This is Mazon Monday post #210. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
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Alethopteris gibbsonii is an extinct seed-fern from the Pennsylvanian Period. It is not as common as its sister taxa Alethopteris serlii and Alethopteris sullivantii.
Lesquereux in 1864
It was named by Leo Lesquereux (1806-1889) in 1870. Lesquereux was a Swiss-born bryologist and a pioneer of American paleobotany. Lesquereux is credited with naming the Mazon Creek fossil deposit in his 1870 report "Report on the Fossil Plants of Illinois", where he referred to the Mazon River as Mazon Creek. “Mazon Creek” appears 104 times in the report, while “Mazon River” never is mentioned.
"At Mazon Creek, the meanders of the stream have dug a broad bed through the bank of shale, and the water, washing for centuries, has uncovered great numbers of concretions and scattered them for miles from their point of origin.”
A. gibsonii can be found on pages 176 and 177 in Jack Wittry's informative "A Comprehensive Guide to the Fossil Flora of Mazon Creek"
Alethopteris gibsonii Lesquereux, 1879
1879-80. Alethopteris gibsoni Lesquereux: p. 183, pl. 28, figs. 4-6; non pl. 33, fig. 1
1938. Alethopteris friedeli (non Bertrand); Bell: p. 68 in part, pl. 62, figs. 3, 4; non fig. 2 Alethopteris lesquereuxii; pl. 63 indeterminate
1958. Alethopteris grandini (non Brongnart); Langford: p. 247, fig. 445
1958. Alethopteris friedeli (non Bertrand); Langford: p. 247, figs. 447, 448
1958. Alethopteris ambigua (non Lesquereux); Langford: p. 247, fig. 446
1958. Alethopteris scalariformis (non Bell); Langford: p. 250, fig. 449
1958. Alethopteris ingbertensis (non Benecke); Langford: p. 250, fig. 451
1962. Alethopteris davreuxa (non Brongnart); Bell: p. 37 in part, pl. 3, fig. 5; pl. 41, fig. 1, non fig. 2 = Alethopteris lesquereuxi
1968. Alethopteris ambigua var. gibsoni Lesquereux; Wagner: p. 38, pl. 4, figs. 15, 16; non figs. 17, 18 indeterminate
1979. Alethopteris ambigua (non Lesquereux); Janssen: p. 145, fig. 133DESCRIPTION: The ultimate pinnae are widest in the middle, then gradually taper to an elongated, triangular lobe. The pinnules are free, but may appear slightly confluent at their bases. They can be either opposite, subopposite, or alternate, narrow, and perpendicular. Their margins gently taper to blunt points at their apices and have a typical aspect ratio of 4.5 times longer than wide. The pinnule lamina is very thick, making it appear inflated. A small, flat rim surrounds the margin, which is enrolled and only visible in strongly compressed examples like those preserved on shale (see Fig. 1). The venation consists of a heavy, sunken, non-decurrent, and straight midvein, which terminates near the apex. The lateral veins rise nearly perpendicular to the midvein, remain straight, are simple or more commonly fork once close to or at the base. They meet the margins at nearly right angles. The margins are enrolled with approximately 30 veins per centimeter.
REMARKS: Alethopteris gibsonii is uncommon. A feature that has been mentioned by Janssen and others as a main diagnostic characteristic is a groove located around the pinnule margins (see Fig. 3 inset). This feature is commonly, but not always, seen. It is made by the compression of the enrolled lamina and a small rim on the margin into the surrounding matrix. The absence of the "compression groove" should not be considered a discounting diagnostic feature.
The use of the name Alethopteris ambigua for the specimens found in the Mazon Creek flora in the past was based on Wagner's (1968) interpretation of this taxon. Since then, Wagner (2008) has united A. ambigua with Alethopteris lonchitica which display features not seen in the Mazon Creek specimens, such as elongated terminal pinnules, and oblique and decurrent midveins and pinnules. Though they are closely related to White's (1899) interpretation of A. ambigua, it is clear that the Mazon Creek examples and A. ambigua should no longer be considered conspecific. The mostly overlooked taxon, A. gibsonii, best encompasses all of the features seen in the Mazon Creek form.
Specimens
From Wittry.
From George's Basement. Langford referred to Alethopteris gibsonii as Alethopteris friedeli
From Andrew Young's beautiful Herrin Coal Gallery... A. gibsonii from M. Angkuw.
