The genus Ufadendron is very similar to the closely related genus Angarophloios in presence of the infrafoliar bladder as well as the wings and heel, but the former genus develops the leaf scar with a central point-like scar containing conductive tissues, disposed at the upper part of leaf cushion (Naugolnykh, 2014), while the latter genus shows only the leaf base buried in the matrix. The new species is different from the type species U. ufaense (Naugolnykh 2014) collected in the Lower Permian of the Cis-Urals, western limits of Angaraland, in its elongated leaf cushions, well developed heel and shorter keel.
The wings and heel are special morphological structures of Angaran lycopsids, which are defined as lateral extensions present in the leaf cushion and/or along the lower limits (margin) of the leaf cushion, which are more distinct than the upper limits, respectively. Occasionally, they can be observed in transverse section and/or in longitudinal section, respectively, such as it was recorded for the species Angarophloios leclercqianus (Meyen, 1972, pp. 155); sometimes the wings and heel could be inferred by the lateral corners and the lower part of the leaf cushion entering the matrix (Meyen, 1976, pp. 133). In our study, the new species clearly shows the wings when the specimen No. TF009HS2 is observed from a slightly inclined plane (Fig. 4b, WG), and the heel can be easily found on the trunk (Figs. 4a, 4b, HL).
Before Ufadendron elongatum is discovered in China, four genera of Angaran elements of lycopsids have been discovsered in the region geographically belonging to Angaran Phytogeoprovince in China (Sun et al., 2010; Huang, 1993, 1977). Angaran elements of lycopsids recorded in China are shown in Table 1, while their localities in Fig. 6.
Genera Species Ages Occurrences References Caenodendron C. primaevum Mississippian Xinjiang Sun et al. (2010) C. karagandense Mississippian Xinjiang Sun et al. (2010) "Angarodendron?" "A.?" baoligeense Pennsylvanian Da Hinggan Range of Northeast China Huang (1993) Eichwaldia E. biarmica Lopingian Da Hinggan Range of Northeast China Huang (1993) E. ulanzhaoensis Lopingian Da Hinggan Range of Northeast China Huang (1993) Viatscheslavia V. sp. Lopingian Xiao Hinggan Range of Northeast China Huang (1977) Ufadendron U. elongatum sp. nov. Lopingian Da Hinggan Range of Northeast China
Table 1. Angaran elements of lycopsids recorded in China
Figure 6. The localities of Angaran lycopsids in China. ① Caenodendron primaevum; ② C. karagandense; ③ "Angarodendron?" baoligeense; ④ Eichwaldia biarmica; ⑤ E. ulanzhaoensis; ⑥ Viatscheslavia sp.; ⑦ Ufadendron elongatum sp. nov. GP. Gondwanan phytogeoprovince, AP. Angaran phytogeoprovince, EP. Euramerican phytogeoprovince, CP. Cathaysian phytogeoprovince. Black and white circles representing Carboniferous and Permian taxa, respectively. The background figure is referenced to Halle, 1937.
Four phytogeoprovinces (paleofloristic realms), i.e., Euramerian, Cathaysian, Angaran, and Gondwanan phytogeoprovinces, were differentiated in the world during the Carboniferous and Permian time (Fig. 6). Taxonomical composition of lycopsids in these realm is different, as it is very well-known and broadly discussed in paleobotanical literature. Compared with the lycopsids from Euramerian Phytogeoprovince, the Cathaysian lycopsids generally have larger leaf scars, most commonly rhomboid-quadrate in shape, and characterized by absence of the lower parichnos. This morphological pattern forms the characteristic morphology of such oriental lycopsids as Cathaysiodendron (Zhang et al., 2006; Li et al., 1995), for instance.
Some Angaran and Gondwanan lycopsids are generally lack of ligule, for example, the genera Angarophloios (Thomas and Meyen, 1984) and Ufadendron. Same or similar morphology is characteristic for some Gondwanan lycopsids, such Bumbudendron, Malanzania (Archangelsky et al., 1981) and Brasilodendron (Chaloner et al., 1979). Nonetheless, there are some cosmopolitan taxa, such as Lepidodendron and Sigillaria, which are widely distributed in Upper Paleozoic (mostly, Carboniferous) floras of the northern hemisphere.
Lycopsids were widely distributed in the Late Paleozoic world. The difference of the leaf cushions and leaf scars among the lycopsids from the four phytogeoprovinces indicates that the paleoclimates were different during the Late Paleozoic. Lycopsids from the Euramerican phytogeoprovince with the most complicated leaf cushions and leaf scars, grew in tropical climatic conditions existed in the Euramerican phytogeoprovince (Boucot et al., 2013; see this paper for further references), and this region was the most comfortable and suitable area for the arborescent lycopsids.
Lycopsids from the Angaran and Gondwanan phytogeoprovinces represent plants well-adapted for existing in the temperate climate, which took place in high latitude regions of both the northern and southern hemispheres, respectively. Some of these lycopsids are similar in absence of ligule, implying in a sort of way that those plants did not develop very well in temperate continent regions because the leaf cushions of lycopsids from the tropical regions, i.e., the Euramerican and Cathaysian phytogeoprovinces, are more complicated and more well-adapted for humid low-latitude tropical climate.
Although both the Euramerian and the Cathaysian phytogeoprovinces were located in the tropical region, the former was actually controlled by tropical continent climate without good air circulation, while the latter was characterized by tropical island climate with good air circulation with possible monsoon effects. The difference of the leaf cushions of the lycopsids, which grew in these two phytogeoprovinces, was probably caused by the different climate conditions. Lycopsids with the lower positioned parichnos in leaf cushion were probably better adapted for highly effective exchange of O2 and CO2, as well as for effective regulation of the content of water in the plant, which was more convenient than for lycopsids without lower positioned parichnos.
The difference between the Euramerian and Cathaysian phytogeoprovinces were reflected not only in lycopsid morphology and anatomy, but also in morphology of ferns and pteridosperms ("seed ferns"), such as uppermost Permian and Triassic Lepidopteris and Permian gigantopterids. For example, the leaves of Lepidopteris with special subepidermal swellings were widely distributed in the Late Permian of Europe, has not been discovered in the Late Permian of China until recently (Zhang et al., 2012). In China, there are abundant taxa of gigantopterids, but in the Euramerian Phytogeoprovince, only several genera of possible gogantopterid affinity were found in the United States (Taylor et al., 2009).
Permian is the last period of the Paleozoic. What happened in the Permian of Gondwanan and Angaran lands, as well as localities surrounding the Tethys Ocean (Baud, 2018) are always a research hotspot in the geology and paleontology. Now it can be regarded as quite possible tendency that at least some of the higher plant taxa were able to migrate along the Tethys coasts both southwards and northwards (Naugolnykh and Uranbileg, 2018; Leven et al., 2011). Most probably such distant migrations were more or less common in Central and East Asia as well, especially if we take climatic reasons in account, which could provoke the considerable changes in the initial areal of formerly endemic taxa. In the case of the lycopsids of the family Tomiodendraceae we can suppose that this group initially were endemic for central Angaraland in Early Carboniferous, then migrated outwards after considerable cooling of the climate in boreal regions in the middle of Carboniferous. The presence of the species Ufadendron elongatum in the Permian deposits of Inner Mongolia is a good illustration of this process resulted from migration of tomiodendroid lycopsids far away from their initial area in Central Angaraland. The plant migrations triggered by climate change also is found in angiosperms. For example, Persea probably migrated from East Himalaya to Southeast Asia in the condition of the uplift of Himalaya through Miocene to Early Pleistocene resulting considerable changes in climate (Khan and Bera, 2016).