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problematic, because of the presence of isolated volcanic mountains and the extension of basalt-capped mesas along the New Mexico-Colorado border from the southern Rocky MountainseastwardtothenorthwestcorneroftheOklahoma
Subsequently,climaticcoolingandincreasedaridity,beginning at around a "terminal Eocene Event," gradually decimated elements of the ancient Cordilleran fauna, north to south, in the continentalinterior,sothatmostoftheseelementsarenow restrictedtoN e w Mexico, Arizona, and regions to the south. O n the other hand, a moderating maritime climate of the Pacific coastal region has allowed a fauna with more of an ancient Cordilleranaspecttopersistthere,from CaliforniatoCanada.
Panhandle. Physiographically, this has been dealt with by Hawley (1986:24) by assigning this area to a Raton Section of the Great Plains Province. T h e question arises as to whether, in terms of molluscan provinces, the area of the Raton Physiographic Section would be assigned better to the Rocky MountainortotheHighPlainsMolluscanProvince.A number of montane snails, typical of the southern Rocky Mountains, occur athigher elevations in the area. However, broad areas of plains, with a depauperate fauna, also occur in the section. Clearly, the area is perplexingly transitional. In an attempt to be
A s the ancient Cordilleran fauna of the interior region shrank
consistent, Iuse an approach similar to that employed, above, for northwesternNew Mexico.Accordingly,thisareaisindicatedin Fig.2B asbeingwithinaRatonSubprovinceoftheRocky Mountain Molluscan Province. A t this point, the epigraph at the beginning of this section m a y bear rereading.
and lower elevations in the southwestern states during times
Presumably, as indicated by itsvery name, the Cordilleran
Cordilleran fauna from the north are species typical of the R o c k y Mountain Molluscan Province. Elements pertaining to these two categories often occur together, especially in higher mountains of the southern part of the state, in such a w a y as to m a k e the defining of molluscan province boundaries a difficult and perhaps unprofitable exercise, although attempted.
region of western North America, to some extent has been
southward, it seems to have been replaced by snails, mostly
small and mostly ofPalearcticor eastern North American origin,
whose descendantscame tomake up alargeproportionofthe
fauna oftheRocky Mountain Molluscan Province. These more
northern species were well adapted to disperse to lower latitudes
equivalent to Pleistocene glaciations, as is amply demonstrated inthelatePleistocenerecordinNew Mexico.
ThepresentfaunaofNew Mexicoseemscategorizableinto t w o p r i n c i p a l a s s e m b l a g e s . T h e a n c i e n t C o r d i l l e r a n f a u n a still survives in mountainous areas mainly in the southern part of the
SUMMARY state,anditsspecieshavebeenusedtraditionallytodefinea Southwestern Molluscan Province. Replacing this venerable
mountainous from itsbeginnings. Successive magmatic arcs and
plate interactions have produced various types of mountains
duringthelaterMesozoic and Cenozoic. Itseems likelythatthe
Cordillera,withitsgenerallymountainous environment and with
its north-south oriented mountain ranges, would have
encouraged development of a biota with m u c h in c o m m o n along
ACKNOWLEDGMENTS
itslength. Such commonality likely was not shared with the less mountainous lands to the east. Differences between the two
AttheNewMexicoMuseum ofNaturalHistory,Iwishfirst of all to thank Paulina V. Ungnade, whose persistence and patience in editing and duplicating multiple draft versions of this paperhaveconvincedme thataltruismreallydoesexist.Without the fruitful discussions and invigorating stimulus provided by Richard A. Smartt, Iprobably would have given up. For insight into problems of early Tertiary paleontology, I thank Spencer G. Lucas,AdrianP.Hunt,andThomasE.Williamson. Inaddition, IwishtothankstaffmembersofNMMNH fornumeroushelpful
regions were, no doubt, accentuated by vicariating interposition of the Late Cretaceous epeiric sea, and of the Great Plains, which latercame to occupy the eastern part of the area covered by the sea. Thus, paleogeography and environment probably contributed to development of a distinctive Cordilleran land snailfauna,termedherethe"ancientCordilleranfauna." No doubt, earlier continental connections, and perhaps accreting terranes, may have had some bearing on thisfauna. However, given s o m e initial stocks to w o r k with, a sizable continent with itssouthern regions in a tropical zone should be quite capable of developing a respectable land-snail fauna on itsown.
services provided, and especially in final preparation of the manuscript. Deserving special mention are Patti Gegick, David Hafner, Jackie McConachie, and Marlene Tanner.
Therefore, it is suggested that in the later Mesozoic the southernmost extension of western North America was a
At the New Mexico Bureau of Mines and Mineral
tropical,mountainouslandboundedbywarm seas,harboringa probably rich and diverse land-snail fauna. Unfortunately, fossils
Resources, I thank Robert H. Weber, and especially John W . Hawley, w h o initiated m e into and gave m e an interest in the QuaternarygeologyofNew Mexico.
Othergeologistsfromwhom IhavelearnedincludeGeorge O.Bachman,DavidP.Dethier,thelateJohnC.Frye,LeeH. thenorth--inthenorthernUnitedStatesandsouthernCanada. Gile,andRogerB.Morrison.Fromthosewhostudyfossilland snails, I have gained much: E m m e t Evanoff, the late A. Byron
that would bear out this hypothesis are lacking for that region and time. Instead, in the Cordillera, the earliestfossils are far to
Given the general tropical aspect of late Mesozoic and early Tertiarybiotaseventhatfarnorth,itseems likelythatthesemore northern snails were outlying members of families that also occurred to the south, as in N e w Mexico and México. Fossils from N e w Mexico ofPaleocene and Eocene age show similarity tothesenorthernfaunas.A widespreadtropical-to-subtropical biotaisstillevidenced intheEocene asfarnorthasWyoming.
Leonard,BarryRoth,andDwightW. Taylor.ArthurH.Harris andRickDumanhavebeenhelpfulinvariousways.My former students, Celinda R. Crews and Karen McCort, were my
mentors, as well, in regard to the paleomalacology of N e w Mexico.