The main characteristics that distinguish mammals from other mammals are as follows: first, they need breastfeeding in the larval stage, even some primitive species, such as platypus, need breastfeeding in the larval stage although they lay eggs; The second type is viviparous (except a few). These two characteristics are considered to be the necessary conditions for it to adapt to the environment, gain more living space and develop to a more advanced state in the process of evolution. In addition, mammals are warm-blooded, but the body temperature of different species is different. The normal body temperature of human is 37℃. There are hairs on the skin surface, but the degree of development is very different. The hair of whales and dolphins living in water has almost degenerated, while mammals living in cold areas on land have thick hair to resist the cold climate.

Mammalian bones can be roughly divided into three parts, namely skull, trunk bone and limb bone (Figure 9- 1 1). Except for the spine and sternum, bones are symmetrical in shape and number. Skull includes skull and mandible. Skull includes maxilla, maxilla (the bone where teeth grow), nasal bone, frontal bone, parietal bone, occipital bone, zygomatic bone, temporal bone, oblique angle temporal bone and so on. (Figure 9- 1 1). These bones are characterized by being flat, thin and irregular. Trunk bones include vertebrae and ribs. Vertebra can be divided into cervical vertebra, thoracic vertebra, lumbar vertebra, sacral vertebra and coccygeal vertebra. The vertebral bodies are angular (with spinous processes), and the connection between the vertebral bodies is flat. Ribs play a role in protecting the chest, flat and long. Limb bones are forelimbs (hands) and hind limbs (feet). The forelimbs have humerus, radius and ulna, carpal bone, metacarpal bone and phalange from proximal to distal, and the hind limbs have femur, tibia and fibula, tarsal bone, metatarsal bone and phalange from proximal to distal. The humerus, radius, ulna, femur, tibia and fibula are all tubular and easy to identify.

Fig. 9- 1 1 skeleton structure of mammalian body (according to Zhang Zhenhong, 1996)

2. Tooth characteristics

Mammalian teeth are highly differentiated. According to the morphological and functional characteristics of teeth, upper and lower teeth can be divided into incisors (I), canine teeth (C), premolars (P) and molars (M). Front teeth are generally shovel-shaped or flat; Canine teeth are conical; However, premolars and molars are complex and generally rectangular and square in shape. Functionally, incisors are mainly used to clamp or cut off food, canine teeth are used to tear food or kill animals, and premolars and molars play a role in breaking food. Therefore, in carnivores, canine teeth are very developed, while in herbivores, canine teeth are degraded. In the evolution of human canine teeth, due to the use of tools, they gradually lost their functions, so they were greatly degraded. Because of the different eating habits of animals, the shapes of premolars and molars are quite different (Table 9- 1). Mammalian teeth go through a deciduous period before they are replaced by permanent teeth, but not all teeth have a deciduous period. Only incisors, canines and premolars develop deciduous teeth, and molars directly develop permanent teeth.

Table 9- 1 Morphology and feeding habits of premolars and molars in mammals

Different mammals have different tooth shapes and numbers, which is the main basis for the identification and classification of mammalian fossils. Some mammals have as many as 40 teeth, but as few as 10. The number of mammalian teeth can be expressed by tooth type:

Quaternary geology and geomorphology

In the formula, I, C, P and M represent incisors, canines, premolars and molars respectively, the numerator represents the number of teeth on the maxillary side, and the denominator represents the number of teeth on the mandibular side. The sum of numerator and denominator multiplied by 2 is the total number of teeth of animals. For example,

Quaternary geology and geomorphology

It means that there are 3 incisors on the maxillary side, 1 canine, 4 premolars, 3 molars, 3 incisors on the mandibular side, 1 canine, 4 premolars and 3 molars. This animal has 44 teeth. Mammals have only 44 teeth at most, and the number of teeth of similar animals decreases with evolution. The tooth types of several mammals common in Quaternary are as follows:

The shape of the crown surface is an important feature of the identification of mammalian fossils, which is mainly composed of the tip and ridge (Figure 9- 12). In different mammals, the characteristics of these tips and ridges are different (Figure 9- 13). Among carnivores, teeth are very sharp, canine teeth are thick, and premolars are sharp blades, which are suitable for cutting meat. The enamel layer of horse premolars and molars has complex folds, and the original tip of upper molars is connected with the anterior ridge. The upper premolar and molar of rhinoceros are π-shaped, while the lower premolar and molar are two curved ridges. The upper molar and molar crown of cattle, sheep and deer are four crescent ridges, but the front and rear tips of cattle are round, while sheep and deer are herringbone, but the former has no tooth column. In elephants, the general evolution trend is that the tooth tip connection is getting better and better, and the number of tooth ridges is increasing (Figure 9- 14).

Fig. 9- 12 dental crown structure of mammalian molars

Fig. 9- 13 Quaternary common mammalian fossils

Fig. 9- 14 evolution of elephant maxillary molars (according to colbert, 1969).

3. Animal horn

There are two kinds of horns in mammals, namely hollow horns and antlers. The angle of the hole is not bifurcated, the stratum corneum is outside and the bone is inside. Only once in a lifetime, it won't grow again if it is broken. For example, the horns of cattle and sheep belong to this category. On the other hand, antlers are forked and cover the skin. They fall off once a year and grow back the next year. Deer belong to this kind of horn.

In the identification of cattle, sheep and deer, the characteristics of horns are very important identification marks. For example, the cross section of buffalo horn is triangular (Figure 9- 15), while the cross section of buffalo horn is nearly circular. The bifurcation of antlers, the position of the first branch (also called eyebrow branch), the included angle with the main branch and the number of bifurcation are all identification marks. For example, if a muntjac has only one fork, the axial deer will be divided into three forks, and if the sika deer is above the three forks, there will be more red deer.

Fig. 9- 15 Evolution of Quaternary buffalo (left) and morphology of antlers (right) (according to China Vertebrate Paleontology Manual, 1979).

4. Formation and burial of mammalian fossils

After the death of mammals, fossils can only be formed by burial and fossilization. Because the Quaternary period is relatively short, especially in the later period, tens of thousands of years ago. Compared with the fossils in the pre-Quaternary strata, the fossilization degree is relatively low, but it is still different from today's animal bones (Table 9-2).

Table 9-2 Differences between mammalian fossils and modern bones

Mammals are buried in situ and in different places after death. In-situ burial refers to the burial of animals after death without transportation or short-distance transportation, such as swamps, caves and other environments are easy to form in-situ burial. This buried fossil skeleton is relatively complete, and sometimes it can be preserved. Burying in different places means that animals are transported for a long distance and then deposited and covered by sediments. Most of the fossils formed in this environment are broken and sporadic, but sometimes "fossil pits" rich in fossils are formed due to hydrodynamic separation. For example, sandy lenses in lakes, rivers and other sediments usually contain fossils.

When looking for Quaternary mammalian fossils in sediments, we should pay attention to the genetic types and sedimentary facies characteristics of sediments. Judging from the probability of finding mammalian fossils, cave accumulation is the most likely, followed by river and lake deposits, followed by flood deposits, and finally slope deposits. Residual deposits generally do not contain mammalian fossils. In lake sediments and river sediments, we should focus on gravel layer or gravel lens. This is just a general principle. In the actual work process, we should make a judgment according to the specific situation.