Growth interruptions in silicified conifer woods from the Upper Cretaceous Two Medicine Formation, Montana, USA: implications for palaeoclimate and dinosaur palaeoecology

Taphonomic studies suggest that most dinosaur bone assemblages in the Upper Cretaceous (Campanian) Two Medicine Formation of Montana originated from drought-induced mass mortalities on the floodplain. This hypothesis is tested further by studying intra-annual growth patterns in silicified woods of cupressaceous/taxodiaceous conifers found in association with dinosaur bone material at a palaeolatitude of 48°N. True growth rings are completely absent, but growth interruptions characterised by concentric, variably persistent zones, 1–8 latewood cells wide are common. Interruptions are irregularly spaced so that, for example, four interruptions may occur along a radius of only 25 cells in one specimen, whilst in another specimen a radius more than 650 cells wide may completely lack interruptions. Irregular growth patterns like this occur in Upper Cretaceous woods across a broad latitudinal belt in the US Western Interior demonstrating that they resulted from regional climatic forcing rather than localised phenomena. Growth interruptions record the occurrence of short-term, aperiodic disturbances to tree growth, under an equable, megathermal climate that was otherwise favourable for continuous, year-round cambial activity. The only probable cause of growth interruption formation were drought-induced fluctuations in the water table and resultant water stress. Growth patterns suggest that sufficient water for uninhibited growth was continuously available for years at a time, but when water stress did occur, it did so repeatedly over a very few months. Comparison with present-day East Africa, where trees show very similar growth patterns, implies that the equable, but erratically humid, environment of the Late Cretaceous Western Interior was exactly the kind of setting where mass mortalities amongst megafauna might be expected. Like East Africa, this environment would have had sufficient carrying capacity to permit large herds of megafauna to develop under normal conditions, but when repeated phases of droughts struck, such populations would have been highly vulnerable.