Page 86 - Land Snails of New Mexico

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mountains. A search for land-snail fossils in cauldron moat sediments,mightbewarranted,althoughsuchfossilsusuallyare not found in areas of igneous bedrock.
Ingersoll et al. (1990:Fig. 3) depicted similar conditions in
north-centralNewMexico.TheyshowedthatintheOligocene
early Miocene period there were three major volcanic fields
flanking the Galisteo-El Rito Basin: San Juan to the north, Taos
Plateau-Latirtotheeast,andEspinasotothesouth.Here,these
volcanichighlandslargelyreplacedoramplifiedtheBrazos
Sangre de Cristo uplift of earlier, Laramide time. Thus, some
type of highland seems to have persisted during the Paleogene
in north-central N e w Mexico, and presumably harbored a land snail fauna more or less ancestral to that of northern N e w
Mexico today. Oligocenevolcanismoccurredwidelyinotherpartsofthe
Rocky Mountain region in addition to N e w Mexico. Volcanism, regionaluplift,climaticchange,and,probably,otherfactors must have been to some extent interrelated, with this interrelationshipimpingingon biotasintheRocky Mountain region. Leopold and MacGinitie (1972:164) presented palynological evidence for changes in the paleofloras of the Rocky Mountain region from a mid-Eocene analogy with the presentflora of southern México and Central America, an early Oligocene analogy with northeastern México, and a Miocene florawhose"primaryrelationship"waswiththatofthepresent Rocky Mountains. In the Western Interior region, Axelrod (1975) found evidence of an increase in a more xeric-adapted flora dominated by sclerophyll, woody plants from the late Eocene into the Oligocene.
Similarly, the Oligocene w a s probably a time w h e n various groupsoflandsnailsoftheWesternInteriorbecameextinctin the northern part of their range, w h e r e s o m e h a d flourished as far north as Canada during the Cretaceous and Paleocene. This seems tohave been the case with such families as: Urocoptidae, Bulimulidae, Humboldtianidae, and Camaenidae, the more tropical helicinids, and the genera Radiocentrum and
Ashmunella.
Evidence of such a change seems to be provided by studies ofPierceandRasmussen(1992)andPierce(1992),whohave reported land snails from the "Cabbage Patch beds" of southwesternMontana.Thesebedsare,accordingtoPierceand Rasmussen (1992:Fig. 2), assignable to the Arikareean Land Mammal Age,ofthelateOligoceneandearlyMiocene.Land snailsweretakenatlevelsofbothOligoceneandMioceneage. Thesebedsareespeciallyrichinsmallersnails.Inthestudyof Pierce and Rasmussen (1992), 10 species of pupillids were described, including 7 species of Gastrocopta, 1 species of Pupoides (Ischnopupoides), and 2 species of Vertigo. Pierce (1992)describedanew speciesineachofthegeneraVallonia, Nesovitrea, Punctum, Polygyroidea, and Oreohelix, and a n e w succineid.Two newspeciesofDerocerasweredescribed.Some shellswereassignedquestionablytothegenusMonadenia. Roth andEmberton(1994)reportedanotherfaunaofArikareeanage from Montana (Deep River Formation), several families of which were shared with the Cabbage Patch beds (Pupillidae,
Valloniidae, Succineidae, and Oreohelicidae), plus the families Discidae, Polygyridae, and Helicinidae. All these families except thelastpresentlyoccurinNew Mexico.TheselateOligocene
early Miocene faunas differ significantly from earlier Tertiary faunas of the northern Rocky Mountain region, comprising genera that are mostly characteristic of the region today. A s viewed by Pierce (1992:617) "one is struck with the modern aspectofthisfauna." Hesuggestedthattherehadbeena "dramatic and relatively swift" change in the nature of snail faunas and climate between lateEocene and late Oligocene, with acoolerandmore seasonalclimatereplacingasubtropical-to warm-temperate Eocene climate. Roth and Emberton (1994:103) posited a mesic paleoenvironment for the Deep River Formation, with mixed-mesophytic forest. Such a Miocene Holarctic forest has been suggested by various authors, as in Leopold and Macginitie (1972).
Land snails enjoying the warmer Eocene climate in the northern Rocky Mountain region might be considered as representing the "ancient Cordilleran fauna," mentioned above. InNew Mexicothereseemstohavebeenapersistenceof diverseelementsofthisancientCordilleranfaunaforvarying periodsoftime,withseveralperseveringtheretoday,mainlyin the southern part of the state: Urocoptidae, Bulimulidae, Humboldtianidae, Radiocentrum, and Ashmunella. Unfortunately, no Oligocene molluscan fossils have been reported from N e w Mexico. The nearest record seems to be that of a Humboldtiana reported from volcanic tuff deposits of Oligocene age in the Indio Mountains, Trans-Pecos Texas (Underwood and Wilson, 1974). Camaenids and the humboldtianid genus Lysinoe still persisted as far north as
Trans-Pecos Texas inlateEocene toearly Oligocene time (Roth, 1984).ItseemslikelythattheOligocenewasatimewhena molluscan fauna with tropical-to-subtropical affinity was replacedbyonemorelikethatofpresentNew Mexico.Thatis, New MexicohadanearlierPaleogenefaunaofrathertropical aspect but, after the "volcanic smoke" of the Oligocene and earlier Miocene had cleared, a fauna of more modern aspect probably was in place, as exemplified by the Jacona fauna, discussed below.
NEOGENE Miocene
The Oligocene/Mioceneboundaryhasbeenascertainedas being at ca. 23.7 M a by Berggren et al. (1985:1415) and at "about24Ma"byBerggrenetal.(1992:42).
By mid-Miocenetime,magmaticarc-relatedtectonismhad
ceasedinNew MexicoandArizona.Accordingtoamodelof
Dickinson and Snyder (1979) and Dickinson (1989:11), the
absence of a subducted slab and development of the San Andreas transform fault and of lateral shear conditions led to
upwellingofmantlebeneaththeregionthathadbeenaffectedby crustal extension. This resulted in another episode of volcanism (basaltic,bimodal)andofthelate,traditionallyrecognizedphase ofBasinandRangetectonism.Thistectonicactivitybeganby 20 Ma in some areas, according to Baldridge and Olsen (1989:241),whonotedthatinthispresentphaseofBasinand Range development, basins are narrower, more north-south oriented, and bounded by faults with steeper angles than in the earlier basins, noted above. Basin and Range tectonic activity has continued to the present, at which time the results of such