The track record of Mesozoic birds: evidence and implications

Abstract

The occurrence, systematics, palaeobiological and evolutionary significance of Mesozoic bird and birdlike tracks are reviewed in the light of recent new discoveries in East Asia, Africa and North America. The most abundant footprint evidence currently comes from Lower Cretaceous deposits of East Asia and North America, where there is now a substantial track record for shorebirds. However, there is also a significant record of pre-Cretaceous bird-like tracks. The Cretaceous Jindong Formation of South Korea has recently yielded over 30 localities and stratigraphic levels, tripling the previous record worldwide and providing evidence of hitherto unknown large waders: Jindongornipes kimi ichnogen. et ichnosp. nov. Like other previously named ichnotaxa, this bird track type probably represents a charadriiform species. It is included with Ignotornis and Koreanaornis in the new ichnofamily Ignotornidae. Lower Cretaceous bird tracks are abundant, typically show high densities (100-300 tracks m 2 ) and often occur in association with invertebrate traces, inferred to have been made by the organisms on which the birds were feeding. Such track evidence suggests that waterbird communities were well-established by the end of the Early Cretaceous. This is about 30 Ma before the purported waterbird radiation inferred from Upper Cretaceous avian skeletal remains, but is consistent with hypotheses that suggest an ancestral shorebird stock. Bird and bird-like tracks from the Jurassic and Late Cretaceous are variable in morphology, still poorly understood, and sometimes attributed to small coelurosaurian dinosaurs. However, current evidence suggests that the Jurassic aviform ichnogenus Trisauropodiscus may be of avian origin. It is sufficiently distinct from typical grallatorid (coelurosaurian) tracks to warrant recognition of a new ichnofamily (Trisauropodiscidae), which includes the newly described ichnospecies Trisauropodiscus moabensis from North America. Bird and bird-like tracks from both the Jurassic and Cretaceous are shown to be much more abundant and distinctive than previously supposed. Moreover, they add significantly to our understanding of early avian evolution and palaeoecology, and provide insights into the timing of events in avian evolution.