Description of Bird Tracks from the Kitadani Formation (Aptian), Katsuyama, Fukui, Japan with Three-Dimensional Imaging Techniques

1. What they did

Fossil bird tracks from the Early Cretaceous are common in South Korea and China, but birds of that age are poorly known in Japan. The Kitadani Formation in Fukui is rich in fossil tracks, and possible bird tracks had been noticed among them.

Seven possible bird tracks were collected from the Kitadani Dinosaur Quarry in 1991. Two showed clear enough relief to be recorded with portable 3D scanners, and the authors described these two in detail:

• Scanning. Both tracks are natural casts (the infill of the original footprint, so they stand up from the rock). The authors scanned them with handheld 3D scanners and turned the scans into contour maps with 0.3 mm contour lines, alongside photographs and line drawings.
• Measuring. They measured track length and width, the length and width of each toe (digits II, III and IV), and the angles between the toes.
• Deciding whether birds made them. They checked the tracks against published criteria for telling bird tracks from those of small meat-eating dinosaurs, and plotted track length against the spread of the toes for their tracks and for published dinosaur and bird tracks.
• Naming the track types. They compared the tracks with known fossil bird track types.

2. What they found

The two tracks

• FPDM-F-74 is a left foot, 60 mm long and 94 mm wide, with slender toes ending in sharp claw traces. The outer toes (II and IV) curve slightly outward at the tips. The total spread between digits II and IV is 117 degrees. There is no back toe (hallux) trace and no web trace, and the heel is a shallow U shape. Three toe pads show on digit IV.
• FPDM-F-75 is also a left foot, smaller at 50 mm long and 71 mm wide, with a total spread of 101 degrees and possible claw traces on digits III and IV. There is no back toe trace. It has likely web traces between digits II and III and between III and IV, reaching less than half way along the toes.

Bird or dinosaur?

The authors assign both tracks to birds because they are small compared with most small dinosaur tracks, have slender toes with indistinct pads, a spread between the outer toes of more than 100 degrees, slender claws, and outer claws curving away from the centre of the foot. The web traces on FPDM-F-75 support this, as webs are not known from non-avian dinosaur tracks. On the scatter plot, both tracks fall within the cluster of bird tracks. Another proposed test, comparing footprint length with pace length, could not be used because both are single tracks, not trackways.

Track types

• FPDM-F-74 has the features of the track type Aquatilavipes and is larger than other Aquatilavipes tracks, though not exceptionally large among Early Cretaceous bird tracks. With only one track, the authors cannot confidently assign it to Aquatilavipes, so it is called cf. Aquatilavipes.
• FPDM-F-75 is placed near Gyeongsangornipes (cf. Gyeongsangornipes). It is larger than most webbed fossil bird tracks and differs from Gyeongsangornipes lockleyi from South Korea in its larger size, smaller toe spread and shorter webs.

One bird or two?

The two tracks differ in toe spread and in webbing, but this does not necessarily mean two different birds made them. The toe spread of a living heron can vary by more than 10 degrees either side of the average, and the missing web in FPDM-F-74 may be due to poor preservation. The authors give them different names but say the differences may be an artefact of preservation.

What the web might mean

The web on FPDM-F-75 is small, and its use for swimming is questionable. Some living birds that do not live by water, such as grouse and some domesticated Galliformes, have small webs like this, so a web does not necessarily mean the bird lived near water.

3. What they concluded

• The two tracks show that possibly two medium-sized kinds of bird lived in the area of the Kitadani Formation, adding to what is known about Early Cretaceous birds in Japan.
• Web traces on FPDM-F-75 and other late Early Cretaceous bird tracks from Asia suggest that partly webbed feet were not uncommon among birds at the time.
• Because no trackways have been found, the track types and shapes of bird tracks in the Kitadani Formation remain uncertain; further excavation could clarify this.

4. How tracking was or was not used

This is a study of fossil footprints (ichnology). The authors identified and measured two isolated footprints, judged whether a bird or a small dinosaur made them from the track shape (size, toe shape, toe spread, claw traces, pads, heel, hallux and web traces), and compared them with published tracks. No trailing was possible: there were no trackways, so stride and pace could not be measured.

5. Tracker or observer certification

Not reported. The tracks were described by the authors, palaeontologists at the Fukui Prefectural Dinosaur Museum and Japanese universities. The tracks were found during the 1st Kitadani Dinosaur Excavation Project, and S. Hayashi is thanked for initial data collection and documentation.

6. Other technologies, skills or innovations

• Portable 3D scanners (Artec Spider and Artec Eva, Artec 3D) to record the tracks’ surfaces, following an earlier recommendation by Arakawa et al. (2002) to use 3D imaging for footprints.
• Image analysis software (VIVID2Rugle and 3D-Rugle, Medic Engineering Corporation) to turn scans into contour (topographic) images. The authors used 3D imaging to document and describe the tracks objectively.
• The 3D scan files are freely available as supplementary data (STL format) on the museum’s website.