Abstract
A newly discovered assemblage of lizard tracks from the Lower Cretaceous Jinju Formation (Sindong Group, Gyeongsang Basin) Korea is the largest yet reported from the Cretaceous. It consists of at least 95 tracks comprising five trackways, including a meter-long trackway (T1) with 50 footprints assigned to the new ichnotaxon Neosauroides innovatus ichnosp. nov. Two other trackways (T2 and T3) are designated N. innovatus paratypes characterized by strong heteropody, relatively wide trackways and small narrow manus tracks. These morphological characteristics distinguish Neosauroides innovatus from the previously reported lizard trackways Sauripes hadongensis from the Hasandong Formation and Neosauroides koreaensis from the Haman Formation, both also from the Gyeongsang Basin. These three lizard track assemblages from the Korean Cretaceous constitute the entire global lizard track record for this period. The Mesozoic record of lizard tracksites is more localized than the lizard body fossil record. This limited distribution suggests bias in the track record and the fossil record more generally. However, due to deposition of fine-grained substrates, suitable for high definition track registration, the Jinju Formation is increasingly well known as an ichnological window on small tetrapod activity and based on diversity, abundance and high-quality preservation, is regarded as an exceptional Konservat-Lagerstätten.
Highlights
(4) one study[33] dealt only with tracks in dry sand, the other three used clay-mud substrates inferred to be comparable in some respect to the pre-lithification substrates preserved in the Jijnu Formation stratigraphic successions (Fig. 1 and SI Fig. 1). (5) Studies of extant lizard tracks and trackways show that they are sufficiently variable in size, footprint morphology and trackway configuration to differentiate morphotypes, but not to identify or discriminate between trackmakers at low taxonomic levels: e.g. species or genera
(6) The same general conclusions apply to the Cretaceous lizard tracks from Korea: i.e., they may be differentiated morphologically as the basis for ichnotaxonomy, but not used to identify trackmakers or trackmaker groups at low taxonomic levels
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Summary
The lizard tracks from the Jinju Formation described in this study were discovered from the 2nd Excavation Project (“Excavation 2”) at the construction site of Jinju Innovation City. The trackways described here originate from a series of grey shales/mudstones, siltstones, alternating fine-grained sandstones and mudstones, and fine-grained sandstones of the Lower Cretaceous Jinju Formation which makes up part of the Sindong Group within the Gyeongsang Supergroup from near Jinju City, South Korea (Fig. 1B and SI 1). These deposits represent a lake basin paleoenvironment that was extremely rich in track-bearing units, with eight track-bearing levels previously reported from a 2.6 meter section that yielded avian (bird) and non-avian, theropod, ornithopod and pterosaur tracks from only one of four excavation sites, in a thick sequence of lacustrine basin sediments[7]. The Jinju Formation is considered Aptian in age[10]
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