The late Triassic-early Jurassic predates Archaeopteryx, the most primitive bird known, which first appears in the fossil record in the late Jurassic fifty million or more years later. Yet here we have convincing traces of modern shore birds in strata that appears to be very well dated to the late Triassic. The authors point out this connudrum and suggest the strata is wrongly dated. However the dating appears to be, so to speak, rock solid. What’s up with that?
Santo Domingo Formation, Argentina
Jorge F. Genise, Ricardo N. Melchor, Miguel Archangelsky, Luis O. Bala, Roberto Straneck and Silvina de Valais
Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 272, Issues 3-4, 15 February 2009, Pages 143-161 | doi 10.1016/j.palaeo.2008.08.014
Abstract: The purpose of this study is to apply neoichnological observations to the behavioural and taphonomic interpretation of a Late Triassic–Early Jurassic track surface from the Santo Domingo Formation (Argentina) containing hundreds of bird-like tracks and trackways. In addition, the factors affecting the formation and preservation of bird tracks in lacustrine settings are particularly addressed. The 5.5 m2 fossil track surface contains different types of trace fossils: the avian ichnotaxa Gruipeda dominguensis (the most abundant), bird-like tracks with elongated hallux impressions, small epichnial rounded pits, and invertebrate traces (Skolithos, Taenidium). The modern environmental analogue chosen for the neoichnological studies was a coastal freshwater pond (the Bajo de los Huesos, Chubut, Argentina) seasonally occupied by sandpipers (Calidris bairdii and Calidris fuscicollis; Charadriiformes). The comparison between the fossil succession and the modern example suggests that they share lithology and sedimentary structures and that sedimentary processes and local palaeoenvironment were fairly similar and do not bias ichnological comparisons. Field observations allowed to distinguish twenty one behaviours that produced distinct traces and four modern footprint types (1 to 4) related to specific substrate conditions. In particular, the preferential formation of bird tracks parallel to the waterline, also confirmed by studies on droppings and invertebrate fauna of the pond, and other associated sedimentary features (ripple marks, wrinkle marks, mud drape thickness) and trace fossils were important for recognition of the shoreline in the fossil example. These observations also allowed us to distinguish the adjacent deeper and shallower parts of the fossil pond and can be applicable to other similar case studies. Contrasting sediment properties and footprint types, some relationships and constraints on the formation and preservation of modern footprints are proposed; although these are very complex processes that will require further studies. Five of the behaviours recognised in the modern pond were inferred from the sixteen trackways distinguished on the fossil track surface, including walking, walking with a zig-zag path, short runs, probing, and landing with legs directed forward (possible trace of flight). The recognition of traces of flight (Volichnia), probing marks, and tracks showing morphology similar to modern shorebirds (G. dominguensis), strongly suggest an avian affinity for the producers of the fossil tracks and, in consequence, the Santo Domingo track site would be younger than supposed.
From Section 2:
The studied fossil track site is located in the Quebrada (gulch) de Santo Domingo situated in the Reserva Provincial Laguna Brava, in the precordillera of La Rioja Province, Argentina. The track surface belongs to the Santo Domingo Formation, which is considered of Late Triassic–Early Jurassic age. This age is based on characteristic fossil wood remains (Caminos et al., 1995), on a 40Ar/39Ar radiometric age from interbedded basalt flows (Coughlin, 2001), and palaeomagnetic studies (Vizán et al., 2005). The formation reaches a minimum thickness of c. 1950 m and is in fault contact with Carboniferous igneous and sedimentary rocks (Caminos and Fauqué, 2001). The Santo Domingo Formation is a red bed succession that displays, from base to top, a transition from alluvial fan, fluvial braided (with calcretes), ephemeral fluvial and shallow lakes, and eolian environments (Vizán et al., 2005). The described track surface with bird-like footprints was recorded from a succession from the upper part of the Santo Domingo Formation, which had been interpreted as an ephemeral fluvial system. The main track surface is located at 1650 m from the base of the section, a second trampled surface with similar tracks has been recorded, although not studied to date.
From the Conclusions:
5 The presence of flight trace fossils (Volichnia), i.e. footprints with elongated hallux impressions that are interpreted as representing low angle landing, associated with probing marks and a similar morphology of Gruipeda dominguensis with tracks of modern shorebirds, strongly suggest an avian affinity for the producers of the fossil tracks. This ichnological evidence is supported by detailed neoichnological observations at the Bajo de los Huesos pond, and cast doubts about the currently accepted Late Triassic–Early Jurassic age of the hosting Santo Domingo Formation, which would be probably younger if it has to match the known body fossil record of birds.