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| Austronaga minuta Wang, Lei & Li, 2023 in Wang, Fraser, Spiekman, Z. Li, et C. Li, 2026. Artwork by Gabriel Ugueto |
Abstract
Archosauromorpha is a dominant reptile clade that primarily occupied terrestrial ecosystems with limited secondarily aquatic invasions that are poorly known. Here we report on a well-preserved Triassic reptile representing the earliest stem-archosaur to evolve unequivocal aquatic specializations. This taxon exhibits a suite of aquatic adaptations previously unknown in stem-archosaurs, such as hyperphalangy, enhanced paddle-like forelimbs, and a loss of articulation between the sacrum and pelvis, indicating an advanced aquatic lifestyle. The fossil exhibits the oldest known stomach, liver, and intestinal preservations in reptiles. Our findings provide unexpected insights into the early aquatic diversification of Archosauromorpha during the Triassic marine ecosystem recovery and highlight a conservative gastrointestinal system in an early Mesozoic vertebrate. In the absence of clear terrestrial members of the other major Mesozoic marine reptile lineages (e.g., ichthyosaurs and sauropterygians), this discovery provides an unambiguous example of such a lineage including both decidedly terrestrial and extensively marine forms, providing tantalizing evidence for the land-to-sea transition in Triassic reptiles.
Systematic paleontology
Diapsida, Osborn, 1903.
Archosauromorpha, von Huene, 1946.
Tanysauria, Spiekman et al., 2024.
Trachelosauridae, Abel 1919 (sensu Spiekman et al., 2024).
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| Skeleton of the holotype of Austronaga minuta (IVPP V18579). Abbreviations are in the Supplementary Materials. Scale bars, 2 cm (A) and 1 cm (B to D). |
Austronaga minuta, Wang et al., 2023.
Type specimen: Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) V18579, a complete articulated skeleton preserved on two slabs (Fig. 1).
Locality and horizon: Waina Village, Luoping County, Yunnan Province, China; Member II of Guanling Formation, middle Anisian (Pelsonian), Middle Triassic.
Emended diagnosis: A tanysaur with the following features: quadrate anteroposteriorly broad and plate-like in lateral view with concave posterior margin; 18 cervical, 24 dorsal vertebrae; tall axe-shaped neural spine on axis; cervical neural spines prominent, with convex dorsal margin and without anterodorsal or posterodorsal projections; chevrons of mid-caudal region hatchet-shaped, with prominent distal expansion and a notch at the midpoint of its ventral margin; forelimb distinctly longer than hindlimb with humerus to femur length ratio over two; hyperphalangy present in both manus and pes.
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| Reconstruction of the long-necked marine reptile Austronaga minuta. Artwork by Gabriel Ugueto |
Wei Wang, Nicholas C. Fraser, Stephan N. F. Spiekman, Zhiheng Li, Olivier Rieppel, Hong Lei, Torsten M. Scheyer and Chun Li. 2026. Highly aquatic marine stem Archosaur with soft tissue preservation. Science Advances. 12(31); DOI: doi.org/10.1126/sciadv.aeb7528 [29 Jul 2026]
