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(Cretaceous) Ammonite Pachydesmoceras Maroccanum from North America

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(Cretaceous) Ammonite Pachydesmoceras Maroccanum from North America N. Jb. Geol. Paläont. Abh. 283/1 (2017), 77–84 Article E Stuttgart, January 2017 First record of the Cenomanian (Cretaceous) ammonite Pachydesmoceras maroccanum from North America Paul L. Sealey and Spencer G. Lucas With 2 figures and 2 tables Abstract: A single specimen of Pachydesmoceras maroccanum COLLIGNON was recovered from the middle Cenomanian Acanthoceras amphibolum Zone in the Paguate Member of the Dakota Forma- tion in Sandoval County, New Mexico. Previous to this report, P. maroccanum was only known from the Cenomanian of the Tarfaya Basin in Morocco. The New Mexican specimen is the first known occurrence of P. maroccanum in North America, with only a few occurrences of Pachydesmoceras reported from the continent. It is also the first Cenomanian record of the genus from North America and indicates a trans-Atlantic migration of P. maroccanum. Key words: Pachydesmoceras maroccanum, Dakota Formation, Paguate Member, Cenomanian, North America, USA. 1. Introduction Mexico. Thus, it is the first Cenomanian record of the genus from North America and the first occurrence of The Cretaceous ammonoid genus Pachydesmoceras P. maroccanum known from the continent. Here, we has not been previously recorded from the Cenomanian document this species in New Mexico and compare it of North America. In the USA, species of Pachydes- to other species of the genus. moceras have only been reported from the upper Al- bian and the upper Turonian (COBBAN & KENNEDY 1991: Abbreviations and conventions: Institution housing fossil collections used in this report: NMMNH refers to the New A2). P. colusaense ANDERSON from the probable upper Mexico Museum of Natural History and Science, Albuquer- Albian of California is the oldest known member of que. All dimensions are given in millimeters; D = diameter, the genus from North America. P. linderi (DE GROS- U = umbilical diameter, Wh = whorl height, Wb = whorl SOUVRE) from the upper Turonian of Colorado is the breadth, c = costal and ic = intercostal. youngest known Pachydesmoceras from the continent. P. maroccanum COLLIGNON was originally described from the middle to upper Cenomanian of the Tarfaya 2. Locality and stratigraphy Basin in Morocco (COLLIGNON 1966: 26, 58-59, pl. 12, fig. 4). A specimen of P. maroccanum was recently The San Juan Basin of northwestern New Mexico ex- recovered from the middle Cenomanian Acanthoceras poses a thick and very fossiliferous succession of Upper amphibolum Zone near the top of the Paguate Mem- Cretaceous marine, marginal marine and nonmarine ber of the Dakota Formation in Sandoval County, New strata that range in age from middle Cenomanian to ©2017 E. Schweizerbart’sche Verlagsbuchhandlung, Stuttgart, Germany www.schweizerbart.de DOI: 10.1127/njgpa/2017/0629 0077-7749/2017/0629 $ 2.00 eschweizerbart_xxx 78 P.L. Sealey and S.G. Lucas late Maastrichtian. The basal portion of this section are strata of the intertongued Dakota-Mancos succession, which record the initial transgression of the Western Interior seaway into northwestern New Mexico (e.g., LUCAS et al. 1998). The most precise age calibration of the Dakota-Mancos strata comes from ammonoid biostratigraphy (e.g., COBBAN 1977a, b). The ammo- noids indicate the entire succession is of middle-late Cenomanian age, encompassing the ammonoid zones of Conlinoceras tarrantense through Calycoceras ca- nitaurinum (Table 1; e.g., COBBAN 1977a, b; SEALEY & LUCAS 2016). This long and well known ammonoid succession has produced few new or rare taxonomic records during the last 40 years, so the rare species of ammonoid reported here is an unusual and singular occurrence. In northwestern New Mexico, the base of the Cre- taceous section is the intertongued Dakota-Mancos succession that consists of sandstone-dominated members of the Dakota Formation interbedded with shale-dominated members of the Mancos Formation. The succession, in ascending order, is Oak Canyon and Cubero members (Dakota), Clay Mesa Member (Man- cos), Paguate Member (Dakota), Whitewater Arroyo Member (Mancos) and Twowells Member (Dakota), and these strata contain ammonoids of middle-late Cenomanian age (e.g., DANE 1960; LANDIS et al. 1973; COBBAN 1977a, b; LUCAS et al. 1998; SEALEY & LUCAS 2003). The specimen of Pachydesmoceras maroccanum documented here came from NMMNH locality 9575 in the southeastern San Juan Basin of northwestern New Mexico, near the village of San Ysidro in San- doval County. This locality is in the upper third of the Paguate Member of the Dakota Formation, stratigraphi- cally above the Clay Mesa Member of the Mancos Formation, which yields ammonoids (notably Acan- thoceras amphibolum) and other marine invertebrate fossils from two stratigraphic levels (Fig. 1). Elsewhere, the Paguate Member also yields ammonoids of the A. Fig. 1. Location map of New Mexico and measured strati- amphibolum Zone (e.g., COBBAN 1977a, b; SEALEY & graphic section at the locality of Pachydesmoceras maroc- canum. The section displays part of the intertongued Dakota- LUCAS 2003). The A. amphibolum Zone is of middle Mancos succession of northwestern New Mexico, the Cubero Cenomanian age (Table 1; COBBAN et al. 2006, fig. 1; and Paguate members of the Dakota Formation interbedded 2008, fig. 2; MEREWETHER et al. 2011, fig. 2). with the Clay Mesa Member of the Mancos Formation. At NMMNH locality 9575, the stratigraphic sec- tion (Fig. 1) begins at the top of the Cubero Member of the Dakota Formation, tabular bedded and bioturbated, meter or more in diameter in its lowermost and upper fine- to medium-grained quartz sandstone. The overly- strata. A calcareous sandstone bed about 5 m above the ing Clay Mesa Member of the Mancos Formation is base of the Clay Mesa Member yields the ammonoid ~ 18 m thick and is mostly gray, slightly sandy shale species Acanthoceras amphibolum, indicative of the with intervals of limestone septarian nodules up to a eponymous middle Cenomanian ammonoid zone. Sep- eschweizerbart_xxx First record of the Cenomanian (Cretaceous) ammonite Pachydesmoceras maroccanum 79 tarian nodules near the top of the Clay Mesa Member as in Pachydesmoceras (MATSUMOTO 1988: 22; WRIGHT et also yield fossils of A. amphibolum. al. 1996: 75). The Paguate Member at NMMNH locality 9575 Faint to weak constrictions are present on the adapical side of primary ribs on the earlier part of the outer whorl, is about 9 m thick (its top is an erosionally stripped thus forming collared ribs. Differentiation of ribs into pri- surface). The entire unit is fine- to medium-grained maries and secondaries does not begin until approximately quartz sandstone that is ripple laminated or wavy bed- after the first one-sixth of the outer whorl. Strong, distant, ded except near the top of the unit, where the sandstone slightly sinuous rectiradiate primary ribs start at the sharply is laminar. NMMNH locality 9575 is in a limestone rounded umbilical shoulder, cross flattened flanks on the earlier part of the outer whorl and slightly convex flanks septarian nodule horizon about 2.5 m below the local on the later part of the outer whorl, where they are strong, top of the Paguate Member. bar-like and high across the venter. The primaries are distant on the earlier part of the whorl and more closely spaced on the later part of the outer whorl, for a total of 13 primaries per whorl. Increase in primaries and decrease in intercala- 3. Systematic paleontology tories in an adoral direction is gradual and fairly consistent. Some intercalatories start outside the umbilical shoulder, but most start on the outside third of the flank. A few secondar- Order Ammonoidea ZITTEL, 1884 ies, mostly on the earlier part of the outer whorl, appear to Suborder Ammonitina HYATT, 1889 split on the outer flank, but it is difficult to make a definite determination. There are a total of 60-61 intercalated ribs. Superfamily Desmoceratoidea ZITTEL, 1895 On most of the phragmocone intercalated ribs crossing the Family Desmoceratidae ZITTEL, 1895 venter are weaker than the primaries but become stronger Subfamily Puzosiinae SPATH, 1922 over the venter and are almost as strong as the primaries on Genus Pachydesmoceras SPATH, 1922 the last preserved portion of the outer whorl. Ribs project only slightly forward over the venter and flare on the ven- Type species: Ammonites denisonianus STOLIczkA, 1865, ter on the last half whorl. The suture is poorly preserved, p. 133, pl. 65, fig. 4; pl. 66; pl. 66a, by original designation but, where visible, is comparable to Puzosia in having long, (SPATH 1922: 127). highly dissected saddles and lobes (e.g., WRIGHT & KENNEDY 1984, text-figs. 2h, m, 4b). Pachydesmoceras maroccanum COLLIGNON, 1966 Fig. 2; Table 2 4. Discussion and conclusions 1966 Pachydesmoceras maroccanum COLLIGNON, p. 26, pl. Members of the subfamily Puzosiinae SPATH, 1922 are 12, fig. 4. uncommon in the Upper Cretaceous of the Western 1987 Pachydesmoceras maroccanum COLLIGNON. – ZABOR- Interior and Gulf Coast (COBBAN & KENNEDY 1991: A1). SKI, p. 36. 1991 Pachydesmoceras maroccanum COLLIGNON. – COB- The only other Cenomanian member of the subfam- BAN & KENNEDY, p. A2. ily from these regions is one unillustrated fragment of Puzosia (Puzosia) sp. from the middle Cenoma- Material: NMMNH P-71327 from locality 9575 in the nian Acanthoceras bellense Zone in Texas (KENNEDY middle Cenomanian Acanthoceras amphibolum Zone in the & COBBAN 1990: 90; COBBAN & KENNEDY 1991: A1). Paguate Member of the Dakota Formation, Sandoval County, New Mexico. The genus Pachydesmoceras ranges from the Albian to the Santonian and has been recorded from Poland Description: The specimen (Fig. 2) is a moderately-pre- (KIN & Niedźwiedzki 2012), Spain, France, Switzer- served mold of Pachydesmoceras maroccanum that ap- land, Germany, Yugoslavia, Romania, Morocco, Zu- pears to be all phragmocone, but because of preservation and luland, Madagascar, Iran, southern India, Japan, New weathering it is difficult to make a definite determination. It is fairly evolute with a moderately wide umbilicus and a Zealand, California and Colorado (COBBAN & KENNEDY low, vertical umbilical wall. Less than one-half of the previ- 1991: A1).
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