Page 78 - Land Snails of New Mexico

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he delimited southward along the Utah-Arizona and Colorado N e w Mexico borders.
Pilsbry (1948:XL-XLI) adopted (see Fig. IB) the basic terminology and boundary definitions of Henderson (1931). However, he suggested existence of a Texas Province, without defining its boundaries. Pilsbry extended Henderson's SouthwesternProvinceeastwardtoincludemostofTrans-Pecos
Texas, and southward into México to incorporate most of Sonora and Chihuahua. South oftheSouthwestern Province, he mapped aMexicanPlateauProvince(1948:Fig.1).InNewMexico, Pilsbry showed the Eastern Division as extending less far to the west than in the rather generalized map of Henderson (1931:Fig.1).
Bequaert and Miller (1973) also retained the basic scheme
ofHenderson(1931,Fig. 1C),recognizingaWesternDivision
thattheysubdividedintoOregon-Washington, California,Rocky
Mountain, and Southwestern Provinces. However, the boundary betweentheCaliforniaandSouthwesternProvinceswasmoved
back to the west to include, in the latter province, arid
southeastern California, southernmost Nevada, and the
northwesternmost part of the Mexican State of Baja California
Norte. They assigned (p. 7) "northeast N e w Mexico" to the
Rocky Mountain Province. Thus, in the latest application of the
by-now traditional land-snail zoogeographic subdivisions, N e w
Mexico is partitioned among the Eastern Division and the
Southwestern and Rocky Mountain Provinces of the Western Division.
Further consideration of the applicability of these divisions andprovincestoNew MexicoisundertakenintheDiscussion section, hereafter.
THE LATER MESOZOIC
In attempting to trace a history of the land snails of N e w Mexico, itisprobablyjustifiabletospeculateaboutconditions as far back as the Mesozoic. Although there are almost no Mesozoic land snails reported from the state, there are Cretaceous records from other western states and Canadian
provinces, and some of these pertain to families presently or formerly found in New Mexico. It is likely that the long continued geologic activity of the Cordilleran region, especially theeffectsof 1)asuccessionofmagmaticarcs,alreadyactive in the Mesozoic, and 2) the presence of Cretaceous epicontinental (epeiric) seas has played a major role in determining the nature and distribution of the land-snail fauna of the general region, and of N e w Mexico in particular.
Jurassic Mountains ofArizona
A l r e a d y d u r i n g m o s t o r all o f t h e J u r a s s i c P e r i o d , a m a g m a t i c arc crossed Arizona south of a northwest-southeast trending line from Parker to Tucson to the southeastern corner of the state, according to Tosdal et al. (1989:Fig. 4). These authors (p. 428) interpreted magmatic activity in the early Jurassic to have formed"avolcanicprovincecomparabletotheNeogene Central AndesofSouthAmericaortheTertiarySierraMadre Occidental of Mexico." Busby-Spera (1988) compared the early Jurassic arc tothatofpresent-dayCentralAmerica.A paleolatitudinaland
climaticmodel ofDickinson (1989:Fig. 9) indicatesthatArizona wouldhavebeenatapaleolatitudeofsome20°N intheEarly Jurassic, or at about the latitude of the present Transverse Volcanic Belt of México, which may be somewhat analogous, biotically, to these early Jurassic volcanic mountains. Tosdal et al. (1989:428) found evidences of transport and deposition of clastic sediments derived from these mountains by "streams or rivers," which suggests that the mountains received considerable precipitation, and were likely mesic and forested, especially on theirPacificcoastalside.
Further evidence of Jurassic mountains is provided by Lipman and Hagstrum (1992), who discussed four volcanic calderas in southeastern Arizona, three of which are in the H u a c h u c a M o u n t a i n s area, a n d s e e m to s p a n a t i m e f r o m m i d - t o late Jurassic. The calderas were large (one being 8 x 16 km), produced extensive ash-flow sheets, and some may have produced resurgent domes. These and other evidences suggest volcanic mountains of considerable height. Probably, long after their eruptive stages, they continued to produce a mountainous landscape.
It is not known what kinds of land snails might have inhabited such Jurassic mountains, in Arizona or elsewhere
along the Jurassic magmatic arc. However, given the existence of such a fauna, it seems likely that there could have been ancestral links between it and the Cordilleran fauna that is
known from Cretaceous fossils. Perhaps there already existed ancestral Jurassic roots of faunas that eventually would be describedastypicalofthemodern Southwestern, California,and Oregon-Washington Molluscan Provinces, noted above. The lattertwoprovinces,intheirmaritimelocations,more likelymay have conserved elements of ancient, Mesozoic Cordilleran faunas than did areas of the Cordilleran interior, with less equable climates.
EarlyCretaceous:SouthernNewMexico and Southeastern Arizona
In the latest Jurassic and continuing on into the Early Cretaceous, southeastern Arizona and southern N e w Mexico
were affected by a kind of geologic activity, different from that discussed above. A t that time, rifting, related to the opening of the Gulf of Mexico, extended northwestward therefrom, forming the Chihuahua Trough. Seager and M a c k (1986:671) discussed Late Jurassic rocks of marine origin in the subsurface in the Las Crucesarea,indicatingthatbythenan arm oftheGulfofMexico had already extended up the Chihuahua Trough to southern N e w Mexico. In the Early Cretaceous, this sea extended across southernNew MexicofromnearLasCrucestotheBigHatchet Mountains and on into southeastern Arizona, where its northwesternmost extension occupied the much-studied Bisbee Basin. Mack et al. (1988:136,140) referred to the "Chihuahua/Bisbee basin" as being an "Early Cretaceous rift basin,"bordered on the north by a riftshoulder, a feature defined and discussed by Bilodeau and Lindberg (1983). The rift shoulder was a source of sediments deposited in the riftbasin to the south. The nature of these sediments indicates presence of exposures of Paleozoic sedimentary and Precambrian crystalline rocksalongtheriftshoulderhighlands(Macketal.,1988:140).