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

Lida Xing,Fengping Wang,Lisa G Buckley,Jianping Zhang,Richard T Mccrea,Laura Piñuela,Hendrik Klein,Martin G Lockley,Peter Dodson

doi:10.1371/journal.pone.0124039

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.

Highlights

In 2006, the Qijiang County Bureau of Land and Resources in Chongqing and the Southeast Sichuan Geological Team discovered dinosaur tracks within the Early to “middle” Cretaceous Jiaguan Formation that outcrops on Laoying Mountain, near the town of Sanjiao, Qijiang County
Given the general morphological similarities between the tracks attributed to large birds and those of small theropods, criteria are required to distinguish the tracks of large avians from those of small theropods
Ichnites attributed to theropod track-makers possess 1), unequal lengths of digits II and IV, with digit IV being longer than digit II, 2) an average total divarication between lateral digits of 90° or less, 3) the theropod “notch”, or indent, in the proximal margin of the print in the region of the metatarsophanageal pad proximal to digit

Summary

Introduction

In 2006, the Qijiang County Bureau of Land and Resources in Chongqing and the Southeast Sichuan Geological Team discovered dinosaur tracks within the Early to “middle” Cretaceous Jiaguan Formation that outcrops on Laoying Mountain, near the town of Sanjiao, Qijiang County. Xing et al [1] assigned most of these tracks to Hadrosauriformes (Caririchnium lotus), and Wupus agilis (Figs 1A and 1B; 2A and 2B), a small tridactyl track type attributed to a small non-avian theropod track-maker [1]. We provide simple quantitative analyses that aid in differentiating between the traces of large avians and those of small non-avian theropod track-makers. For the sake of nomenclatural clarity, hereon “theropod” refers to non-avian Theropoda outside of Aves, whereas “avian” refers to those Theropoda within Aves, following Gauthier [4]. This is necessary to clarify, as all Aves are Theropoda, but not all Theropoda are Aves

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Conclusion