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elements of the North American biota.
The southern end of the Cretaceous magmatic arc and its associated mountains were located in southern California and
adjacent México, west of where its Jurassic antecedent had been inArizona.However,tothenorth,theCretaceousarcmusthave occupied a position similar to that of the Jurassic arc and of the presentSierraNevadaMountains(Dickinson, 1989:Figs.4,5). The Cretaceous magmatic-arc mountains in this region have been termed "a belt of great batholiths" by Hamilton (1988:4). NorrisandWebb (1990:85)speculatedthatthesemountains were as high as or higher than the present Sierra Nevada. Hamilton (1988:17) has suggested an analogy of these ancient mountainstopartsoftheAndesMountainsover5kmin elevation. Comparison to the Andes also has been suggested by Nilsen (1987:82). There seem to be similarities both in terms of physiography and of plate tectonics. Both ranges have been related to subduction of oceanic plates (Nazca and Farallon) under continent-bearing plates (South American and North
American).
Perhaps, then, the Cretaceous magmatic-arc mountains of
California were of Andean aspect, and also at a paleolatitude comparabletotheAndes ofcentralChile(Dickinson, 1989:Fig. 9).Itappearslikelythattheyweremesic,forestedrangesthat provided favorable habitats for land snails, at least on their seaward slopes. Such Cordilleran mountains, extending from México to Alaska with probable connections to northeastern Asia at times, must have provided a remarkably long although narrow corridorforoccupancyby landsnailsadaptedtomontane habitats.Assuch,thesemountainsmayhavebeenofmajor significance in the development of a distinctively western North American land-snail fauna during the later Mesozoic. Such a fauna, ina broad sense, also may have had some bearing on the nature of the land-snail fauna of N e w Mexico, even though removed geographically.
EastoftheCretaceousmagmatic-arcmountains was aregion located in present Nevada and northwestern Utah, which has been termed the "Hinterland" by Armstrong (1972) and most later authors, in contradistinction to the m o r e eastern Cordilleran foreland. Coney and Harms (1984:552-553) interpreted the Hinterland as an "overthickened crustal welt," produced by compressional "crustal telescoping," and suggested that the regionmighthaveresembleda"TibetanorAndean altiplanolike plateau" in the lateMesozoic. The altiplano analogy conforms well with an Andean model for the magmatic-arc mountains to the west. Perhaps the Hinterland w a s within a rain s h a d o w of the magmatic-arc mountains, as they were probably atthattime in a zone of prevailing westerlies (Dickinson, 1989:Fig. 9).
East of the Hinterland belt was the Cordilleran fold and thrust belt (also termed as the overthrust belt, foreland thrust belt,orSevierthrustbelt).Thiswas aremarkablylong,sinuous, mountainous belt extending from Alaska southward to the eastern Mojave Desert region of California. Levy and Christie Blick (1989) indicated that this belt formed within the time frameof150-50Ma andElison(1991)asbetween165-55Ma (Early Cretaceous to Eocene in both cases). Thus, in part, it would be time-equivalent to the Laramide Orogeny, discussed below. This belt was caused by intracontinental compression related,atleasttosome extent,toeventsinvolvingtheboundary
oftheNorth American Plate (Elison, 1991:1235). Estimates of minimum crustalshorteninginthefold-thrustbeltcalculatedby Levy and Christie-Blick (1989:Table 1) range from 104 to 135 km. Clearly, such horizontal compression must have been converted by folding and thrusting into vertical planes that produced imposing mountain ranges. Such mountains are shown inpaleogeographic reconstructions, as for western Montana by Rice and Shurr (1983:Fig. 17) and for central Utah by Ryer and McPhillips (1983:Fig. 14).
Eaton and Nations (1991) summarized studies concerning
Cretaceous sediments deposited along the western margin of the Cretaceous sea in southwestern Utah and northern Arizona.
Theyfoundevidencesofdepositionbystreamsflowingeastto northeast from the Sevier Thrust Belt mountains, and also by streams flowing north to northeast from the Mogollon Highlands to the south (Eaton and Nations, 1991:Fig. 6). The Sevier and Mogollon highlands, thus, bordered a roughly V-shaped western extension of the sea, which has been termed the "Grand Canyon Bight" (Eaton and Nations, 1991:1). Fluviatile deposition indicated presence of perennial streams (Schmitt et al., 1991), which suggests that the adjacent mountains received considerable precipitation and were probably well vegetated.
Itseems likelythatthefold-thrustranges,nottoodistant from the epeiric sea in Cretaceous time, provided favorable opportunities for land snails. In some ways, itmay have been analogoustothepresentSierraMadre OrientalofMéxico, also w e s t w a r d f r o m a w a r m sea. T h e fold-thrust m o u n t a i n s m a y h a v e provided a long "highway" offering opportunities for dispersal oflandsnails,northwardorsouthward.Familieswithmembers utilizing such a highway likely may have included the Urocoptidae, Oreohelicidae, and Helminthoglyptidae, known from Cretaceous fossils in the region, and still existing there. Several other families are reported as Cretaceous fossils in the region and m a y have inhabited the fold-thrust belt west of the epeiricsea,althoughtheynow existtothesouthorsoutheastof the Cordilleran region. Included are the families Helicinidae,
Cyclophoridae, Subulinidae, and Camaenidae. In the fold-thrust mountains, snails belonging to such families might have found habitats similar to those in the present Sierra M a d r e Oriental, as suggested above.
Although narrow longitudinally, the sources cited above
indicate that the lands west of the epeiric sea provided a variety
of habitats in regard to elevation, climate, and vegetation, and
presumably supported a correspondingly diverse land-snail
fauna, which must have contributed in some ways to producing
Lateandpost-CretaceousfaunasofNew Mexico.Unfortunately, the nature of the Late Cretaceous land-snail fauna of N e w
Mexico isnot known. Hartman (1981b) referred to Planorbis
chacoensis Stanton, 1917, as being a terrestrial species, and he
reports (pers comm., 9 July 1992) continuing work on nonmarine Cretaceous mollusks from northwestern New
Mexico. A discussion of the taphonomy of nonmarine invertebrates,and vertebrates of latest Cretaceous age in the FruitlandandKirtlandFormationshasbeenprovidedbyHunt (1991), although no specific terrestrial snail taxa are mentioned.
In the Rocky Mountain region, with desiccation of the epeiric sea, the more western area that had been covered by the sea joined with the land to the west and took on itsCordilleran
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