Page 104 - Land Snails of New Mexico

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usually found together in modern faunas. Living Gastrocopta
armifera and Vallonia parvula occur as far west as the eastern
foothillsof the Sangre de Cristo Mountains, and G. armifera in
the upper Pecos Valley. The Gilman fauna indicates that they
formerly occurred farther to the west. At present, Pupoides
albilabris, Gastrocopta cristata, and Gastrocopta pellucida
would be expected at lower elevations, where they m a y occur in
riparian floodplain habitats. They would not be expected to
occur with such presently montane species as Vallonia
cyclophorella, Euconulus fulvus, and Nesovitrea hammonis
electrina. The Gilman Pleistocene site probably comprised a
well-watered floodplain, perhaps with both wooded and grassy
habitats, and attractive to species n o w found at both higher and lower elevations.
Exposures located ca. 4 k m (2.5 mi) N N W up the Rio Guadalupe Canyon from Gilman, northwest and southeast of the mouth of Deer Creek Canyon, seem to consist of mixed alluvial and colluvial sediments. From these were collected Vallonia
cyclophorella,succineids,Nesovitreahammonis electrina,and Oreohelix strigosa depressa.
Recently, a fossil fauna, dated at being close to the Pleistocene-Holocene boundary has been found in sediments along the Arroyo del Coyote at the base of the Manzanito Mountains, Bernalillo Co. (courtesy of Thomas E. Williamson and Richard A. Smartt). The fauna includes shells of Pisidium (fingernail clams) and aquatic snails of several species. With these occur the following land snails: Carychium exiguum, Pupoides hordaceus, Gastrocopta armifera, Gastrocopta pentodon form tappaniana, Vallonia cyclophorella, Helicodiscus eigenmanni, Discus shimekii, Euconulus fulvus, Oxyloma cf. retusum, succineids, Nesovitrea hammonis electrina, Hawaiia minuscula, and Zonitoides arboreus. As with the Gilman fauna, the presence here of Gastrocopta a r m i f e r a i n d i c a t e s a p a s t e x t e n s i o n o f r a n g e w e s t o f its p r e s e n t known confinesinnorthernNew Mexico. Overall,thefauna suggests presence of some kind of aquatic habitat such as marshes or beaver ponds.
Smartt and Hafner (1989) reported a microfauna of fossil terrestrial snails and associated microtine rodents from
sediments in Whut Cave on the Mesa del Oro, Cibola Co., southwest of Albuquerque. Snails w e r e m u c h m o r e c o m m o n in sedimentsatlowerlevels,which was interpretedasindicativeof a cool, mesic environment. Snails became rarer, upward, during w h a t w a s considered to involve a shift to drier (although still cool) conditions. Species taken included Gastrocopta pilsbryana (only at one level), Pupilla blandi and Vallonia gracilicosta (concentrated at the lower levels), and the following, which occurred at numerous levels: Pupilla muscorum, Pupoides hordaceus, Succinea sp., Vallonia cyclophorella, and Zonitoides arboreus. With the possible exceptionofthesuccineid,noneofthespeciesisknown from the immediate areaofWhut Cave today.
DISCUSSION
Much ofNew Mexico andthelandstoitswesthavebeen a p a r t o f t h e C o r d i l l e r a n r e g i o n o f N o r t h A m e r i c a s i n c e f a r b a c k
into the Mesozoic. As such, geologic history of the region has been shaped largely by the activities and migrations of the successive magmatic arcs associated with the Cordillera. These conditions are reflected in physiography and climate and, ultimately,inbiotas.
The presence of (1) Cordilleran highlands related to magmatic-arc tectonics and (2) the proximity of warm seas surely played major roles in development of biotas of the mid-to late Mesozoic. Mountain building, related to the Jurassic magmaticarc,wasoccurringatleastasclosetoNew Mexicoas southwestern Arizona and, from there, southeastward into México, Caldera and lava deposits s h o w that volcanic mountains
of considerable heights existed in several areas in southeastern Arizona during the mid-to-late Jurassic. In the Cretaceous, other h i g h l a n d s , s u c h a s a rift s h o u l d e r a n d t h e M o g o l l o n H i g h l a n d s , have been describedasexistinginsouthwesternNew Mexico and southeastern Arizona. In regard to seas, it is probably biogeographicallysignificantthat,duringthelaterMesozoic, the area of México was greatly reduced, eastward and southward, with its southern shores bathed by the circumglobal Tethyan Panamanian Sea. México comprised, in the paleogeographic models of Enos (1983:Figs. 5 and 6), a northern peninsula of varying size, temporally, and a southern archipelago with large islands. Thus, in the late Mesozoic, northern México and the borderregionofArizonaandsouthwesternNew Mexicoseem to have formed a southward extension of the western North American continent. This mountainous extension was
surrounded by seas to the east, south, and west.
In this southern extension of the continent, mountains and
adjacentwarm seaspresumably produced tropical lowlands and lushly vegetated highlands, perhaps analogous to present Central America. As thesouthernmost and most tropicalpartofwestern North America of the time, and with a varied topography that would have encouraged endemism and speciation, this extension probably supported a rich and varied land-snail fauna and m a y havebeenacenterinwhichsometaxaevolvedandfromwhich
some may have dispersed northward towards the continental interior of western North America.
The sea became especially significant in the Cretaceous. First, an arm extended northward along the Chihuahua Trough intosouthwesternNew MexicoandsoutheasternArizona,where itseems to have been flanked by block-faulted highlands of a rift shoulder to the north. (Such a mountainous shoreline could have producedearlyresort-propertypossibilitiesintheDeming Plains area!) Later, this restricted southern sea merged into the Cretaceous epeiric sea, which covered New Mexico. Its transgressions and regressions, interacting with mountains along its western shores, must have produced a dynamic, changing physiography with islands and peninsulas appearing and disappearing. Such conditions probably enhanced development offaunasadaptedtoenvironmentsrangingfrom coastallowlands to adjacent highlands with tropical vegetation. Present eastern México,fromtheGulfofMexicototheSierraMadreOriental, providesalikelymodern analog.As inthatpartofMéxico, today, an annual dry season may have prevailed, causing land snails to undergo dry-season dormancy.
With the oscillating final recession of the epeiric sea, the w e s t e r n p a r t o f N e w M e x i c o w a s u n c o v e r e d first, s o it s e e m s