Page 82 - Land Snails of New Mexico

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the Cretaceous, had been covered by the epeiric sea.
As described by Dickinson et al. (1988:1024), the entire Cordilleran foreland in the central Rocky Mountain region was
"brokenupintodiscretelocalbasins,"whichwereseparatedby "strongly emergent" uplifts. As these Laramide highlands developed, itislikelythat they gradually became populated with land snails, probably predominately by nearby western, Cordilleran colonists.
Uplifts degraded to contribute sediments (variously of latest
Cretaceous, Paleocene, and Eocene age) to nearby basins,
thereby leaving indirect evidence of their existence preserved in
basin fill. In N e w Mexico, both larger and smaller basins
existed,withthemore significantbeingnortheasttosouthwest:
the Raton, Galisteo-El Rito, San Juan, Carthage-La Joya, Baca,
Sierra Blanca, and Cutter Sag-Love Ranch Basins (Lucas and
Ingersoll, 1981). S o m e of the sediments filling these basins are fossiliferous.TheRatonBasinisnotableforitsPaleocenefloral
(leaves and pollen) fossil record. Fossil aquatic snails have been reported from Paleocene beds of the Raton Basin and both aquatic and terrestrial snails from the Paleocene of the San Juan Basin (Cockerell and Henderson, 1912, Cockerell, 1914, Henderson, 1935, Hartman, 1981a).
In north-central N e w Mexico, the broad Brazos-Sangre de Cristo uplift of Laramide age occupied areas approximately located where ranges bearing those same names occur today. Later the Nacimiento-Gallina-Archuleta uplift arose to the west and partitioned offthe eastern part of the former San Juan Basin toform the Galisteo-El Rito Basin (Ingersoll et al., 1990:1288). These extensive uplifts already may have harbored the beginnings of what would become the Rocky Mountain molluscanfaunalelementinnorthernNewMexico.However, the fauna of such interbasinal uplifts is not known.
PALEOGENE
In the Paleocene and Eocene, the landscape of western and
northernNewMexicoseemstohavebeendominatedbythe
upliftsand basins produced by Laramide deformation. Although
thepersistenceofmountainshasbeenstressedherein,itistobe
supposed that they rose from low base levels in the earlier
Paleogene. A t the beginning of the Paleocene, easternmost N e w
Mexico, having emerged above the sea not long before, still
wouldnothaverisenmuch abovesealevel.Itseemslikelythat lowlands and basins elsewhere in the state were also at low
Basins ofthePaleocene in a grouping of "swampy (coal) basins and broad floodplains at or close to sea level" supporting vegetation of tropical to subtropical requirements, although he foundsomeevidenceformorexericfloralcommunitiesinhillier areasawayfrombasinsandfloodplains.Sloan(1994:882)used data from various reports to derive an estimated elevation of the Paleocene-Eocene Green River Lake system in the Utah Wyoming region of 300 m (985 ft). He estimated that surrounding highlands could have reached an additional 500 m inheight,or800m (2,625ft)abovesealevel.
Berggren and Prothero (1992:5) presented evidences for "an anomalously w a r m global climate o p t i m u m spanning s o m e 4-5 m y during the early Eocene.", which was followed by a "gradual, stepwise cooling during the remainder of the Paleogene, . . ." From botanical evidence, Wing et al. (1991:1191)assessedearlyEocene(59-50Ma) climatesofthe BighornBasinofnorthwesternWyoming aslikelytohavebeen the warmest of the Cenozoic, with estimates of annual average temperaturesduringthis9-million-yearperiodrangingfrom 13° to 18°C. In the strata studied, tree ferns, palms, and cycads occurred along with other cold-sensitive plants.
From palynologicalstudiesintheRockyMountainregion,
Leopold and MacGinitie (1972:166-167) recognized two moist
and two dry subtropical floral phases encompassing most of the
Eocene, with a development in the latest Eocene of a dry phase
associated with an "essentially temperate flora." A tropical,
humid, forested environment has been suggested as
characterizing the Eocene San Juan Basin of northwestern N e w
Mexico,basedonflorasandfaunasoftheSanJoseFormation
(Tidwell, et al., 1981; Smith and Lucas, 1991). Fossil palm leavesoccurintheEocenePalmParkFormationinnorthern
Doña Ana Co.
The aboveassessmentssuggestthatlandsnailsinhabiting
Paleocene and early-to-mid Eocene basins must have enjoyed climates like those of present tropical or subtropical lowlands. Inhabitants of nearby uplifts are not known, nor isthe nature of
the climatic conditions that they encountered at these higher elevations.
ThePaleoceneandEocenefaunallistofNewMexicoland snails is scant. Several taxa of terrestrial and freshwater
gastropods were described and reported from the Paleocene of northwesternNewMexicointhelate1800sandearly1900s (White, 1883,1886; Cockerell and Henderson, 1912, Cockerell, 1914, 1915). Hartman (1981a) identified the four localitiesfrom which these earlier collections were obtained as being in SandovalCo.,onenearCubaandthreeintheareaofTorreon. These and subsequent collections of Paleocene snails seem to have been derived from one mollusk-bearing facies that occurs
elevations. In the western United States, low regional elevation,
among other factors, probably contributed to the climatic
warmth thatgenerallyisascribedtotheearlyPaleogeneup tothe
mid or lateEocene. By the latestEocene there are evidences of
increasedregionalelevation,asnotedbelow. intheNacimientoFormationoftheTorreonianland-mammal
Paleocene-Eocene Biotas
Studies of leaf physiognomy by Wolfe (1978) indicated a Paleoceneinwhichtropicaltoparatropicalrain-forestvegetation occurred widely in North America. Axelrod (1975:291) interpreted the Paleocene climate of the southern United States, in general, as having been mild, frostless, and monsoonal with asummerrainyseason.HeincludedtheRatonandSanJuan
age of the Paleocene in geomagnetic anomaly zone 27 normal, dated at62 M a (S. G. Lucas and T. E. Williamson, pers, comm., 6 July 1992). Hartman (1981a:Table 1) provided a useful summary of the species that were reported from the Nacimiento Formation in the early publications, cited above. Some comments concerning taxa comprised in Hartman's listfollow:
Helix adipis White, 1886, and Helix chriacorum Cockerell, 1914, are small shells of uncertain affinity. Solem (1979:283) suggested that H. adipis might be a helminthoglyptid and that H.