This is Mazon Monday post #53. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
-----------------------------------------------------
This week, we are going to take a look at a fairly common fern fossil of the Mazon Creek biota. That fern is Diplazites unita. We featured D. unita back in Fossil Friday #35 with specimens from ESCONI member Andrew Young.
Diplazites unita belongs to class Filocopsida (ferns). A good explanation of the ferns of Mazon Creek appears on page 81 of "The Comprehensive Guide to the Fossil Flora of Mazon Creek".
The members of the ferns described here have pinnate leaves, which have been shown to be homosporous and produced sporangia on the underside (abaxial surface) of their leaves. This includes problematic species with unknown fertile structures thought to be ferns based solely on having pecopteroid foliage. However, as more knowledge has been gained about the fossils, better-defined fossil genera have been developed based on a combination of pinnule vein pattern, sporangia characteristics, and overall pinnule form (see Appendix D). The genus Pecopteris is still used, but only on less-understood species, or species with a combination of features which do not easily fit into a better-defined genus. One of the taxonomic casualties is the genus Asterotheca, once commonly used by earlier authors for any pecopterid fern which, when fertile, produced radial sporangia in a line on each side of the midvein and divided into four or five sporangia. It was later found that the interpretation used by these authors for Asterotheca was not that of the original author thus the name was invalid applied. Presently, only one Late Carboniferous species is left of Asterotheca.
Some of the ferns, like the ones that grew Corynepteris and Nemejcopteris foliage, were shrub-like and belonged to the extinct order Zygopteridales. The Zygopteridales had unique vascular structures and grew banana-shaped sporangia on the underside of the leaves or at branch tips. Senftenbergia and Oligocarpia are early members of the extant order Filicales. The plant that produced Oligocarpia is poorly understood and has not been confidently assigned to a family. Senftenbergia is a common Pennsylvanian-age genus which grew as a tree fern, and whose characteristics are well understood. It has been assigned to the extinct family Tedeleaceae. Stellatheca is thought to be a possible very early member of the extant order Osmundales, which produces some of the most widely recognized species today, such as the royal fern, cinnamon fern, and interrupted fern. Most of the species in the following genera-Acitheca, Cyathocarpus, Diplazites, Myriotheca, Crenulopteris, and Pecopteris-grew as tree ferns and belong to the extant order Marattiales. Also, possibly belonging to this group is Radstockia, but it was likely herbaceous. The earliest known examples of this group are from the Carboniferous Period and reached their peak of diversity in the Permian. The extant members of Marattiales are not arborescent and produce more than one trunk, unlike their ancient Carboniferous ancestors.
With Pennsylvanian-age ferns, the name Psaronius is used for both the name of the fossil taxon of the trunks of marattialean tree ferns as well as the name for the entire plant. Psaronius was opportunistic and rapidly occupied any areas of disturbed soil away from flooded habitats of the swamp. It grew up to 30 feet tall, with a trunk that was unbranched until near the top, where it produced a crown of large, up to 10 feet long, pinnately divided fronds. Psaronius had a trunk covered by a wide, fibrous root mantle which gave it strength,
and visible frond scars only near the top.The fronds attached to the trunks of Psaronius formed distinctive frond scars when shed. The generic names Artisophyton, Megaphyton, and Caulopteris are used for these scars. When present, the scars are diagnostic for the different species of Psaronius.
Diplazites unita has had a bit of a tortured past with quite a few different names over the years. This list is from "The Comprehensive Guide to the Fossil Flora of Mazon Creek".
- 1836. Pecopteris unita Brongniart: p. 342, pl. 116, figs. 1-5
- 1833. Pecopteris longifolia Brongniart: p. 273, pl. 83, fig. 2
- 1870. Alethopteris longifolia. Lesquereux: p. 469
- 1870. Alethopteris emarginata Göppert; Lesquereux: p. 398, pl. 13, fig. 4
- 1879-80. Pecopteris emarginata. Lesquereux: p. 225, pl. 39, fig. 11
- 1879-80. Pecopteris unita Brongniart; Lesquereux: p. 223, pl. 40, figs. 1-7
- 1884. Callipteridium sinuatum? Brongniart; Lesquereux: p. 745, pl. 99, fig. 3
- 1925. Ptychocarpus unita Brongniart; Kidston: p. 548, pl. 131, figs. 1-9
- 1925. Pecopteris unita Brongniart; Noé: pl. 35, fig. 1; pl. 36, figs. 2-4
- 1969. cf. Desmopteris longifolia. Darrah: p. 145, pl. 31, fig. 1
- 1969. Pecopteris unita Brongniart; Darrah: p. 143, pl. 14, figs. 2, 3; pl. 15, figs. 1, 2
- 1971. Asterotheca (ex Pecopteris) pfefferkorn: p. 17, fig. 8, non fig. 7; pl. 8, figs. 1-7
- 1979. Ptychocarpus unitus Brongniart; Janssen: p. 139, fig. 122
- 2015. Diplazites unita Brongniart; Cleal: pp. 1-12
The "ESCONI Keys to Mazon Creek", published in 1990, has some nice simple descriptions to help identify this species. At that time, D. unita was referred to as Pecopteris unita.
Immature pinnules are placed alternately along the rachis and arc united. The midvein extends nearly to the apex where it forks. Veins are opposite, do not branch, and curve upward to the margin.
Mature pinnules are separate and broadly attached to the rachis. The midvein is strong and extends nearly to the apex where it forks. Veins are opposite each other, straight to the margin, and do not fork. The mature pinnules were formerly called Ptychocarpus.
The text from ""The Comprehensive Guide to the Fossil Flora of Mazon Creek" is also very descriptive and helpful for identification.
DESCRIPTION: The pinnae are linear-lanceolate and slightly contracted near their bases; they taper near the apex to an obtuse point. The pinna rachis is broad and striated. The pinnules are slightly decurrent with parallel sides and united for some distance from the base; the apex can be obtuse (see Fig. 8) or subacute (see Fig. 3). The midveins are decurrent, extending to the tip where they sometimes divide into two veinlets. The lateral veins are simple, nearly opposite, and arch toward the pinnule tip, especially the lower ones, which are strongly oblique. The sori have an asterothecoid appearance with typically 6 sporangia. The isolated spores are correlated with Laevigatosporites minutus (Pfefferkorn et al., 1971 and Jennings and Millay, 1979).
REMARKS: Diplazites unita is one of the most common fossils in the Mazon Creek flora and the second most common true fern. In the Mazon Creek flora, about two out of every three are found in a fertile state (see Fig. 2) which is an unusually high percentage. Around the world, several different spore types have been isolated from fertile D. unita foliage, strongly suggesting there are more than one biological species represented by this form. Only one spore type has been found on D. unita from the Mazon Creek area Jennings and Millay, 1979), suggesting only one biological species is present.
Specimen photos
Illinois State Museum - Langford (Ptychocarpus unita)
Terminal foliage - Andrew Young
Fertile form - Andrew Young
Mature - Andrew Young
Terminal foliage - Jann Bergsten
