Page 88 - Land Snails of New Mexico

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related to lower elevation, as sediments of the Tesuque Formationhavebeenupliftedsome 1200-1500m, subsequent to deposition, according to Axelrod and Bailey (1976:249).
InastudyutilizingfossilseedsindepositsoftheMiocene PlioceneOgallalaFormationinwesternTexas,some60-125km east of N e w Mexico, Frye and Leonard (1957:16) found evidenceofa"mixedprairieflora,apparentlydominatedby grasses," although fossil hackberry seeds also were found. In subsequent investigations, their group found sparse, but similar, fossilseedassemblagesfartherwest,ineasternNew Mexico.In theirinvestigationsinTexasandNew Mexico,theyfoundalmost no localities with fossil mollusks of Pliocene age, in contrast to theirearlierstudiescarriedoutfarthernorth,intheHigh Plains of Kansas. Frye, Leonard, and Glass (1978:12) noted that, after several field seasons studying the Pliocene Ogallala Formation ineasternNew Mexico,theyhadfound"onlyonesignificant fauna of fossil mollusks." This fauna was from a locality ca.
The fossil land-snail fauna, discussed above, does not
include distinctively tropical or subtropical elements, such as the
camaenids found in Eocene strata. In fact, the fauna appears
much likethatofpresentNew Mexico,justasthough some
major faunal reorganization had taken place between the
glimpsesaffordedby earlierPaleocene and Eocene assemblages
and the one afforded by the Jacona, mid-Miocene fauna. Even
t h o u g h t h e f a u n a is c o n s i d e r e d b y t h e a u t h o r s c i t e d a b o v e t o h a v e
existed in a warm, lowland environment, it is much like the
fauna of areas of moderate heights (1525-2135 m/5,000-8,000
ft.) today in N e w Mexico. Of course, these modern montane
snailsmightbequitehappytooccupyawarm,humidhabitat, 11.3km(7.0mi)southofClayton,UnionCo.,whichthey with palms, ifone were offered to them nearby, as along a river
termed the "Clayton South" locality. This fauna was discussed furtherby Leonard and Frye (1978).
At Clayton South, they found 1 species of sphaeriid clam, 10 speciesofpulmonateaquaticsnails,and 15taxaoflandsnails. The assemblage contained 2 extinct species (Gastrocopta debilis and G. arena) described and known only from this locality. The fauna included several species commonly associated with grasslands today, including Gastrocopta cristata,Pupoides spp.,and Hawaiia minuscula. Pupilla blandi occursingrasslandsfartherweston thehighgrasslandsorvegas near Las Vegas. T w o species of succineids were listed. At least one species of succineid, assigned to Succinea vaginacontorta by Franzen(1971),iscommon inprairiehabitatsoftheHigh Plains today. The presence of Vertigo milium suggests habitat marginal to a prairie marsh or pool, presumably inhabited by the
aquaticspecies.Leafdetritusunderagroveoftreesassociated with such a marsh could have been inhabited by Gastrocopta pilsbryana,Euconulusfulvus,and Valloniaperspectiva,which are species common in various habitats in cordilleran New Mexico today. All in all, the habitat seems conformable to a prairie or savannah interpretation, but with ranker grasses than those of the short-grass prairie at the Clayton-South locality today.Thehabitatofthevegassome 175km tothewest,with their taller grasses and greater rainfall, might provide a better present analog.
Another molluscan fauna of Pliocene age has been found in
According to studies discussed above and to a review
incorporatingdatafromvariouspartsoftheworld(Croninand Socorro,SocorroCo.Thefaunaoccursinsedimentsbelowa
valley. Similarly, their mid-Miocene antecedents may have accommodated themselves to various habitats at both lower
(probably restricted) and higher (probably more widespread) elevations. At present, introduced Washingtonia palms survive in the Rio Grande Valley as far north as Truth or Consequences (1290 m/4,240 ft).
It seems that by mid-Miocene time a "Rocky Mountain fauna" was already in existence in northern New Mexico, marked by presence of the genus Oreohelix, the "Mountain snails," and of a variety of smaller species that are poorly represented in older Tertiary strata. Roth (1986:260) noted that Miocene land-snail faunas in the western United States were
preponderantly composed of genera still living in the same regions.
Pierce and Rasmussen (1989) studied sediments and
vertebrate and molluscan fossilsfrom a middle Miocene (early
Barstovian) locality in western Montana. They interpreted the mid-Miocene climate there to have become cooler and more
seasonal than that of the late Eocene to early Oligocene. Pliocene
Berggren et al. (1985:1415) supported a Miocene-Pliocene boundary date of ca. 5.3 Ma, Rio et al. (1991) proposed a younger date ofca. 4.93 Ma.
Dowsett, 1991:v-vi),Plioceneclimatewas stillwarm relativeto succeeding Quaternary climates, although a pattern of global cooling had commenced as early as the latest Eocene. For example, Matthews and Ovenden (1990) analyzed Pliocene pollen records that indicated presence of trees on shores of the ArcticOcean at80°N. For thewestern United States,Thompson (1991:126-127) assembled evidences indicating a milder climatic regime with m o r e effective moisture during the first 4/5 ofthePliocene,whichheindicatedtobe4.8–2.4Ma,followed byacooler,drierintervalat2.4-2.0Ma andasecond,moister and probably warmer interval at 2.0-1.8 Ma. H e interpreted (p. 125) evidence from vertebrate paleontology in the Great Plains (mainly, Kansas and Oklahoma) as suggesting an open, grassy savannah with a "subhumid-subtropical" climate in the late Miocene and earlyPliocene.
volcanic-ash deposit. Itcomprises only Pupilla blandi, Vallonia gracilicosta, Discus whitneyi, succineids, and Zonitoides arboreus. All of these are kinds (although the succineids are not assigned to a species) that occur in faunas of younger, Pleistocene age along the Rio Grande Valley, and all occur in New Mexico today. This suggests considerable long-term conservatism in regard to the land-snail fauna of the Rio Grande
Valley.
THE QUATERNARY
Compared to the paucity of fossil land-snail localities in Tertiary sediments in N e w Mexico, Quaternary localities are relatively numerous. However, precise chronological control is usuallylacking.On theeasternplainsandinintermontanebasins
the Rio Grande Valley, in the northwest part of the City of