Page 105 - Land Snails of New Mexico

reasonabletosupposethatthefirstandmajorcoloniststomove
into the tabula rasa that was produced came from the west.
Although relatively narrow, Cordilleran North America west of
the epeiric sea was a region of varied physiography, which
relatedtocontinuedmagmatic-arcactivities.Thiswas MioceneJaconafaunaofnorthernNewMexico,noneofthe
presumably also evidenced in varied climatic conditions and in
the environments of biotas. In the coastal states and provinces
and in the fold-thrust belt west of the epeiric sea, mountains
seem likely to have provided long narrow pathways offering
travel opportunities northward and southward from the
southernmost extension, noted above, to Alaska and, presumably
at times, beyond to northeastern Asia. All of this suggests a
fertile mix of land-snail groups comprising an ancient
Cordilleran fauna, during the latest Cretaceous and earliest
Tertiary,from which were drawn western propagules spreading into N e w Mexico.
After recession of the epeiric sea and imposition of any
vicissitudes caused by a terminal Cretaceous event or events,
there may have survived a relatively uniform "ancient Cordilleran fauna" in western North America. As discussed
a b o v e , fossil r e c o r d s o f t h e P a l e o c e n e a n d E o c e n e s h o w f a m i l i e s such as the Helicinidae, Urocoptidae, Bulimulidae, Camaenidae, and Humboldtianidae and the genera Ashmunella and Radiocentrum existing much farther north than they do at present (Wyoming and beyond) in the Rocky Mountains region. Itseems likely that a fauna ranging from tropical to subtropical affinity existed in the Cordilleran region of the United States (especially at lower elevations) during the Paleocene and most of the Eocene. Probably, diversity would have been greater towards the more tropical south ifpresent patterns prevailed in which there is greater biodiversity towards the tropics.
During the remainder of the Tertiary, these halcyon conditionsmore orlesssteadilydeteriorated,northtosouth,so that groups requiring warmer conditions were progressively eliminated in a north-to-south pattern. Climatic change in the r e g i o n m a y h a v e b e e n c a u s e d b y v a r i o u s f a c t o r s , b u t it s e e m s that increased regional elevation, evidenced already in the late Eocene, as atFlorissant,Colorado, probably played a major role.
As summarized by Lucas (1983:187), "The Eocene Oligocene transition in N e w Mexico was a time of significant changeinmagmatism andsedimentationthroughoutthestate." BetweenlateEoceneandlateMiocenetime,themountainous aspect of N e w Mexico was accentuated as a result of widespread volcanism, regional uplift, and earlier phases of Basin and Range-like extensional activity. Paleobotanical evidence indicates a related climatic cooling with a long-term pattern of decrease in tropical and subtropical floral elements, and an increase in plants adapted to cooler and more xeric conditions. There is evidence also of the existence of high-elevation montaneforestsofmodernaspectinNewMexico.Itislikely that a similar situation prevailed in regard to land snails with, as suggested above, the more tropical species that were stillpresent in the Eocene becoming increasingly extirpated as a wave or waves of extinction involving ancient Cordilleran taxa progressed in the region, north to south. Urocoptids, bulimulids, and Radiocentrum, which formerly had ranged much farther north, must have succumbed gradually, north to south. Evanoff etal.(1992) recorded Ashmunella and the humboldtianid genus
more tropical indicators is present.
InNew Mexico,atpresent,severalgeneraoflargersnails deemed "ancient Cordilleran" reach their northernmost limits.
These limits might be considered lines of retrenchment
southward. Such groups include Humboldtiana and
Radiocentrum, relegated only to the extreme southern part of the
state, urocoptids and bulimulids, which are holding out in the
southern one-third of the state, and Ashmunella, which still
occurs in the southern three-fourths. These groups have been
considered mainly as defining a Southwestern Molluscan
Province, which is reasonable and practical. However, as
employedhere,anotherway ofcharacterizingthesetaxamight
b e i n e m p h a s i z i n g t h a t t h e y a r e a n c i e n t C o r d i l l e r a n g r o u p s , still
lingering in what m a y be the southern part (and quite possibly
the original part) of their former domain--hence, "southwestern"
by virtue of shrinkage of their range. Hochberg et al. (1987) traced a distributional restriction southward of Radiocentrum
since the Paleocene, and suggested a similar pattern for the Urocoptidae. Probably, extirpation of an "ancient Cordilleran fauna" southward was more marked in inland areas (Montana, Wyoming, etc.) than in the coastal regions, where descendants of an ancient Cordilleran fauna might still be considered predominant.
T h e r e s e e m s to h a v e b e e n a trickle o f dispersalists, a l m o s t all of smaller land-snail species, which entered the northern Cordillera from Eurasia, perhaps even as representatives of the Cordilleran fauna were being decimated in the north, and perhaps in response to the same conditions. That is to say that cooling to the north could have produced conditions inimical to Cordilleranspecies,butinvitingtosome Palearcticimmigrants. Such invasionsprobablyoccurredatvarioustimesduringthe Cenozoic, just as they did in the better-documented cases of woodyplantsandvertebrates.Some taxaprobablyenteredthe northern Cordillera from Eurasia during times when Asiamerican Miocene mesophytic forests existed in the far north. O t h e r s , w h i c h still a r e c o n s p e c i f i c w i t h P a l e a r c t i c t a x a , p r o b a b l y arrived during times of Pleistocene accessibility, as in the case ofsome species offish and mammals. Pilsbry (1948:XLII) and Bequaert and Miller (1973:64–74) have listed a number of species with Holarctic distributions, most of which seem to be of Palearctic origin.
With desiccation of the Cretaceous epeiric sea, access to the area that ithad covered was opened to the fauna of adjacent eastern North America. However, the Great Plains, occupying muchofthesameregionashadtheepeiricsea,nodoubt continued to act as a barrier of sorts between eastern and
western faunas. This was suggested also by Henderson (1931:186), who supposed that "the present unfavorable conditions in a broad area east of the Rocky Mountains, which now preventtheminglingofthefaunas,haveexistedwithbut littlechangesinceEocenetime." Thereislittleevidencethat elements of an ancient Cordilleran fauna were inclined to
venture eastward onto the plains, except for an invasion of
Skinnerelix(ofEvanoffandRoth, 1992)asoccurringasfar north as Wyoming in the late Eocene (Chadronian), indicating thatAshmunella and the Humboldtianidae, also, have undergone a pattern of progressive extirpation, north to south. In the
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