Macginitiea

''Macginitiea'' is an extinct genus in the family Platanaceae ranging from the Late Paleocene to Late Eocene of North America, known from the Clarno Formation of central Oregon. The genus is strictly used to describe leaves, but has been found in close association with other fossil platanoid organs, which collectively have been used for whole plant reconstructions. ''Macginitiea'' and its associated organs are important as together they comprise one of the most well-documented and ubiquitous fossil plants, particularly in the Paleogene of North America.Myers, J. A. (2003). Terrestrial Eocene-Oligocene Vegetation and Climate in the Pacific Northwest. ''From Greenhouse to Icehouse: The Marine Eocene-Oligocene Transition. Columbia University Press, New York'', 171-185.

Because paleobotanical material is often found in disarticulation, different species names are often used to refer to different organs (e.g. leaves, fruits, wood) even if those organs might have belonged to the same plant. When these organ species are considered together as a whole plant, the study is known as a whole plant reconstruction. Some localities have enough co-occurrences of different fossil plant organs that whole plant reconstructions are possible, one example being the ''Cercidiphyllum''-like ''Joffrea'' from the Joffre Bridge locality of Alberta, Canada.

The earliest definite occurrences of ''Macginitiea'' have been observed from the Comstock Flora of southern Willamette Valley, Oregon, which has been dated to be younger than 38-40 million years old (Bartonian), though similar ages are observed throughout the Pacific Northwest, with occurrences around 38-39 Ma.

Description

''Macginitiea'' leaves are readily recognizable by their three to nine, equally spaced, palmate lobes. Like other Platanaceae, ''Macginitiea'' has palinactinodromous primary venation, a type of venation wherein primary veins fork multiple times. ''Macginitiea'' also has an inflated petiole base, which in modern counterparts encloses the underlying axillary buds for the next year, indicating their deciduous nature. The branching of primary veins often occurs basally, rather than suprabasally (as encountered in some modern species). Although the secondary veins of modern ''Platanus'' are usually straight, the secondary veins of ''Macginitiea'' are so regular and prominent compared to other platanoids that this recognizable "chevron" pattern has been considered a primary characteristic of the species.Wolfe, J. A., & Wehr, W. (1987). Middle Eocene dicotyledonous plants from Republic, northeastern Washington. Margins are usually entire in all ''Macginitiea'' species, but can sometimes have minor teeth.

''Macginitiea'' differs from modern ''Platanus'' with its often greater number of lobes and narrower angle between adjacent primary veins.

Different species of ''Macginitiea'' can be distinguished on the basis of lobe depth, extent of chevrons, prevalence of teeth, and partly lobe number. In total, 5 species of ''Macginitiea'' leaves have been described.

Table adapted from Pigg & Stockey (1991).

Whole plant reconstructions

The Clarno Plane

The "Clarno Plane" was established as an informal name to refer to the whole plant recognized from five fossil species: ''Macginitiea angustiloba'' ( leaves), '' Plataninium haydenii'' (wood), '' Macginicarpa glabra'' ( infructescences), '' Platananthus synandrus'' ( staminate inflorescences), and '' Macginistemon mikanoides'' (isolated stamen clusters). The Clarno Plane is known from the west coast of North America across several states, including central California, Oregon, and northern Washington.

Like the modern ''Platanus,'' ''Macginicarpa'' has clusters of five carpels per floret. However, in striking contrast to the modern, ''Macginicarpa'' flowers have a well-developed perianth and fruits lack the prominent dispersal hairs characteristic of the modern ''Platanus''. Modern carpel numbers are more variable than the consistent five of ''Macginicarpa,'' ranging from four to nine.

Staminate flowers (''Platananthus synandrus'') have an even more developed perianth than pistillate flowers, but similarly have more consistent numbers of parts than the modern, with 5 stamens per floret. ''P. synandrus'' pollen appears to be smaller than pollen from modern ''Platanus''. ''Platananthus synandrus'' is also distinctive from extant ''Platanus'' for the elongation of its connectives, extensions of filament tissue that cover or divide an anther. In ''Platananthus'' and in modern ''Platanus'', peltate (shield-like) connectives cover the tops of anthers, but the connectives of ''Platananthus'' are 4 to 5 times the length of the modern. Stamens are connate (fused) within each floret, causing them to be shed in clusters of stamen bundles, rather than one at a time as in modern ''Platanus'' species. Stamen bundles associated with ''Macginitiea'' have been put under the genus ''Macginistemon''.

''Plataninium'' wood is similar, but has wider rays and more scalariform (ladder-like) perforations than that of recent species.

As of 1986, Dr. Steven Manchester said that "The Clarno Plane is currently the most completely documented fossil angiosperm species, known morphologically and anatomically from wood, leaves, pistillate and staminate inflorescences, fruits, and pollen." Associations of multiple organs of the Clarno Plane in various combinations have been found in "more than ten localities" throughout western North America, as of 2008. Other platanoid leaves have since been found in association with reproductive structures, including leaves such as '' Platimeliphyllum'', '' Ettingshausenia'', '' Evaphyllum'', ''Platanus neptuni'', ''Platanus nobilis'', and '' Sapindopsis''.

The Joffre Plane

The Paleocene fossil leaf species ''Platanus nobilis'' was established as a species intermediate between ''Macginitiea'' and modern ''Platanus''. However, differences between ''P. nobilis'' and ''Macginitiea'' were later considered too minor to justify placing ''P. nobilis'' in a different genus, particularly since ''P. nobilis'' was associated with ''Macginicarpa'' inflorescences. As such, ''P. nobilis'' was reassigned to ''Macginitiea nobilis.''

The "Joffre Plane" as a whole plant reconstruction includes leaves from ''Macginitiea nobilis'', pistillate inflorescences and infructescences from ''Macginicarpa manchesteri'', and staminate inflorescences of ''Platananthus speirsae''. ''Macginitiea nobilis'' is set apart from other ''Macginitiea'' species by its fewer number of lobes (usually 3, instead of 5-9) and less distinct "chevron" venation pattern. ''M. nobilis'' has been found in various stages of development from the Joffre Bridge locality, from seedlings with cotyledonous leaves to mature, true, trilobate leaves.

Discovery and naming

The genus ''Macginitiea'' was named after the prominent paleobotanist Harry Dunlap MacGinitie, who was the first person to suggest the close relationship of ''Macginitiea'' to the family Platanaceae. Though fossils of ''Macginitiea'' were originally classified under the genus ''Aralia,'' MacGinitie noticed that the venation patterns of ''Aralia angustiloba'' more closely resembled that of ''Platanus'' rather than ''Aralia'', and reclassified ''Aralia angustiloba'' as ''Platanophyllum angustiloba.'' However, Wolfe & Wehr found that the genus ''Platanophyllum'' was problematic, and therefore made the new genus ''Macginitiea'' for these leaves.

Ecology

''Macginitiea'' has been known to be the dominant element in many Eocene floras. The Clarno Plane is thought to have occupied open, riparian habitats, similar to that of modern ''Platanus''. They also have been found several times in co-occurrence with ''Cercidiphyllum''-like plants. The association with ''Cercidiphyllum'' suggests that the Clarno Plane was tolerant of full sunlight and poorly developed soils.

References

External links

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{{Taxonbar, from=Q21396968

Platanaceae
Eocene plants of North America
Eocene United States
Paleogene geology of Oregon
Eocene genus first appearances
Eocene extinctions
Clarno Formation