Sciponocerasgracile (Shumard) GommonUpper Cretaceous guide fossil in NewMexico byWillian A. Cobban,Paleonlologist, US. GeologicalSurvey, Denver, Colorado

kiponocuas graclleis an ammonite that belongs to a group of straight well preserved and are mostly uncrushed intemal molds of lime- or nearly straight forms known as baculites, a name derived from stone. Diameters at the larger end of the shells are usually 10 to 15 the Upper Cretaceousgenus Baculites (Latin, baculum,a rod or staff, mm, but can be as much as 25 to 30 mm. Eagle Ford specimensare and ites, stone). Sciponocerasis of early Late Cretaceousage (middle straight and have subelliptical to nearly circular cross sections. Or- and late Cenomanian). Members of the genus are fairly small, and nament on the intemal molds consists of fairly uniform ribs that most are less than 20 mm in diameter at their larger end. The shell arise at midflank and arch forward on crossing the venter, where has a very low angle of taper, usually a degreeor two. Omament, they are strongest. These ribs usually number 5 to 7 in a distance other than ventral ribs, may or may not be present, but all specimens that is equal to the diameter of the shell. Constrictions, spacedat 1 have constrictions at some growth stage(Fig. 1). Theseconstrictions, to 11/zshell diameters, parallel the ribs. On some individuals, the which represent thickened parts of the shell, are only visible on the constrictions completely encircle the intemal molds and cross the intemal molds. Fig. 1E is a good example;most of the specimenhas dorsal side with faint forward arching. The suture is rather simple retained its shell material, and only two constrictions are visible, with somewhat rectangularlobes and saddles.Wright and Kennedy where the shell is missing. (1981,fig. 38) have presentedsome excellent photographs of seven Sciponocerasgracile, originally described as Baculitesgracilis Shlu- good examplesof S. gracilefrom the Eagle Ford of Texas. mard (1860,p.596), camefrom the EagleFord Formationof northern Many localities of S. gracilehavebeen found in New Mefco (Fig. Texas,where the speciesis abundant. The whereabouts of the type 2). The New Mexico specimens are like those from Texas, except or type specimensis unknown. Specimensfrom the Eagle Ford are none has been collected that exceeds20 mm in diameter. An occa-

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FIGURE l-Sciponocerasgracile (Shumard). All natural size except H, which is twice natural size. A-C, ventral, lateral, and dorsal views of hypotype USNM 356912,from USGSMesozoic locality D5780in SE1/+sec. 8, T5S,R2E, Socorro County, New Mexico; D and E, ventral and lateral views of hypotype USNM 387358,from USGSMesozoic locality D11760in center of sec.24, T17S,R13W, Grant County, New Mexico; F and G, ventral and lateral views of hypotype USNM 356913,from USGS Mesozoic locality D11529in NE1L sec. 11, T18S, R18W,Grant County, New Mexico; H, lateral view ( x 2) of hypotype USNM 387359that has an initial coil (arrow) on the small dislocated end, from USGSMesozoic locality 951 near Kanab, Utah; I-K, ventral, lateral, and dorsal views of hypotype USNM 387360,from same locality as F and G; L and M, ventral and lateral views of hypotype USNM 356911,from same locality as A-C; N-P, dorsal, lateral, and ventral views of hypotype USNM 387361,from USGSMesozoic localltyD6793 in NW1/qsec. 23, T7N, R8W, Cibola County, New Mexico; Q and R, ventral and lateral views of hypotype USNM 387362,from same locality as A-C.

November 199{J New Mexico Geology sional specimenhas a completeaperture, which is curved away from the ventral side and slightly flared (Fig. 1M) like a specimen from the Greenhorn Limestone near Pueblo, Colorado (Cobbanand Scott, 1972,pl. 17, figs.23,24). Specimenssmaller than 2 mm in diameter are not present in the collections from New Mexico. A very small juvenile from a limestone concretion from the Tropic Shaleof south- ern Utah reveals that the speciesbegins with an initial coil of about 0.7-mm diameter (Fig. 1H). This coil consists of a protoconch and one complete whorl. Initial coils will probably be found in the lime- stone concretions in the Bridge Creek Limestone Member of the Mancos Shalein westem New Mexico. Similar initial coils, although of smaller diameter (about 0.5 mm), are present on juvenile Baculites codyensisReeside collected by me in 1940from a limestone concretion in the Marias River Shale in northern Montana (Cobban, 1951b,p. 13, loc. 33). The Montana specimens,which are of Santonian age, i l-*--t o*rcco i i are younger than S. grncile.Landman (1982)described initial coils o lr of baculites that may have come from the concretion found by me. rL-l- so""|,oo *oo"u Sciponocerasgracili was indicated as a guide fossil to the bise of i f----*r"-l ,,1o i- i : rocks of Turonian age in the Western Interior of the United States i L-r*1 i Y+/ --l--_1 i.n:o!nLrnaorn i ClClesjf.."". | in 1951(Cobban, L95la, fig.2) and soon after was recommendedas ifi r i f- '.--- '-lt rlir I t;^ t--- -*- -,--- I I I I i lb T--- I I Le' l---- i i ls'e',r .l I I Ir-ll/}l-'.-*---'- iiii o.,"-,ol arit i ilt i l.!1I,6*: t:-,'- ,,.,,ttsrc Ii i i l*: ri--..r------i L_*_"j rdcy cerasgeslininnum Zone of southem England and correlatives in Bed I I i**"o l ! !1{iI i-* C in Devon, perhaps also occurring at the baseof the Middle Chalk. iilio,i! Widespread elsewhere at the same level in France (Sarthe, Nor- I| I| I Lrn*Llni |f '-*---fI I mandy), southem Germany, the United States,Mexico and perhaps i*-;li.i Li -'* i i-'- i l-4ica'qo f"- *r -- ---,-: l Angola." il iln In New Mexico, S. gracileoccurs at many localities in the Green- '1",'j""Y"'mrl horn Limestone or in calcareousrocks of Greenhom age (Fig. 2). In - O IOO 2oo km ll the Raton Basin, the specieshas been found in limestone beds in the basal part of the Bridge Creek Member of the Greenhorn Lime- FIGURE where gracilehas been collected in stone. Farther west at Lamy, the specieswas 2-LocalitiesO Sciponoceras New recorded as Baculites Mexico. gracilisby Rankin (L9M, p. l0). Sciponocerasgracile occurs in chalky limestone beds in the Greenhom Limestone Member of the Mancos Shale at many localities along the east and south sides of the San References Cobban, W. A., 1951a,Colorado shaleof central and northwestem Montana and equiv- alent rocks of Black Hills: American Associationof Petroleum Geologists,Bulletiir, v. 35, no. 10, pp.217O-2198. Cobban, W A., 1951b,Scaphitoid cephalopods of the Colorado group: U.S. Geological Suruey, ProfessionalPaper 239,42 pp. basalpart of the Bridge CreekLimestone beds (e.g., Hook and others, Cobban, W A , 1977, New and little-known ammonites from the Upper Cretaceous (Cenomanianand Turonian) of the Westem Interior of the United States:U.S Geo- 1983,p. 25). East and southeastof Socorro, S. gracileis found in the logical Suruey, ProfessionalPaper 699,24 pp. basal part of the Bridge Creek Limestone Member of the Mancos Cobban, W. A , and Reeside,l.8.,lr.,1952, Conelation of the Cretaceousformations Shale(Hook, 1983,p. l7l;Tabet, L979,p.1.3). Farther southwest, in of the Westem Interior of the United States:Geological Society of America, Bulletin, the Truth-or-Consequencesarea, the speciesoccurs in concretionary v.63, no. 10,pp. 1011-1M3. Cobban, W. A., and Scott,C. R.,1972,Stratigraphy and ammonite fauna of the Graneros limestone in the Bridge Creek Limestone beds of the Mancos Shale. Shale and Greenhorn Limestone near Pueblo, Colorado: U S. Geological Survey, In the Deming-Silver City area farther west, S. gracilehas been col- ProfessionalPaper 545, 108 pp. lected from limestone beds and limestone concretions in the Bridge Hook, S. C.,1983, Stratigraphy,paleontology, depositional framework, and nomencla- Creek Member of the Colorado Formation (Hook and Cobban, 1981, ture of marine Upper Cretacmus rocks, SocorroCounty, New Mexico: New Mexico GeologicalSociety, Guidebook to 34th Field Conference,pp. 165-172. fig. 3). Hook, S C., and Cobban, W. A., 1981,Iate Greenhorn (mid-Cretaceous)discontinuity surfaces, southwest New Mexico: New Mexico Bureau of Mines and Mineral Re- sources,Circular 780, pp.5-27. Hook, S C, Molenaar, C M, and Cobban, W. A., 1983,Stratigraphy and revision of nomenclature of upper Cenomanian to Turonian (Upper Cretaceous)rocks of west- central New Mexico; in Contributions to mid-Cretaceouspaleontology and shatig- raphy of New Mexico, part II: New Medco Bureauof Mines and Mineral Resources, Circular 185, pp.7-28 Landman, N H , 1982,Embryonic shells of Bcculifes;Journal of Paleontology,v 56, no. 5, pp. l23s-r24r Pike, W S , Jt , 1947,Intertonguing marine and nonmarine Upper Cretaceousdeposits of New Meico, Arizona, and southwesternColorado: Geological Society of America, Memoir 24, 103pp. Rankin, C. H , 194J,Stratigraphy of the Coloradogroup, Upper Cretaceous,in northem New Mexico: New Mexico Bureau of Mines and Mineral Resources,Bulletin 20. 27 locally. pP. - The specimensshown in Fig. l were photographed by R. E. Burk- Shumard, B F, 1860, Descriptions of new Cretaceousfossils from Texas:Academy holder formerly of the U.S. Geological Survey,-Denver, Colorado. ScienceSt Louis Transactions,v. 1, pp 590-510. These specimens,all hypotypes, are stored in the National Museum Tabet, D E , 1979, Geology of Jomada del Muerto coal field, Socorro County, New Mexico: New Mi:xico Bureau ofMnes andMinerafRemrces, -158, - of Natural History in Washington, D.C., Greular 2epp and have USNM catalog Wright, C. W., and Kennedy, W. J., 1981,The Ammonoidea of the Plenus Marls and numbers. the Middle Chalk: PalaeontographicalSociety [Monographl, 148pp r-.1

Naa Merico Geology November 1990