Purported Triassic angiosperms
|In the 1970s there was a rush to find angiosperms that were older than the Mid
Cretaceous fossils. None of these discoveries was wholly convincing. However, current
phylogenetic evidence suggests a deep split between angiosperms and gymnosperms,
so it is not impossible that angiosperm fossils might be identified from the
Triassic (250-200 Mya).|
Many purported angiosperm precursors have been singled out because of net veins in leaves, a form that has arisen several times independently in different clades. Furcula is one candidate: its leaves are net-veined: a forked midrib, with secondary veins coming off it with intercostal veins between these that form a reticulate pattern.
Clues for discovering the true origin of angiospermy may be found in a plant group that can be considered to be a living fossil...
How palaeoecology of the Triassic may have led to the evolution of angiosperms
The Triassic saw the demise of the great supercontient Pangaea,the single global landmass. The Earth was slowly drying. There were no polar icecaps. The ground was dominated by ferns, while conifers and tree-ferns made up the forests. Cycads and ginkgos (both gymnosperms) were increasing in importance. On land, the main animals were archosaurs, a reptile group that led to dinosaurs in the Late Triassic
Pangaea began to break up in the Triassic, with flooding of the continents occurring as the prolonged dry spell of the Permo-Triassic ended. Seasonal weather patterns took hold through the Jurassic and Cretaceous.
Unfortunately, the remains were not in good enough condition to prove its affiliations, so the Cretaceous origination theory of angiosperms held. Then, in 1977, another Sanmiguelia fossil was discovered. This time, it was in much better condition, and whole, with roots,leaves, stems and most excitingly of all, structures resembling flowers. However, the fossil is still controversial; its affiliations to gnetales are still under debate, and thus the Cretaceous/Triassic argument rages on.
Triassic angiosperm-like pollen: Even if the macroplant fossils such as Sanmiguelia are debated, distinctive angiosperm-like pollen grains have been reported from the Newark Supergroup of eastern North America, in sediments of Carnian age (Late Triassic, some 228 million years ago).
The pollen was placed in the Crinopolles group. It was found with many other intermediate forms, between ginkgo pollen and angiosperm-like pollen, so the Carnian might represent an explosively experimental period. The Carnian angiosperm-like plant that produced the pollen might have failed to survive because of its rarity. It made up less than 2% of the palynoflora. One of its key characteristics is that it has many apertures, that key angiosperm feature. Under the evolutionary pressure of the time, however, those with a single aperture survived more easily and prevailed until the genes resurfaced in the Cretaceous. Wolfe et al. (1989) have suggested that angiospermy evolved early on and then disappeared from evolutionary view for some time.