Fire, Flowers, and Dinosaurs

The Mesozoic Era is the geological interval comprising the Triassic, Jurassic, and Cretaceous Periods, and it is best known for the rise and fall of the dinosaurs. The Mesozoic began around 250 million years ago and continued to around 66 million years ago—a not inconsiderable chunk of geological time, and framed by mass extinctions at its beginning and end. Fifty years ago there were very few published papers on fire in deep time, but the most important one, which I’ve touched on before, was ‘Forest fire in the Mesozoic’, by Tom Harris of the University of Reading. Tom was an important scientist, one of the leading palaeobotanists in the world. Energetic and passionate about his fossil plants, he was a scientist with broad interests, and given to experimentation and lateral thinking. The evidence that Tom used in his paper on fires in the Mesozoic was limited to only a couple of charcoal occurrences in these rocks. The Permian Period ended with the biggest known mass extinction in Earth history, when life was almost wiped out. Whole ecosystems collapsed. So what would the world have looked like at the start of the Triassic? Among whole groups of plants that had become extinct were the giant club mosses that had been the major coal-forming plants of the late Paleozoic, and the glossopterids that had dominated southern continental vegetation. In the first few million years after the extinctions, plant diversity appears to have been low, but some new plants became prominent, including the pole-like spore-bearing lycopod called Pleuromeia, and the scrambling seedplant called Dicroidium, which had fern-like foliage. The first 10 million years of the Triassic are thought to have been a time of ecosystem recovery. According to Berner’s model, the Triassic started with very low levels of oxygen in the atmosphere. Researchers had noticed that there were no coals found at the beginning of the Triassic, and this interval was called the ‘coal gap’. The problem, therefore, was that charcoal in coal could not be used as a proxy for atmospheric oxygen for this time interval.