The Sangamon interglacial vertebrate fauna from Rancho la Brisca, Sonora, Mexico.

Abstract

Bones of 51 species of vertebrates including fish, amphibians, reptiles, birds, and mammals were recovered from a sedimentary deposit at Rancho la Brisca in north-central Sonora, Mexico. The fauna was preserved in a marshy cienega habitat with 49.0% of the species and 87.2% of the identified bones representing aquatic and semiaquatic animals. The most common animals in the fauna are Kinosternon sonoriense (Sonoran Mud Turtle) and Rana 'pipiens'-complex (Leopard Frog). The presence of Bison species, Mammuthus species, Equus cf. tau, and Camelops species places the fauna in the Rancholabrean Land Mammal Age. Bugo cf. kelloggi, B. mazatlanensis, Leptodactylus melanonotus, Pternohyla fodiens, and Masticophis cf. mentovarius are subtropical thornscrub animals that presently occur to the south and/or west of la Brisca. Bufo alvarius cf. Callisaurus draconoides and Sceloporus cf. clarkii are other animals that suggest a paleoclimate with warm winters and a well-developed summer monsoon. The fauna represents an interglacial environment subsequent to the appearance of Bison about 150 000 years ago in the Sangamon Interglacial and with a climate similar to that of the Late Holocene of the last 4000 years. The distribution of fish and mud turtles suggest past stream connections between the Gila River drainage in southeastern Arizona and the rivers in Sonora. In contrast, Pseudemys scripta (Yaqui Slider) apparently never entered Arizona, although it reaches north-central Sonora today. -from Authors