Page 92 - Land Snails of New Mexico

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combinedasone taxon,andPupoideshordaceusiscombined withPupoidesinornatus.TherecordsofPupillasyngenes may well pertain to Pupilla blandi or P. muscorum. This pattern suggests that dispersal onto the eastern plains w a s less m a r k e d towards the south. However, even in the south a Cordilleran source for such dispersal was likely dominant, although the e a s t e r n o u t l y i n g m o u n t a i n s a n d h i g h l a n d s t y p i c a l o f n o r t h e a s t e r n New Mexico were lacking, except for the relatively low MescaleroEscarpment,
Althoughdistancesfrommountainstoplainswerenotgreat inNew Mexico,nevertheless,itisclearthatsomemovement that could be termed as "dispersal" did take place, however m o d e s t it m a y h a v e b e e n . O n e m i g h t v i s u a l i z e c e r t a i n s p e c i e s a s extending their range from montane relictua onto the plains in response to climatic and vegetational changes that produced conditions m o r e favorable to t h e m at l o w e r elevations, only to g o extinct there as these conditions reversed. Such species might conveniently be termed "dispersalist" and could include Pupilla blandi, P. muscorum, Vallonia cyclophorella, Discus whitneyi, Euconulus fulvus, Nesovitrea hammonis electrina, and possibly Gastrocopta armifera and Helicodiscus eigenmanni. Some or most of these species m a y have been adapted especially to the scattered aquatic-marginal habitats, discussed above, and aquatic birds might have been involved in dispersal.
Since almost all species that occurred on the eastern plains ofNew MexicointhePleistocenealsooccurredintherelatively nearby Cordillera to the west, it is difficult to m a k e a case for m u c h dispersal from the eastern leg of the arch described above. However, Strobilops texasianus, reported by Hester (1972:Table 7) from the Blackwater D r a w Locality, Roosevelt Co., is a southeastern species, which extended onto the High PlainsasfarwestaswesternKansas,Oklahoma,andtheTexas Panhandle during the Pleistocene (Hubricht, 1985:Fig. 104). The distributionalpatternofValloniaparvulasuggeststhatit dispersed onto the High Plains from the east. Ithas managed to persisttothepresentasa component oftheHigh Plainsfauna of northeastern New Mexico and sparsely in the Sacramento Mountains. If Helicoduscus parallelus is, in fact, a species distinct from H. eigenmanni, its occurrence in Wisconsin age depositsinnortheasternNew Mexico(Table1)mightrepresent another species of eastern origin.
Inabroader,longer-termsense,otherspeciesmay traceback to more eastern origins. Henderson (1931) and Bequaert and Miller(1973)placedtheeasternplainsofNew Mexicoatthe western margin of their Eastern Division of zoogeographic regions pertaining to North American land snails. In terms of ultimateoriginsasfarbackasLateCretaceoustime,thismay be a reasonable assignment, because some of the present or Pleistocenespeciesoftheplainsprobablyhadancestralrootson the eastern side of the Cretaceous epeiric sea. However, during the long intervening period of Tertiary time, there surely has been a history of distributional complexity that is probably
impossibletounravel.Someeasternspeciesmayhavedispersed westwarddirectlyacrosstheHighPlainstoNew Mexico, perhaps using riparian corridors, and established themselves in theCordillera.Some may have enteredtheRocky Mountains at more northerlylatitudes(asmembers ofthe"boreal"element discussedabove)anddisseminatedsouthwardtoNew Mexico,
comprisingpartofa"RockyMountain"fauna.Inbothcases, suchtaxanowwouldbeconsideredCordilleranintermsofthe
arch model proposed above. In some cases such Cordilleran species might be considered as consubspecific with eastern relatives, or they might have speciated into different subspecies orsiblingspecies.Repeatedinvasions,orpossibleintrogression b e t w e e n still c l o s e l y r e l a t e d s p e c i e s , c o u l d h a v e g i v e n r i s e t o taxonomic complexities, perhaps reflected in the difficulty experiencedinmakingspecificallocationforsome snailstreated herein.Examplesthatmightapplyhereinclude: 1)Gastrocopta pilsbryanaandG.pentodon(notincluding"form"tappaniana), 2) the Gastrocopta armifera, G. ruidosensis, and G. abbreviata c o m p l e x ; 3 ) s o m e p o p u l a t i o n s o f l a r g e r v a l l o n i a s , v a r i o u s l y assignedtoValloniagracilicosta,V.cyclophorella,V. albula, and V. sonorana; 4) Punctum minutissimum and P. californicum, and 5) Helicodiscus parallelus and H. eigenmanni.
DuringtimesofPleistoceneglaciationswhen theHigh Plains environment seems to have been generally more hospitable to land snails, dispersal from various parts of the proposed arch onto the plains must have produced something of a genetic mixing bowl, as populations with varying degrees of interrelationshipprobably came into contact with each other. In
a historical zoogeographic context, the situation seems to be highly complex rather than showing a simplistic east-to-west dispersalpattern.
Q U A T E R N A R Y FOSSILS F R O M BASINSAND MAJOR REVERVALLEYS
The intermontane basins and major river valleys of N e w M e x i c o share with the eastern plains (just discussed) relatively low elevation, low gradients, and similarity in regard to fossil faunas. Similarity is especially close between the eastern plains andthecontiguousPecosRiverValleyofsouthernNew Mexico.
Pecos River Valley
Although the Pecos Valley is well demarcated from the eastern plains in Chaves Co. by the Mescalero Escarpment, the distinction becomes less obvious to the south as the escarpment diminishes and disappears in southern Eddy Co. Leonard and Frye (1975) and Leonard, Frye, and Glass (1975) included a number of localities from the Pecos Valley in their studies, respectively, of east-central (14 localities) and southeastern (10 localities)New Mexico.Inthesestudies,mostspeciesoffossil land snails reported were shared by both the eastern plains and the Pecos Valley. Localities on the adjacent, eastern plains, treatedinthesepapers,areindicatedinTable 1.Derivedfrom the two studies,Table 2 indicatesnumbers oflocalitiesinthe Pecos Valley from which species were taken, as well as species and numbers of specimens reported by Henderson (1917) from a fossiliferous deposit along Spring River near Roswell.
The headsofwesterntributaryvalleysleadingintothe southernPecosValleyareinthefoothillsandhigherreachesof the Guadalupe, Sacramento, and other mountains to the west. This western, montane influence seems reflected in some Pleistocenefossilassemblagescollectedalongsuchtributaries. Speciesandnumbersofspecimenstakenatseverallocalities