Page 89 - Land Snails of New Mexico

ofCordilleranNewMexico,fossilshellsusuallyoccurinalluvial deposits related to former aquatic habitats such as marshes, ponds,lakes,springs,andstreams.Inmontaneareas,fossilsare found both in alluvial and colluvial deposits, these latter occurring on mountain slopes, often along the walls of canyons.
Throughout New Mexico, it appears that there was a depression of lifezones to elevations lower than those prevailing at present, during times equivalent to the latest Wisconsin glaciation, and presumably during earlier glaciations as well. During such times, ranges of several species of land snails s e e m tohaveextendedfrommontanesourcesintolowerintermontane
basins and onto the plains of the eastern part of the state.
QUATERNARY FOSSILS FROM THE EASTERN PLAINS
The major sources of information concerning Pleistocene faunas of the plains of eastern New Mexico are from the malacological work of A. B. Leonard incorporated in three publications dealing with northeastern, east-central, and southeastern New Mexico, respectively: Frye, Leonard, and
Glass (1978), Leonard and Frye (1975), and Leonard, Frye, and Glass(1975).InnortheasternNew Mexico,Frye,Leonard,and Glass (1978) found no fossilfaunasthattheyjudged tobe pre Wisconsin in age, but reported (p. 5, Fig. 14) 48 collections from 43 localities considered as being of Wisconsin or Holocene age. In east-central N e w Mexico, Leonard and Frye (1975) listed31PleistocenelocalitiesfromtheHighPlainsand 14from thePecosRiverValleytothewest. Similarly,insoutheastern New Mexico, Leonard, Frye, and Glass (1975) treated assemblages from 11 localities from both the eastern plains and thePecosValley. Speciesreportedinthethreepublicationsare
summarizedinTable 1.Precisedatingislackingformost ofthe New Mexicolocalitiestreatedbytheaboveauthors,although they considered most to be of Wisconsin age. Dates that are available range from ca. 13,000 to 27,500 B.P.
From Harding Co. in northeastern N e w Mexico, McMullen and Zakrzewski (1972) reported a molluscan fauna in association with late Pleistocene mammalian fossils. Their
listingisgiveninTable 1. Intheeast-centralpartofthestate, Hester (1972:25) reported a fauna from the Blackwater D r a w localities, dated at 10,490+900 B.P. Species listed from this locality include Gastrocopta armifera, G. procera, Strobilops texasiana, Hawaiia minuscula, Vallonia gracilicosta, Pupoides marginatus (probably P. albilabris), and Retinella electrina
To thesouth,atNash Draw inEddy Co.,Ashbaugh and Metcalf(1986)reportedafaunaassociatedwithaformerspring complexinthisnowaridarea(Table1). Associationwith fossils of Camelops indicates a Pleistocene age. A fauna of latest Wisconsin age (ca. 11,000 BP) reported by Pierce (1987:Table6-1)fromtheLubbockLakestudyarea(substratum
1 B-C), Lubbock Co., Texas, is summarized in Table 1. This localityisincludedbecauseofthedetailedchronostratigraphyof the siteand toprovide comparison with a plains fauna some 105 km eastofNewMexico.Itislikelythatmostoftheabovefaunas are of Wisconsin age. Itis assumed thatspecies represented at numerous localities were important components of the eastern plains fauna in Wisconsin time. In Table 1, species that are found at 5 or more localities, excluding Roosevelt Co. and the L u b b o c k L a k e site, i n c l u d e t h e f o l l o w i n g s p e c i e s , a r r a n g e d b y number of localities from which each species is reported (in parentheses):
Succineids, sp. indet (76) Hawaiia minuscula (43)
Vallonia gracilicosta (38) Pupoides albilabris (28) Pupilla muscorum (23) Gastrocopta cristata (22) Pupilla blandi (21) Gastrocopta armifera (19)
Vallonia cyclophorella (16) Helicodiscus eigenmanni (13)
Vallonia parvula (11) Gastrocopta procera (8) Helicodiscus parallelus (8) Gastrocopta pellucida (7) Pupoides inornatus (6) Vertigo ovata (6)
Vertigo milium (5)
Nesovitrea hammonis electrina (5)
Before returning to specific points suggested by Table 1, some general remarks concerning recent assessments of the late Pleistocene environment of the High Plains seem in order. Wells and Stewart (1987), on the basis of plant (tree), land snail, and mammalian fossils, proposed that the High Plains of Kansas and Nebraska supported a taiga-like vegetation during the time of the late Wisconsin (Woodfordian) Glaciation. In contrast, Holliday (1987:238) suggested, in regard to the southern High Plains in the general region of Lubbock and Plainview,Texas,andClovis,New Mexico,that"Availabledata suggest that the region was primarily an open grassland or
[=Nesovitrea hammonis electrina]. Drake (1975) reported on
13 assemblagesoffossilterrestrialandfreshwatersnailsfrom
localitiesinnorthernRooseveltCo.Mostofthesewerefrom grasslandwithsomenonconiferoustreesthroughmostofthe
diatomites at the well-known Blackwater D r a w paleontological and archeologicallocality and atAnderson Basin, a few miles to the east. Ages ranged from ca. 8,500-15,750 B.P., hence, latest Pleistocene and early Holocene. Several tens of thousands of shellswererecovered,themajorityofwhichwere aquatic.From the 13 assemblages in Roosevelt Co. more than 19,000 specimens were of the aquatic genus Gyraulus, while less than 3,000 were of terrestrial species. Total numbers of shells reported for each species from the 13 assemblages, excluding those listed as "fragments," are indicated in Table 1.
Quaternary." InnortheasternNew Mexico,itisprobablethat some elements of forests such as those presently existing on mesas and scattered mountains of the area extended to lower
elevations, onto the plains. However, in reports of Pleistocene land snails from eastern N e w Mexico, cited above, half the
species that Wells and Stewart (1987:Table 1) considered as indicativeofforestswithsomecordilleranaffinitiesarerareor
lacking; Cionella lubrica, Vertigo binneyana, V. gouldii, Columella alticola, Discus shimekii, and Punctum minutissimum lacking and Vertigo modesta rare. That reliable
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