Page 96 - Land Snails of New Mexico

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Zonitoides arboreus. These species do not inhabit the Rio Grandefloodplainheretoday,but,exceptforG. armifera,are associatedwithfoothillormontanehabitatsover1,675m (5,500 ft)in elevation. Hackberry seeds (Celtis) are associated with the snails, and several kinds of trees may have contributed to a riparianwoodland thatproduced leaflitterinhabitedby snails.
Snails from deposits containing partially decomposed plant material, exposed in banks of the Rio Grande at the foot of the Robledo Mountains south of Fort Selden, Dofia Ana Co., provide further information concerning Holocene aspects of the Rio Grande floodplain fauna. These outcrops comprise Localities R-34 to R-37 reported by Metcalf (1967:60). A
charcoal sample from Locality R-34 yielded a radiocarbon determination of "less than 195 years" (Metcalf, 1967:34). It seems very likely that all localities are comparably young, and indicative of conditions prevailing along the floodplain just before Europeans began modifications of the valley. Species of land snails obtained at these localities are listed in Table 3
(under "Rio Grande F."). The fauna consists mainly of aquatic
species and of terrestrial species that typically inhabit the
vegetated margins of marshes and pools. The associated fossil
plant material also indicates a marshy habitat. Historical
accounts suggest that marshes were common along the Rio
Grande floodplain in this area until European settlement
(Metcalf, 1967:32). The floodplain would have provided a
narrow stripwith aquatic and moist habitats, quite differentfrom
thexericenvironmentofthevalleyareasflankingthefloodplain. This seems to be reflected in the contrast between the diverse
fauna of aquatic and marsh indicators from late Holocene sediments of the Rio Grande floodplain and the sparse fauna of xeric indicators found in alluvium deposited along tributaries entering the valley from the east and west.
D e t h i e r e t al. ( 1 9 8 8 ) d i s c u s s e d t h e g e o l o g i c d a t i n g o f e r o s i o n surfaces in the Española area, using several dating methods, including amino-acid ratios that were determined for fossil land snails occurring below three such surfaces. Dates indicated (pp. 934-935) for times of deposition of the snails were "between 700 and 500ka," "younger than 250 ka," and "younger than 130 ka"(ka=thousandsofyears).DethierandMcCoy (1993)further discussed utilization of aminostratigraphic relations of Quaternary deposits in the Española area. They noted that amino-acid ratios were determined for gastropods (succineid and
Valloniashells)from44 sitesindepositsthattheycalculatedto rangeinagefrom 620 ka tomodern. Some broad chronological groupings have been provided by Dethier for gastropods submitted to m e for identification. These groupings, together with taxonomic determinations, are indicated in Table 3. A g e
groupings range from 620-650 ka to 40-80 ka.
Faunas of various ages and from various geographical
localities in the Rio Grande Rift Valley show considerable similarity. In Table 3, comparing faunas from the Dethier localities(northern)withthoseofDoña Ana andSierraCounties (southern), 15 species are recorded as both northern and southern. The dominant species in assemblages from both areas are similar, including Gastrocopta cristata, Pupilla blandi, Vallonia gracilicosta, and succineids. In collections, Pupilla muscorum and Vallonia cyclophorella were relatively more common to the north and Pupoides albilabris, Gastrocopta
pellucida, Vertigo ovata, Helicodiscus singleyanus, Hawaiia minuscula,andZonitoidesarboreustothesouth.Threespecies arerecordedonlyinthenorth:Vertigomodesta,Euconulus fulvus, and Oreohelix strigosa. However, the first two species do occur in mountains in the southern part of the state. Fourteen species are recorded in the south, but not in the north, although mostoftheseprobablyoccurredinnorthernNew Mexico. Their absence, here, is probably merely attributable to the smaller number oflocalitiessampled inthenorth and tothe lackoflatest Pleistocene or earliest Holocene assemblages associated with marshes, such as seem to have occurred at the Placitas Arroyo
andB-2 alluviumsites(Table3).
In general, itseems that a Pleistocene land-snail fauna with
much similarity in regard to component species extended along the length of the Rio Grande Valley across N e w Mexico. Fossil asssemblages comprising this fauna differ in their composition from those presently occurring in the valley. This is surely attributable, in part, to the changes wrought by Europeans in theirprofound modification of an area so inviting for agriculture and as human habitation. The young fauna noted from Rio Grande floodplain sediments (Table 3) indicates much change in the lastcentury. Apart from this, ithas been contended, above, that life-zone depression was involved in the case of the Tortugas and Picacho alluviums and of the faunas from the Española area. It seems that the Placitas Arroyo spring and marsh-related habitat had disappeared by some time in the Holocene, as, above the older beds, there are younger Holocene deposits that contain only scant numbers of shells, all n o n
hygrophilous.
Gila River Valley
Along the Gila River Valley in extreme western New Mexico (Hidalgo Co.), there are valley-flanking terraces with associated alluvial deposits, which generally resemble some of the alluvial terraces along the Rio Grande Valley. Morrison (1965) designated the lowest of these as "Stream-terrace g r a v e l s " Q t - 1 a n d Q t - 2 , a n d t e n t a t i v e l y c o r r e l a t e d ( i n litt., 2 8 July 1967) Qt-2 with the Picacho alluvium of the Rio Grande Valley in the Las Cruces area. In sediments under Morrison's Q t 2 terrace, immediately north of Virden, Hidalgo Co., fossil molluskswerefound,includingthefollowingterrestrialspecies: Pupoides albilabris, Gastrocopta a. armifera, G. cristata,
Vallonia cyclophorella, Helicodiscus singleyanus, succineids, and Hawaiia minuscula.
Although additional Pleistocene fossil localities have not beenfoundalongtheGilaValleyinNewMexico,somemaybe revealed by further search. N o t far down-valley in Greenlee Co., Arizona, several fossiliferous exposures have been found in road cuts along Arizona Highway 75, between Sheldon and the junctionofHighway 75 with U.S. Highway 666. Itislikelythat these species also were inhabiting the valley immediately upstreaminNew MexicoatthattimeinthePleistocene.The list from three Arizona localities includes the following land snails (arranged by declining total number of specimens, shown in parentheses):
Gastrocopta cristata (136) Succineidae (111)
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