Reanalysis of Wupus agilis (Early Cretaceous) of Chongqing, China as a Large Avian Trace: Differentiating between Large Bird and Small Non-Avian Theropod Tracks

Abstract

Trace fossils provide the only records of Early Cretaceous birds from many parts of the world. The identification of traces from large avian track-makers is made difficult given their overall similarity in size and tridactyly in comparison with traces of small non-avian theropods. Reanalysis of Wupus agilis from the Early Cretaceous (Aptian-Albian) Jiaguan Formation, one of a small but growing number of known avian-pterosaur track assemblages, of southeast China determines that these are the traces of a large avian track-maker, analogous to extant herons. Wupus, originally identified as the trace of a small non-avian theropod track-maker, is therefore similar in both footprint and trackway characteristics to the Early Cretaceous (Albian) large avian trace Limiavipes curriei from western Canada, and Wupus is reassigned to the ichnofamily Limiavipedidae. The reanalysis of Wupus reveals that it and Limiavipes are distinct from similar traces of small to medium-sized non-avian theropods (Irenichnites, Columbosauripus, Magnoavipes) based on their relatively large footprint length to pace length ratio and higher mean footprint splay, and that Wupus shares enough characters with Limiavipes to be reassigned to the ichnofamily Limiavipedidae. The ability to discern traces of large avians from those of small non-avian theropods provides more data on the diversity of Early Cretaceous birds. This analysis reveals that, despite the current lack of body fossils, large wading birds were globally distributed in both Laurasia and Gondwana during the Early Cretaceous.

Fig 1. Footprints of Wupus agilis and Limiavipes curriei.

Wupus agilis (A, B) and Limiavipes curriei (c). A, Print A6-1 (see S1 Table for footprint labels) of W. agilis. B, Print A6-2 of W. agilis. C, Replica of holotype RTMP 1998.089.0011 of Limiavipes curriei deposited at the PRPRC [3], [12]. Scale in centimeters.

Fig 2. Trackway comparison of Wupus agilis and Limiavipes curriei.

Two trackway segments (A-B) of Wupus agilis from the Lotus Stockade Tracksite. Note the short pace and stride relative to footprint length. C, PRPRC 2005.07.002, trackway of Limiavipes curriei (modified from McCrea et al. [3]). Scale = 10 cm.

Fig 3. Data collection schematic for reanalysis of Wupus agilis.

Diagrammatic representation of linear and angle measurements collected directly from individual prints (A,B) and trackways (C) of Wupus agilis (S1 Table and S2 Table). A, Footprint measurements: II, digit II; III, digit III; IV, digit IV; FL, footprint length; DLII, digit II length; DLIV, digit IV length; DWII; digit II width; DWIII, digit III width; DWIV, digit IV width. B, DIVII–III, digit divarication II–III; DIVIII–IV, digit divarication III–IV; FW, footprint width; C, Trackway measurements: PL, pace; PA, pace angulation; SL, stride. DIVTOT (not shown), total divarication, summed from measurements of DIVII–III and DIVIII–IV.

Fig 4. Trackway map of the Lotus Tracksite.

Trackway map of the lower layer of the Qijiang locality, showing individual footprints and trackways of Wupus agilis, also showing pterosaur trackways of Xing et al. [2].

Fig 5. Geographic location of the Wupus agilis locality.

Geographic map of the location (footprint icon) of the Qijiang locality (Lotus Tracksite) in Qijiang District, Chongqing Municipality, China. Modified from Xing et al. [2].

Fig 6. Stratigraphic section of the Wupus agilis locality.

Stratigraphic section of the Jiguan Formation (left) with a detailed stratigraphic section of the Qijiang locality (right). Modified from Xing et al. [2].

Fig 7. Discriminant analysis morphospace plot comparing Limiavipedidae (Wupus and Limiavipes) to prints of small- and medium-sized theropods and large wading birds.

Discriminant analysis scatterplot comparing log₁₀-transformed and mean removed linear data (footprint length, FL; footprint width, FW; digit II length, DLII; digit IV length, DLIV; pace length, PL; stride length, SL) and mean removed angular data (total divarication, DIVTOT; pace angulation, PA) of Limiavipedidae (Wupus agilis, dark blue; Limiavipes curriei, dark brown) to ichnotaxa of Cretaceous theropods (black), Mesozoic avians (green), Cenozoic avians (pink), and traces of extant avians (orange). The scatterplot shows that Limiavipedidae, as well as the Cenozoic avian ichnotaxa and ichnites from extant avians, do not share morphospace with Cretaceous theropod tracks. Axis 1 is interpreted as the size–total divarication axis; as size and pace angulation increase (as the size of the trackmaker increases and as the trackway narrows), total divarication decreases. This is consistent with the observations of theropods having a smaller total divarication, as well as a larger size and narrower trackway. Theropods group positively along Axis 1, while birds, with their smaller size and higher total divarication, and more “toed-in” footprints, group negatively along Axis 1. Axis 2 is interpreted as the relationship between FW and the lengths of the lateral digits to FL, PL, and SL; footprints with longer lateral digits (DII, DIV) are relatively shorter in length, and are found in trackways with shorter PL and SL. Avian prints are interpreted to have subequal lateral digits and a higher L/W ratio, and the discriminant analysis correlates with the interpretation that avian prints belong to trackways with a relatively shorter pace length (Tables 4–5).