Aquatilavipes: a name for fossil footprints

Aquatilavipes is an ichnogenus, a name for a distinctive kind of fossil footprint, rather than a biological genus established from bones. C.M. Currie described hundreds of small tracks from an Aptian slab of the Gething Formation in British Columbia. Their size and toe arrangement were interpreted as bird-like, and several trackways suggest multiple animals crossed soft sediment. Later tracks assigned to the same ichnogenus in Alaska are much younger. The extinct bird catalogue therefore treats each assignment as a footprint comparison, not proof of one bird species persisting across millions of years.

Quick facts

Aquatilavipes: fossils and first description

In 1981, ichnologist C.M. Currie named Aquatilavipes from small tracks preserved in the Gething Formation at Peace River Canyon, British Columbia. The type is a surface bearing more than two hundred footprints, not a bird skeleton. The tracks are generally about 20 to 44 millimetres long, and their three forward-pointing toes give them a familiar bird-like outline.

The original slab records at least four trackmakers, based on track size and patterns across the surface. Some trackways appear to move through an area as the animals stepped across damp sediment. The density of prints makes the slab unusually informative about activity at one place, but it does not show a flock’s social relationship or exact number of animals with certainty: tracks can overlap, disappear or be repeated by one individual.

Because the footprints are the evidence used to define the name, Aquatilavipes is an ichnotaxon. Ichnotaxa classify traces such as tracks, burrows and feeding marks. Their names are separate from the formal biological names used for bodies. A trace name is useful even when the exact trackmaker cannot be identified.

Aquatilavipes: classification and species

Currie interpreted the tracks as those of birds. The small size, toe arrangement and trackway shape support a bird-like maker, and the finds are among the early records used to discuss birds in Cretaceous ecosystems. But a footprint does not preserve a skull, feathers, wing, or diagnostic bone. It cannot show whether the maker belonged to a known biological genus or to an extinct lineage not represented by skeletal fossils.

Even when the trackmaker was certainly a bird, linking a track to a particular species is difficult. Foot shape changes with substrate, depth, speed and posture. A bird pressing into soft mud may leave a wider, more splayed impression than it would on firm ground. Sediment can also erase details between toes. Similar-looking tracks can be produced by animals with different anatomy.

For that reason, the correct distinction is: the trace genus is named and diagnosed from track morphology; its biological trackmaker remains uncertain. Calling the animal itself “the bird Aquatilavipes” turns an evidence category into a body genus and overstates what the slab preserves.

Aquatilavipes: anatomy and ecology

Trackways can offer clues about movement. The length of a stride, orientation of prints and spacing between left and right steps may be compared to body size and to modern birds. On Currie’s slab, some tracks were interpreted as having been made while the animals foraged or fed. Such a reading depends on their pattern and setting, rather than on food preserved in the tracks.

A footprint cannot directly reveal what the animal ate. It records pressure and motion at the sediment surface. No prey remains, stomach contents or bill anatomy is attached to the trace. The shore or wetland interpretation comes from the sedimentary context and the small bird-like tracks, not from proof that the trackmaker hunted in water.

Angles between toes and the width-to-length proportions can help compare tracks, but they are not biological fingerprints. Currie reported an average toe divarication near 113 degrees for the type sample. That value describes a group of impressions; it does not by itself determine species, exact habitat or feeding method.

Aquatilavipes: environment and interpretation

Later fossil tracks from Alaska have also been referred to Aquatilavipes. Upper Cretaceous footprints in the Cantwell Formation, described by Fiorillo and colleagues, include tracks assigned to A. swiboldae. Some are about a quarter larger than the British Columbia material. The assignment rests on similarities in form, while the Alaska fossils are tens of millions of years younger than the Aptian type tracks.

This time difference is especially important. Similar footprints in rocks of different ages do not demonstrate that one biological species survived from the Early to Late Cretaceous. They show that researchers consider the track morphology close enough to use the same ichnogenus name. A broad trace category can include tracks from different species, perhaps even distinct groups that converged on similar foot proportions.

As with other trace fossils, the best account keeps three questions apart: what features are preserved, what kind of animal could have made them, and what behaviour might be inferred from their arrangement. The first is directly observable, the second is a biological interpretation, and the third usually carries greater uncertainty.

Aquatilavipes: what the evidence supports

Aquatilavipes is valuable precisely because it preserves activity that bones rarely record: animals walked across a surface, leaving repeated steps in sediment. The type slab preserves a large collection of small tracks and supports the presence of small bird-like animals in the Cretaceous environment. It does not preserve a portrait of the animal or a direct species identification.

A careful reconstruction may show a small bird-like trackmaker at a wet sediment margin, but its plumage, body dimensions, call and exact behaviour cannot be derived from the footprints alone. The name belongs to the trace. The biological identity remains a question for comparison with other evidence.