Gastropods of Howells Ridge, Grant County, New Mexico: A Fauna in the Process of Extinction?

Abstract

Ten species of gastropods are listed from Howells Ridge, a limestone escarpment in Grant County, New Mexico. Probably only five of these species presently live on the Ridge. One species was found only in cave deposits. Four species occur as fossils in colluvial hillslope deposits and probably have become extinct in Holocene time. It is suggested that much of the fauna seems to be in a process of extinction. Chief source of the fauna is probably the Big Hatchet Mountains to the southeast. Howells Ridge is a southeast-northwest aligned linear escarpment located in extreme southwestern Grant County, New Mexico, in the northern part of the Little Hatchet Mountains. The ridge is 6 mi long and 0.4-0.7 mi wide. The base of the ridge is 4900-5000 ft in eleva- tion, the summit generally 5500-5 700 ft, with its highest point 5 738 ft. The Little Hatchet Mountains were mapped as pertaining to the Upper Sonoran Life Zone by Bailey (1913: P1. 1), with the surround- ing plains mapped as Lower Sonoran. The climate is arid. At Hachita (4500 ft elevation), 7 mi NE of Howells Ridge, average annual rain- fall for a 30 year period prior to 1941 was 10.77 inches (U.S. Dept. Agri., 1941: 1012). Extensive geological work by Robert A. Zeller, Jr. (1970) showed the area of Howells Ridge to be structurally and stratigraphically com- plex. The ridge is capped along most of its length by a massive, marine reef-limestone of the Formation of the Lower Cretaceous. Cliffs of this limestone commonly reach heights of 60-80 ft along the crest of the ridge; they are referred to hereafter as the U-Bar Cliffs. The cliffs are best developed in the northwestern part and in the south- eastern half of the ridge, being poorly expressed over a distance of two miles between these two areas. Occurring below the Forma- tion and forming most of the northeastern slope of the ridge, are (1) in the southeastern part, the Lower Cretaceous Hell-to-Finish Forma- tion, older than the Formation, and (2) in the northwestern part, the Upper Cretaceous Ringbone Formation, younger than the