Page 94 - Land Snails of New Mexico

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along these western tributary valleys are shown in Table 2. These localities include:
ArroyodelMacho: ChavesCo.,banksofArroyodel Macho,immediatelyW ofU.S.Hwy.285,SE%,NE%,sec.1, T8 S,R23 E, 1,126m (3,695-ft)elev.
RioFelix: ChavesCo.,banksofRioFelix,ca.0.8km (0.5 mi)W ofitsconfluencewiththePecosRiver,7.4km (4.6mi) SEofDexter,immediatelyEofwestsectionlineofsec.35,T 13 S,R26E, 1,033-m(3,390-ft)elev.
RockyArroyo: EddyCo.,banksofRockyArroyobetween Rocky Arroyo Cemetery and Indian Big Spring along N M R o a d 137;NEA,NE%,sec.26andNEA,NW/4sec.27,T21S,R24
E;1,082-1,097m (3,550-3,600-ft)elev. MosleyArroyo:EddyCo.,bankexposuresonwestsideof
MosleyArroyo,NWW,SE%,sec.26,T23S,R25E,1,067m (3,500-ft) elev.
Black River: Eddy Co.; terrace with buff-colored sediments,immediatelyS ofBlackRiver,inroadcutalongroad leading to U.S. Hwy. 62/180, 3.2 k m (2 mi) to the east, near center sec. 24, T 25 S, R 2 4 E , ca. 1,097-m (3,600-ft) elev.
Several larger species of land snails taken in the greater Pecos Valley were not part of the fauna of the eastern plains. Two polygyridsnailsoccurinQuaternarysedimentsalongthe valley. Linisa texasiana is fairly common as a fossil from R o s w e l l s o u t h w a r d i n t h e v a l l e y , i n d i c a t i n g t h a t it w a s f o r m e r l y morewidespreadtherethanatpresent,when itseemsrestricted to the Bitter Lake area east of Roswell. Fossil shells of
Ashmunella rhyssa have been taken in bank sediments of Rocky Arroyo (NE%, NW/4, sec. 27, T 21 S, R 24 E) at a locality indicated in Table 2, above, and of Black River 12 k m (7.5 mi) west of Malaga. It seems that this presently montane species lived at lower elevations along Rocky Arroyo and Black River inthePleistocene,asitdidatKeen SpringintheTularosaBasin, discussed below.
Along the western margins of the Pecos Valley, a fauna transitional to that of the foothills (exemplified by Dry Cave, discussed hereafter) seems to have occurred. Such an assemblage is represented by materials from a terrace along upper Black River, listed in Table 2. The species Rabdotus dealbatusneomexicanus,now typicallyamontanesnailinthe area, is c o m m o n in deposits at this site, as are several other species n o w found at higher elevations in the Guadalupe and
Sacramento Mountains: Pupilla blandi, Gastrocopta a. armifera, Gastrocopta procera, and Vallonia perspectiva. The occurrencetogetheratthissiteofRabdotusd neomexicanus with Vertigo ovata is of interest. Both species still persist in Eddy Co.: the Rabdotus only in mountainous areas, and the Vertigoonlyathousandmeterslowerindamp habitatsalongside the springbrook issuing from Blue Spring. However, along the upper Peñasco River in the Sacramento Mountains, above Mayhillatelevationsoffrom2,042-2,195m (6,700-7,200ft), R. d. neomexicanus lives on hillslopes alongside the floodplain of the stream, which is marshy in places and offers habitats suitable for Vertigos. These, and several other species that occur inPleistocenefaunasalongthePecosValley,exhibitlife-zone depressions when compared with faunas presently found at higher elevations in mountains to the west.
Rio Grande Rift Valley
Topographic basins and valleys are, of course, found throughouttheBasin and Range Physiographic Province, which incorporates some one-third of the area of N e w Mexico. The m o s t extensive of these in the state is the R i o G r a n d e Rift Valley, which, by its north-south axis, bisects the state. This valley has been the most intensively investigated of the various basins in regard to Quaternary fossil mollusks. Metcalf (1967, 1969) reportedfaunasfromthevalleyinsouthernNew Mexico,and David P. Dethier has made available for study assemblages he collected in the Española area, in the northern part of the valley.
InsouthernNew Mexico,collectionsofPleistoceneagewere made atlocalitiesin two widespread alluvial units: the Tortugas andPicachoalluviums(Metcalf,1967).A compositelistingof species taken in these units is given in Table 3. In the assessment of Gile et al. (1981:48-49:Fig. 8), s o m e sediments of the Picacho and Tortugas alluviums were considered likely to pertain, respectively, to the first and second interglaciations preceding the Wisconsin Glaciation (oxygen isotope stages 5
and 7). It is likely that these units also contained sediments
pertaining to times of the waning glaciations preceding these
interglaciations (oxygen isotope stages 6 and 8, in part). The
faunas themselves are more indicative of glacial-equivalent than
of interglacial-equivalent conditions, as they suggest a lowering
oflifezones. The listinginMetcalf(1967:Table 1)shows that
one-half of the terrestrial species in the Picacho and two-thirds
of those in the Tortugas alluvium are species presently living in
montane habitats, implying a life-zone depression offrom 6 1 0
910m (2,000-3,000ft).Almostallspeciesinvolvedinthis
apparent colonization of lower-elevation areas were smaller
species. The larger montane species seemingly retained a long
continued preference for highlands. Larger species were found
only in the case of the Tortugas alluvium, which yielded
Ashmunella hawleyi at two localities and a fragment of a Rabdotus atone locality. •
Ashbaugh and Metcalf (1986:13) discussed a fossiliferous alluvium exposed in banks of Placitas Arroyo, ca. 3 k m southwest of Hatch, Doña Ana Co. Species obtained are indicated in Table 3 (under "Placitas Alluvium"). No age determinations were available for this deposit. However, from its stratigraphic position below Holocene alluvium, degree of induration,andnatureofthefauna,itseems likelytobe oflatest Pleistocene or earliest Holocene age, quite possibly of an age nearthatoftheB-2 alluvium, discussed below. The assemblage reflects a habitat very different from the present arid conditions at the site. Ashbaugh and Metcalf (1986:14-16) interpreted faunas from two lower units at the locality to suggest presence of a springbrook with an associated riparian stand of trees, while the fauna of an upper unit was inferred to show evidence of pools and marshes related to spring outflow.
A deposittermed"B-2alluvium"ofearlyHoloceneagewith a radiocarbon date of 9,360+150 B.P. was reported by Metcalf (1969:160)frombanksoftheRioGrandeca.2.5kmWSW of Garfield, Doña Ana Co. Species found in this alluvium are indicated in Table 3. The fauna includes Cionella lubrica, Gastrocoptaa armifera,Valloniaperspectiva,V gracilicosta, Helicodiscus eigenmanni, Glyphyalinia indentata, and