Page 91 - Land Snails of New Mexico

Mexico,insofarasitimpliesapopularimage ofvast,featureless, grassy plains. Although such expanses dominated by grasslands do exist,theyprobablywereneverashospitabletomolluscan life as were scattered aquatic-related situations and areas of more broken terrain within the general Plains region.
There isabundantevidenceindicatingexistenceofaquatic
habitats on the High Plains during the Pleistocene. This is to be
expected, given the lack of external drainage where the Plains
are undissected, and assuming an enhancement of effective
precipitationduringPleistocenetimesequivalenttoglaciations.
Certain terrestrial species must have flourished in habitats
marginal to marshes, pools, ponds, and small lakes. Vegetation
aroundsuchbodiesofwaterprobablyincludedrushes,sedges,
cattails, a n d s o m e p o o l s i d e s t a n d s o f d e c i d u o u s trees. I n fact, t h e
majority of the localities summarized in Table 1 seem to have
been associated with aquatic habitats. This is shown clearly by
the presence of aquatic snails in association with terrestrial snails. Of the 43 localities indicated from northeastern New
Mexico in Table 1, 36 contain aquatic species. The aquatic, planorbid snail Gyraulus parvus occurs at 16 localities, suggesting that pools with aquatic vegetation, favored by this species,werepresent.Ofthe31 localitiesfromeast-centralNew Mexico (Table 1), 16 contain aquatic species, and of the 11 localities from the southeastern part of the state, 5 have aquatic species.
Terrestrialspeciesthatwere associatedmost likelywith such aquatic habitats include Vertigo ovata, V. milium, Gastrocopta pentodon form tappaniana, Hawaiia minuscula, Oxyloma sp., and Deroceras laeve.
Referring to the terrestrial species obtained from the BlackwaterDraw andAndersonBasinlocalitiesinRoosevelt
Co. (Table 1),Drake (1975:203) noted that "damp environments are indicated by practically all the forms of terrestrial snails determined...." Preferencefordamp habitatsisindeedtypical ofsome ofthe commonest species in Drake's assemblages, such as Carychium exiguum, Gastrocopta pentodon, form tappaniana, and Vertigo ovata, as well as Vertigo milium and Deroceraslaeve,whichwerelessabundant.Morethan200 specimens of the tappaniana form of Gastrocopta pentodon were taken in each of two assemblages. (See discussion of Gastrocopta pentodon in Metcalf and Smartt, this volume.) In another assemblage, 598 specimens of Gastrocopta holzingeri were recorded, suggesting a microhabitat at this stratigraphic level,which was especiallycongenialforthisspecies.
AtthestratigraphicleveloftheLubbock Lake siteindicated in Table 1 (Pierce, 1987), which quite clearly was marginal to an aquatic habitat, the most common species were Vertigo ovata, Gastrocopta pentodon form tappaniana, and Hawaiia minuscula, with the first species comprising 37% of the specimensrecoveredandtheothertwo, 12% each.Thesethree species were also moderately common in the spring-related d e p o s i t s s t u d i e d b y A s h b a u g h a n d M e t c a l f ( T a b l e 1). N o d o u b t , aquatic habitats were numerous, although of scattered occurrence during Wisconsin times on the High Plains, but most have disappeared during the Holocene. Snails associated with such habitats m a y have had an advantage in dispersal provided by the transportation services of aquatic birds visiting the scattered prairie ponds and marshes, which are inferred as
havingexisted. -
Topographically, breaks in the High Plains surface have
been produced in various ways. Although the High Plains surface itself, formed on the Miocene-Pliocene Ogallala Formation, is internally drained, there are a number of stream systemsthattraversetheeasternplains,incisingvalleysflanked by bluffs with rock outcrops, and supporting woody vegetation (often the shrub Rhus trilobata). In northeastern N e w Mexico, there are extensive areas in which basaltic lava deposits surmount the High Plains surface and support different plant associations and assemblages of land snails than do the plains elsewhere.Also,notfardistantinmuchofnortheasternNew Mexico, higher mountains and mesas often arise from surroundingplains,withtheMesa deMaya andSierraGrande in Union Co. being prime examples. In general, there is an
altitudinaldecreaseinnumber ofspecies,which can be observed today, grading from montane to lower bluffs and valley habitats. Fossil evidence indicates fluctuation of these habitat zones up and down in response to Quaternary climatic changes. Such a patternoflife-zonedepressions and elevations can be visualized best in this part of the state. Apropos of these altitudinal fluctuations, involving eastern outliers of the Cordillera, the followingobservationsseem germane.
For purposes here, the High Plains (Fenneman, 1931:11) fromNebraskatoTexasandNewMexicomightbeconsidered a s fitting w i t h i n a n e l o n g a t e a r c h , w i t h its a p e x t o w a r d s t h e n o r t h and its two legs towards the south. During times of glaciation related cooling, floral and faunal elements seemingly dispersed inward from various parts of the periphery of the arch. Such a model was devised by Wells and Stewart (1987) for the High Plains ofKansas and Nebraska. They referred to taxa dispersing from the western leg of the arch as "cordilleran," those from the eastern leg as "eastern deciduous forest," and those from the northernapexas"boreal." Utilizingthesedistinctions,they pointed out arboreal, land-snail, a n d m a m m a l i a n taxa that likely
dispersed inward onto the High Plains, from one or more of
thesesources.Some taxawereascribedtoonlyone source,some
totwo,andafewconstruedasconvergingfromallthreesides and termed "Cordilleran-boreal-eastern deciduous forest"
species.
Pierce (1987:43) categorized fossil land snails occurring at
the Lubbock Lake site on the Texas High Plains (=Llano
Estacado) in a similar manner, referring to some species as "northern or montane" and to others as "eastern or northeastern."
I n t e r m s o f s u c h a n a r c h m o d e l , it s e e m s l i k e l y t h a t m o s t o f thespeciesthatdispersedontotheeasternplainsofNew Mexico during times of Pleistocene glaciations were derived from the Cordilleran part of the arch. This seems especially likely in northeastern N e w Mexico where mesas and isolated mountains
surely must have acted as routes and stepping-stones during times of dispersal (glaciations) and as refugia for relicts during timesofretrenchment(interglaciations).Probablythiswas also the case farther south in N e w Mexico. Overall, there was a marked diminution in numbers of species southward. Thus, in the three studies of A. B. Leonard cited above, I interpret the following numbers of species as being indicated: 24 from the northeastern, 19 (or 18) from the east-central, and 10 from the southeastern plains of N e w Mexico. Species of succineids are
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