EARLY CRETACEOUS FLORA OF MONGOLIA

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Phytostratigraphy In Mongolia, the Early Cretaceous localities contain sparse fern fragments only while the Jurassic floras are dominated by ferns. These types of assemblages appear to be easily distinguishable, but the fern criterion needs further practical corroboration. The fern beds may comprise also the lowermost Cretaceous. Czekanowskia is abundant in the Jurassic, but has not hitherto been recorded from the Cretaceous. Four phytostatigraphic units can be provisionally recognized in the Cretaceous: (1) The Baiera manchurica zone represented at Tsagan-Tsub, Modon-Usu and Bon-Tsagan. Baiera manchurica is common in the Urgal Formation (Valanginian) of the Bureja basin (KRASSILOV, 1972). (2) The Otozamites — Pseudolarix erensis zone, comprising the localities Bon-Tsagan, 23—22,Gurvan-Eren 236—29 and Manlaj 2,3 and 92. Apart from Otozamites and Pseudolarix, the correlation of Bon-Tsagan, 23—22, and Gurvan-Eren is supported by Brachyphyllum sulcatum, while Gurvan-Eren and Manlaj contain the angiosperm fossils. This community has no close analogues outside Mongolia. The flora is of generalized Wealden aspect and does not seem younger than Neocomian. (3) The Baierella hastata (and its cones Karkenia mongolica) — Araucaria mongolica zone includes localities of the Shin-Khuduk — Anda-Khuduk level, mostly in the paper shale facies at Bon-Tsagan, 73, 74, 75, 87, 193, 194, Kholbotu-Gol 196, 197, 199, 216, Khuriltu, 210, Erdeni-Ula 213, Shin-Khuduk, 1, 117, 118, 119, and Modon-Usu, 132. Baierella hastata is closely related to B. uninervis (SAMYL.) KRASSIL. from the Tschemchuco (Aptian) beds of the Bureja basin and Ginkgoites mongolica also represent a leaf morphotype common in the Tschemchuco beds. Neozamites occurs in the Aptian of Jacutia and Primorye. This community is comparable with the Aptian flora of Primorye (KRASSILOV, 1967) on account of Araucaria and other conifers. (4) The Limnothetis-Limnoniobe zone is confined to the Khulsyn-Gol Formation at Bon-Tsagan 45—19. It is destinctive on account of the mass occurrence of endemic cryptogamous plants (new genera). However the gymnospermous species are the same as in tne third zone, suggesting the age not younger than Aptian. Palaeoecology The Early Cretaceous flora of Mongolia is ecotonal between the temperate (Siberian) and subtropical vegetation comprising such genera as Phoenicopsis, dominant in the Siberian realm, and Otozamites, typical representative of the Ptilophylletum communities. Judging by their frequencies, Siberian plants might have grown on slopes, while subtropical plants dominated the lowland vegetation. The Mongolian flora differs from both Bureja (temperate) and Primorye (subrtropical) floras by much lower fern content. Irrespective of their actual frequence in a flora, ferns are rare fossils except in the fern marshes. The abundance of ferns in the Jurassic localities of Mongolia suggests fairly extensive fern marshes on the deltaic mud flats. At the beginning of the Cretaceous, the fern marshes had been reduced and the fern fossils became rare. This happened sumultaneously with the development of meromictic lakes and deposition of seasonal varvites — the paper shales. Some reduction of forest vegetation can also be assumed. In the Manlaj localities, conifers are represented mostly by winged seeds and cone scales. It appears that a conifer forest was at some distance from the lake, presumably on slopes, while on the flat shores grew the primaeval reed- and sedge-like monocots. They possibly colonized the mud flats exposed when the lake shranked during a dry season. The appearance of angiosperm fossils in the lower horizons while they are lacking in the overlain strata, seems paradoxical, but it can be related to the reduction of indigenous Mesozoic vegetation. A bennettite Otozamites lacustris accompanied them, and judging by the leaf anatomy (see below), it was adapted to periodical submergence. In Bon-Tsagan, 23—22, the conifers are better represented, and in Gurvan-Eren, Pseudolarix erensis is most aundant and Brachyphyllum is also common. The Pseudolaix forest grew perhaps on nearby slopes. An angisperm Gurvanella might also occur on slopes, because the winged fruits only were fossilized. At the close of the Neocomian, the Otozamites-angiosperm community was replaced by Baierella-Podozamites shrubs, and the forest vegetation also underwent considerable changes. Araucaria mongolica and Brachyphyllum densiramosum are abundant in practically all Aptian localities. They are represented by shoots, microstrobili Williostrohus latisaccus, Stenomischus sp., megastrobili and seeds (Samaropsis). The conifer forests dominated by Brachyphyllum and
iraucaria were apparently proximal to the sites of deposition. These communities can be conceived as a continuaon of the European-Central Asian brachyphyllous forests (i.e., characterized by the Brachyphyllum shoot morhology assumed convergently by conifers of different affinites, mostly the Araucariaceae, Taxodiaceae and Chei- Dlepidiaceae). The brachyphyllous forests extended through Mongolia to Primorye and Japan. The northern limit f Araucaria was at the Tyrma River, 50° North (KRASSILOV, 1978 a). The expansion of brachyphyllous forests in Lptian time can be related to the putative amelioration of climate. Another type of the forest vegetation is represented by prolific samaras Pityospermum amplexum of Abietasous affinities. There are also less numerous leaves (PityophyHum) and microstrobili (.Pityanthus), presumably of le same plant. The abundance of samaras and their even distribution in major localities suggest an extensive slope )rests dominated by an Abietaceous conifer mixed with a few representatives of the Siberian floristic realm, such > Phoenicopsis angustifolia which appeared in the fossil record at the close of the Aptian. In the Shin-Khuduk varvites, the lighter coloured lamellae (spring layers) are densely covered with pollen rains distinguishable under low magnification. I picked these pollen grains by needle and after a few minutes sposure to HCL and HN03 counted them to obtain a rough estimate of the contribution of various conifers to le pollen rain (the figures obtained by this technique are presumably less biased than in the standart palynoloical procedure dealing with volumes rather than surfaces). The numerical representation per 100 pollen grains is > follows: ytyanthus microsaccus (Pinaceae) — 40, yilliostrobus latisaccus (Araucaria mongolica) — 36, darneya angusta (Podozamites) — 6. For the remaining 18, the megafossils were not recognized. The rise of aquatic or semiaquatic Limnothetis id Limnoniobe at the close of the Aptian (Bon Tsagan, 45—19) can be related to eutrophication of the lakes, lass occurrence of the Limnothetis gametophytes suggests that the megaspores germinated in water and the imetophytes floated before settling on a muddy ground. There is much evidence of the animal-plant interactions. The most interesting are the caddis fly cases built of arkenia seeds (Text-Fig. 7J). According to SUKATSHEVA (in press), these cases rank among the most sophisticated ructures ever built by caddis flies. The inner tube is made of fusenized plant tissues, resin bodies of Karkenia seeds id occasional megaspores of Limnothetis. The outer sheath is built of the Karkenia seeds which are very carefully id, their micropyles facing outside. The Karkenia seeds have been produced by a plant with Baierella hastata leaves, is conceivable that Baierella grew in thick bushes along the shores, shedding abundant seeds in water. Other caddis ies have occasionally used the anthers of Pityanthus (PL 17, fig. 228). The cuticles and rasin bodies of Karkenia found in coprolites evidence the feeding of birds or small reptiles i these fleshy seeds. The samaras appear less attractive for seed-eaters, but there is a single coprolitic body Э mm in diameter with four damaged seeds of Samaropsis aurita (PL 14, fig. 174). Plant fossils resembling aquatic angiosperm fructifications are known from the Manlaj locality only. This >cality is also especially rich in chironomid remains which literally cover the bedding planes. Some functional Jations between the unusual diversity of aquatic plants and the abundance of chironomids may be iggested by the role of chironomid larvae in the phosphorus cycle. In modern eutrophic lakes, the phosphorus mtent of water is increased by chironomids (GALEPP, 1979). Phylogenetic comments In the Early Cretaceous flora of Mongolia, Swedenborgia and Darneya, hitherto recorded mostly from the riassic, associate with primaeval angiosperms and peculiar endemic plants. One can assume that the environlents in Mongolia were favorable for both survival of relics and production of new forms in various plant -oups. Limnothetis with solitary sporangia terminal or pseudoterminal on specialized short-shoots constitute another sample (complimentary to Synlycostrobus KRASSILOV, 1978 b) of evolutionary experimentation in the Mesozoic ycopsida, leading to reproductive structures analogous to those of conifers. Limnoniobe represents hitherto irecorded group of Mesozoic heterosporous plants possibly related to the Pennsylvanian Polysporia. New data on the leaf anatomy of a bennettite Otozamites lacustris with aerenchymous spongy mesophyll suggest
Page 1 and 2: EARLY CRETACEOUS FLORA OF MONGOLIA
Page 3: 75—5, 87—8, 193—3, 194—1 wi
Page 8 and 9: axis, dense on the side branches, w
Page 10 and 11: meshes 7—8 mm wide, resembling th
Page 12 and 13: Text-Fig. 4 (Legend see p. 13)
Page 14: Otozamites sp. Text-Fig. 4L Remarks
Page 17: Remarks : The leaf is more deeply d
Page 20 and 21: Czekanowskiales Phoenicopsis angust
Page 22 and 23: Leptostrobus sp. PL 7, figs. 80, 81
Page 24 and 25: adaxial side and a few abaxial stom
Page 26 and 27: Description: The largest shoot from
Page 28 and 29: Localities: Bon-Tsagan, 23—22, 34
Page 30 and 31: Remarks: GRAUVOGEL-STAMM & SCHAARSC
Page 32 and 33: Gurvanella dictyoptera sp. nov. PL
Page 34 and 35: flat, with parallel margins, but sl
Page 36 and 37: Typhaera fusiformis sp. nov. PL 19,
Page 38 and 39: Plate 2 Figs. 16—24. Limnothetis
Page 40 and 41: Figs. 115, 116. Brachyphyllum sulca
Page 42 and 43: Fig. 236. Stigma showing short hair

EARLY CRETACEOUS FLORA OF MONGOLIA

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