Page 101 - Land Snails of New Mexico

Table 5 (continued)
Zonitoides arboreus
Vitrinapellucida Deroceras laeve
Ashmunellajamesensis Ashmunella kochii sspp.
Ashmunella pasonis sspp.
Ashmunella rhyssa sspp. Ashmunella ruidosana
Ashmunella tegillum Ashmunella tularosana Thysanophora hornii
X
X
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85
129 5
Vertigo elation seems to be very rare at present. However, it appears that, in general, smaller species are less inclined to go extinct than larger species in these and other mountains of the state. This m a y b e related to the abilities of small species, noted above, to have behaved as dispersalists in the Pleistocene. B y contrast, larger species seemingly were loathe to leave their m o n t a n e habitats, w h i c h is n o t s u r p r i s i n g if their l i n e a g e s i n d e e d have been adapted to mountain environments during periods of time extending back to the earlier Tertiary or even to the Mesozoic.
Ranges ofSouth CentralNew Mexico Westward across the Tularosa Basin from the Sacramento Mountains are the predominantly calcareous San Andres
Mountains. In this range, colluvial hillslope deposits at a few localities have yielded Quaternary fossils, again, these include Oreohelixneomexicana.A compositelistingoffossilspecies from the San Andres Mountains is given in Table 5.
The Franklin Mountains lie south of the San Andres-Organ
Mountainchain.SpeciesofQuaternaryfossilsknown from these
mountains are listed in Table 5. Although mainly in Texas, the
Franklins seem to deserve mention because colluvial deposits,
especiallythose on the north-facingwall of Vinton Canyon in the
northernpartoftherange,haveprovidedadiversePleistocene
fauna, reported by Metcalf and Johnson (1971), and greatly
augmented recently by collections of Richard D. Worthington.
Oreohelix neomexicana and Radiocentrum ferrissi occur in the
same colluvial unit along Vinton Canyon. Sympatry of these
genera israre. WestwardacrosstheJornadadelMuertoBasinfromtheSan
Andres Mountains are the Caballo Mountains, another block faulted range with massive limestone outcrops. Quaternary fossil snailshave been found in colluvial slope deposits in canyons on the west side of Brushy Peak and in low, outlying foothills in the southwestern part of the range (Metcalf, 1977:55-56). Species taken are listed in Table 5. An endemic subspecies of the Caballo Mountains, Ashmunella kochii caballoensis, occurs both living and as a fossil. K n o w n only as a fossil, Oreohelix
caballoensis seems close, conchologically, to species of Oreohelix occurring to the west (Black Range and other mountains).
The Caballo and Franklin Mountains, similar in size and elevation and both arid ranges today, were compared by Metcalf (1977:55-57). The limestone and rhyolitic outcrops of the Franklins are better producers of talus than outcrops of the Caballo Mountains. For land snails inhabiting such arid mountains, accumulations of talus seem to have been a last refuge into which they could withdraw as aridity increased in Holocene time. Probably because they provide fewer such refuges, the Caballo Mountains have fewer species, both living and fossil, than the Franklins. Eighteen fossil and seven living species have been obtained in the Caballos. F r o m the Franklins, including recent collections m a d e by R. D. Worthington, noted above, the numbers are 24 fossil and 16 living. Most of the species n o w extinct in the Caballo and Franklin Mountains still occur along wooded canyons of the more massive and higher Sacramento, Organ, and Black ranges of south-central N e w Mexico, where moisture conditions are sufficient to support forest. Thus, these fossil faunas are indicative of a Pleistocene d e p r e s s i o n o f life z o n e s i n t h e r e g i o n . T h e o c c u r r e n c e o f V i t r i n a pellucida alaskana as a fossil in the Franklin Mountains is e s p e c i a l l y i n d i c a t i v e o f a v e r y d i f f e r e n t c l i m a t e . T o d a y , it o c c u r s
onlyinforestedhabitatsaboveca,2,195m (7,200ft)inmoist
habitats, as along floodplains of mountain streams. The fossil faunafromtheCaballoMountainscontainsnumerousshellsof
Gastrocopta quadridens, a species of scattered occurrence in New Mexico, which lives along the crest of the Sandia Mountains at ca. 3,230-m (10,600-ft) elevation. Perhaps the present environment of Sandia Crest resembles that which prevailed along the crest of the Caballo Mountains when G. quadridens flourished in that range. In both ranges, outcrops of massivelimestoneoccurinareaswhere G. quadridens isfound, livingorfossil.
Stillfarther westward, across the Rio Grande Rift Valley, is another complex of ranges, some of which have yielded fossil land snails. North to south, these comprise the Cuchillo Mountains, Black Range, Cooke Peak, the Florida Mountains, and the Tres Hermanas Mountains. In each of these ranges, one ormore speciesoffossilOreohelix has been found, as well as in the Sierra Rica Mountains to the west of the Tres Hermanas
range.WiththeexceptionofOreohelixsubrudisintheBlack
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