The discovery of Wupus agilis in South Korea and a new quantitative analysis of intermediate ichnospecies between non-avian theropods and birds

Abstract

Recently, Wupus agilis-like footprint (NHCG a10955) was discovered in South Korea. Morphological comparison with Wupus agilis showed that NHCG a10955 could be assigned to the first occurrence of Wupus agilis from South Korea, representing the largest bird footprint in South Korea. In addition, this study performed a subsequent quantitative analysis to find a better solution to identify the trackmaker of intermediate ichnospecies. Principal component analysis (PCA) and novel probabilistic method relative bird-likeness percentage (RBP) in FL-included condition and FL-excluded condition was performed between unwebbed Mesozoic bird, theropod, and intermediate ichnospecies. The result showed that intermediate ichnospecies generally show high morphological similarity with Mesozoic bird ichnospecies if foot length was excluded from the analysis. Besides Wupus agilis, some large intermediate ichnospecies (foot length over 200 mm), such as Magnoavipes lowei and Laramie Saurexallopus, tend to have higher RBP in FL-excluded conditions that the avian affinity of these intermediate ichnospecies is hard to dismiss. Furthermore, this study also found that FL/FW ratio is an essential criterion for identifying intermediate ichnospecies since it is tied to the intrinsic foot width of the trackmaker. Although the existence of large Mesozoic birds is relatively difficult to prove due to the scarcity of skeletal records, the high bird-likeness of large intermediate ichnospecies could represent the ichnological record of large Mesozoic birds. Adding a numerical probability along with qualitative identification could be more practical rather than making a solid conclusion (e.g., 100% dinosaur or bird footprint) for objectively identifying trackmakers until the proper skeletal evidence is discovered.