Page 87 - Land Snails of New Mexico

tectonism are seen in m u c h of western N e w Mexico. B l o c k
pupillid shows the apertural denticles of Gastrocopta armifera. A f e w specimens, in which denticles are obscured, have the shell shapeofGastrocoptaproceraorG. cristataandlikelybelong to the subgenus Gastrocopta. A number of shells show the indented outer lip typical of many Vertigos. Several have the sizeandshapeofVertigomilium.Some largerspecimenshave thewideshellandaperturalshapeofVertigoovata,whilesome higher, more slender specimens are similar to Vertigo berryi. These three Vertigos occur in Pleistocene deposits in N e w M e x i c o .
faulting has uplifted rocks of many ages, including those associatedwithsomeoftheprocessesdiscussedabove,suchas Laramide basinfill and earlier-phase Basin and Range volcanics. In north central N e w Mexico, Ingersoll et al. (1990) noted evidence of the initiation of late-phase Basin and Range block faulting and activity associated with the Rio Grande Rift in the early to mid-Miocene period, 21-15 Ma. The Santa Fe and Nacimiento blocks were produced east and west, respectively, of the Española Basin (earlier, the Galisteo Basin). Later, in the period 15-8Ma, the Taos Range became recognizable, north of theSantaFeblockintheareaoftheformerTaosPlateau-Latir
3) Valloniidae. The fauna includes a well-preserved series of Vallonia in the size range of Vallonia cyclophorella and with
Questa Caldera volcanic field. The Taos Range and Santa Fe Block became integrated into the present Sangre de Cristo Mountains.AtthesouthendoftheNacimientoblock,theJemez volcanicfielddevelopedatabout13Ma andcontinuedoninto the Quaternary. In overview, this reconstructed paleogeographical sequence of Ingersoll et al. (1990) indicates persistence of mountains or highlands of one kind or another in north-central New Mexico, from Laramide uplifts through Oligocene-Miocene volcanicfieldstoMiocene and laterblock faulted uplifts, with later ones associated with rifting. Itwould seemthatmontanelandsnailscouldhavefoundsuitablehabitats
the relatively thin lip of that species.
4)Zonitidae.Afew minuteshellsresemble thoseofHawaiia
inmountainsinthisareathroughoutmuch oftheCenozoic--at leastfromthetimeofextensionaldeformationonward.Probable
6) Polygyridae. A number of specimens are of a small species of polygyrid, in which mature shells range from 9 to 11 mm indiameter.Thereisathickened,reflectedlipthatiswell developed; however, in specimens in which the aperture is cleared of matrix, no parietal or lip denticles are seen. On zoogeographic grounds (as in the case of Polygyra petrochlora, discussed above), these fossils might be placed most reasonably in the genus Ashmunella. They are small for the genus, but within the size range of Ashmunella nana Metcalf and Fullington, 1976, a Pleistocene fossil species from the Guadalupe Mountains of Texas.
descendants of such species are components of the fauna of a Rocky Mountain Molluscan Province, as defined by Henderson (1931).
The above highlands flanked, east and west, a well-defined depression thatpersistedfrom Laramide time onward, firstas the Galisteo-El Rito Basin, and lateras the San Luis-Española Basin in which sediments of the Santa Fe Group would accumulate. Ultimately itbecame, structurally, a graben of the Rio Grande Rift and, physiographically, the Rio Grande Valley of today. Patterns of Miocene sedimentation in this basin are highly complex, with sediments derived from surrounding highlands of diverse lithologies. At times, sandy eolian sediments were deposited also. Fossil plants are preserved in some of these Miocene sediments in the Rio Grande Valley, and one unit, the Tesuque Formation, has yielded a fauna of land snails.
7) The most abundant fossils in the assemblage are of a relativelysmalloreohelicid,reachingsome18mm inmaximum width. Sculpture of the apical whorls is visible on a few specimens. The diameter of individual apical whorls is short, and about the same as that seen in specimens of Radiocentrum. However, such short diameters are also typical of shells of the OreohelixneomexicanacomplexofnorthernNewMexico. Well-defined radial ribs of the embryonic whorls, such as occur in Radiocentrum, are not observable. These could have been e r o d e d a w a y , a l t h o u g h it s e e m s m o r e l i k e l y t h a t t h e y w e r e n e v e r present. As observed on a few, better-preserved shells, embryonic shells seem to comprise about two whorls, as in Oreohelix, not the ca. 1.5 whorls comprised in Radiocentrum. O n e well-preserved, small shell displays spiral striae ventrally, as in O. neomexicana and some specimens exhibit spiral undulations on the whorls, dorsally, as in this species. These charactersseem toargueforassignmentofthismaterialtothe
Through the good offices of Dan Chaney, U.S. National Museum, I have been able to examine fossil gastropods he obtained from the Jacona microfauna. Chaney (1988) indicated that this microfauna occurs in the Pojoaque Member of the TesuqueFormationoftheLateBarstovianLandMammal Age, whichisofmiddleMiocene age.The localityisinSantaFe Co. intheRioGrandeRiftValleyofnorth-centralNew Mexico.The whitish matrix in which the shells occur covers the surface of
most shells, fills apertures, umbilici, and hinders identification. Nevertheless, several families of terrestrial snails seem to be recognizable, as follows:
Oreohelix neomexicana/yavapai group rather than to Radiocentrum.
1)Cionellidae.A singleshell,consistingonlyoftheapical threewhorls,seemstobeaCionellaofthesizeofmodernC. lubrica of N e w Mexico.
Lymnaeidfreshwater snails occur with the above terrestrial species indicating a paleoenvironment near water. A warm climatecanbeinferredfrompresenceofpalmwood andfronds in the Pojoaque Member, as well as in the underlying Skull Ridge Member of the Tesuque Formation (Chaney, 1988, Axelrod and Bailey, 1976:238). The latter authors considered the biota of the Skull Creek Member to imply a probably subtropical climate with mild winters. This may have been
2) Pupillidae. Several shells, some with distinctive rib sculpture preserved, are assignable to the Pupoides (Ischnopupoides) hordaceus/inornatus complex, which still occurs in northern New Mexico. One specimen of a large
minuscula. There are other small, discoid shells whose relationships are not clear.
5) Succineidae. There are numerous well-preserved shells of succineids, just as is often the case in collections of Pleistocene agemadeinNew Mexicoatlowerelevations.
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