Araucaria mirabilis (Spegazzini, 1924) - fossil araucariacean cone from the Jurassic of Argentina. (public display, Field Museum of Natural History, Chicago, Illinois, USA)

Plants are multicellular, photosynthetic eucaryotes. The oldest known land plant body fossils are Silurian in age. Fossil root traces of land plants are known back in the Ordovician. The Devonian was the key time interval during which land plants flourished and Earth experienced its first “greening” of the land. The earliest land plants were small and simple and probably remained close to bodies of water. By the Late Devonian, land plants had evolved large, tree-sized bodies and the first-ever forests appeared.

Seen here is a quartz-replaced (silicified) fossil cone from an ancient Araucaria tree. Araucaria is a type of conifer, with about 20 species living today in the Holocene. Araucariaceans have a fossil record dating back to the Triassic. During the Mesozoic, they occurred in both Southern Hemisphere and Northern Hemisphere continents. Today, they are Southern Hemisphere plants.

This specimen was accompanied by no specific stratigraphic or locality information, but it is likely from the famous Cerro Cuadrado fossil forest of Patagonia, Argentina. Silicified cones were once abundant there, up to six inches long. The site has been overcollected and new specimens are scarce. Cones are covered with helically-arranged, cone scale complexes (= combination of bracts and ovuliferous scales). Longitudinal sections of these fossil cones are similar to those seen in modern araucariaceans. Fossil cones may have seeds in them, about 80% of which have embryos inside. Embryonic forms are just like modern araucariaceans. Pollen cones have not been found at Cerro Cuadrado - just seed cones. The seed cones don't contain pollen grains, or they are rare. Pollen grains are erosive - enzymes in the pollen grains eat away at tissues and create spaces in the pollen chamber of a seed. The same erosive tendency results in pollen tubes not being preserved. This is the case in modern araucariaceans. In the fossil seed cones, pollen chambers are never eroded away and no pollen tubes are present. Nor are there empty spaces where such structures should have been, but were subsequently eroded away. Preserved pollen cones are very rare in all araucariacean fossils, which is odd. Those that have been found are debatable, in terms of their araucariacean identity. It is possible that these fossil Araucaria trees had embryos developing without pollen (= apogamy), which is a condition found in ferns and lycopods.

The Cerro Cuadrado fossil forest was preserved by a volcanic deposit. The Patagonian forest is close to several cinder cone volcanoes. Silicification of the fossil cones occurred because of the high silica content of the volcanic ash.

Classification: Plantae, Pinophyta, Pinopsida, Pinales, Araucariaceae

Stratigraphy: unrecorded/undisclosed Jurassic unit

Locality: Patagonian Desert, Argentina

See reference:

Stockey, R.A. 1977. Morphology and Reproductive Biology of Cerro Cuadrado Fossil Conifers. Ph.D. dissertation. Ohio State University. Columbus, Ohio, USA. 141 pp.

See info. at: <a href="https://en.wikipedia.org/wiki/Araucaria_mirabilis" rel="noreferrer nofollow">en.wikipedia.org/wiki/Araucaria_mirabilis</a> and <a href="https://en.wikipedia.org/wiki/Araucaria" rel="noreferrer nofollow">en.wikipedia.org/wiki/Araucaria</a> and