Araucaria mirabilis

''Araucaria mirabilis'' is an extinct species of coniferous tree from Patagonia, Argentina. It belongs to the genus ''Araucaria''.

''A. mirabilis'' are known from large amounts of very well preserved silicified wood and cones from the Cerro Cuadrado Petrified Forest, including tree trunks that reached in height in life. The site was buried by a volcanic eruption during the Middle Jurassic, approximately 160 million years ago.

Discovery

Fossils of ''Araucaria mirabilis'' are found in great abundance in the Cerro Cuadrado Petrified Forest of Patagonia, Argentina. They were the dominant species of a forest buried by a volcanic eruption about 160 million years ago.

The petrified forests of ''A. mirabilis'' were first discovered in 1919 by the German-Argentinean botanist Anselmo Windhausen. Noting that petrified cones were being kept as souvenirs by local farmers in the area, he explored the region and discovered the site of the petrified forests in 1923. He sent the specimens he collected to the German botanist Walther Gothan in Berlin in 1924. Gothan named them ''Araucaria windhauseni'' in honor of Windhausen in 1925.

However, the Italian-Argentinean botanist Carlo Luigi Spegazzini had also acquired specimens from the petrified forest from various sources. He tentatively identified the specimens as ''Araucarites mirabilis'' in 1924.

An American paleontological expedition led by Elmer S. Riggs (1923–1924) of the Field Museum of Natural History also discovered the petrified forests. The numerous specimens Riggs collected (who identified them as ''Araucaria'') were later described by the American paleontologist and paleobotanist George Reber Wieland as ''Proaraucaria elongata'' (1929), ''Proaraucaria mirabilis'' (1935), and ''Proaraucaria patagonica'' (1935). Wieland and Gothan interpreted the absence of separate petrified seeds as evidence that the cones did not shed their scales at the final growth year. This was originally stated by Wieland as a justification for its classification under a new genus ''Proaraucaria''.

An amended description was published by the Scottish paleobotanist Mary Gordon Calder in 1953. Calder questioned the conclusions of Wieland and Gothan. She also discarded the earlier classification of Spegazzini of '' Araucarites''. The latter is a form genus, usually used for incomplete plant fossil specimens that resemble ''Araucaria'' but lack enough preserved details for more accurate classifications. Citing striking similarities with the extant ''Araucaria bidwillii'', Calder reclassified the specimens as ''Araucaria mirabilis''.

Taxonomy and nomenclature

''Araucaria mirabilis'' belongs to the genus ''Araucaria''. It is classified under the family Araucariaceae of the order Pinales. Its has previously been assigned to the section ''Bunya'' of ''Araucaria'', which contains the living Australian ''bunya-bunya'' (''Araucaria bidwillii''). However, this has subsequently been questioned, with other studies finding it to have a basal position within ''Araucaria''.

The genus name ''Araucaria'' is derived from the Spanish exonym ''Araucanos'' ("from Arauco"), referring to the Mapuche people of Chile and Argentina. The specific name '' mirabilis'' is from Latin for "wondrous" or "amazing".

Description

A large number of petrified tree trunks of ''A. mirabilis'' are found still standing in the Cerro Cuadrado Petrified Forest. Preserved in volcanic ash, some of the specimens measure in diameter and were at least in height when alive. The trees were preserved just as the cones had finished maturing.

The cones are spherical, ovoid to ellipsoidal in shape, with a wide axis. They average in length. The largest specimens are nearly in diameter. The center of the cones consist of a parenchymatous pith surrounded by fused vascular bundles (two for each bract-scale complex, with each vascular bundle containing resin canals).

The bracts have thick and wide woody wings tapering towards the base. They are around long and wide, including the wings. They are overlain by fertile scales containing one seed each, embedded on the upper surface. They are arranged helically. The "ligules" (the free tip of the fertile scale characteristic of ''Araucaria'') are wide, high, and long.

The mature seeds of ''A. mirabilis'' are about long and wide. The seed integument has three layers of tissues - the sarcotesta, the thickened sclerotesta, and endotesta. It is fused to the nucellus (central portion of the ovule) only at the base. The sclerotesta (the "shell") also exhibits a zigzag pattern of sclereids. The fossilized seeds are exquisitely preserved, showing both mature and immature stages. They often contain well-developed dicotyledonous embryos, indicating that they were capable of lying dormant. The size of the cones did not indicate maturity as small cones around in diameter can be found with fully formed embryos. Most of the cones have been preserved before their seeds could be dispersed. Some cones, however, do not contain embryos and the naked axes of cones have also been recovered (described by Wieland as a separate species - ''Proaraucaria patagonica''). It is believed that ''A. mirabilis'' shed only its seeds but not the scales at maturity. Nevertheless, no separate petrified seeds or bracts have been recovered.

Small woody corm-like structures have also been found. Initially identified as "seedlings", are now known to be lignotubers.

Paleobiology and paleoecology

''A. mirabilis'' exhibits two characteristics shared only by ''A. bidwillii'' among extant ''Araucaria'' species. First is the separate origins of the vascular bundles of the bract and fertile scales; second is the highly vascularized "ligule". They also both have dicotyledonous embryos. On this basis, ''A. mirabilis'' is classified as belonging to the section ''Bunya''.

However, the seeds of ''A. bidwillii'' are much larger, long and wide, than the seeds of ''A. mirabilis''. ''A. bidwillii'' also exhibits cryptocotylar hypogeal germination (the cotyledons are non-photosynthetic and remain in the shell on germination), while there is evidence that indicates that ''A. mirabilis'' and other extinct members of the section ''Bunya'' exhibited epigeal germination (the cotyledons are photosynthetic and expand above-ground). In addition, no reliably identifiable fossils of members of the section ''Bunya'' have been recovered from Australia, the native range of ''A. bidwillii''. The only existing species of ''Araucaria'' in South America today are ''Araucaria angustifolia'' and ''Araucaria araucana''. Both belong to the section ''Araucaria'' of the genus. Setoguchi ''et al.'' (1998) have recommended that the extinct members of the section ''Bunya'' (which includes '' Araucaria sphaerocarpa'' of the United Kingdom) be treated as a separate group.

''A. mirabilis'' is found in association with other conifers, including '' Pararaucaria patagonica'' (not to be confused with the synonym ''Proaraucaria patagonica'' of ''A. mirabilis''), and '' Araucarites sanctaecrucis''. ''P. patagonica'' is also known from cones. It has no living descendants and its closest living relatives appear to be members of the modern family Cupressaceae (cypresses). ''A. sanctaecrucis'' fossils consist of foliage and branches.

The fossils of the putative bracket fungus ''Phellinites digiustoi'' are also found in the Cerro Cuadrado Petrified Forest. The latter was initially believed to be the oldest known mushroom-forming fungus (Agaricomycetes). Later examinations now make it likely that ''P. digiustoi'' was, in fact, part of the periderm of the fossilized bark of ''A. mirabilis''.

It is believed that the long necks of sauropod dinosaurs may have evolved specifically for browsing the foliage of the typically very tall ''A. mirabilis'' and other ''Araucaria'' trees. The energy-rich ''Araucaria'' leaves required long digestion times and were low in protein. This and the global distribution of vast forests of ''Araucaria'' makes it likely that they were the primary food sources for adult sauropods during the Jurassic. Juveniles, however, which lacked the bulk of the adults and required larger amounts of proteins for growth, probably subsisted on other plants.

''A. mirabilis'' fossils have been found with damage resulting from beetle larvae. These beetles are believed to be the ancestors of the most ancient lineage of bark beetles in the weevil family (Curculionidae) - the members of the tribe Tomicini, which are still serious pests of conifers today. They were probably host-specific to members of the genus ''Araucaria'' since the Mesozoic. Modern ''A. bidwillii'' are also hosts to primitive weevils from the family Megalopodidae and leaf beetles from the family Nemonychidae.

Distribution and geologic time range

''Araucaria'' forests were distributed globally and formed a major part of the woody flora of the Mesozoic era. The Cerro Cuadrado Petrified Forest is part of the La Matilde Formation, dated to the Bathonian to Oxfordian ages (164.7 to 155.7 million years ago) of the Middle to Upper Jurassic. The area was once part of the subtropical and temperate regions of the southern supercontinent Gondwana in the Mesozoic era, a more or less continuous landmass consisting of what is now modern South America, Africa, Antarctica, India, Australia, New Zealand, and New Guinea.

See also

* Paleobotany
* '' Araucaria haastii''
* ''Araucarioxylon arizonicum''
* ''Nothofagus''

References

{{Taxonbar, from=Q4784457

mirabilis
Jurassic plants
Mesozoic trees
Prehistoric plants of South America
Jurassic Argentina
Fossils of Argentina
Fossil taxa described in 1924