How did the initial diversification of flowering plants transform the world's forests? Flowering plants (angiosperms) comprise most of the biomass and diversity of temperate and tropical forests today, but we know from the fossil record that this was not always the case. In a recent issue of IAWA Journal, my co-authors and I presented the results of a study aimed at understanding the role of angiosperm trees in Cretaceous forests (Jud et al. 2017).
The Cretaceous Period lasted from 145 Million years ago to 66 million years ago. Fossil assemblages from the Lower Cretaceous reveal a dramatic shift in the diversity plant communities, from a world virtually without flowering plants to a world in which they make up most of the diversity of plant species. The oldest fossils attributed to the angiosperms are rare pollen grains from about 135 million years ago (there may be older fossils, but the affinities of these candidates are still disputed), but in collections from the mid-Cretaceous, about 100 million years ago, we find that most of the different types of leaf fossils and pollen fossils can be confidently identified as angiosperms. By contrast, assemblages of fossil wood tell a slightly different story. Throughout most of the Cretaceous rocks worldwide, fossil gymnosperm wood (think conifers and ginkgo) is much more common than angiosperm woods. Could this mean that Cretaceous angiosperms were mostly herbs and shrubs, and rarely large trees?
As an undergraduate student in 2007, I spent some time collecting fossils on Vancouver Island. We visited an old gravel quarry with fossils that date to ~89 million years ago. Fossil ferns and gymnosperms had already been identified, and Steve Karafit (UCA) had already identified small fragments of angiosperm wood as well. As we collected we found a large permineralized log in the wall of the quarry. It was about 38 cm (15 inches) in diameter and at least 2 meters long. We assumed that it was a conifer and collected a few chunks to analyze back in the lab.
Under the microscope it is easy to distinguish the wood of most gymnosperms and angiosperms. Angiosperms have vessels, a type of water-conducting cells typical of flowering plants, but they are not found in conifers nor most other gymnosperms. We were surprised to see that this log had wide vessels typical of an angiosperm tree. In order to understand the significance of this discovery, I set to work reviewing previous discoveries of fossil woods from the Cretaceous rocks in North America.
We assembled a data-set of 178 angiosperm wood occurrences from the Cretaceous of North America. Plots of the data show a strong latitudinal gradient in both the diameter and frequency of angiosperm wood fossils from Cretaceous deposits across the continent, but large conifer woods are widespread. Although flowering plants were distributed worldwide before 100 Million years ago, evidence for angiosperm trees (more than 10 cm in diameter) before ~95 million years ago is very rare. The wood we found on Vancouver Island is one of the northernmost occurrences of angiosperm wood on the continent.
In modern forests flowering plants dominate in terms of species diversity and in terms of biomass or cover. This has been the norm for the last 66 Million years, but our analysis revealed two interesting patterns for the Cretaceous of North America. First, fossil angiosperm woods are most common in the uppermost Cretaceous rocks, which date to ~84-66 million years ago. Angiosperm dominance in the forest canopy seems to be something that emerged slowly, long after first appearance of flowering plants ~135 million years ago. Second, there was a strong latitudinal gradient in the distribution of angiosperm trees. Fossil wood from large angiosperm trees is exceedingly rare in the Cretaceous rocks of Canada and Alaska, even though conifer wood is fairly common. All of this is consistent with the larger story that angiosperm dominance lagged millions of years behind the angiosperms diversification. For more information see the article here, or you can request a copy from me directly.