现生河狸的起源

ORIGINS OF MODERN BEAVERS

本文通过对来自欧洲、亚洲、北美洲的河狸属（Ｃａｓｔｏｒ）化石和现生种的头骨形态学之研究，认为河狸属的起源可上溯到中新世的Ｓｔｅｎｅｏｆｉｂｅｒ。现生欧洲河狸Ｃ．ｆｉｂｅｒ与中国化石河狸Ｃ．ａｎｄｅｒｓｓｏｎｉ有密切关系；而北美现生河狸Ｃ．ｃａｎａｄｅｎｓｉｓ则可能源于美国的化石河狸Ｃ．ｃａｌｉｆｏｒｎｉｃｕｓ。约六百多万年前，河狸属从欧亚大陆穿越白令陆桥向北美的扩张，导致了现生河狸两个种的出现。这六百多万年的隔离是造成两种现生河狸在基因库、生态、头骨形态方面分异的根本原因。

Modern beavers include Castor jiber and Cattor canadensis. C. fiber is known from Eurasia. C. canadensis is native to North America and was introduced into Europe in 1930's[6]. C. fiber can be distinguished from C. canadensis by the following characters: (1) nasals in C. fiber are longer and anteriorly broader than in C. canadensis; (2) maxilla-jugal suture in C. fiber is more posterior than in C. canadcnsis; (3)maximum breadth of basioccipital depression in C. fiber is greater than maximum length, and the breadth is smaller than the length in C. canadensis, (4) posterior spine of palatine in C. tibcr is shorter than in C. canadensis, (5) chromosome of C. fiber is 2N=48, and that of C. canadentit is 2N=40[2-5]. Skull morphology of C. anderstoni from a Nihewanian site in China is similar to modern European beaver C. fiber, but the maxilla-jugal suture of the latter is slightly more posterior than the former, basioccipital depression of the latter is rectangular (anteroposterior diameter smaller than transverse diameter), and that of the former is rounded. The skull of C. californicus from Hagerman (Blacan Land Mammal Age) in idaho is similar to that of C. anderssoni, but the maxilla-jugal suture is slightly more anterior than in the latter. The skull morphology of C. canadensis is different from skulls of other three species by its most anteriorly, located maxilla-jugal suture, rectallgular basioccipital depression (anteroposterior diameter greater than transverse diameter), and well-developed anterior masseter ridge. Skull morphology of the four species reveals two evolutionary trends. From C. anderssoni via C. calijornicus to C. canadcnsis: (1) maxilla-jugal suture gradually moves anteriorly; (2) basioccipital depression becomes anteroposteriorly elongated; (3) anterior masseter ridge grows stronger through time. From C. anderssoni to C. jiber: (1) maxilla-jugal suture moves slightly posteriorly; (2) basioccipital depression becomes transversely elongated; (3) anterior masseter ridge is enlarged slightly; (4) nasals become broader anteriorly and longer (Fig.1 and Plate1). Castor is derived from Steneofiber around 9-10 Ma B.P. in Europe[4] and dispersed to North America through Bering Land Bridge around 6.6Ma (the early Hemphillian Land Mammal Age)[16]. The first occurrence of Castor in North America is C. californicus with the age earlier than 6 Ma B.P[9]. The early Hemphillian Castor incisors reported by Shotwell[18] are doubtful because of lacking of more complete material, and the early Hemphillian specimen described by Wilsont[17] is Steneofiber because it is of the cheek tooth pattern of Steneofiber hesperus. Xu[4] thinks that previous Eucastor hesperus in North America[19], Monosaulax tongurensis and Monosaulax changpeiensis from China [20,21] belong to Steneoliber hesperus based on dental pattern. Modern North American beaver C. canadensis is derived from C. californicus,modern European beaver C. fiber is derived from C. anderssoni. North American C. californicus came from a Castor-dispersal from Eurasia through Beringia. The origins and speciations of modern beavers were the result of the changing Beringia from sea strait to land bridge and back to sea strait.