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Podocarpus from the Upper Cretaceous of Eastern Asia and its bearing on the theory of conifer evolution

Podocarpus tzagajanicus sp. nov. from the Uppermost Cretaceous (Tzagajan beds) of the Bureja River augments the Mesozoic record of the northern hemisphere Podocarpaceae. 'Northern' and 'southern' conifers grew side by side in Mesozoic and Tertiary forests. The distribution of conifers has been more deeply affected by climatic changes than by continental drift. IN a review of the Cambridge symposium on the biogeographical aspects of continental drift, Jardine and McKenzie (1972) quoted among selected examples Florin's theory of conifer distribution. They claimed (p. 24) that 'the history of the conifers provides another striking example of the action of drifting continents as agents of dispersal. Florin showed that from the Late Carboniferous (about 300 my) to the early Eocene (about 50 my) each of the conifer genera (with the exception of Araucaria) had either a "Gondwana" distribution . . . or "Laurasian" distribution. . . . The present disjunct distribution of Podocarpus and other Gondwana genera may be the product of the break-up of Gondwanaland, and the Tertiary spread of them into Indonesia and Southeast Asia may result from the northward drift of Australia.' Such views on the history of conifers are widely accepted. Rudolf Florin, an outstanding palaeobotanist and a great authority on gymnosperm taxonomy, came to his ideas of conifer evolution when describing Tertiary conifers from Chile (Florin 1940). He held then that both lineages of conifers, northern and southern, had been perfectly separated through time and space. Paranocladus, Walkomiella, Buriadia, Araucariaceae, Podocarpaceae, and Athrotaxis constituted the main body of the southern group and the rest of the conifers the northern one. Twenty years later Florin (1963) reiterated his views. The only exception made was for the Araucariaceae (but not for the genus Araucaria) which had been recorded from several northern localities. The works of R. Krausel on fossil woods and of R. A. Couper on microfossils provided additional evidence in favour of Florin's theory. Although several authors (Buchholz 1948; Ferguson 1967; Krassilov 1967, 1971) objected to this theory, it became fairly popular among botanists and earth scientists and was cited in many textbooks (e.g. Stebbins 1967). In recent years it has been used as a confirmation of continental drift. It is worth mentioning that Florin himself opposed the drift theory and relied upon 'continental bridges' as pathways of conifer distribution. The 'southern' palaeozoic conifer Walkomiella is hardly distinguishable from the 'northern' Lebachia. The early Triassic northern family Voltziaceae was represented in the southern hemisphere by the genus Voltziopsis. Other Mesozoic families such as Cycadocarpidiaceae and Cheirolepidiaceae were also distributed in both northern and [Palaeontology, Vol. 17, Part 2, 1974, pp. 365-370, pi. 49.] 366 PALAEONTOLOGY, VOLUME 17 southern continents. Cycadocarpidium has been recorded from the Upper Triassic of Argentina and the generic name Tomaxellia was recently proposed by Archangelsky (1968) for a southern cheirolepidiaceous conifer with pollen grains of Classopollis-type. He also suggested the cheirolepidiaceous affinities of Patagonian Jurassic conifer Pararaucaria, as well as Indostrobus from the Cretaceous of India. The 'southern' taxodiaceous genus Athrotaxis (or its nearest approach Athrotaxites) has been repeatedly recorded from the Lower Cretaceous of Canada and U.S.S.R. (Bell 1956; Krassilov 1967). All the above-listed facts contradict the division of ancient conifers into Gondwanian and Laurasian groups. But even more important is the evidence of the Laurasian distribution of Mesozoic Podocarpaceae. Pollen grains of the podocarpaceous type are known from many northern localities. However, most of them have been recently attributed to artificial genera. I referred to Podocarpaceae several megafossils from the Lower Cretaceous of the Primorye (near Vladivostok, Far East of the U.S.S.R.). One of them, Podocarpus sujfunensis Krassilov, displays the characters of the Nageia section of the genus Podocarpus (Krassilov 1967). The leaves are 120 mm long and 2-2 mm broad, with numerous veins, amphistomatic. Stomata all over the surface in longitudinal files separated by several cell files, amphicyclic; the polar encircling cells shared by adjacent stomata or absent; subsidiary cells papillate. Another species, Podocarpus harrisii Krassilov has linear-lanceolate, shortly petiolate, single veined, hypostomatic leaves about 40 mm long and 4-6 mm wide. The abaxial epidermis with broad central stomatic band is occasionally divided into two or three parts by narrow and irregular nonstomatiferous zones. Stomata arranged in files, longitudinally orientated, amphicyclic; subsidiary cells papillate. The topography of the abaxial epidermis is rather unusual for conifers, with flat single-veined leaves. However, several living species of the subgenus Stachycarpus show stomata over the vein. The division of stomatic bands into 'Teilstreifen' is also known among Stachycarpus species (Florin 1931). Bilaterally flattened leaves have been recorded from the Lower Cretaceous of Primorye under the name Paracmopyle florinii Krassilov. Primorye certainly was not the only Laurasian territory where Podocarpaceae flourished during Mesozoic time. Gomolitzky (1962) described leaves with podocarpaceous cuticle characters from the Jurassic of Central Asia. The fossil wood Podocarpoxylon triassicum has been found in the Keuper of Central Europe (Selmier and Vogellehner 1968). I suggested the podocarpaceous affinity of the Wealden species Tritaenia (Abietites) linkii (Roem.) Magdefrau and Rudolf as evidenced by stomata organization and topography of abaxial epidermis with three Teilstreifen' (Krassilov 1967, 1971). Podocarpus tzagajanicus sp. nov. from the uppermost Cretaceous of the Amurland augments the record of Mesozoic podocarps. As far as I know it is the only Late Cretaceous representative of the family. E X P L A N A T I O N OF PLATE 4 9 Figs. 1-8. Podocarpus tzagajanicus sp. nov. Upper Cretaceous, Bureja River. 1, 2, leaf fragments, x 1. 3, 4, abaxial cuticle, parts of stomatal band (slightly retouched), x 70. 5, 6, cells of adaxial epidermis, x 58 and 146. 7, 8, stomata, x 395.