September 22. Triassic plants

Plants during the Triassic were much like those of the preceding periods, although much diminished in extent by the Permian extinction. I mentioned the coal gap a few days ago, the early Triassic time from which no coal beds are known, with the implication that large coal-making forests were decimated by the extinction event, but compare that to the forests known in Antarctica, that we talked about yesterday. The earth is wildly varied, and was in the Triassic, too. 

The ferns, conifers, and other plants of the Triassic were largely gymnosperms, plants that make seeds without fruiting bodies to enclose them. The name gymnosperm means “naked seed.” Angiosperms’ seeds are enclosed in fruits and other protective matter, and they also have flowers. The name means “seeds enclosed in a protective vessel.” Angiosperms are today’s flowering plants. But there were no flowers during the Triassic – the oldest likely possible angiosperms are from the Jurassic, about 160 million years ago, and they really aren’t abundant and confidently identifiable until the Cretaceous, 125 to 130 million years ago. But the ancestors of the angiosperms were probably beginning to diverge from the gymnosperms by Late Triassic time. 

To an extent, flowering plants burst on the scene in the fossil record quite suddenly. Charles Darwin saw this as a problem for his theory of evolution, and called it an “abominable mystery.” But more recent discoveries have extended the story so that we can see much of the step-wise evolution of flowering plants, though much of the story remains obscure.

The genus Sanmiguelia, ferns found in late Triassic rocks of northwest Texas, has sometimes been suggested as the first angiosperm. The fossils, from about 230 million years ago, preserve the plants in growth position, allowing for more accurate interpretations of their lives. It was probably a herbaceous plant that grew in swampy wetlands that had angiosperm-like flowers. If it was not a primitive angiosperm, it seems likely that it was a close ancestor to them. There are also some spores that seem to be angiosperm-type pollen grains, dated to the middle Triassic, about 245 million years ago in Switzerland.

The question remains, if they existed, why did angiosperms not diversify and expand for almost 150 million years more, until the mid-Cretaceous, when they underwent a huge radiation. That question still stumps researchers, and we can’t be entirely certain when flowering plants, angiosperms, began. If they are as old as Triassic, or even Jurassic, they certainly were not common then.

—Richard I. Gibson

Links:
Sanmiguelia 
Purported Triassic angiosperms

Image from R.W. Brown, USGS Prof. Paper 274-H (public domain)