Uhangrichnus: webbed bird tracks from Cretaceous Korea

Uhangrichnus is an ichnogenus – a name for a distinctive kind of trace fossil – established for webbed bird footprints from the Late Cretaceous of South Korea. Its type species, U. chuni, comes from the Uhangri Formation, where many impressions occur on ancient lake-margin surfaces. Broad web traces connect the three forward digits; later work also documented occasional marks from a small rear toe and a short trackway. The name applies to the footprint pattern, not to a bird skeleton. Similarity to a duck's foot therefore does not establish that a duck, or any other modern family, made it. The extinct bird catalogue distinguishes this trace-based evidence from species known through bones.

Quick facts

Naming a track rather than a bird

Researchers formally named Uhangrichnus chuni in 1995 from a large set of footprints at Uhangri in southwestern Korea. The type series was represented by impressions on a surface, with a silicone cast and replica designated as the name-bearing record. The original field material came from several nearby track-bearing patches, not from a single animal preserved while walking. It records a repeated foot shape across many steps and individuals, although the exact association between every impression and a single trackway is not always recoverable.

The type surface was studied before it could be lost to continued weathering and coastal exposure. A cast preserves a three-dimensional copy of the original relief, making the diagnostic margins available for later comparison. It is still a replica of a trace, not a body fossil. There is no skull, wing or leg bone directly tied to the footprints, so the trackmaker cannot be given a skeletal species name.

The original series contains dozens of prints. Measurements compiled from the better-preserved impressions give a typical footprint roughly 3.7 centimetres long and 4.6 centimetres wide when the occasional rear-toe mark is excluded. The track is wider than it is long because the front toes spread and the web impressions extend between them. Individual dimensions vary with sediment, pressure, erosion and the way the margins are interpreted.

Digits, webbing and walking sequence

The dominant impressions are digits II, III and IV. Broad web traces fill the angle between each adjacent pair, giving the footprint a palmated outline. The middle digit projects forward and the outer toes diverge. In well-preserved prints their margins curve slightly inward. These details distinguish the ichnotaxon from narrower, unwebbed bird tracks and from traces in which the rear toe is consistently prominent.

The type description did not require a visible hallux mark. A later trackway from the same locality showed a small rearward impression in some steps. The diagnosis was consequently discussed and emended to account for the occasional fourth mark. A toe can be present anatomically yet fail to touch the ground on every step; whether it registers also depends on how soft or wet the surface is. Variable visibility is therefore not by itself proof of different trackmakers.

The short topotype trackway provides several consecutive placements. The feet fall close to a narrow midline, and the middle toe turns inward relative to the direction of travel. Step measurements describe this particular sequence, not a universal stride for the animal. Without a securely known hip height, leg length and speed model, the trackway cannot produce a reliable running speed. It shows a walking pattern more directly than it shows body size or lifestyle.

An ancient lake margin in Korea

The Uhangri Formation is part of the Cretaceous Gyeongsang Basin record. Its sedimentary rocks include mudstone, siltstone and sandstone deposited in and around a lake system. Ripple marks, drying cracks, plants, invertebrate traces and vertebrate footprints point to shallow margins that were sometimes submerged and sometimes exposed. Bird-like, dinosaur and pterosaur tracks occur in the regional assemblage. Their proximity reflects a shared depositional landscape, not proof that the animals met or used the same patch at the same moment.

Volcanic rocks above and below the track-bearing levels constrain the age of the relevant beds. Published dates bracket a broad Late Cretaceous interval, roughly from the Cenomanian toward the Campanian, rather than providing a precise year or a single age for every footprint. The broad bracket matters because the formation spans changing environments and may preserve surfaces of different ages.

A webbed foot is mechanically suited to distributing load on soft sediment and can assist movement in water. That functional interpretation fits a lakeshore setting, but the track itself does not prove how often its maker swam, what it ate or whether it was aquatic. No beak impression, feather, stomach contents or prey remains belong to the type trackway. Those questions remain outside the direct footprint record.

Why the bird's identity remains open

Broad webbing is familiar in living waterbirds, including ducks, but this shape is not a unique family marker. Similar foot structures can evolve independently when birds move on wet ground or through shallow water. The Uhangrichnus tracks lack the skull and wing characters used to establish relationships among birds, so an identification as a duck would go beyond the evidence. It is safer to describe the maker as a bird with a webbed foot, based on the overall track morphology and context.

Other Korean Cretaceous ichnotaxa preserve different combinations of toe length, webbing, heel and trackway pattern. Comparisons can separate named footprint forms, but the number of trace names is not necessarily the number of biological bird species. One species may leave different impressions on different substrates; unrelated birds may leave similar traces. Trace taxonomy is a practical system for documenting repeatable shapes, with uncertainty about the organisms behind them.

Several authors have questioned whether Uhangrichnus can be distinguished consistently from other webbed-foot ichnogenera, including Phoenicopterichnum, and whether some named forms represent synonyms. Other tracks have been compared with U. chuni without being assigned confidently to the species. These proposals concern the boundaries and priority of trace names. They do not erase the fossil surface or its webbed impressions; they reflect the difficulty of classifying animal traces when preservation varies.