大熊猫颅骨、下颌骨及牙齿特征在进化上的意义

THE SKULL, MANDIBLE AND DENTITION OF GIANT PANDAS (AILUROPODA):MORPHOLOGICAL CHARACTERS AND THEIR EVOLUTIONARY IMPLICATIONS

本文对大熊猫颅骨形态及牙齿结构进行了研究,认为大熊猫与始熊猫的裔征不同于熊科成员,应单列一大熊猫科(Ailuropodidae)。在大熊猫种系进化中,它们的形态与机能是统一的。不同种类的形态特征,又是在适应环境中逐渐产生的。根据这个认识,我们把大熊猫的进化历程可划分为始发期、成长期、鼎盛期和衰败期。

The Chinese giant panda as the black-and-white bear, was firstly recorded by Pere David as Ursus melanoleucus based on its skin in 1869. In 1870, Milne-Edwards wrote: 'In its external form, indeed, it very much resembles a bear, but the osteological characters and the masticatory system clearly distinguish it from the bears. It must constitute a new genus which I have called Ailuropoda'. In 1871 this French zoologist thought the generic name should be Ailuropus. The taxonomists later changed the name back again to Ailuropoda and the giant panda is today known as A. melanoleuca (black and white). Information gathered by our paleontologists during years of excavation shows that the giant panda had a very wide prehistoric distribution in most of the regions of eastern China. Its remains have even been found outside China, in the northern parts of Burma, Thailand, Laos and Vietnam. For more than one hundred years scholars have never had a common opinion on the phylogenetic position of the giant panda. Some scientists hold that it should be placed in the bear family, others classify it as a member of the raccoon family, while still others maintain that the giant panda belonging to a distinct taxonomic family——Ailuropodidae. The present paper discusses the morphological characters of giant panda and their implications for evolution. In recent years, with the discovery of Ailurarctos and the increase of collections of Ailuropoda, great progress has been made in understanding the phylogeretic history of the giant panda. The material dealt with in the present paper includes cranial and dental remains of Ailurarectos, Ailuropoda and Ursus, which were excavated from Late Miocene through Pleistocene deposits in South China. The giant panda definitely belongs to the order Carnivora in taxonomic terms, but it differs from bears and dogs in morphology and ecology. In this paper I plan to explore the relationship between the morphology of the panda skull and its function. The morphological characteristics of the skull of pandas are as follows: 1) the snout is shorter than that of bears or dogs; the zygoma are strongly developed and widened, the bizygomatic breadth (the distance of zy. -zy.) is 200—216 mm in the extant panda, 245—250 mm in subspecies baconi, 220—230 mm in A. microta, and in the black bear it is the narrowest at only 140—150 mm; 2) the bone plate of the zygoma is wide, the maximum breadth of zygoma is 52 mm in extant panda, 62mm in A. m. bacom and only 15mm in the black bear; 3) owing to a well-developed masticatory muscle group, the root of the zygoma of the panda moves forward to P^4-M^1, this muscle group is smaller and weaker in black bears than in pandas, the position of zygomatic root in the black bear is above M^1-M^2. A well-developed sagittal crest is also a nothable characteristic on the skull of the panda. The height of the crest is 15—20 mm in the extant panda and 20—25 mm in A. m. baconi. The frontal sinus is large in extant pandas, being about 100—120 mm in length and 50—55 mm in height. The flat occipital plane for neck muscle attachment and developed mastoid process increase the strength of connection between the skull and the trunk. The mandibular ascending ramus is at right angle with the mandibular corpus, the rami diverge more, so the bicondylar width is widened. The symphysis joint surface is greatly enlarged. The mandibular condyles are long transversely, thus making them mechanically more effective for the masticarory muscle group. Three cusps (parastyle, paracone and metastyle) are present on the premolars of the giant panda (extant, bacobi, microta). An anterior entocone appears on the third and fourth premolar of Ailurarctos, but it is very small on the third premolar. Only two cusps are present in the premolars of black bears. The molar width is larger than length in giant pandas and Ailurarctos, the black bears eexpress the oposit condition. The tuberculum is well developed on the occlusal surface and the internal cingulum is wide on the molars of the giant panda. All the characteristics mentioned above show that Aihtrarctos is close relative of both the bear and giart panda, and can be regarded as a transitional form. On the basis of morphological characteristics of fossil giant pandas, the evolutionary history can be divided into four development stages: 1) The origin stage: Late Miocene in age. Ailurarctos from Lufeng, Yunnan Province is a representative of this stage. Its premolars have three cusps and double roots, an antero-internal cusp is on the p^4, the developed zygoma is located above P^4-M^2. Molar length is larger than width with a rare tuberculum and undeveloped cingulum, which show that Ailurarctos differs from the members of Ursidae and is evolving toward Ailuropoda. 2) The developmental stage: Early Pleistocene in age. The members of this stage are composed of A. microta. Their characteristics are: small body size, well developed cheek teeth that resemble those of A. m. baconi, the base of the dygoma is above P^4—M^1, and mandibular symphysis is long. The giant panda of this stage can be regarded as Ailuropoda. 3) The expanding stage: Middle Pleistocene in age. Representative type is A. m. baconi. Its chief characteristics are: the body size became larger, about two times of that of A. microta. The face breadth (bizygomatic breadth) is 250 mm, the lingual cingulum of the molars expand and is cuspidate with many dental facets, the mandibular symphysis is longer and more butterssed than that in A. microta, which indicates the masticatory muscle group is more developed. During this stage the panda's living radius became larger because of a more suitable climate and more abundant supply of bamboo. This is the expansion stage for the giant panda. 4) The declining stage: Late Pleistocene in age. Representative type is the extant giant panda, A. melanoleuca. The body size is reduced and 1/7—1/8 smaller than that of A. m. baconi; the face breadth is only 216 mm, the mandibular symphysis is shorter and more buttressed than that of A. m. baconi, which indicates the masticatory muscle group is declining as compared with that of A. m. baconi. Many factors have conspired to make the number of pandas gradual reduction. The mammalian faunas and climatic changes during late Pleistocene and Holocene in eastern China have been reviewed. Variations in climate were accompanied by rapid sea-level fluctuations during the latest glacial and post-glacial periods. Studies of the Penghu, Hsiaonanhai, and Shinnongjia faunas reveal that there was a widespred drop in air temperature during the late Pleistocene (15,000 to 10,000 year B. P.). During the cold stages mammals of North China, such as the woolly rhinoceros (Coelodonta antiquitatis), mammoth (Mammuthus sp.), horse (Equus ferus przewalskii), roe deer (Capreolus sp.), Scaptochirus sp., and male-rat (Myospalax sp.) migrated Southwards (Huang, 1985). The cooling of the climate and the competition with mammalian species of North China slowed the development of the giant panda during this periods (Late Pleistocene to Holocene). The encroachment of human activities in particular has also greatly damaged the environment of the panda's former habitat and has thus reduced its existence range. According to recent investigations Chinese zoologists have known that it distributs in a limited area of the South of the Qin Ling Mountains in Shaanxi and Balang Mountains, the Great and the Lesser Xiang Hills and Liang Mountains in Sichuan now.