Fossil Record of the Araucariaceae

Araucariaceae: fossil record

The fossil record of Araucariaceae starts with the conifer ancestor, Voltziales. This group is considered a transition between conifers and Cordiates, and some of the species belonging to that group resembled Araucarian conifers. Those Voltziales recognized as the "walchias" were trees resembling the extant Norfolk Island Pine, and like them, the walchias have spirally arranged, needlelike leaves. Walchias were abundant in the Upper Permian. One other Permain-Triassic genera of the Voltziaceae, Ullmannia, has a bract-scale complex that is similar to that of Araucaria in general morphology. Though one must realize the records from the Paleozoic are very scarce, so we have to rely on several different specimens in which only some parts of the plant are well-preserved, so we don't know the whole picture.

Scientist have shown that the wood of the Permo-Carboniferous Glossopteris is araucarioid. If one were to examine an isolated fragment of the wood from a stem of the Jurassic Pentoxylon, there would be no hesitationin assigning it to Araucaria. There are no ray tracheids, wood parenchyma cells, or resin canals. The radial walls of the tracheids show bordered pits that are circular or slightly flattened. Yet, using characteristics other than those of secondary wood, Glossopteris and Pentoxylon are not close relatives of the conifers. Whether the araucarioid type of wood evolved independently in these groups or originated from a common ancestral type remains to be determined.

Since there is not a lot known about representation of Araucariaceae during the Paleozoic due to the lack of well-preserved specimens, we have to look into more recent fossil record. Dadoxylon-Araucarioxylon type of wood is best correlated to species of Araucariaceae. Among the specimens assigned to the Mesozoic form genus Araucarioxylon, the most famous are the permineralized trunks of the Petrified Forest National Monument of Arizona. Thousands of these logs have been discovered weathering from the Chinle Formation of the Upper Triassic.

It is clear that paleobotanists have difficulty in assigning the specimens to families, to say nothing of genera of present-day conifers. Three Mesozoic genera, Brachyphyllum, Pagiophyllum, and Geinitzia, are shown to have leaves similar to present-day Araucariaceae. All three genera are represented in the Middle Jurassic beds of Yorkshire. It has been shown that Brachyphyllum mamillare has pollen cones and pollen of the Araucaria type. According to Harris, the seed cone Araucarites phillipsii and the pollen cone bearing Brachyphyllum mamillare belong to the same plant. For what they describes as the "whole plant", the name Araucaria phyllipsii was proposed. Pagiophyllum kurri has the general aspect of shoots described as Araucaria, but it is now known, however, that shoots of Pagiophyllum kurri have seed cones that are not Araucaria. The leafy shoots of Geinitzia bear helically arranged leaves with the free part spreading. They are needle-like in form and of equal thickness in their horizontal and vertical dimensions. Fossils similar in appearance have been placed in Araucarites and Pagiophyllum, as well as other genera. Among living conifers, Araucaria heterophylla has similar shoots. Geinitzia-like shoots of the Permian have been placed in the genus Walchia and those of the Triassic in Voltzia.

The last thing that we have to consider is the reproductive structures of fossil Araucariaceae. As shown above, it is impossible to determine from wood and leaf fossils exactly when in geological time the Araucariaceae made its appearance. We get assistance, however, in making more accurate determinations to families, even genera, of conifers. For example, cone scales similar to those of Araucaria have been found in beds as old as the Triassic.

The idea of a Lower Triassic or Upper Permian origin of the family is consistent with the Permian occurrence of Ullmannia Frementaria, whose foliage and wood are similar to Araucaria, and whose bract-scale complexes consist of a single inverted ovule on a large scale. Based on present evidence, it is reasonable to suggest that the Araucariaceae originated from Ullmannia-type representative of the Permian Voltzailes. Two Mesozoic genera are considered here. One is Araucarites, whose cone-scale complexes are typically wedge-shaped with a broad base. As in living Araucaria, the complex is composed of a conspicuous bract with a beak-like apophysis. When the cone matures the cone scale complexes become unarticulated. Thus, most specimens of Araucarites are incomplete and only show the bract with a median depression that indicates the former position of the seed.

Ruth A. Stockey has made a detailed analysis of the permineralized cones from the famous Cerro Cuadrado petrified forest of Patagonia. Although the geological age of the petrified forest is in doubt, present evidence indicates that it is Middle to Late Jurassic. As a result of her studies of these beautifully preserved cones Stockey was able to relate the structures of Araucaria mirablilis cones to that of living Araucaria bidwillii. There are numerous accounts of araucarian cone-scale complexes and permineralized cones from many widely separated geographical locations that are Jurassic and Cretaceous in age. Many of the Jurassic localities are from India, Australia, and England, while those of the Creatceous are from Europe, Africa, and North America. Other localities of Tertiary age are confined to the Southern Hemisphere. Based on their frequency of occurrence and wide geographical distribution, it is clear that the Araucariaceae became an abundant and important element of Upper Triassic, Jurassic, and Cretaceous floras both in the Southern and Northern Hemispheres. By the onset of the Tertiary, the family was in a state of decline, becoming more or less restricted to the Southern Hemisphere.



Source:
Stewart, Wilson N. Paleobotany and the Evolution of Plants. Cambridge University Press, New York, p.405, 1983.
Kendall, A. "Jurassic member of the Araucariaceae." Ann. Botany. v.13:151-161, 1949.
Stockey, Ruth A. "The Araucariaceae: An evolutionary perspective." Rev. Paleobotany & Palynology. v.37:133-154, 1982.