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Oreohelix eastward into Iowa during the Pleistocene ultimatelyPalearcticoreasternNorthAmericanelementsofthe
(Henderson, 1931:184, Pilsbry, 1948:XLV). Rocky Mountain fauna. Save for a few exceptions, such as the Therewereapparentlysomegroupsformerlyofbroader occurrenceofanAshmunellaandRabdotusinsedimentsofthe
east-west distribution, which did not survive in the Cordilleran
region. Roth and Emberton (1994) discussed the occurrence of
the genera Hendersonia and Euchemotrema, in Arikareean beds
(Oligocene-Miocene)inMontana.Thesegenerapresentlyoccur
in the eastern United States, and were interpreted by these
authors (p. 93) as having been "displaced eastward off the North
American cordillera."As inthecaseofPalearcticimmigrants, the eastern North American land snails that do seem to have
dispersedintothepost-CretaceousCordillerawere mainly small species, especially towards the south (apparently less so in the northern plains). This suggests that snails that are relatively small throughout life are more easily dispersed than those that reachrelativelylargersizes.Thisphenomenon alsowaspointed out by Henderson (1931:180), w h o noted that smaller snails are "easily transported in various ways" and tend to be of "wide geographicaldistribution." BequaertandMiller(1973:47-48) also discussed the more widespread distribution and less tendencytowardsendemism insmallerspeciesascontrastedto larger species in the Southwest. They suggested that strong winds of the southwestern region might be involved in aerial transport of small snails. They also suggested that dispersal might take place by floods or by avian transport. In aquatic and paludal species, itseems likely that transport by aquatic birds does, indeed, play a role in dispersal.
In the region comprised by the present Rocky Mountains, most elements of an earlier Cordilleran fauna no longer occur. Cordilleranelementsmay includethegeneraMicrophysulaand some species of Discus, but are represented mainly by the "mountain snails" of the genus Oreohelix. Itseems likely that this live-bearing genus evolved in the northern Cordillera, perhaps from an earlier, oviparous oreohelicid stock like Radiocentrum. Perhapsitsovoviviparityparallelsthatofother high-latitude poikilotherms, as in some species of Natrix, Thamnophis, and Lacerta.
During atleastthelastglaciation-equivalenttimeofthe P l e i s t o c e n e , it s e e m s t h a t t h e r e w a s a n i n c u r s i o n s o u t h w a r d i n t o
Tortugas alluvium of the Rio Grande Rift Valley, the ancient Cordilleran elements seem to have retained their proclivity to be
just that: Cordilleran.
The QuaternaryfaunaoftheeasternHigh PlainsofNew
Mexico, traditionally assigned to the Eastern Molluscan Division,isconsideredheretobe ofmixed origin.During times of Pleistocene glaciations, it seems to have been inhabited mainly by derivativesfrom themore modern (not ancient) Rocky Mountain fauna, which was, itself, of mixed origin, as suggested above. At the present time, only the hardiest, remnant species of the Pleistocene fauna persevere on the eastern plains of N e w Mexico. Some oftheseprobablycanbe consideredbona-fide eastern derivatives that did pioneer westward onto the plains. However, atpresentthefauna,overall,issodepauperateasto
impart few intimations of zoogeographic significance. Molluscan Provinces Redux
In the absence of insuperable faunal barriers, any attempt to establish zoological provinces very rigidlymust fail.--JuniusHenderson (1928:88).
Inlightoftheprecedingdiscussion,itmay be appropriateto reexaminetheallocationofNewMexicotovariousmolluscan
Zoogeographic provinces. O f course, in terms of the long-term historical point of view emphasized herein, this is only an ephemeralsituation.From ashort-termhistoricalpointofview, the very exercise of proposing zoogeographic provinces is mainly associated with traditional zoogeography of the past and seems tobeofscantconsequenceatthepresenttime.Despite such reservations, a short analysis m a y be justifiable.
As notedearlier,BequaertandMiller(1973)hadplaced muchofNewMexicoinaSouthwesternMolluscanProvince
(see Fig. 1C), except for a small north-central to northeastern area that was allocated to a Rocky Mountain Molluscan Province, and approximately the eastern one-third of the state, which was placed in an "Eastern Division."
The Southwestern Molluscan Province was viewed by Henderson (1928, 1931) and Bequaert and Miller (1973) as easternNorthAmericanderivatives,Oreohelixseemstohave beingdefinedprimarilybypresenceofthegeneraHolospiras.l., Ashmunella, Oreohelix (then including Radiocentrum), and
southernNew Mexicoofa"RockyMountainfauna,"including someCordilleranelements,butmuch enrichedbyPalearcticand
thrivedinmountainsacrossCordilleranNew Mexicoduringthe
time of the Wisconsin glaciation (and probably other
glaciations), occurring to the southernmost mountains such as
the Guadalupe, Franklin, and Sierra Rica ranges. In the Black
Range and Sacramento Mountains, Oreohelix underwent
r a d i a t i o n s , still e x e m p l i f i e d b y l i v i n g s p e c i e s i n t h e f o r m e r a n d
by fossil taxa of the O. oterana complex in the latter. The
extension of Oreohelix southward, apparently during times of
glaciation-related cooling, seems to have itsreciprocal in the
patternofextinctionofthegenusinsouthernNewMexico mappedbyBequaertandMiller(1973:Fig.1--seeFig.1C,
duringthewarmer timeoftheHolocene. Similarly,Pleistocene a n d H o l o c e n e e v i d e n c e a m p l y d e m o n s t r a t e s t h a t a n u m b e r o f other,presentlymontanespeciesextendedtheirrangesfarther south and to lower elevations during times of Pleistocene glaciation, and subsequently suffered extinction there. These were mostly smaller species, many of which were probably
herein)aspertainingtotheSouthwesternMolluscanProvince.
T h e s e g e n e r a a r e l a c k i n g , h o w e v e r , i n a l m o s t all o f t h e C o l o r a d o
Plateau,includingthatportionofitinnorthwesternNew Mexico andinArizonanortheastoftheLittleColoradoRiver.Withstrict
adherencetothepresenceoftheabovedistinguishinggeneraas defining the Southwestern Province, these areas would not be
Sonorella. Bequaert and Miller also included, as distinguishing genera, Chaenaxis and Eremarionta in the western and Humboldtiana in the eastern part of the Province.
Holospira s.l., Ashmunella, Radiocentrum, and Humboldtiana have been characterized, herein, as components of an ancient Cordilleran fauna, formerly more widespread towards the north, and n o w restricted to the southern part of their former range. This southern part includes much of the area