Page 81 - Land Snails of New Mexico

character. That is, Tertiary magmatic, tectonic, and volcanic activitieswere to produce an environmental and biotic diversity muchincontrasttothatofthePlainsregion,whichdevelopedto theeast.New Mexico, locatedastridetheCordilleran-Plains interface,surelywascolonizedbybothwesternandeastern immigrants attracted by the tabula rasa presumably produced u p o n progressive desiccation of the epeiric sea.
A s has been recognized in various faunal and floral groups, in Sundering North America into eastern and western parts, the C r e t a c e o u s e p e i r i c s e a a c t e d a s a m a j o r i s o l a t i n g b a r r i e r b e t w e e n biotas. Leopold and MacGinitie (1972:148-149) discussed this in regard to floras. This ancient allopatry, east and west of the epeiricsea,may bereflecteddimlyinpresentfaunas,although Cretaceouseffectshavebeenobscuredbysome65Ma of subsequentCenozoichistory.Theeasternpartoftheseawasto be replaced in the Cenozoic by the Great Plains, which, in their t u r n , c o n t i n u e d t o a c t a s a b a r r i e r t o e a s t e r n a n d w e s t e r n p l a n t s andanimalsthatwereunabletoadapttothePlainsenvironment.
Henderson(1931:185)andPilsbry(1948:XLV)relatedthe existence of the epeiric sea to origin of Eastern and Western Divisions in the North American land-snail fauna, with this being variously reinforced by subsequent development of the Great Plains. A s noted, these authors allocated easternmost N e w Mexico(intheGreatPlains)totheirEasternDivision,andthe western, Cordilleran part of the state to a Western Division, Henderson (1931:186) took special note of the role of the Great Plains as a continuing isolating mechanism between eastern and western faunas.
Although diversity of the Plains fauna has fluctuated in the
past,asdiscussedhereafter,itseems nevertohavebeen asrich
asthatinCordilleranNewMexico,inwhichmanygroupshave western rather than eastern roots. This could be termed an
"Ancient Cordilleran Fauna," and probably included most of the taxa (families or genera) indicated as distinguishing the present Southwestern Molluscan Province (Henderson, 1931; Pilsbry, 1948, Bequaert and Miller, 1973).
Cretaceous Termination
It is probable that a process of post-epeiric sea recolonizationofNew Mexicobylandsnailswaswellunder way, from west to east, at the termination of the Cretaceous.
Current models strongly support the hypothesis of a terminal eventcaused by one or more asteroid or comet-related impacts, one of which impacted as near as the Yucatan region (Hildebrand et al., 1991; Swisher et al., 1992). It is not k n o w n what effect such an impact might have had on land snails of the western United States; however, palynological studies provide information concerning the plants with which snails presumably were associated. Tschudy and Tschudy (1986) analyzed pollens occurringacrosstheK/T boundary atoutcropsinMontana and WyomingandintheRatonBasinofColoradoandNew Mexico. They concluded (p. 670): "It is clear that the Western Interior terrestrialCretaceousflorareceivedaprofoundecologicalshock atthe KIT boundary." However, despite indications of "massive devastation of plant life," they found no evidence in the western interior region of "total extinction of any key Cretaceous genus.
..". T h e y a s s u m e d t h a t t h e r e w e r e r e f u g i a , w h i c h e n a b l e d m a n y
plant species to survive. Perhaps a similar scenario applies to landsnails.InsofaraslandsnailsofK/Ttimewereabletoenter
dormancy, asdo many modern species,thiscouldhavepre adaptedthemforsurvivingan"impactwinter"ofsomeweeksor months. Wolfe (1991) has suggested, on the basis of paleobotanical evidence from Wyoming, a major bolide impact in early June and a resulting impact winter of less than 2-months duration. Ifthiswerethecase,andifNewMexicohada subtropical climate with a spring-to-early-summer dry season l i k e t h a t o f m u c h o f M é x i c o t o d a y , t h e n l a n d s n a i l s still m i g h t have been in dry-season dormancy at the time of impact.
Tschudy and Tschudy (1986:668) noted that, in sections analyzed in their study, there was a zone above the KJT "boundaryclay"thatyielded"abundantsapropelandfusinite suggestiveofdecayingvegetationandfire." Sheehanand Fastovsky (1992:558) have discussed forest-soil food chains b a s e d o n d e t r i t u s c o m p o s e d o f l e a f litter, d e c a y i n g w o o d , r o o t s , andbranches,andonassociatedfungi.Among groupsutilizing such food resources, they included the gastropods. These authors, and Sheehan and Hansen (1986), contended that detritus feeders probably enjoyed an advantage over those animals, which fed on living plant materials at the time of the terminal Cretaceous event. They pointed out that the death of plants and animals actuallymay have temporarily increased amounts offood available to detritvores. Land snails might have found that dead vegetation or fungi attracted to decaying plant materials were a source of life-sustaining nutrients during a time of scarcity of living plant food. It is apropos to note Solem's (1979:286) suggestion that ever since the late Paleozoic "Land Snailspresumably kept contentedly chewing away on dead plant matter." Attheleveloffamiliesoflandsnails,atleast,there seemtohavebeennoidentifiableextinctionsattheendofthe CretaceousintheCordilleranregion,andsome generareported from the Upper Cretaceous also are recorded in the Tertiary.
LARAMIDE COMPRESSIONAL DEFORMATION
DuringthelatestCretaceousandearlyTertiary(Paleocene to Eocene), there occurred yet another highland or mountain buildingepisodeinNew Mexico,theLaramidedeformationor orogeny. In Dickinson's model (1989:8-9) the magmatic arc, which had been located along the western edge of the continent during most of the Cretaceous, migrated eastward during Laramide time. This is inferred to have been caused by a flattening of the angle of dip of the subducted Farallon slab, so that melting occurred where the slab penetrated the asthenosphere. Dickinson (1989:9) indicated that Laramide activityinsouthern Arizona involvedfolding, thrust-faulting, and general regional uplift, beginning about 80-75 Ma. The arc spread eastward, south and east of the Sevier fold-thrust belt, into Wyoming, Colorado, and N e w Mexico. Dickinson et al. (1988:1030,1032) suggested that Laramide deformation in southernColoradoandnorthernNew Mexicobegan75-65Ma, andterminated40-35Ma.InNewMexico,themagmaticarc spread approximately as far eastward as where mountains occur today.Inthenorthernpartofthisregion,theLaramideorogeny initiatedbuildingofantecedentsofthepresentRocky Mountains, which, in a dramatic turnabout, rose in a region that earlier, in
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