A newly discovered dinosaur from southwestern China is shedding light on the early evolutionary history of ornithischian dinosaurs, marking it as one of the earliest-diverging members of this group found in Asia. This discovery, made through a multi-institutional investigation in China, introduces Archaeocursor asiaticus to the scientific community. The research suggests that this dinosaur might be part of a previously unrecognized wave of ornithischian dispersal from Gondwana to Laurasia, independent of the migration of armored dinosaurs that is often associated with the early Jurassic period.
Ornithischians are a significant clade of dinosaurs that would eventually give rise to some of the most iconic and diverse groups, such as ankylosaurs, stegosaurs, hadrosaurs, ceratopsians, and pachycephalosaurs. While much is known about the later members of this clade, the early evolutionary history of ornithischians remains relatively obscure. The fossil record of early ornithischians is scarce, and much of what we know about their origins and dispersal patterns is still based on limited evidence.
To understand this early phase of ornithischian evolution, it is essential to consider the geographical and geological context of the time. During the early Jurassic period, the Earth was divided into two primary landmasses: Gondwana and Laurasia. Gondwana included what are now South America, Africa, India, Madagascar, Australia, and Antarctica, while Laurasia consisted of North America, northern Asia, Greenland, and Europe. This division had significant implications for the distribution of prehistoric life, including dinosaurs.
Fossil discoveries of early Jurassic ornithischians are more abundant and diverse in regions that were once part of Gondwana. In contrast, only a few similar-aged ornithischian fossils have been found in Laurasia, suggesting that Gondwana may have been the original home of these dinosaurs. This pattern has long been interpreted as evidence for an origin of ornithischians in Gondwana, with subsequent migrations leading to the spread of these dinosaurs into other regions.
The recent discovery of Archaeocursor asiaticus adds an intriguing twist to this narrative. The fossil was unearthed in the Lower Jurassic Ziliujing Formation in southwestern China, a region that was once part of Laurasia. The find includes a nearly complete left femur, which measures 93 mm in length. Based on the size of the femur, scientists estimate that the dinosaur was a small-bodied individual, roughly 1 meter in length. The femur’s features provide essential clues about the dinosaur’s development and growth. Osteohistological analysis reveals that the bone has parallel-fibred tissue and tightly packed lines of arrested growth, indicating that the individual was a young adult at the time of death. The histological thin-sectioning further revealed information about the dinosaur’s growth rates and skeletal maturity.
One of the most important aspects of this discovery is the detailed morphological features visible in the femur. High-resolution CT scans allowed paleontologists to observe distinct features that set Archaeocursor asiaticus apart from other contemporaneous ornithischians, such as Yuxisaurus kopchicki. These features include a ridge on the proximal surface of the femur, a broad fourth trochanter, and a uniquely shaped medial condyle. These characteristics suggest that Archaeocursor asiaticus was more closely related to Gondwanan ornithischians than to other species found in Laurasia, which further strengthens the hypothesis of an early dispersal event from Gondwana.
The significance of this discovery lies in its implications for our understanding of the early migration and diversification of ornithischians. Until now, the idea that ornithischians from Gondwana migrated to Laurasia primarily through the spread of armored dinosaurs had been the dominant theory. However, the presence of Archaeocursor asiaticus in southwestern China suggests that ornithischians may have dispersed into East Asia through an independent event. This discovery supports the hypothesis that ornithischians underwent a rapid diversification during the Early Jurassic, potentially due to multiple dispersal events from Gondwana to Laurasia.
This finding is not only important for its potential to reshape our understanding of ornithischian evolution but also for its contribution to the broader narrative of dinosaur migration patterns. The fact that Archaeocursor asiaticus appears to be an early-diverging ornithischian in Asia challenges the previously held notion that the majority of early ornithischian evolution occurred exclusively in Gondwana. Instead, it suggests a more complex scenario in which ornithischians dispersed across both major landmasses during the Early Jurassic, contributing to the clade’s diversification.
Archaeocursor asiaticus also provides valuable insights into the environmental and ecological conditions of the early Jurassic period. The Ziliujing Formation, where the fossil was found, is an important geological site that preserves evidence of the diverse ecosystems that existed in the region at the time. The discovery of this dinosaur in such a location adds to our understanding of the types of habitats that early ornithischians inhabited and the ecological niches they occupied. The fossil evidence suggests that these dinosaurs lived in a variety of environments, ranging from coastal plains to forested areas, and were likely small, fast-moving herbivores.
The ongoing study of Archaeocursor asiaticus and its place in the evolutionary tree of ornithischians is still in its early stages. However, the fossil provides a wealth of information that will undoubtedly lead to further discoveries and refinements in our understanding of early dinosaur evolution. As scientists continue to investigate this and other fossils from the same formation, it is likely that more evidence will emerge to support or refine the hypothesis of multiple dispersal events of ornithischians into Laurasia.
Reference: Xi Yao et al, New evidence for the earliest ornithischian dinosaurs from Asia, iScience (2024). DOI: 10.1016/j.isci.2024.111641
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