Didactyl theropod track from Lower Cretaceous of Japan + thunnosaurian ichthyosaur from Lower Cretaceous of Colombia

[Ben Creisler]

Ben Creisler

bcre...@gmail.com

Recent papers:

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Yuta Tsukiji, Soki Hattori & Yoichi Azuma (2025)
First didactyl theropod track from the Lower Cretaceous Kitadani Formation, Tetori Group, Fukui, Japan
Cretaceous Research 106249
doi: https://doi.org/10.1016/j.cretres.2025.106249
https://www.sciencedirect.com/science/article/abs/pii/S0195667125001727


A small theropod track was discovered in the Lower Cretaceous Kitadani Formation at the Kitadani Dinosaur Quarry in Fukui, Japan. Its distinctive didactyl morphology suggests that the trackmaker was a deinonychosaurian theropod. The track measures 8.0 cm in length and 4.7 cm in width, with a divarication angle of 20° between digits III and IV. These features are consistent with those of the ichnogenus Velociraptorichnus. The track exhibits a centrally positioned metatarsophalangeal pad impression along its longitudinal axis, a feature also observed in other ichnotaxa interpreted as having been made by arctometatarsalian theropods. Given that the arctometatarsalian condition is restricted to troodontids within Deinonychosauria, the trackmaker is inferred to be a troodontid. This specimen represents the first record of a deinonychosaurian track from Japan and provides new ichnological evidence for the presence of deinonychosaurian theropods in the Kitadani Formation. Furthermore, it reinforces the previous hypotheses suggesting a close relationship between the dinosaur ichnofauna of the Kitadani Formation and other dinosaur ichnofaunas of the Lower Cretaceous in East Asia.

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Daniel Eduardo Pomar, Cristian David Benavides-Cabra & José Alejandro Narváez-Rincón (2025)
The first record of a thunnosaurian ichthyosaur from the upper Albian of South America
Journal of South American Earth Sciences 168: 105824
doi: https://doi.org/10.1016/j.jsames.2025.105824
https://www.sciencedirect.com/science/article/abs/pii/S0895981125004869


Highlights

First Albian ichthyosaur from Colombia, expands South American records.
Preservation features suggests anoxic bottoms in the Hilo Formation.
Colombian Lower Cretaceous deposits may offer key insights into Cretaceous ichthyosaurs.

Abstract

This study describes the first thunnosaurian ichthyosaur specimen discovered in the upper Albian beds of the Hilo Formation at Guayabal de Síquima (Cundinamarca, Colombia), representing the first record of an ichthyosaur from the Albian of Colombia and South America. The specimen comprises a segment of the axial skeleton, including 31 articulated and 2 disarticulated vertebral centra, several ribs, gastralia fragments, and disarticulated appendicular elements. We assign the specimen to Thunnosauria based on the presence of ribs with 8-shaped cross-section. Nearly all Cretaceous thunnosaurians are ophthalmosaurids, with the exception of Malawania anachronus. The specimen is comparable in size to some ophthalmosaurids and is larger than M. anachronus. However, no diagnostic features support its inclusion within Ophthalmosauridae. Therefore, despite its Albian age and size consistency with Ophthalmosauridae, we conservatively limit the systematic position of the new specimen to Thunnosauria. The abundance of organic matter, pyrite and high articulation of the specimen, suggest deposition in anoxic waters with high organic matter content, which limited benthic fauna and prevented disarticulation by scavengers. This interpretation aligns with previous analyses of the Hilo Formation's depositional environment.

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