Hyatt is an ammoniacal family in Tibet, which was founded in 1900. Up to now, 12 genera have been included in this family (Sepkoski 2002). Hyatt Regency (1900) was the first one to establish the Tibetan ammonia family. On the basis of the three Himalayan genera (Tibet, Anatibetites and ParaTibet) established by Mojsisovics (1896), he established the Tibetan Ammonia Family. Since then, new genera of amphibole (Diener, 1908), hornless amphibole and amphibole (Wang, He Guoxiong, 1976) have been established in Himalayan region. Tibetan ammonites belong to Pali Cites (Gemmellaro1904) in Sicily, Italy, and Neotibeites (Krumbeck1913) in Timor in the western Pacific, both of which were at low latitudes at that time. Tozer (1994) also classified Oxytropis Tozer 1994, Oxytropis Tozer 1994, Oxytropis Tozer 1994 and Oxytropis spath 195 1 as Tibetan. Tozer (1988) thinks that these Tibetan ammonites are representative of low latitudes, which well explains the block migration in western Canada. Therefore, the Late Triassic ammonites in Tibet, as a typical warm temperate fauna, are of special significance for the study of paleoclimate and paleoocean.
Within the East Tethys of China, the ammoniacal and ammoniacal fossil groups (Mojisisovics,1893; Dina, 1906), Nyalam, Tibet (Wang et al., 1976), Jianchuan, Weishan, Lanping (Liang Xiluo, 1976), Qamdo, Tibet (Wang et al.,19865433). Zhiduo area in Qinghai (He Guoxiong et al., 1985), Yushu area in Qinghai (He Guoxiong et al., 1990), Quemocuo area in Qinghai (Niu Zhijun et al., 2003) (illustration 1).
The Indian submarine model was built by Douglas (1929) using Iranian materials. The geographical distribution of this genus includes Iran, East India and Timor Island (Krumbeck,1913; 1924) 。 Later in Hungary, Oman, Thailand, Russia, Caucasus and Tajikistan (Krumbeck1924; Parisova,1947; Falahi,1983; Waltman, 200 1). In China, it is reported that Indian sea fans are produced in southern Qinghai (Zhang Zuoming et al., 1985), Mount Qomolangma in Tibet (Wen et al., 1976) and Kangma area (Chen Jinhua et al., 1976).
The ammonite fossil material reported in this paper comes from the section of Upper Triassic TuPamela Group near Amdo 109 Daoban. The TuPamela Group on this profile is a set of TuPamela Group deposited by transgression and regression, the upper part of which is covered by timely sandstone with a thickness of more than 400 meters, and then the Kemochuo Group with a thickness of more than 700 meters ... No fossils have been found in the two strata covered by TuPamela Group, and it is presumed that it is late Triassic-early Jurassic. Chrysanthemum stone is located in the stratum with a thickness of about 100 m in the upper part of Tumenjiang Formation. The ammonite has two horizons, about 50 meters apart (thickness). Laying hens H4- 1 produce Placites oldhami. In H6- 1 horizon, there are ammonite minerals Acutus and Placitescf. Quintarra. The bivalves born together with these ammonites are Indocten himalayaensis Wenetlan, Myophoriopsis sp., Skeletonema costatum, Skeletonema latifolia, and quasi-cultured algae. ; Indopecten himalayaensisWen and Costatoria napengensis Healey were produced in the top fossil horizon.
Illustration 1 Amdo 109 ammonite horizon of coal-bearing clastic rock formation of Tumenla Group of Upper Triassic in Daoban (section 26P7)
Wang et al. (1976) discovered five ammonite faunas in Everest. Pointed snout is a molecule in the ammonite zone in the lower part of Middle Norris. Therefore, according to the fossil group and sedimentary facies of 109 Daoban, a set of timely sandstone overlying the ammoniacal tine-shaped anatase layer is likely to represent regressive deposits, and there will be no marine deposits later than the middle Norris period in this area. Recently, Tibetan ammonite (inset 1) was also found in Quemocuo area west of 109 Daoban (origin 9151",latitude 33 45' 56"). There are two genera, no diabetes and paradiabetes (Niu Zhijun et al., 2003). They established the assemblage of ammonites and nodulites. Wheeleri, and think that its era is also the early and middle period of the Norrie era. He Guoxiong et al. (1985) also described Placites oldhami found in Zhiduo County and P. Polydactylum found in Zaduo County, in the middle of Norrie. The significance of bivalve dating is far less than that of ammonite, but the late Triassic bivalve assemblage in Amdo 109 Daoban section can be compared with the early and middle Nori bivalve fauna in western Yunnan (Ma Qihong et al., 1976) and the late Triassic Nori fauna in Naa, northern Myanmar (Healy, 1976).
The differentiation of Mesozoic biogeography at the level of "domain" only began in the Middle Jurassic, and they were the northern domain and the Tethys domain (Westermann, 2000). The former stands for cold climate, while the latter stands for warm climate. Prior to this, the global sea area was a homogeneous warm environment in the Tethys Sea, so that many genera and species of bivalves and ammonites showed global geographical distribution. In Triassic, although the warm temperate zone was very broad, even reaching the high latitude of the polar region (above 70), there was no real tropical rain forest (Haram, 1994). Compared with the warmest Middle Cretaceous, the northern boundary of the warm temperate zone only reached 30 north latitude to 40 south latitude.
The geographical distribution of late Triassic ammonia in Tibet and Indian sea fans also reflects the above-mentioned Mesozoic biogeographic differentiation stage. In the longitudinal direction (longitude direction), Tibetan ammoniacal family or Indian sea fan is distributed in Qinghai, western Yunnan and northern Tibet, in the Himalayan region on the south side of the East Tethys Sea, or in Timor Island on the ancient equator in the Late Triassic. Horizontally (latitudinally), ammoniacal plants are found not only in tibetes on the east side of the Pacific Ocean, but also in western Canada on the west side of the Pacific Ocean, such as Oxytibetes, Prodrepanites and Mesohima-vatites (Tozer 1994) (Figure 2). Tozer (1982) thought that these three genera should have been low-latitude tropical fauna, but because the terrain bearing these ammonites moved northward, the original latitude of the fossil producing area was changed, and they reached the present high-latitude area. Compared with the first-class geographical distribution, if the same endemic species exist in the southern and northern waters of the East Tethys Ocean, their geographical distribution can better explain the way of biological diffusion.
Illustration 2 Geographical distribution of Indian sea fan (Indopecten) and Tibetan ammonites (tibetids) and modern ocean current Tibetids (2) duck green stone; ③ Parailmenite; ④ Metamorphic sandstone; ⑤ Nodular diabetes; ⑥ porphyry; ⑦Pali cites； (8) New fiber iron ore; Pet-name ruby binaural ceramics; ⑩﹣oxytibetites； ⑾﹣proderpanites； ⑿ Middle Himalayan rocks; ● Indole pectin
According to the concept of modern biogeography (Brown and Gibson 1983), the controlling factors of modern marine biogeography are geographical isolation caused by land-sea distribution pattern, climate difference caused by different latitudes and the movement path of ocean currents. Among them, ocean currents play a key role in the geographical distribution of marine life. Ocean current is the main way for most mollusks with planktonic larval stage, such as ammonites and sea fans to spread and radiate. The formation of modern ocean surface circulation is based on three factors: ① wind stress, including wind friction and wind pressure on ocean currents; ② Pressure gradient force and friction force in seawater; (3) the earth's rotation deflection force (that is, an object with a horizontal velocity component on the ground in the non-equatorial region of the earth, also known as the Coriolis effect). The general laws of modern ocean surface circulation are ① anticyclone ocean circulation centered on subtropical high at middle and low latitudes; (2) Cyclonic ocean circulation centered on the low pressure area in the middle and high latitudes of the northern hemisphere; (3) west wind drift in the mid-latitude region of the southern hemisphere; ④ Circumpolar circulation around Antarctic continent; ⑤ Monsoon circulation forms in the northern Indian Ocean. The formation of all these surface circulation is related to the fact that the earth's climate is divided into tropical zone, temperate zone and cold zone due to different latitudes. For example, the latitude difference between the equator of modern earth and 30 north latitude leads to the formation of monsoon (trade wind), which directly produces the formation mechanism of blowing or drifting (Figure 3). Modern biogeographic flora also shows that due to the existence of equatorial circulation, symmetrical and different biogeographic divisions are usually formed on both sides of the equator. Therefore, both the equator and the ocean basin have become effective barriers to the spread of modern marine life.
Explanation 3 Modern atmospheric circulation and ocean current generation mechanism and possible Triassic atmospheric circulation.
Kristan-Tollmann and Tollmann (1983) and Kristan-Tollmann (1988) proposed a tropical trans-Pacific diffusion model of Triassic or Mesozoic tropical biota based on echinoderms and microfossils, and Newton (1988) proposed a model based on bivalves. She (he) believes that tropical creatures on the east and west sides of the Pacific Ocean can spread through equatorial circulation. If we admit that biological diffusion is spread through equatorial circulation, it means that there is low latitude circulation on both sides of the equator, which is the same as the modern ocean current mechanism. Then this model is actually no different from the modern ocean current model (see Kristan-Tollmann and Tollmann, 1983, page 2 15, Figure 9). Due to the barrier of equatorial circulation, biological communication between the southern hemisphere and the northern hemisphere should be very difficult. In fact, in the Late Triassic, the ammoniacal family in Tibet and the sea fan in India all had the same species, which were distributed in the Himalayan region around 30 south latitude and Qiangtang region around 30 north latitude. It is difficult to explain the distribution law of these organisms by equatorial circulation model. In addition, Megalodon Clam (bivalve), an important reef-building species in the Late Triassic, is not only distributed in the geographical range between 30 north latitude and 30 south latitude, but even distributed in the northern hemisphere to reach a higher latitude (Tamura 1983), indicating that they can freely spread in the Tethys Sea with almost no geographical obstacles. At the same time, these warm molecules can cross the equator and reach the high latitudes in the southern hemisphere and the northern hemisphere respectively.
Therefore, it is obvious that the principle of modern biogeographic zoning cannot be fully applied to Mesozoic. In Mesozoic, the global climate was quite warm, and there was no ice sheet in the polar regions. The temperature difference between high latitudes and low latitudes on the earth is very small, and the climate will not be obviously zoned because of latitude. Without the temperature difference, there is naturally a lack of the mechanism of atmospheric circulation that produces monsoon, and it is also likely that there is a lack of the mechanism of equatorial circulation, so the exchange and diffusion of fauna will not be hindered by the existence of the equator. In this case, the driving force of ocean currents at that time may be mainly the rotation bias of the earth. The higher the latitude, the greater the deviation of the earth's rotation, and the ocean currents produced are mainly coastal ocean currents. Under the action of two-way coastal currents in the northern and southern hemispheres, a clockwise coastal current may be formed, which can pass through the equatorial region, thus becoming the main way for ammonites and bivalves to spread in the offshore environment. In addition, under the action of sunlight, the atmosphere above the Pangu United ancient continent, which was very open in the Late Triassic, was heated more easily than the atmosphere of the open Tethys Ocean, resulting in upwelling, which needed to be supplemented by airflow with lower temperature from the ocean. Under the action of this large airflow, it is possible to produce a force mechanism to accelerate the speed of coastal current and become the carrier of marine life diffusion in the late Triassic. Tibetan ammonites, Indian sea fans and other Triassic fauna show their biological relationships on both sides of the Pacific Ocean. It is likely that they can migrate freely with the help of the Pacific coastal current, so many creatures have the characteristics of distribution along the east and west sides of the Pacific Ocean. Another possibility is that in the Late Triassic, the breakup of the Lauya and Gondwana continents just started, and the East Tethys still maintained a wide enough ocean surface, and the Paleo-Tethys ocean was closed, while the Middle Tethys ocean only cracked, without forming a deep and wide ocean basin. Because there is no deep-sea basin, the impact on the transoceanic communication of fauna is much less (Figure 4), thus forming the distribution of Tibetan ammonites and Indian sea fans around the Pacific Ocean.
Illustration 4 East Tethys Current and Biological Diffusion Route ① New Megalodon; (2) Napeng fauna; ③ Indomethacin; ④ Tibetan mastiff
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Distribution pattern of late Triassic Tibetan antelope and bivalve in Qiangtang area and its paleogeographic significance
According to their distribution patterns, Tibetan antelope (ammonite) and scallop (scallop) are considered as indicators of subtropical or tropical animals. In recent years, various kinds of titanium and bivalves including anatase, anatase, anatase and anatase have been found in southern Qinghai and western Yunnan. . Endemic species of these fauna, such as sauropod and Himalayan cheetah, Wenhelan, appear in southern Qinghai and western Yunnan. The distribution pattern of these late Triassic molluscs shows that although the equatorial circulation and the vast East Tethys Ocean are usually considered as the main obstacles to the spread of modern fauna, the radiation or spread of fauna is from south to north (or the opposite direction). This may also indicate that there was a warm climate in the vast area of East Tethys during the Late Triassic. Further consideration is that the latitude climate zone in the Late Triassic may have been very blurred. On today's earth, there is no obvious climate and temperature difference between high latitude and low latitude areas. It is speculated that there is no mechanism for the formation of modern monsoon climate. Therefore, continental marginal current, regional warm current and equatorial circulation generated by atmospheric circulation may not exist at that time. The spread of animals may depend on the continental marginal flow caused by the rotation of the earth and the blocking force of land masses. This may be the reason why Tibetan antelopes and other taxa are distributed along the edge of the ancient Pacific Ocean.
Keywords late Triassic, ammonites, paleogeography, diffusion