Tatarornipes: bird tracks from Cretaceous Inner Mongolia

Tatarornipes chabuensis is an ichnospecies, a name for a recurring form of fossil footprint, from the Early Cretaceous Jingchuan Formation of Inner Mongolia. It was described from dozens of three-toed tracks and trackways, not from bones. The impressions reveal the shape of the foot where it pressed into sediment and the spacing of successive steps. They do not show a beak, feathers, wings or the body of the bird that made them. The extinct bird catalogue includes this track-based name alongside birds known from skeletal fossils.

Quick facts

Discovery at Chabu

The Chabu area lies in the Ordos Basin of Inner Mongolia, where at least sixteen tracksites have been numbered. Chabu site 4 is the type locality for Tatarornipes chabuensis. Martin Lockley, Jianjun Li, Masaki Matsukawa and Rihui Li formally described the ichnotaxon in 2012 after earlier reports had already recognised bird-like prints from the region.

The diagnosis was based on a large sample, with more than fifty measured and well-preserved tracks from five samples and representative trackways. Repetition matters in ichnology: a single mark can be distorted or incomplete, while a series lets researchers separate recurring anatomy-like traits from cracks, erosion or a slipping step. Additional samples were reported from Chabu sites 1, 5 and 15. Similar prints from Shandong were discussed as related, but a wider range for the exact ichnospecies requires careful comparison of morphology and preservation.

The name refers to the Chabu region. It applies to the recurring trace form, not to a biological species established from a skeleton. Trackmakers can be named even when the animal that made them remains unidentified.

Shape and movement recorded in the prints

The tracks are medium-sized and functionally three-toed, with digits II, III and IV directed forward. The toes diverge widely and are broad near their bases. In the comparative table used by the describing study, a representative print is about 52 millimetres long and 60 millimetres wide, with an angle near 110 degrees between the outer toes. Those values summarise the measured sample rather than define every individual print.

Several trackways preserve the sequence of steps. They show relatively long strides and little rotation of the feet with respect to the direction of travel. That is direct spatial information about foot placement, and it is consistent with steady bipedal walking. It is not a measured running speed. Without detailed parameters for a particular trackway and a biomechanical analysis, assigning a speed would add unsupported precision.

Wide toe bases may partly reflect how wet sediment deformed and later weathered. Experiments and comparisons among fossil track series show that a soft or collapsing surface can broaden the apparent outline. The original authors considered a large sample useful for separating persistent features from such variation, yet proximal toe width remains a feature that should be assessed across multiple prints.

The original comparison with other Early Cretaceous bird traces focused on diagnostic proportions as well as overall size. Tatarornipes is larger and more robust than small, gracile forms such as Koreanaornis. That difference is useful for recognising separate trace morphologies, but does not prove that the trackmakers belonged to distant biological branches. Two birds with similar anatomy could leave differently sized traces, and related birds could behave differently on one sediment surface.

A footprint is also a three-dimensional interaction between a foot and a substrate. A shallow impression may omit a toe tip; a deeper step may distort the margin or preserve pads that are invisible nearby. Researchers therefore rely on repeated impressions and trackway context rather than treating one unusually clear print as a perfect anatomical mould.

The age and setting of the Jingchuan Formation

The Jingchuan Formation contains mudstones, fine-grained sandstones, locally limestone and tuffaceous material. Its river and lake deposits preserve track surfaces along with footprints of sauropods and non-avian theropods. At Chabu, bird tracks occur within these broader vertebrate assemblages. Their association means different animals crossed surfaces preserved in the same formation; it does not demonstrate a mixed flock or an encounter between a bird and a dinosaur.

Older literature commonly placed the formation in the Barremian, roughly 130–125 million years ago. More recent work on ostracods, small crustaceans with fossilising shells, has proposed a younger Aptian age. The safest summary is Early Cretaceous, with Aptian favoured in newer interpretations and older Barremian assignments still appearing in the literature. The ichnotaxon itself does not resolve this stratigraphic debate.

The fluvio-lacustrine environment tells us where the sediment surface formed. It does not prove that the trackmaker lived only beside water. A bird could have crossed a wet margin while using a broader landscape that left no trace in that bed.

What cannot be reconstructed

Three forward-pointing toes, repeated print proportions and a bird-like gait support identifying the trace as avian. They do not reveal the whole leg length, mass, wingspan, bill or feather covering. Early Cretaceous skeletons such as Sapeornis provide comparisons for the diversity of birds at the time, but matching age and region is not enough to identify an individual trackmaker.

No diagnostic webbing is visible in the named prints, so an aquatic foot should not be drawn solely because the tracksite formed beside a river or lake. Likewise, a print contains no direct dietary evidence. Insects, fish or plant material could have been eaten by birds crossing the area, but no beak, stomach content or prey association assigns one of those diets to the unknown trackmaker.

The ichnofamily remains unresolved because the track characters do not provide a secure basis for assigning the trace to Koreanaornipodidae, Jindongornipodidae or Ignotornidae. Leaving the family open is a statement about the limits of footprint evidence, not a defect in the name.