Page 99 - Land Snails of New Mexico

in Dry Cave, Harris (1987:148) observed that most presently extralimital species of the fauna were "drawn predominantly fromnearbyhighlands." Thismaywellapplytotheland-snail fauna as well.
and are superposed on stil lolder deposits. It seems, then, that the latestepisode of downcutting must have been no younger than e a r l y H o l o c e n e . A s n o t e d a b o v e , it s e e m s l i k e l y t h a t f o s s i l i f e r o u s hillslope colluvium w a s deposited during times of Pleistocene cooling. There may be several such colluvial units in a single canyon. Fossiliferous exposures in these mountains are numerous, occurring along almost every canyon that has been i n v e s t i g a t e d . E v e n t u a l l y , it s h o u l d b e p o s s i b l e t o c o r r e l a t e s u c h units from one canyon to another. However, such geological study of these sediments has not been undertaken yet, so that chronology and correlation of the fossil faunas are not understood. A composite listing from colluvial and alluvial deposits of the Sacramento Mountains is provided in Table 4.
Gallinas Canyons).
In the Guadalupe Mountains, extending southeastward from the Sacramento Mountains, Oreohelix neomexicana and two species of Ashmunella (A. rhyssa and A. nana) occur as fossils in Pleistocene colluvial deposits (Metcalf and Fullington, 1976). I n a d d i t i o n , t h e r e a r e t w o s p e c i e s o f A s h m u n e l l a still l i v i n g i n
Higher Mountains
F o s s i l m o l l u s k s h a v e b e e n f o u n d i n m o n t a n e Q u a t e r n a r y deposits only in mountains with calcareous bedrock, not in those with igneous bedrock. This can be observed, for example, in crossing from the calcareous Sacramento Mountains to the adjacent igneous Sierra Blanca Mountains. Fossiliferous deposits occur both as alluvium and colluvium. Alluvial deposits containing Quaternary fossils may occur as floodplains of mountainstreams, as marsh or other ponded sediments, or as silt or travertine deposits associated with springs and springbrooks. Fossilsmaybeexposedwhensuchsedimentsaresubsequently incised by erosion or downcutting by streams. Fossiliferous colluvial deposits formed on hillslopes, and exhibit varying degrees of calcareous cementation, this generally increasing with
A suggestion of multiple ages for various colluvial and alluvial units is provided by fossils of the genera Ashmunella and Oreohelix. Metcalf (1973) described four new species of Ashmunella with distinctive denticulation from fill in four
age.Inmany cases,colluvialslopedepositsseem tohavebeen related to periglacial phenomena such as solifluction and frost related processes. Deposits often have an appearance of initially having been a silty slurry that contained varying amounts of angular stones, interpreted as being frost rubble. Such rubble deposits are massive in some places, and seem to have originatedastalusorrockglaciers.Astheycommonlydonow in southern New Mexico, snails seem to have occupied intersticesofthisancienttalus,presumably attimeswhen they were dormant. Subsequently, calcareous silty deposits accumulated andfilled these interstices, entombing the snails or their shells. Sometimes small groups of fossil snails appear in t h e s e interstitial c a t a c o m b s , w h e r e t h e y m a y h a v e d i e d t o g e t h e r . Colluvial slope mantles are not forming at the present time and seem to provide, in themselves, an indication of a markedly different hillslope regimen during glacial-equivalent times.
differentcanyons. Inaddition,fossilsofAshmunellaswithvery poorlydevelopedteeth,orno teeth,arecommon infilldeposits in various canyons. All of these m a y be variants of Ashmunella rhyssa,presentlytheonlyspeciesofthegenusintheSacramento Mountains. However, fossils from different localities vary considerably in shell morphology, ranging from small to large, depressed to elevated, and from more conical to more angular. Some ofthesevariantsmay representstillotherextinctspecies. A similarlydiversegroupoffossilOreohelixoccursinthe mountains,someoftheminassociationwithvariousofthe above kinds of Ashmunella. In treating this complex, Metcalf (1982) described a n e w species, Oreohelix oterana, comprising three subspecies. Oreohelix penascana, described by Metcalf and Crews (1982), is a very different Oreohelix, morphologically, found at one locality in Rio Peñasco Canyon. Oreohelix neomexicana, a more widespread species, occurs in Pleistocene and some earlyHolocene deposits in the Sacramento Mountains,butisextincttherenow, sofarasisknown (Crews,
M o s t of the mountains with calcareous bedrock that provided favorable conditions for preservation of Quaternary molluscan fossils are in southern N e w Mexico. However, such conditions alsoexisttothenorth,intheNacimientoMountainsandinsome Southern canyons of the Sangre de Cristo range (as in Pecos and
1981). Oreohelix nogalensis occurs as a fossil in a northern extension of the Sacramento Mountains, east of the Sierra Blanca Mountains, although living populations are restricted to the Sierra Blanca range.
Sacramento Mountains
Preeminent among the southern mountains in richness of
QuaternarymolluscanfossilassemblagesaretheSacramento Ashmunellacarlsbadensisalsohasbeenfoundasafossilin
Mountains and their southeastern extension, the Guadalupe MountainsofNewMexicoandTexas,aBasinandRange-type mountain complex, formed on uplifted Paleozoic strata. Along canyons,fillmay consistofalluvialand/orcolluvialdeposits with the two sometimes interdigitating. Such canyon fill is extensive in these mountains, oftenforming thick deposits. Older deposits occur higher up on walls of canyons, in which there m a y have been multiple episodes of cutting andfilling. T h e latest timeofdowncuttingoccurredsome timebeforedepositionofthe presentfloodplains along canyon floors. Radiocarbon dates from these alluvial floodplain deposits are as old as mid-Holocene,
colluvial deposits in the eastern foothills of the Guadalupe Mountains and in several caves, its type being a fossil specimen fromacavenearthemouthofDarkCanyon,Eddy Co.,at1,067 m (3,500-ft)elevation.
the Guadalupes: A. amblya amblya and A. carlsbadensis.
In addition to the fossil fauna of larger land snails in the Sacramento-Guadalupe mountain chain, there is a good representationofsmallerspecies.Few ofthesesmallerspecies have gone extinct in the Sacramento Mountains, although this seems to be the case with Pupoides hordaceus, P. inornatus, andNesovitreahammonis electrina(thelatterhasbeentaken, living, at one locality in the nearby Sierra Blanca Mountains).
93