Dinosaur-dominated footprint assemblages from the Cretaceous Jindong Formation, Hallyo Haesang National Park area, Goseong County, South Korea: Evidence and implications

Abstract

The track-rich Cretaceous Jindong Formation comprises part of an intra-arc basin, fluvio-lacustrine succession that represents a late stage in the evolution of the Kyongsang Basin. This formation is replete with track-bearing levels indicating the activity of many generations of dinosaurs and birds. The track-rich beds occur in the upper part of the Hayang Group (Kyongsang Supergroup), which also contains other, underlying dinosaur-track-bearing formations. However the Jindong Formation and underlying formations have produced few age-diagnostic body fossils. Altered volcaniclastic sediments such as are found the Jindong Formation complicate interpretation of the age of the tracks as discussed in the accompanying companion paper. Nonetheless such settings provided near optimal conditions for the formation and preservation of abundant track assemblages (ichnofaunas), and the Jindong Formation has become an ichnological “cause celebre” producing impressive statistics on the number of track-bearing sites, number of track-bearing levels and number of measured trackways. These data allow various inferences about certain aspects of the population structure, behavior and distribution of the dinosaurian track makers in these dinosaur-dominated paleocommunities. The Jindong Formation and underlying Haman Formation have also yielded many bird tracks. The complete lack of avian body fossils in Korea and the rarity of dinosaur skeletal remains means that the footprint record currently provides the vast majority of the Mesozoic vertebrate evidence available for the entire Korean peninsula. Thus, the tracks represent a highly significant addition to the national paleontological heritage of Korea, as well as being a very important component of the East Asian and global footprint records. Detailed studies of a 100–200-m-thick succession at the Sangjok Dinosaur Tracksite National Monument in the Hallyo Haesang National Park area in Kosong County reveal an average of about two track-bearing levels per meter, making it one of the richest track-bearing sections on record and providing evidence of the activity of hundreds of individuals. Many other track sites are found locally in the Jindong Formation in Kosong County (about 500 km 2) including one described herein from near Gohyeon village where the Jindong Formation type section is situated. Other track sites can be traced laterally over larger distances within the Gyeongsang Basin. The composition of ichnofaunas throughout this region appears remarkably consistent. The Jindong Formation is one of the few localities where sauropod, ornithopod, and bird tracks all occur in abundance, probably due to latitudinal/climatic controls. The sauropod tracks, which include wide-gauge forms allied to Brontopodus, form the largest brontosaur trackway sample yet reported but are characterized by a high proportion of small individuals. Such unusual size-frequency distributions raise interesting ecological and taphonomic questions about the biasing of the body fossil record towards large individuals by various physical (preservational) or biological/ ecological controls. The most abundant dinosaur trackways are those of iguanodontids (cf. Caririchnium or Iguanodontipus) that often traveled in herds. By contrast, sauropod tracks show little or no evidence of gregarious behavior and rarely occur on the same bedding planes as ornithopod trackways. This suggests a pattern of mutual exclusion or geological segregation between these two herbivore groups, which indicates that they probably frequented the area at different times. Sauropod and ornithopod track size-frequency distributions are also fundamentally different, suggesting that the ornithopods were mainly sub-adults and adults, whereas the sauropods were predominantly juveniles. Theropod tracks are uncommon suggesting a low predator:prey ratio of 1:20. Bird tracks including the large ichnospecies Jindongornipes kimi, an intermediate-sized form, Goseongornipes markjonesi ichnosp. and ichnogen. nov., and a small ichnospecies Koreanaornis hamanensis occur at several dozen stratigraphic levels in association with nematode trails ( Cochlichnus) and other invertebrate traces. These three ichnospecies are assigned to the respective ichnofamilies Koreanornipodidae ichnofam. nov., Ignotornidae, and Jindongornipodidae ichnofam. nov. All these avian footprints are typical of bird track assemblages in lake shoreline deposits, and indicate the activity of many generations of waders or shorebirds. We also recognize other, much less common, small footprint types tentatively attributed to a perching bird or a diminutive theropod. Collectively the bird tracks indicate the considerable potential of avian ichnites to provide insight into avian paleoecology at an early stage in the evolution of Class Aves.