This is Mazon Monday post #308.  What’s your favorite Mazon Creek fossil?  Tell us at  email:esconi.info@gmail.com.

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Acitheca polymorpha is an extinct species of the class Filcopsida. It is one of the rarer species of fern found in the Mazon Creek fossil localities. The classification of Acitheca polymorpha (Pecopteris polymorpha) has been problematic. Leo Lesquereux figured Pecopteris polymorpha as Pecopteris miltoni in 1879.

A. polymorpha was described by  Adolphe-Theodore Brongniart (1801-1876) as Pecopteris polymorpha in 1831. Brongniart, a French paleontologist, is considered by many to be the father of paleobotany. He described many Carboniferous plants.

P. polymorpha was moved to the genus Acitheca in 1925 by Robert Kidson, who was a Scotish botanist and paleobotanist. Acithica was originally erected by Wilhelm Philippe Schimper in 1879. Schimper (1808 – 1880) was an Alsatian botanist with French and later German citizenship. Schimper was a geology professor and made significant contributions in geology. In 1874 while studing the Paris basin, he proposed the Paleocene Era as a new subdivision of geologic time. He based on paleobotanical findings.

From the paper “Taxonomic revision of the Palaeozoic marattialean fern Acitheca” by Erwin Zodrow, et. al. we find an explanation of the reclassification.

Schimper based his diagnosis on two fertile specimens figured by Grand’Eury (1877): a small compression from the Kasimovian–Gzhelian (Stephanian) of the St. Étienne Basin (exact locality details not given), and a petrifaction from the Assise de Millery (Autunian) of Champ des Espargeolles near Autun (Fig. 1B). Kidston (1925, p. 537) wrote: “To Renault we are indebted for a detailed description of the synangia of Pecopteris polymorpha Brongniart, which forms the type of Schimper’s genus Acitheca”. In this paper Kidston extended the concept of Acitheca polymorpha to complete fronds (p.540), but restricted the description of the genus Acitheca (p. 536) to fertile pinnules. The latter was redescribed by Renault (1883), and Lesnikowska and Galtier (1991). It is widely stated that the Grand’Eury petrifaction is, therefore, the type of Acitheca but this is not the case. ICBN Article 10.1 and 10.2 (Greuter, 1988) stated that the type of a name of a genus is the type of a name of a species, and that the species must be one of those that the original author definitely includes within the genus. The only species that Schimper definitely combined with Acitheca (in the figure caption) is A. polymorpha (Brongniart) (basio-nym P. polymorpha Brongniart, 1834), and so this must be taken as the type species of the genus.

This immediately introduces a difficulty as the syntypes of Pecopteris polymorpha (and thereby of the genus Acitheca) described by Brongniart (1834) are all sterile (or at least all of the figured examples are). This in itself does not invalidate the genus, as ICBN Article 7.2 (Greuter, 1988) stated that the type does not have to be necessarily “the most typical or representative element of a taxon”. However, it is clearly necessary to show that the plant that produced the sterile types of P. pol ymorpha would also have had reproductive structures that correspond in nature to those described in Schimper’s diagnosis of Acitheca. The Grand’Eury (1877) specimens cannot be used here as in neither case are they well enough preserved to confirm in fact conspecifity with Acitheca polymorpha, in our evaluation.

Acitheca polymorpha can be found on page 22 of Jack Wittry’s “A Comprehensive Guide to the Fossil Flora of Mazon Creek”

Acitheca polymorpha (Brongniart) Schimper, 1879

1831. Pecopteris polymorpha Brongniart: p. 331, pl. 113, figs. 1-6
1925. Acitheca polymorpha Brongniart; Kidston: p. 539, pl. 128, fig. 1; pl. 129, figs. 1-4; pl. 130, figs. 3, 4
1969. Pecopteris polymorpha Brongniart; Darrah: p. 141, pl. 19, fig. 1
2006. Acitheca polymorpha (Brongniart) Schimper; Zodrow et. al.: pp. 239-280

DESCRIPTION: The ultimate pinnae are linear-lanceolate and terminate in an obtuse terminal lobe. The pinnules alternate and rise from the undulate, finely striate rachis at an oblique angle. The pinnule margins are parallel and often convex, becoming bluntly rounded at the apex. Larger pinnules near the base of the frond are lobed (see Fig. 3), while pinnules near the middle of the frond are slightly contracted at the base (see Fig. 1). The venation is generally distinct. The midvein is thick, straight, and extends to the apex where it divides twice. The lat-eral veins divide at the base, rise at a wide angle, arch, then both divide again before meeting the margin at an angle. When fertile (see Fig. 2), the pinnules often lie in hollow and inflated gaps in the matrix because of the large, elongated, and often protruding Acitheca synangia.

REMARKS: Acitheca polymorpha is rare. This taxon has been confused with Pecopteris miltoni by some authors-most notably by Lesquereux who, in 1879, figured Pecopteris polymorpha as P. miltoni. The reason for Lesquereux’s confusion was that Brongniart’s description for his P. polymorpha and his interpretation of Artis’s P. miltoni are nearly indistinguishable. This was an early problem in resolving the true affinities of Artis’s P. miltoni.

Specimens

Field Museum PP 58046

Field Museum PP 26863

Field Museum PP 46055 -From Wittry’s “A Comprehensive Guide to the Fossil Flora of Mazon Creek”

Field Museum PP 25608 -From Wittry’s “A Comprehensive Guide to the Fossil Flora of Mazon Creek”

ESCONI specimen