https://doi.org/10.1016/j.gr.2023.02.003