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Cycad Distribution and Continental Drift

In 1915 Alfred Wegener, a German climatologist and geophysicist, published an expanded version of his 1912 book The Origin of Continents and Oceans. This novel hypothesis, which has now been confirmed by other scientists, states that continents are not fixed; instead, their relative positions and the positions of the oceans have changed over time. Wegener's hypothesis led to the science of plate tectonics, where scientists study the gradual movement of large plates of the earth's crust along major fault zones. About 200 million years ago, the continents were joined into one supercontinent that Wegener called Pangea (Figure A). By 135 million years ago, Pangea had divided into two large subcontinents called Gondwanaland and Laurasia (Figure B). In the great southern subcontinent Gondwanaland, South America and Africa were connected with each other and with Antarctica, India, and Australia. This is roughly the time when the first flowering plants began to appear on earth. By 65 million years ago, about the time when dinosaurs became extinct, the continents had divided into positions resembling the present-day configuration (Figure C). As of April 2000, the continents resembled the configuration shown in Figure D.

The widespread distribution of certain plant groups correlates rather nicely with plate tectonics. For example, there are three families of cycads, the Cycadaceae, Stangeriaceae and Zamiaceae, which include a total of 185 species in 11 genera. They are widely distributed throughout tropical and temperate continents of the world. Prior to the theory of continental drift, the widely scattered distribution of present-day cycads in Australia, South Africa, Malaysia and the Americas was an enigma. The large, rounded seeds are typically dispersed short distances by gravity and their ability to roll downhill. This movement is undoubtedly enhanced by animals feeding on the disintegrating seed cones, particularly species with large, colorful seeds.

Most species of cycads tend to grow in regions distant from the coast, and have relatively limited areas for seed dispersal. Their viable seeds are not dispersed by ocean currents because they sink in water. How could all these species of cycads have such a worldwide distribution when their seeds are too large, heavy and dense to be carried great distances by birds, wind, or ocean currents? When cycads and extinct cycadeoids (the ancient precursors of cycads) once thrived on earth, the continents were united into an enormous supercontinent named Pangaea. The gradual movement of large plates of the earth's crust has resulted in today's isolated continents and the widespread distribution of cycads. Although they once represented a dominant and very successful plant line during the days of dinosaurs, many of today's relict cycad populations are seriously threatened with extinction due to extensive collecting and diminishing habitats. Indeed, the plight of today's cycads is shared by many endangered plant and animal species that once flourished in vast, pristine ecosystems.

Although most species of cycads tend to grow in regions distant from the coast and have relatively limited areas for seed dispersal, there are several notable exceptions. At least three species of Cycas (C. circinalis, C. rumphii and C. thouarsii) are found primarily in coastal regions of eastern Africa and southeast Asia, and have seeds that are buoyant in seawater. Their exceptionally large seeds have a thick, spongy flotation layer not found in other species, and undoubtedly accounts for their distribution on distant islands of the South Pacific.

Petrified Forest National Park in northeastern Arizona contains hundreds of acres of perfectly preserved logs from an ancient conifer forest that dates back to the late Triassic Period (approximately 225 million years ago). The trees of this forest coexisted with dinosaurs. Most of the petrified logs were previously assigned to an extinct species Araucarioxylon arizonicum, a presumed distant relative of Araucaria; however, new evidence indicates that these fascinating deposits of petrified logs represent a broad diversity of conifer species. Streams carried dead logs into this once swampy lowland region where they were buried in sediments rich in volcanic ash. Over countless centuries, the woody tissue has been replaced by minerals and gradually turned into stone. These ancient trees flourished during a time when all of the continents were united into the vast supercontinent Pangea. The area of Petrified National Park was located near the equator, at approximately the latitude of present-day Central America. Many of the reddish, agatized logs do not show cellular detail; however, there are some permineralized specimens in which minerals permeated the porous cell walls and filled the cell cavities (lumens). Thin sections of these samples viewed under a microscope show remarkable cellular detail.

Although the binomial Araucarioxylon arizonicum has been used in the literature for more than a century, Rodney A. Savidge of the University of New Brunswick (Bulletin of Geosciences Vol. 82 No. 4: 301-328, 2007) states that it is superfluous and therefore an illegitimate name (nomen superfluum). He examined thin sections of the original three specimens (syntypes) housed at the Smithsonian Institute upon which the species was first described by F.H. Knowlton in 1889. He concluded that they represented different species within two new genera of extinct trees. According to the International Code of Botanical Nomenclature, a valid species can have only one type specimen or holotype. Consequently, only one of the three specimens was retained as the new type or lectotype Pullisilvaxylon arizonicum. Savidge examined several other logs previously identified as A. arizonicum and concluded that they also represented additional new genera and species. His extensive anatomical studies indicate that the majority of logs at Petrified Forest National Park do not belong to a single species. It appears that the superfluous name "A. arizonicum" actually refers to a complex of extinct conifers. Based solely on the tracheid structure of permineralized wood (including resin canals, rays and tracheid pitting), and without DNA evidence, it is difficult to be certain which trees in the complex are ancestral relatives of the araucaria family. According to Dr. Savidge (personal communication, 2008), an immense amount of research into petrified woods is needed before the ancestries leading to modern trees will be clearly understood. At this time it would be purely conjectural to assign a scientific name to most logs in Petrified Forest National Park without detailed microscopic examination of the wood.

Trees of "Araucarioxylon arizonicum" grew to a height 200 feet (61 m) with a trunk diameter from 4 to 9 feet. According to Sidney R. Ash and Geoffrey T. Creber (Paleontology Vol. 43 No. 1: 15-28, 2003), the living tree did not closely resemble any of the present-day Araucaria trees of the southern hemisphere as postulated in past reconstructions. The branches did not occur in whorls as they do in most conifers, instead they grew irregularly along the trunk. Sydney Ash and Rodney Savidge also studied the bark anatomy of Araucarioxylon arizonicum ("The Bark of Late Triassic Araucarioxylon arizonicum from Petrified Forest National Park, Arizona," IAWA Journal 25 No. 3: 349-368, 2004), and concluded that it was quite unlike the banded bark of extant Araucaria heterophylla. These ancient conifers grew in a tropical rain forest with marshes and river lakes, in an environment very different from today's Arizona landscape.

There are other closely-related plant species with widespread distibutions; however, most of these plant groups did not exist during the time frame of Pangea. Several theories have been proposed to explain the occurrence of two remarkably similar species of copal-bearing trees of the genus Hymenaea on the isolated continents of Africa and South America, including continental drift and dispersal of seed pods by ocean currents. During the Cretaceous and Jurassic Periods, South America and Africa were once connected in a great subcontinent called Gondwanaland. Although the time frame for Gondwanaland is probably too early for Hymenaea, there may have been an ancestral forest throughout this vast land mass that gave rise to today's isolated copal species. A more plausible explanation involves the ocean dispersal of woody Hymenea seed pods from Africa. Adding credibility to the idea of pods floating across the Atlantic Ocean from Africa via the Equatorial and Brazil Currents are the large brown seeds inside which may remain viable even after many months at sea.

There are more than 100 species of coral trees (Erythrina), mostly found in Mexico, Central and South America, and Africa. It is doubtful that the genus Erythrina existed when Africa and South America were connected because the time frame when these continents were united is too early for Erythrina. Many of the present-day species have seeds that do not float in seawater, although some species, such as the tropical Asian and Polynesian E. variegata, have buoyant seeds that may have been dipersed by ocean currents. In fact, the latter species or its ancestral progenitor may have given rise to native populations of E. tahitensis on Tahiti or possibly wiliwili (E. sandwicensis) on the Hawaiian Islands.

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