Exploring the Early Evolution of Ornithomimosaurs

Location where Tyrannomimus was discovered

Ornithomimosaurs are a distinctive group of theropod dinosaurs. They typically possessed relatively small heads and long necks, and their overall body form resembled a long-tailed ostrich. Many members of this lineage also show anatomical features associated with herbivorous diets. During the Cretaceous period these dinosaurs achieved remarkable diversity, yet the early stages of their evolutionary history remain poorly understood.

In recent years, continued fossil excavations have uncovered numerous ornithomimosaur remains within the Kitadani Formation in Fukui, Japan. These fossils represent multiple individuals. When researchers compared the material, they discovered that the specimens share several morphological traits that had not previously been observed in other ornithomimosaurs. This combination of features indicated that the fossils belonged to a previously unknown taxonomic unit. After detailed anatomical description and phylogenetic analysis in 2023, the material was formally recognized as a new genus and species.

Tyrannomimus fukuiensis

Reconstruction of Tyrannomimus（Image source：Sino-Pteryx， CC BY-SA 4.0 ）

The name Tyrannomimus fukuiensis literally means "tyrant mimic from Fukui." The genus name refers to certain anatomical features that resemble those of tyrannosauroids. These similarities are interpreted as ancestral traits shared by both tyrannosaurids and ornithomimosaurs.

Tyrannomimus lived in Japan during the Early Cretaceous, about 125 million years ago. It inhabited an ecosystem alongside a diverse assemblage of dinosaurs from the same formation, including Fukuisaurus, Fukuititan, Fukuiraptor, Fukuivenator, Fukuipteryx, and Koshisaurus.

Phylogenetic analysis indicates that Tyrannomimus belongs to Deinocheiridae, a subgroup within Ornithomimosauria. At present it represents the oldest confirmed member of this family. The analysis also revealed an unexpected relationship: the Mongolian species Harpymimus appears to be the closest known relative of Tyrannomimus. In contrast, Shenzhousaurus, discovered in northeastern China and geographically closer to Japan, is more distantly related.

Interestingly, this pattern is not unique to ornithomimosaurs. Hadrosauroid dinosaurs from the Kitadani Formation also appear more closely related to species from northern Asia than to those from northeastern China. Even the composition of fossil cockroach species from the same deposits shows a similar pattern.

These findings suggest that the faunal composition of the Kitadani ecosystem was more closely aligned with northern Asian biotas than with those of northeastern Asia. The results also serve as an important reminder for paleobiogeographic studies: simple geographic proximity in space and time does not necessarily indicate closer evolutionary relationships between fossil assemblages.

A Tyrannosaur or an Ornithomimosaur?

Another intriguing aspect of the study concerns the dinosaur Aviatyrannis. This species, named in 2003, is known from fragmentary fossils found in Late Jurassic deposits in Portugal and has traditionally been classified as an early tyrannosauroid.

Researchers noted that certain fossil elements of Aviatyrannis closely resemble those of Tyrannomimus. Because of this similarity, they conducted a more detailed comparative analysis to clarify its taxonomic placement. The results showed that although Aviatyrannis possesses some traits associated with tyrannosauroids, those anatomical regions are either poorly represented among known ornithomimosaurs or contain features that also occur within Ornithomimosauria. As a result, the classification of Aviatyrannis remains uncertain.

When Aviatyrannis is included in phylogenetic analyses, it emerges as the sister lineage of Tyrannomimus. If this interpretation proves correct, Aviatyrannis would represent the earliest known member of Deinocheiridae and one of the earliest ornithomimosaurs. Such a scenario would fill a roughly 20-million-year gap in the fossil record of Ornithomimosauria. At present, the earliest confirmed ornithomimosaur fossils come from South Africa and date to about 140 million years ago. Additional discoveries will be necessary to test this possibility.

Conclusion

This study describes a new early ornithomimosaur species and provides valuable insight into the early evolution of the group. It also demonstrates that similarities between fossil faunas cannot always be predicted from geographic proximity alone. Finally, the reanalysis of Aviatyrannis introduces a new perspective on the evolutionary history of ornithomimosaurs and may help bridge a long-standing gap in their fossil record.

Author: Bai Leng

Reference:

Hattori, S., Shibata, M., Kawabe, S., Imai, T., Nishi, H., Azuma, Y. (2024). New theropod dinosaur from the Lower Cretaceous of Japan provides critical implications for the early evolution of ornithomimosaurs. Scientific Reports.

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