麻豆淫院

A newly identified dinosaur from southwestern China is revealing what appears to be the earliest-diverging ornithischian dinosaur in Asia. A multi-institution investigation in China has introduced the world to Archaeocursor asiaticus, and the research suggests an earlier dispersal event of ornithischians from Gondwana to Laurasia, independent of the migration of armored dinosaurs.

Ornithischia is a clade of dinosaurs that, from its origins, would diversify into many later forms, such as ankylosaurs, stegosaurs, hadrosaurs, ceratopsians, and pachycephalosaurs. While the results of these later forms are well known, the early evolutionary origin of the clade itself remains poorly understood.

Gondwana and Laurasia were once two separate landmasses on Earth comprising most of the dry land. Gondwana was made up of South America, Africa, India, Madagascar, Australia, and Antarctica, while North America, northern Asia, Greenland and Europe were part of Laurasia.

Early Jurassic ornithischian fossil finds are more abundant and diverse in former Gondwana lands with only a few finds of similar age known from Laurasia sites, suggesting Gondwana as the origin.

A journal pre-proof in iScience, titled "New evidence for the earliest ornithischian dinosaurs from Asia," details a nearly complete left femur from the Lower Jurassic Ziliujing Formation, revealing what appears to be the earliest-diverging ornithischian dinosaur in Asia.

The femur measures 93 mm in length, suggesting a small-bodied individual with a body length of approximately 1 meter. Osteohistological analysis revealed parallel-fibred bone and tightly packed lines of arrested growth, indicating that the specimen was a young adult at the time of death. Histological thin-sectioning provided information about growth rates and skeletal maturity.

High-resolution CT scans allowed detailed morphological observations of distinct features, such as a ridge on the femoral proximal surface, a broad fourth trochanter, and a uniquely shaped medial condyle. These features distinguish Archaeocursor asiaticus from contemporaneous ornithischians like Yuxisaurus kopchicki and align it more closely with the Gondwanan taxa.

Attempts to reconstruct the evolutionary relationship placed Archaeocursor asiaticus at a primitive position within Ornithischia, possibly preceding armored dinosaur dispersals to the region. This provides evidence for a potential additional dispersal event of ornithischian dinosaurs into East Asia, reinforcing hypotheses of a rapid diversification during the Early Jurassic.

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