A Fossil Equisetum Sp. (Family Equisetaceae, Subgenus Hippochaetae) from the Late TertiaryAsh Hollow Formation of NebraskaAuthor(s): Joseph R. ThomassonReviewed work(s):Source: American Journal of Botany, Vol. 67, No. 1 (Jan., 1980), pp. 125-127Published by: Botanical Society of AmericaStable URL: http://www.jstor.org/stable/2442544 .

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Amer. J. Bot. 67(1): 125-127. 1980.

A FOSSIL EQUISETUM SP. (FAMILY EQUISETACEAE, SUBGENUS HIPPOCHAETAE) FROM THE LATE

TERTIARY ASH HOLLOW FORMATION OF NEBRASKA

JOSEPH R. THOMASSON' Division of Science and Mathematics, Black Hills State College, Spearfish, South Dakota 57783

ABSTRACT

Silicified leaf-sheath fragments of Equisetum sp. are described from Late Tertiary strata in west-central Nebraska. The close affinity of these fossils to the subgenus Hippochaetae is supported by the presence of sunken stomata arranged in two regular rows on each leaf-sheath segment. This is the first report of the genus Equisetum from the late Tertiary Ash Hollow Formation of central North America.

ALTHOUGH the genus Equisetum is abundantly represented in the fossil record from the Car- boniferous to the Pleistocene, there are few reports in the literature of such fossils exhib- iting well preserved micromorphological or anatomical features. Harris (1961) used epi- dermal features to distinguish certain Jurassic fossil Equisetum and Gould (1968) described the Jurassic E. bryanni from stems and leaves showing micromorphological and anatomical details. Gomez (1978) reported a fossil Equi- setum impression from late Tertiary strata in Costa Rica. He referred the fossil to the sub- genus Hippochaetae on the basis of poorly preserved stomatal features. The only account of a fossil Equisetum from North American deposits showing preservation of anatomical details is that of Brown (1975). He described E. clarnoii from the Eocene of Oregon based on petrified root and stem fossils and conclud- ed that the fossils could be assigned to the subgenus Hippochaetae. In the present paper silicified fossils of Equisetum exhibiting mi- cromorphological detail are described from Miocene-Pliocene strata in Nebraska.

STRATIGRAPHY-The fossils were collected by the author during the summer of 1978 from a freshwater marl deposit approximately mid- way in a vertical exposure of 55 meters of flu- vial silts, sands, and conglomerates. The strata

1 Received for publication 23 April 1979; revision ac- cepted 20 August 1979.

This study was supported by grants from the National Geographic Society (1883), the National Science Founda- tion (DEB-7809150), and the Black Hills Foundation.

The author thanks Drs. N. R. Lersten, C. N. Page, R. W. Pohl, and F. Rich for reviewing the manuscript and Mr. Sherman Fuerniss and Ms. Debbie McBride for their able assistance in the field. The Bessey SEM facilities at Iowa State and the metallurgical engineering SEM facilities at South Dakota School of Mines and Technology were used in the preparation of this report.

from which the fossils were recovered belong to the Ash Hollow Formation (Nebraska sense) of Late Miocene to Pliocene age. The locality has yielded numerous and diversified plant (angiosperm and algae) and animal (vertebrate and invertebrate) remains which are currently the subject of separate studies. Detailed lo- cality information is available to qualified in- vestigators in the files of the Black Hills State Herbarium or from the author.

MATERIALS AND METHODS-Eleven speci- mens were found in a sandy-silty, freshwater marl deposit at JRT site 50 in Garden County, Nebraska. Fossils were recovered in situ from the sediment by degaging. Herbarium and fos- sil specimens selected for scanning electron microscopy were mounted on brass discs with high purity silver paint and coated with carbon and 28 cm of .008 in. diam 99.9% gold in a Varian vacuum evaporator. The specimens were then observed at 20 Kv in a JEOL JSM- 35 (Fig. 1-3, 5-6) or at 10 Kv in a Coates and Welter Qwickscan 100 (Fig. 4) scanning elec- tron microscope. Specimens were numbered in a BP (brass plate) series. A photographic record was kept on Polaroid 665 positive/neg- ative film. All specimens are deposited in the herbarium at Black Hills State College, Spear- fish, South Dakota.

DESCRIPTION-The fossils consist of frag- ments of leaf-sheaths devoid of scale leaves (Fig. 1, 2) and derive probably from aerial stems (C. N. Page, personal communication, 1979). The fragmentary leaf-sheaths generally consist of 2 to 4 segments, each 0.5-0.6 mm wide and 2.4-3.5 mm long. The individual seg- ments, which are slightly convex in the center, are separated by a furrow about one-tenth the width of each segment. One nearly complete leaf-sheath fragment is 2.5 mm wide and con-

125

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126 AMERICAN JOURNAL OF BOTANY [Vol. 67

* .'4~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

* 4'.' ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~~~~~~~~~~~~~~~~~~~~~~~-4 ~~~~~ ~~~~~~~~ .~ ~ ~ ~ ~~~~~1.Z

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. -~~~~~~~b

Fig. 1-6. Fossil and modern Equisetum leaf-sheaths. 1. E. sp., fossil leaf-sheath fragment showing rows of stomata (arrows), BP288. x33. 2. E. sp., fossil leaf-sheath fragment consisting of 5 segments, BP290. x22. 3. E. laevigatum (BHSC 8433), leaf-sheath fragment showing rows of stomata (arrows) and scale leaves (1), BP291a. x33. 4. E. sp., oblique enlargement of area outlined in Fig. 1, showing bar-shaped thickenings (t) suffounding stomatal openings, BP288. x 130. 5. E. sp., enlargement of small area outlined in Fig. 1, showing round projections (p), bar-shaped thickenings (t) and guard cells (g) at the bottom of stomatal passages, BP288. x470. 6. E. laevigatum (BHSC 8433), enlargement of area outlined in Fig. 3 showing round projections (p), bar-shaped thickenings (t), and guard cells (g) at the bottom of stomatal passages, BP 291a. x470.

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January, 1980] THOMASSON-FOSSIL EQUISETUM 127

sists of 5 segments. (Fig. 2). The micromor- phology of each segment is characterized by the presence of two rows of sunken stomata (Fig. 1, 4). Short passages leading to the sto- matal complex can be clearly seen (Fig. 4, 5). The external opening of each stomatal passage is surrounded by bulky, bar-shaped thicken- ings and small round projections (Fig. 4, 5). The flanks of the segments immediately adja- cent to each furrow are heavily thickened (Fig. 4).

DIscUSSION-The Equisetum sp. fossils most closely resemble modern E. laevigatum A. Br. (Fig. 3, 6), but, because of the relatively small amount of material available, this rela- tionship is tentative. Nevertheless, a clear ref- erence of the fossils to the subgenus Hippo- chaetae is indicated by the sunken stomata arranged in two regular rows on the flanks of each segment and by the massive development of sheath ornamentation (Hauke, 1963; Page, 1972). This is only the second report of a mem- ber of the subgenus Hippochaetae from the fossil record of North America and is the first report from the widespread Late Tertiary Ash Hollow Formation.

The fossils were found along with a rich an- giosperm flora which included specimens of Berriochloa, Leersia, Oryzopsis, Nassella, and Panicum (Gramineae); Biorbia and Pro- lappula (Boraginaceae); Carex (Cyperaceae);

Potamogeton (Zosteraceae); and Polygonum (Polygonaceae). In addition, several inverte- brate (ostracods and molluscs) and vertebrate fossils (gomphotheres and rhinoceroses) have been recovered from the deposit. The excellent preservation of the fossil flora and fauna ap- pears to rule out any long distance transport of the plant and animal remains, but rather would seem to indicate that the plants and an- imals lived at or very near the final site of deposition. The associated flora and fauna sug- gest a mesic, probably lacustrine, paleoenvi- ronment for the Equisetum sp. fossils.

LITERATURE CITED

BROWN T. M. 1975. Equisetum clarnoii, a new species based on petrifications from the Eocene of Oregon. Amer. J. Bot. 62: 410-415.

GOMEZ, L. D. 1978. Preliminary note on a fossil Equi- setum from Costa Rica. Fern Gaz. 11: 401-403.

GOULD, R. E. 1968. The morphology of Equisetum lat- erale Phillips, 1892, and E. bryanii sp. nov. from the Mesozoic of Southeastern Queensland. Aust. J. Bot. 15: 153-176.

HARRIS, T. M. 1961. The Yorkshire Jurassic flora. I. Thallophyta-Pteridophyta. British Museum of Natural History, London.

HAUKE, R. 1963. A taxonomic monograph of the Equi- setum subgenus Hippochaetae. Beih. Nova Hedwigia 8: 1-123.

PAGE, C. N. 1972. An assessment of inter-specific rela- tionships in Equisetum subgenus Equisetum. New Phytol. 75: 355-369.

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