Lemna

Smith, MacKenzie A., Greenwalt, Dale E. & Manchester, Steven R., 2023, Diverse Fruits And Seeds Of The Mid-Eocene Kishenehn Formation, Northwestern Montana, Usa, And Their Implications For Biogeography, Fossil Imprint 79 (1), pp. 37-88 : 49

Genus Lemna L.

Text-fig. 4e

M a t e r i a l. USNM PAL 620300.

L o c a l i t y. Park.

D e s c r i p t i o n. Leaves 0.9 mm long, 0.5–0.6 mm wide; elliptical; aerenchyma present; pseudoroot 0.4 mm long, 0.2 mm wide with slanted terminus.

R e m a r k s. The small size, elliptical leaves, presence of a pseudoroot and lack of venation are characters of the aquatic araceous subfamily, Lemnoideae. The lack of brown pigment cells, which we might expect to preserve in these cellularly preserved compressions (and which do preserve in the Spirodela fossils), rules out its placement in Spirodela or Landoltia and the broad elliptical shape of the fronds contrasts with the more narrow ellipses of Wolffia HORKEL ex SCHLEID and the narrow fronds of Wolfiella ( Armstrong 2021) . Note the name of the distinctive surficial cells is brown pigment cells. While they are brown in modern plants we would not necessarily be able to determine the color in the fossils. Thus, the leaf shape and lack of brown pigment cells support the placement of this specimen in Lemna . In North America, Lemna fossils are known from the Miocene of Mississippi ( McNair et al. 2019). The Kishenehn fossils are the oldest record of Lemna , predating Oligocene occurrences in Eurasia ( Dorofeev 1963). Another notable occurrence of Cenozoic Lemnoideae is that of the Limnobiophyllum leaf and root fossils from the Late Cretaceous through Oligocene of western North America ( Hoffman 1995, Kvaček 1995, McIver and Basinger 1993, Stockey et al. 1997). Limnobiophyllum differs from our fossils by having veined leaves, branching roots and size (leaves are 1–2 cm wide) ( Kvaček 1995, Stockey et al. 1997).

Figures

Text-fig. 4. Lemnoideae from the Kishenehn Formation. a: USNM PAL 625611 Spirodela sp. showing fronds, prophyllum and daughter plant with brown pigment cells. Scale bar 2mm. b: Close up of USNM PAL 625611 prophyllum and brown pigment cells. Scale bar 1mm. c: USNM PAL 768304 Spirodela sp. showing fronds, prophyllum and daughter plant with brown pigment cells. Scale bar 2.5mm. d: Close up of USNM PAL 768304 daughter plant with brown pigment cells. Scale bar 2.5mm. e: USNM PAL 620300 Lemna sp. showing fronds with aerenchyma and pseudoroot. Scale bar 1mm. f: USNM PAL 626140 Wolffiella sp. “tongueshaped” multicellular frond with brood pouch at the base. Scale bar 2mm. g: Modern Wolffiella caudata frond with brood pouch and daughter plant from Pagliuso et al. 2018. Scale bar 1 mm.