A new species of Pseudotorellia Florin (Ginkgoales) from late Cretaceous deposits of the Lena-Vilui depression (Eastern Siberia)

The replacement of mesophytic flora, with a predominance of gymnosperms and ferns, by cainophytic flora, with a prevalence of angiosperms, took place in the middle of the Cretaceous Period.

The investigation of this phenomenon is of great importance to the understanding of evolution patterns of the major systematic groups, and palaeofloras as a whole. The processes whereby angiosperms appeared and diversified, as well as the processes whereby gymnosperms and ferns met extinction, appear to have different features, depending on the region studied. In Eastern Siberia (the Lena-Vilui depression), the main floristic turnover happened near the Albian-Cenomanian boundary, and was characterized by the abrupt disappearance of all mesophytic elements: the cycadophytes, czekanowskialeans, ancient ginkgoaleans and conifers.

Examination of the newly collected fossil plants fr om the Turonian-Coniacian deposits of the Lena-Vilui depression reveals the remains of Pseudotorellia leaves. This finding is the first evidence of the survival of mesophytic relicts in the Late Cretaceous Period within Eastern Siberia, and it changes our knowledge about the last evolutionary stages of the genus Pseudotorellia in Northern Asia.

The Lena-Vilui depression is a large basin of non-marine sedimentation, where an almost continuous succession of palaeofloras from the Late Jurassic up to the Coniacian can be traced. Numerous localities of fossil plants are known within the Lena, Vilui, Tyung, Linde, and Lepiske river basins (Budantsev, 1968; Sveshnikova, 1967; Kirichkova, 1985; Golovneva, 2005а,b). The modern stratigraphy of the Mesozoic deposits of this region was created on the basis of investigations carried out by V. A. Vachrameev and Yu. M. Pushcharovsky (1952, 1954). The Lower Cretaceous deposits were divided by those authors into the Batylykh, Eksenyakh and Khatyryk Formations, and the Upper Cretaceous deposits were divided into the Timmerdyakh and Linde Formations.

Phytostratigraphy of the Jurassic and Lower Cretaceous deposits were studied by A.I. Kirichkova (1985), and the Upper Cretaceous deposits studied by L. B. Golovneva (2005а,b). The floral development of the Albian and Late Cretaceous was divided into three stages (Golovneva, 2005а): Khatyrykian (Albian), Boskhian (Cenomanian) and Viluian (Turonian-Coniacian).

Khatyrykian flora comes from the Khatyryk Formation. It is the last mesophytic flora of the Lena-Vilui depression. It is characterized by high species diversity and consists of more than 100 species. This assemblage comprises mostly ferns, cycadophytes, ginkgoaleans, czekanowskialeans, and conifers like other Early Cretaceous floras of Northern Asia. Angiosperms are rare, and are represented by several small-leaved species from the genera Trochodendroides and Morophyllum (Kirichkova, Budantsev, 1967; Golovneva, 2005a).

The Boskhian flora comes from the lower part of the Timmerdyakh Formation. The species composition, and ratios between the main systematic groups, in this flora are significantly different from those of the Khatyrykian flora. Cycadophytes, czekanowskialeans, ancient ginkgoaleans (Pseudotorellia, Sphenobaiera), and conifers (Podozamites) disappeared in the Boskhian time. Overwhelming, the majority of ferns also disappeared and, on the whole, this group loses its dominant position. In Boskhian flora, taxodiaceous conifers and angiosperms began to dominate. The angioperms are represented by large-leaved forms from the genera Menispermites, Trochodendroides, Pseudoprotophyllum, Araliaephyllum, Cinnamomophyllum, Scheffleraephyllum, Celastrophyllum, and Liriodendropsis. The absence of any early Cretaceous survivals  is a distinguishing feature of the Siberian Cenomanian floras; this is in contrast to the Late Cretaceous floras of Northeastern Russia, wherein many Early Cretaceous elements persisted up to the end of the Cretaceous (Vakhrameev, 1988).

The Viluian flora comes from the upper part of the Timmerdyakh Formation. This assemblage includes about 80 species, and is characterized by a predominance of angiosperms from genera Menispermites, Araliopsoides, Paraprotophyllum, Platanus, Pseudoprotophyllum, Trochodendroides, Araliaephyllum, Magnoliaephyllum, Cissites, Celastrophyllum, Dalembia, Hollickia, Nordenskioldia and Quereuxia. As of yet, no relicts of Early Cretaceous taxa are recorded in the observed Viluian flora. However, lanceolate leaf fragments with parallel venation were discovered during the latest investigation of the newly collected Viluian fossil plants. The study of epidermal characteristics allows assigning these remains to the genus Pseudotorellia of the order Ginkgoales.  This genus was erected by R. Florin (1936) for leaves from the Lower Cretaceous deposits of Spitzbergen. Later, the generic diagnosis was amended several times (Watson, 1969; Bose, Manum, 1990).

In this paper, the findings of Pseudotorellia from the upper part of the Timmerdyakh Formation are described as a new species, P. insolita (N. Nosova et Golovn., sp. nov). Although the main morphological and epidermal features of these leaves are consistent with a diagnosis of the genus Pseudotorellia; the new species differ in some features from all known species of this genus.

To date, the genus Pseudotorellia includes about 38 species. V. A. Krassilov (1972) described megastrobiles Umaltolepis, which were associated with the leaves of P. angustifolia Dolud., and erected for these remains the family Pseudotorelliaceae belonging to the order Ginkgoales. The majority of the Pseudotorellia species were found in Jurassic and Lower Cretaceous deposits. At that time, the genus Pseudotorellia was practically an obligatory element of all the palaeofloras from the eastern areas of the Euro-Sinian paleofloristic realm and the western areas of the Siberian realm.

From the Late Cretaceous deposits, only one finding of Pseudotorellia is known. This is P. postuma Samyl. from the Arkagala Formation of northeastern Russia (Samylina, 1988). On the basis of the latest data, the age of the Arkagala flora is defined as the Santonian-Campanian  (Herman, 1999). Therefore, P. postuma is younger than P. insolita. This species comes from the vicinity of the Okhotsk-Chukotka volcanogenic belt. The latter region is characterized by the survival of different Early Cretaceous relictual taxa in the Late Cretaceous paleofloras (Samylina, 1988). But in Siberia, the finding of P. insolita is the first evidence of survival of the Early Cretaceous relicts. Other records of the genus Pseudotorellia in Siberia are known only from the Albian (Kirichkova, 1985). Leaves of P. tjukansis were found in deposits of the Khatyryk Formation. Therefore, the findings of P. tjukansis and P. insolita are separated from each other by both a long time interval, and by significantly different epidermal characters.

The remains of P. insolita are represented only by fragments which come from a plant debris layer. This type of preservation, and the scarce abundance of these remains, allows us to conjecture a rather long transport of Pseudotorellia leaves to a burial place. It seems probable that Pseudotorellia trees may grow in more elevated environments.

Such an interpretation is consistent with accepted theories which suggest that early angiosperms in the middle of the Cretaceous preferred lowland environments and inhabited mainly river valleys and coastal plains (Retallack, Dilcher, 1981), while a considerable quantity of mesophytic gymnosperms may have occupied upland environments wh ere they may have survived up to the end of the Cretaceous (Krassilov, 1985).