Saturday, April 30, 2011

Plants of the Cretaceous by Kat Combs

The above picture is of the bark of an ancient relative of the common Cycads belonging to the order Bennettitales. These, along with most plants present in the Cretaceous were gymnosperms. Here's a bit of background: there are two main groups of plants, the more primitive gymnosperm, and the angiosperms. Gymno means naked and sperm means seed, examples of these are your typical conifer trees who's seeds, or cones, are not covered in an ovary. Angio meaning covered and sperm meaning seed, angiosperms are also known as the flowering plants. Most plants one thinks of now and days is an angiosperm such as Magnolias, tulips, apple trees, banana trees, etc. all of these having two key similarities: flowers and a seed covered in an ovary, fleshy like apples, or hard like nuts.

Before the lower Cretaceous, gymnosperms were the only plants around, but at the start of the lower Cretaceous, the first angiosperms began to show up. Angiosperm populations and diversity exploded after their first appearance. They had the advantage over gymnosperms in that they could reproduce and grow much faster when a disturbance, such as fire, killed off all the plant life in that area leaving room for the next generation to take over. That next generation being angiosperms. Though they were quick to take over, angiosperms didn't dominate until after the Cretaceous. So gymnosperms were still the dominating plant species found during the Cretaceous. Despite their relatively slow growth rate as compared to angiosperms, gymnosperms were diverse and found everywhere in the Cretaceous.

There is nothing today that can truly be compared to what it might have been like for the dinosaurs, though some areas like New Caledonia or eastern Australlia where they have the towering Araucaria trees could give you some idea, that mixed with a wide variety of vines and ferns much like you would find in tropical rain forests today; just not the same types of species. During the Cretaceous period the earth was much warmer than normal with raised sea levels. During our trip we found several fossilized tree deposits just in the site we were working, as shown in the picture below. Gypsum was present in the tree as was a core of coal (the black seen in the center of the tree trunk).

As the plants thrived so did the dinosaurs. One might think, given the enormous size of the herbaceous dinosaurs and the typical slow growth rate of gymnosperms, how could they possibly have had enough to eat? A possible hypothesis is that the presence of a fast growing, parasitic, gymnosperm leafy vine that would cover the trees in a green, edible blanket.
Some plants typical in the ancient rain forest would be small amounts of angiosperms, adding a splash of flowery color like magnolia, ancient towering conifer trees with needles and others with broad, flat leaves like the Ginkgo trees still found today, blankets of ferns covering the ground, vines twisting and spiraling up trees toward the sky; everything green and warm.

This trip was a blast and very educational (and sunny, with not enough sunscreen). Picturing how it might have been when the great reptiles were wandering the planet left me wishing I could see it, or at least go to Washington state and New Caledonia to get a mixed sense of what was going on.

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