The panda is a unique mammal in my country and the most familiar species to people. One of the reasons why pandas are so loved is that there are not many of them left in the wild. It is understood that there are no more than 1,000 pandas, and they are already on the verge of extinction. However, looking at the "family history" of pandas, they also have a prosperous experience. In this regard, since the late Miocene, pandas have generally experienced the following evolutionary patterns: initial stage, growth stage, peak stage, and decline stage.
1. Originating period: Late Miocene (Original stage)
The initial period started in the late Miocene (7-8 million years ago), represented by the Eo-panda.
As mentioned earlier, pandas evolved from the ancestral bear (Ursavus) in the Miocene Epoch. The first type to appear was called the panda. This panda is nearly the size of an obese dog. As its teeth adapted to eating bamboo, the premolars were developed, and a small amount of enamel bulges appeared on the chewing surfaces of the molars. In order to meet its functional needs, the "lobes" disappeared, and the base of the zygomatic arch moved forward to the position from the fourth premolar to the second molar. Ecological observations of living pandas by the China Panda Conservation Research Center (1987) revealed that this type of animal mainly relies on the tearing and cutting functions of its premolars in the wild. In addition to these bamboo-eating traits, the teeth of the primitive panda also retain some primitive shapes, such as the sparse arrangement of premolars, molars that are longer than wide, and the lower third molar is nearly round, etc. These changing and unchanged morphological characteristics of the Eo-Panda just show that it is between the Ancestral Bear and the Panda in terms of systematic relationship. It is the ancestor and representative of the initial stage of the panda-like animals in China.
2. Growth stage: Early Pleistocene (Developmental stage)
The growth period roughly started in the Early Pleistocene (2 million years ago), represented by the small species of panda. Its physical form has the following characteristics:
(l) The body size is slightly larger than that of the panda; (2) the anterior cusps of the premolars are well developed, the anterior inner cusps of the upper third premolars are thick, and the fourth premolars have a tendency to become molars; (3) the width of the molars is slightly larger than the length; (4) ) The diameter of the tooth enamel column decreases and the density increases; (5) The temporal fossa is deeply concave.
Points (l) to (5) show that the evolutionary level is far more advanced than that of the primitive panda in the initial stage. What is particularly outstanding is the molar teeth, which have completely developed in the direction of eating bamboo, that is, they have strong crushing force. and grinding power. The geographical distribution of small species of pandas is also wider than that of primitive pandas. As far as is known, their footprints can be found in Anhui, Guangxi, Guizhou, Hubei, Shaanxi, Chongqing and other regions.
However, the skull, teeth, masticatory muscles, mandibular symphysis length and other structures of the small species of panda are far less developed than those of the Bartholin subspecies and living species. In contrast, at the evolutionary level it can only be placed in the growth stage.
3. Heyday: Late Early Pleistocene (Expanding stage)
The heyday roughly started in the late Early Pleistocene (1 million years ago), represented by the Babi panda.
Since the Pleistocene, as the Qinghai-Tibet Plateau has risen again, the Yunnan-Guizhou Plateau in the west and the Qinling Mountains in the north have risen, hindering the expansion of the dry and cold northwest monsoon to the southeast, and the ecological environment has undergone great changes. The Pasteurian panda, which evolved from the small species of panda, is 1/3 larger than the small species of panda, and its food demand has increased accordingly, thus changing the isolated and closed distribution state, starting from the plateau (Yunnan, Guizhou, Sichuan) and gradually The open areas spread to the Pearl River Basin in South China, the Yangtze River Basin in central China, and a small number of areas in North China, and to South Asia and the Mekong River Basin in Vietnam.
In its heyday, pandas had thick, short and wide skulls. The height of the zygomatic arch bone plate was more than 75 mm, which was twice as high as that of smaller species of pandas. The forward base position of the front end of the zygomatic arch focused the power of biting and cutting food on the first panda. On the fourth premolars and the first molar, this chewing function can concentrate the forces of the masseter muscles, joints and mandible in one place, saving the bite force of the muscles. The tongue-shaped posterior process of the mandibular joint and the semicircular articular surface greatly enhance the fixation between the joint head and the socket; the well-developed sagittal ridge, nuchal ridge and coronoid process expand the attachment of the strong masticatory muscles and occipital muscles. noodle. Compared with the bears that progressed in the later period, the molars of the Papilion panda are particularly wide. If compared with the size of the chewing surface, the white bear is very small. The chewing surface size of its first molar is 1.9 square centimeters, while that of the Papilion panda is 5.46 square centimeters. cm, while the white bear is more than twice as heavy as the panda (Zhang Heyu et al., 1960). This shows that although they are all carnivores, the chewing surfaces of bears' teeth have not changed much since they evolved from their carnivorous ancestors to omnivores. On the contrary, pandas are further specialized, producing wide molars and well-developed internal tooth belts adapted for grinding bamboo or hard plant roots.
It can be speculated that these physical structures of the Babi panda developed simultaneously with its habit of eating bamboo. This allows it to develop greatly once it gets a suitable living space and more new food sources, reaching the heyday of the panda's evolution.
4. Declining stage: Late Pleistocene (Declining stage)
The period of decline began at the end of the Late Pleistocene (since 20,000 years ago) and is represented by the living panda.
Climatic factors play a certain role in the decline of pandas. Research data shows that during the glacial maximum of the last glacial period, sea levels dropped by 130-140 meters, and the average annual temperature was about 5 degrees lower than today. As the temperature drops, some mammals from North China migrate southward, such as the Przewalski's horses, Nama elephants, buffalo, etc. found on the seabed between Hujing Island and Wang'an at the southern end of the Penghu Waterway in Taiwan Province. Their cheek teeth have a high-crested structure. Milling type. Migrating from north to south is obviously to maintain the relative stability of their living environment and their food needs. Changes in the natural environment have also affected the panda's territory. Judging from the fossil materials found in various places (referring to the area south of the Qinling Mountains), there has been a relatively reduced number.
Entering the early Holocene, about 8,000 years later, new caves such as Shuanglong Cave in Jinhua, Zhejiang 7,800 years ago, Xiawanggang in Xichuan, Henan 7,000 years ago, Laibin Bala Cave in Guangxi 6,500 years ago, and Dashi Cave in Wushan, Chongqing 6,000 years ago were discovered. There are also panda subfossils unearthed from Stone Age sites, but the number is even smaller. According to historical research, during the Kangxi period, when my country's population reached 100 million, primitive agriculture experienced a major development period. Large areas of forest land were reclaimed, and the living space became smaller and smaller. Pandas living in open areas gradually disappeared; their ancestral homelands (Yun, The pandas from Guizhou and Sichuan have also declined. Their body size is about 1/8 smaller than that of the Papilion panda. The connection between the sagittal ridge and the frontal ridge has moved backward. The attachment surface of the temporalis muscle has been relatively reduced, and the wrinkles on the chewing surfaces of the teeth have also been relatively reduced. Panda skulls, mandibles, teeth, etc. unearthed from burial pits in Puding, Guizhou, and Hannan Mausoleum in Shaanxi have these characteristics.
In addition to the above factors, the decline of pandas also has its own reasons, such as low reproduction rate and cub survival rate, poor self-defense, etc.
The above evolutionary model reveals the entire process of pandas from their origin to their growth to their peak and gradually decline. I hope pandas can get out of their predicament and coexist with humans.
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