Ammonites del Aptiano inferior (Cretácico Inferior) de los estratos basales de la Formación La Peña del Estado de Nuevo Léon, noreste de México: implicaciones biocronoestratigráficas

Ricardo Barragán^1,* and Florentin J–M. R. Maurrasse²

¹ Departamento de Paleontología, Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, Delegación Coyoacán, 04510 México D. F, Mexico.* ricardor@geologia.unam.mx

² Department of Earth Sciences, Florida International University, University Park Campus, Miami, Florida, 33199, U. S. A.

Manuscript received: June 7, 2007 ]]> Corrected manuscript received: November 7, 2007
Manuscript accepted: November 10, 2007

ABSTRACT

Detailed paleontological studies of the basal strata of the La Peña Formation from a stratigraphic section of the State of Nuevo León, northeastern Mexico, reveal a rich ammonite assemblage representative of the local biozone indicative of the uppermost level of the early Aptian corresponding to the Dufrenoyia iustinae Taxon Range Zone. The Lower Aptian assignment of this Mexican biozone is corroborated by its distinctive ammonite assemblage characteristic of both uppermost early Aptian standard ammonite zones of the stage; namely the Dufrenoyia furcata Zone, of the tripartite Mediterranean standard scheme, and its equivalent Tropaeum bowerbankii Zone of the northeastern Europe bipartite scheme. Thus, in Mexico, the shift of deposition from a shallow carbonate platform locally represented by fades of the Cupido Formation, to the deeper fades of the La Peña Formation took place sometime during the late early Aptian andnotin the late Aptian as previously assigned. To avoid diachronic interpretations of the Cupido–La Peña formational contact, the ammonites analyzed in this study for biochronostratigraphic purposes, were compared to other coeval assemblages reported from the basal strata of the La Peña Formation on a regional scale.

Keywords: Ammonites, Lower Cretaceous, early Aptian, La Peña Formation, Mexico.

RESUMEN

Estudios paleontológicos detallados de los estratos básales de la Formación La Peña de una sección estratigráfica del Estado de Nuevo León, en el Noreste de México, revelaron una asociación de ammonites representativa de la Mozona local indicativa del nivel más superior del Aptiano temprano correspondiente a la Zona de Rango del Taxón Dufrenoyia iustinae. La asignación al Aptiano temprano de esta Mozona mexicana es corroborada por su asociación distintiva de ammonites característica de las dos Mozonas estándar de ammonites de la parte más superior del subpiso; la Zona Dufrenoyia furcata. del esquema tripartita para la Provincia Mediterránea, y su equivalente, la Zona Tropaeum bowerbankii del estándar para el noreste de Europa en el esquema bipartita del piso. Así, en México, el cambio de régimen de depósitos de plataforma carbonatada somera representada localmente por fades de la Formación Cupido, a fades pelágicas más profundas de la Formación La Peña ocurrió en algún momento durante el término del Aptiano temprano y no en elAptiano tardío como ha sido previamente asignado. Para evitar interpretaciones diacrónicas del contacto formacional Cupido–La Peña, los ammonites analizados en este estudio para propósitos biocronoestratigráficos, fueron comparados con otras asociaciones similares reportadas de los estratos básales de la Formación La Peña a escala regional.

Palabras clave: Ammonites, Cretácico Inferior, Aptiano temprano, Formación La Peña, México.

INTRODUCTION

The sedimentary cover of northeastern Mexico consists mainly of Upper Jurassic to Lower Cretaceous marine facies that crop out along the Sierra Madre Oriental fold belts. Excellent outcrops of Barremian–Aptian sequences are particularly well–exposed within the Sierra Madre Oriental Transverse near Monterrey, Nuevo León State (Figure 1). This area includes well established lithostratigraphic units formally described as the Cupido Formation (Imlay, 1937; Humphrey, 1949) of Barremian–Aptian age, and the La Peña Formation (Imlay, 1936; Humphrey, 1949) of late Aptian age. In Nuevo León State, the Barremian–Aptian rocks of the Cupido Formation are thick–bedded (decimeter to meter scale), shallow–water deposits known as the Cupido platform carbonates. These facies are consistently overlain by the La Peña Formation, characterized by thinly bedded (centimeter to decimeter scale) limestones, marly limestones, marls, and calcareous shales with abundant ammonites. The La Peña Formation facies represent a transgressive unit that crops out over an extensive area of northeastern Mexico, and was assigned a late Aptian age on the basis of ammonite occurrences (Cantú–Chapa, 1963, 1976; Young, 1969; Contreras–Montero, 1977). This original assignment of age was based on the bipartite subdivision of the stage into lower and upper Aptian in accordance with the biochronostratigraphic standard scheme established for northeastern Europe (Casey, 1961). However, the exact age of this Mexican Aptian stratigraphic unit remained uncertain, because most of the published fauna lacked precise stratigraphic control impeding the recognition of standard biozones and consequently broad regional correlations (Barragán, 2000).

Further stratigraphic works by Wilson and Pialli (1977), Smith(1981), Goldhammer et al. (1991) and Wilson and Ward (1993) argued that the carbonate deposition of the Cupido platform was abruptly terminated by terrigenous input that changed previous conditions into those that led to facies of the La Peña Formation. This event can be correlated with a global transgressive episode that began in the late early Aptian, which eventually drowned the Mexican carbonate platforms that had been flourished since the Barremian, as is recorded in other parts of the world (Barragán, 2001).

Recent bed–by–bed sampling of a stratigraphic section of the Cupido–La Peña formational contact has been carried out at La Huasteca canyon in northeastern Mexico, situated in the western part of the state of Nuevo León, southwest of Monterrey City (25° 37.109' N – 100° 28.869' W) (Figure 1). The studied section is located along the north–exposed limb of an open anticline at the eastern end of Sierra de Los Muertos. It is part of the tightly folded and thrusted tectono–sedimentary terranes that include the structures of the Sierra Madre Oriental, related to the Laramide orogeny in northeastern Mexico.

High–resolution sampling and careful recovery of the ammonites at the basal strata of the La Peña Formation in the studied section yielded a distinctive assemblage of index ammonite species within the first levels immediately in contact with the infrajacent Cupido Formation.

The aims of this work are to present the studied ammonite faunal assemblage of these strata, and its age interpretation in regards to the local biostratigraphic scheme (Barragan–Manzo and Méndez–Franco, 2005). Our study also establishes the biochronologic correlation of this ammonite assemblage with the newly revised standard biostratigraphic framework for the Tethyan Realm [Reboulet and Hoedemaeker (reporters), 2006]. Our results further provide more precision on the timing of the event characterized by a shift in conditions of deposition from the Cupido carbonate platform to the La Peña transgressive facies in northeastern Mexico.

The present research focused on detailed lithological and paleontological characteristics of the different facies recorded in the formational contact between the Cupido and the La Peña Formations in the stratigraphic section at La Huasteca canyonin Nuevo León State, northeastern Mexico.

The study included four dedicated field seasons in order to complete detail observation of variations of individual strata, as displayed in the column shown in Figure 2. The section was measured on a bed–by–bed scale, up to a total thickness of 28 m, from which the lower 22 m represent facies of the Cupido Formation, while the upper 6 m correspond to facies of the La Peña Formation. Rock samples from each individual stratum were recently prepared in thin sections for the characterization of the microfacies, inorganic, and organic carbon variations throughout the stratigraphic sequence (Barragán, 1999; Méndez–Franco, 2003; Maurrasse et al., 2006; Ponton–Guerrero, 2006). In the present work, three hundred and thirty seven specimens were collected all throughout the facies of the La Peña Formation, and the total collection represents a well–diversified assemblage, which in general consists of partly deformed but well–preserved casts. However, special emphasis was given to the ammonite assemblages of the basal strata of this lithostratigraphic unit, which make a total of eighty six specimens. Ammonites were prepared, studied, and those illustrated in this work were deposited in the invertebrate collection of the Museum of Paleontology at the Institute of Geology, UNAM under catalogue numbers IGM 8838 through IGM 8845. On account of a well–documented taxonomic provinciality that was already established for this time interval (Barragán, 2000), the systematic study of the ammonites focused on index species that served to recognize recently proposed local biostratigraphic units on the basis of these macrofossils (Barragán–Manzo and Méndez–Franco, 2005). Timing of the shift in environment of deposition, from the shallow–water Cupido carbonate platform to the more terrigenous and deeper waters sediments of the La Peña Formation, was interpreted through biostratigraphic correlation of the local ammonite biozone recorded at the base of the La Peña with the standard zonal scheme for the Tethyan Realm [Reboulet and Hoedemaeker (reporters), 2006]. Since chronostratigraphic units may not be isochronous, in order to avoid diachronic interpretations of the Cupido–La Peña formational contact at the studied section, the ammonites analyzed in this study for biochronostratigraphic purposes were compared to other coeval assemblages reported from the basal strata of the La Peña Formation on a regional scale (Cantú–Chapa, 1963; Cantú–Chapa, 1976; Contreras–Montero, 1977; Barragán, 2000; Méndez–Franco, 2003).

LITHOLOGY AND PALEONTOLOGICAL CONTENT

The Barremian– Aptian section exposed at La Huasteca canyon and subject of this study is essentially composed of marine carbonates (Figure 2). At the field scale these rocks show a sharp contrast between facies of the Cupido limestones that are competent beds rich in rudists, and those of the La Peña Formation composed of thinner beds characteristically more ductile, extremely compressed, often sheared, and rich in ammonites. As stated above, these sedimentary facies and their total inorganic carbon (TIC) and total organic carbon (TOC) variations throughout the section were previously investigated (Barragán, 1999; Méndez–Franco, 2003; Maurrasse et al., 2006; Ponton–Guerrero, 2006). The lowermost 22 m of the section are dominated by an alternation of extensively bioturbated pale grey biocalcirudites with rudists, and biocalcarenites with ooids, abundant benthonic foraminiferal assemblages mainly miliolids, orbitolinids, and textularids. These limestones also include common pellets, pelecypods, gastropods, algal fragments and scarce ostracods. Their calcium carbonate content (CaCO₃ = TIC equivalent calculated from the total carbon) varies between 78 and 99 %, while total organic carbon (TOC) values remain constantly below 1 %. These facies are typical of the shallow–water carbonates of the Cupido platform, an important paleogeographic element of the Barremian–early Aptian times of northeastern Mexico (Böse, 1923; Imlay, 1936, 1937, 1938, 1944; Kellum, 1944). The uppermost 6 m of the measured section assigned to the La Peña Formation consist of a rhythmic alternation of grey to black biocalcilutites, which, by contrast with the underlying Cupido limestones, show extreme variations in TIC (from a low of 17% to a high of 92.18 %) as well as in TOC (between 0.34 and 3.2 %). Thus, their sharp contrasting physical appearances reflect their composition, as the Cupido limestones are consistently higher in TIC, whereas the La Peña rocks always have higher organic carbon and terrigenous components (Barragán, 2000; Maurrasse et al., 2006; Ponton–Guerrero, 2006). Also, one of the most important hallmarks the La Peña beds is the faunal change associated with these facies. They include common planktonic foraminifera, while benthic taxa are very rare or absent, although there are other epibenthic fauna such as mollusk pelecypods and echinoid fragments. These facies represent the transgressive unit referred to as the La Peña Formation. All stratigraphic levels of this unit at La Huasteca canyon are dominated by ammonites, which represent 96 % of the macrofauna, and the rest of the accompanying macrofossils comprises mostly echinoids and pelecypods. The great abundance of partly deformed, but well–preserved ammonites that constitute the assemblage include index Aptian species within six genera: Pseudohaploceras, Burckhardtites, Dufrenoyia, Colombiceras, Kazanskyella, and Penaceras.

SYSTEMATIC PALEONTOLOGY

In the present work, the ammonite taxonomy used to identify taxa to the genus level is after Wright et al. (1996), whereas subsequent identification of species within genera used mostly the works of d'Orbigny (1840–1842), Kilian (1889), Hill (1893), Anthula (1900), Burckhardt (1925), Riedel (1938), Scott (1940a), Humphrey (1949), Stoyanow (1949), Cantu–Chapa (1963,1976), and Contreras–Montero (1977). The scientific nomenclature rules used follow the conventional Linnean hierarchy as prescribed by the 4^th edition of the International Code of Zoological Nomenclature (ICZN, 1999), which took effect on January 1, 2000.

This study gives basic information about the genus or species characteristics. The most significant specimens are illustrated in Figure 3a–3h.

Type Species. Ammonites liptoviensis Zeuschner, 1856, p. 181: by original designation.

Diagnosis. Moderately involute ammonites with slightly compressed shells and convex sides. Regular straight or sinuous constrictions, sometimes collared. Fairly fine, sharp or rounded, branching ribs between constrictions, originating on umbilical edge, extending across the flanks and crossing the venter.

Pseudohaploceras reesidei (Humphrey, 1949) Figure 3a

Material. Seventeen specimens. The specimen illustrated is identified as IGM–8838.

Description. Shell discoidal, moderately involute. Last whorl embracing about two thirds of preceding whorls. Whorl section subellipsoidal. Flanks flattened. Umbilicus broad. Venter narrowly rounded in inner whorls, squarish and much wider on last part of outer whorl. Ornamentation consists of flexuous well–marked flanges, constrictions, and finer intercalated ribs.

Remarks. Pseudohaploceras reesidei was originally described by Humphrey (1949, p. 152) as Uhligella reesidei from one specimen collected in the La Peña Formation of Coahuila, Mexico. Subsequently Cantú–Chapa (1976) found similar forms in a stratigraphic study of the La Peña Formation in the area of Nuevo León, Mexico, and proposed the new genus Caseyella to include his specimens and those classified as Uhligella aguilerae and Uhligella jacobi by Burckhardt (1925). Cantú–Chapa (1976) emphasized the differences in the ornamentation pattern between the Mexican specimens and the genus Uhligella, particularly because the Mexican specimens displayed strong and flexuous flanges on their shells, a morphological feature that is not observed on specimens of the genus Uhligella from Europe. Although his observations were correct, the characteristics of the ornamentation pattern displayed by the Mexican specimens are rather typical of the genus Pseudohaploceras defined by Hyatt (1900). In view of this fact, Wright et al. (1996) grouped specimens of this species under the name oí Pseudohaploceras reesidei (Humphrey), and they reassigned the rest of the Mexican species previously labeled as Uhligella to this genus. Present authors concur with Wright et al. (1996) opinion.

Suborder Ancyloceratina Wiedmann, 1966
Superfamily Deshayesitoidea Stoyanow, 1949
Family Deshayesitidae Stoyanow, 1949
Subfamily Deshayesitinae Stoyanow, 1949 ]]> Genus Dufrenoyia Kilian and Reboul, 1915

Type Species. Ammonites furcatus J. de C. Sowerby, in Fitton, 1836, p. 339: by original designation.

Diagnosis. Moderately evolute ammonites with compressed shells and flat sides and venter. Ribs sinuous, fine or coarse, commonly broad and flat, branching or long and short, interrupted on venter in inner whorls, later continuous, and raised into ventrolateral clavi.

Dufrenoyia justinae (Hill, 1893) Figure 3b–3c

Acanthocerasl justinae Hill, 1893, p. 38, pi. 7, figs. 1–3.
Hoplites roemeri Cragin, 1893, p. 234, pi. 44, fig. 4–5.
Ammonites justinae (Hill), Hill, 1901, pi. 21, fig. 6.
Dufrenoyia justinae (Hill), Burckhardt, 1925, p. 17, pi. 10, fig. 14–15; Riedel, 1938, p. 48–49, pi. 8, fig. 15–19; Scott, 1940a, p. 1022–1025, pi. 60, fig. 7–8; pi. 62, fig. 9; Humphrey, 1949, p. 122, pi. 6, fig. 1–7; pi. 7, fig. 1–9; Cantú–Chapa, 1963, p. 57–58, pi. 4, fig. 6; Cantú–Chapa, 1976, p. 9, pi. 1, fig. 3,5,11–11a; Renz, 1982, p. 18–19, pi. 1, fig. 8 a–b; Barragán, 2000, p. 102–107, pi. 53, fig. 5–10; pi. 54, fig. 1–12; pi. 55, fig. 1–6; Barragán, 2001, p. 193, fig. 3 (1–6); Méndez–Franco, 2003, p. 59–62, pi. 5, fig. 7–13; Bogdanova andHoedemaeker, 2004, p. 205–207, pi. 16, fig. 6; pi. 17, fig. 1–4; pi. 18; pi. 19, fig. 1.

Material. Thirty one specimens. The specimens illustrated are identified as IGM–8839 and IGM–8840.

Remarks. Dufrenoyia justinae (Hill) is easily recognized by the characteristics of its ventral region, particularly by the squared venter and the prominence of the ribs when they cross it transversely. Based on differences in the ventral region of the shells of specimens from Mexico, as well as on comparison of specimens from Texas, and Colombia, Biirgl (1956) proposed several varieties oí Dufrenoyia justinae that were identified as Dufrenoyia texanaby Burckhardt (1925). Specimens studied in the present work have been compared to those reported by Burckhardt (1925) and Humphrey (1949) from northeastern Mexico, as well as those studied by Stoyanow (1949) and Scott (1940a) from Arizona and Texas, respectively, and those illustrated by Riedel (1938) and Bogdanova and Hoedemaeker (2004) from Colombia, and Renz (1982) from Venezuela. All the forms from these different areas are here considered conspecific with Dufrenoyia justinae, and the differences observed on the ventral region on all of them can be easily considered within the range of variation of a single species.

Genus Burckhardtites Humphrey, 1949

Type Species. Neocomites nazasensis Burckhardt, 1925, p. 14: by original designation.

Diagnosis. Very similar to Dufrenoyia from which it differs by its more rapidly increasing whorl height and a finer, irregular, biconcave ribbing that never crosses the venter.

Burckhardtites nazasensis (Burckhardt, 1925) Figure 3d

Neocomites nazasensis Burckhardt, 1925, p. 4, pi. 3, fig. 4–7.
Burckhardtites nazasensis (Burckhardt), Humphrey, 1949, p. 130–131, pi. 10, fig. 1–5; Cantó–Chapa, 1976, p. 9, pi. 1, fig. 1; Renz, 1982, p. 24, pi. 1, fig. 17 a–b, 18; Barragán, 2000, p. 89–92, pi. 51, fig. 4–7; Barragán, 2001, p. 194, fig. 4 (1–2); Méndez–Franco, 2003, p. 54–56, pi. 5, fig. 1–3.

Material. Seventeen specimens. The specimen illustrated is identified as IGM–8841.

Remarks. The genus Burckhardtites was introduced by Humphrey (1949, p. 130) to include all forms described by Burckhardt (1925, p. 14, plate III, figs. 4–7) as Neocomites nazasensis from the Aptian of Rio Nazas, Durango, Mexico. As argued by Humphrey, "the genus Neocomites disappeared in the Hauterivian and is characterized by very involute, much compressed forms with ribs inclined towards the apertural zone, equally spaced, and which bifurcate from tubercles located at mid flanks". Burckhardtites nazasensis (Burckhardt) is an Aptian species more related to Dufrenoyia, than it is to Neocomites (Wright et al., 1996). Nonetheless, Burckhardtites can be easily separated from Dufrenoyia by its whorl height that increases more rapidly, and its fine, irregular, biconcave ribs.

Burckhardtites ehlersi Humphrey, 1949 Figure 3e

Burckhardtites ehlersi Humphrey, 1949, p. 136, pi. 12, fig. 4–6.

Material. Six specimens. The specimen illustrated is identified as IGM–8842.

Description. Shell discoidal, moderately evolute, embracing about one–half of preceding whorls. Whorl section of subequal height and width, thickest in lower third; greatly expanding anteriorly. Flanks convex, flattened, and inclined toward a broadly truncated venter. Umbilicus moderately wide and deep; umbilical wall almost straight; umbilical shoulder strongly rounded. Posterior portion of last whorl bears regularly alternating, coarse, widely spaced primary and secondary ribs in a crowded manner. Primaries begin on umbilical wall and are radial across umbilical shoulder. They curve forward on inner half of flanks, bend backward, and are flattened and falciform on outer half of flanks, curving forward again to terminate in prominent clávate tubercles on ventro–lateral angles. Secondaries originate well down on inner half of flanks, and are equal in strength to primaries across the rest of the flank. At this stage, venter smooth or marked slightly by faint swellings between opposed clavi. Ornamentation on the anterior portion of last whorl changes in a marked manner. Ribs are less crowded and attenuated, and cross the venter transversely. Ventro–lateral tubercles become less clávate and tend to disappear anteriorly.

Remarks. Humphrey (1949) erected this species from his observations on one specimen from the La Peña Formation at Cañón de San Antonio, Sierra de Los Muertos, Coahuila, Mexico. Even though its inner whorls are not preserved, the single specimen of this study assigned to Burckhardtites ehlersi shows morphological similarities to the specimen described by Humphrey.

Family Parahoplitidae Spath, 1922
Subfamily Acanthohoplitinae Stoyanow, 1949 ]]> Genus Colombiceras Spath, 1923.

Type Species. Ammonites crassicostatus Orbigny, 1840, p. 64: by original designation.

Diagnosis. Rather evolute ammonites with slightly compressed shells. Inner whorls with convex sides and flat–topped ribs branching at umbilical shoulder or mid–flank, where one tubercle may be present. Outer whorls compressed, with flat sides and flat to subrounded venter and less flat–topped ribs.

Colombiceras spathi Humphrey, 1949 Figure 3f

Colombiceras spathi Humphrey, 1949, p. 151–152, pi. 18, fig. 7–8; Barragán, 2000, p. 117–119, pi. 57, fig. 3–10; Barragán, 2001, p. 194, fig. 4 (5); Méndez–Franco, 2003, p. 69–72, pi. 7, fig. 1–3.
Colombiceras sp. Cantú–Chapa, 1976, pi. 1, fig. 8–8a.

Material. Eight specimens. The specimen illustrated is identified as IGM–8843.

Description. Shell discoidal, compressed, and moderately evolute; with last whorl embracing about one–fourth of preceding whorls. Flanks convex. Venter narrow and flattened. Whorl section subquadrate higher than wide. Umbilicus wide. Ornamentation consists of irregularly alternating primary and secondary ribs. Primaries rise on umbilical wall, quickly gain prominence on umbilical shoulder and pass radially or very slightly flexuous across flanks, widening rapidly toward venter. On last whorl, several primaries bifurcate on the inner third portion of flanks or from tubercle–like swellings on umbilical shoulder. Two or more primaries may follow each other or alternate with single secondaries. Secondary ribs originate over the inner third portion of flanks, and may simulate bifurcation by rising very close to an adjoining primary rib near umbilical area. On the outer third portion of flanks, all ribs present typical flat–topped, steep–sided appearance characteristic of the genus. Ribs are widest and have most relief on venter, where they are evenly spaced, equal in size, and transverse.

Remarks. Humphrey (1949, p. 151) described Colombiceras spathi based on observations made on a single tectonically–distorted specimen from Coahuila State, northeastern Mexico. As discussed by Humphrey, this species is morphologically similar to Colombiceras subtobleri (Kasansky) from Daghestan, northern Caucasus of the western Caspian Sea Region. The main difference is that C. subtobleri is more involute and the bifurcation of its primary ribs takes place on the third portion of the flanks closer to the venter, whereas in C. spathi the bifurcation occurs on the third portion of the flanks closer to the dorsum. The eight specimens studied herein are morphologically similar to that one illustrated by Humphrey (1949, p. 151–152, plate 8, figs. 7–8).

Type Species. Hypacanthoplitesl rursiradiatus Humphrey, 1949, p. 142: by original designation.

Diagnosis. Like Colombiceras, but primary ribs rectior rursiradiate on inner half of side, then branching and rur–siradiate, flattening toward venter; periodic ribs enlarged on early whorls. Perhaps synonymous with Colombiceras.

Penaceras rursiradiatus (Humphrey, 1949) Figure 3g

Hypacanthoplites? rursiradiatus Humphrey, 1949, p. 142, pi. 14, fig. 11–12.
Penaceras rursiradiatus (Humphrey); Cantú–Chapa, 1963, p. 54–55.
Pegnaceras rursiradiatus (Humphrey); Etayo Serna, 1979. p. 53.
Penaceras rursiradiatus (Humphrey); Wright et al., 1996, p. 275; Barragán, 2000, p. 122–125, pi. 58, fig. 1–3.

Material. Three specimens. The specimen illustrated is identified as IGM–8844.

Remarks. Penaceras rursiradiatus was originally described by Humphrey (1949) as Hypacanthoplitesl rursiradiatus based on observations made on one specimen from the La Peña Formation in Coahuila State, northeastern Mexico. Humphrey (1949, p. 142), assigned the specimen to Hypacanthoplites with some degree of doubt because of the flattened venter and transverse swellings of the ribs on the ventro–lateral margins, characters that are not diagnostic of the genus. The issue of morphological differences with Hypacanthoplites was raised by Cantú–Chapa (1963) who pointed out the marked curving of the ribs toward the protoconch on the outer half of flanks, and across ventral area, characteristic that also gives the name of the type species. Hence, Cantú–Chapa (1963) proposed the new genus Penaceras. Further emendations of the genus name as Pegnaceras and Penaceras, were introduced by Etayo–Serna (1979), and Wright et al. (1996), respectively.

The specimens assigned in this work to Penaceras rursiradiatus are morphologically similar with the one identified as Hypacanthoplites? rursiradiatus by Humphrey (1949).

Subfamily Parahoplitinae Spath, 1922
Genus Kazanskyella Stoyanow, 1949

Type Species. Kazanskyella arizonica Stoyanow, 1949, p. 100–101: by original designation.

Diagnosis. Rather involute ammonites with whorl section oval or rectangular. Ribs normally rigid and rectiradiate throughout.

Kazanskyella aff. arizonica Stoyanow, 1949 Figure 3h

Material. Four specimens. The specimen illustrated is identified as IGM–8845.

Remarks. Assignment of the specimens collected at La Huasteca canyon section to Kazanskyella arizonica is based on comparison with Stoyanow's work (1949) from Arizona. We decided to leave the nomenclature open due to the fact that the four specimens assigned to this species display ribs bent forward on venter through the outer third of last whorl, an index characteristic of the genus Parahoplites.

DISCUSSION ON THE AMMONITE ASSEMBLAGE AND ITS AGE

Taxonomy and biostratigraphy of Aptian ammonite assemblages of the La Peña Formation in northeastern Mexico have been useful conventional media used in strati–graphic correlation in the region as advocated in the works by Burckhardt (1925), Humphrey (1949), Cantú–Chapa (1963), Young (1969), Cantú–Chapa (1976), Contreras–Montero (1977), and Barragán (2000). However, very little attention has been given to the detailed sedimentological aspects to address the issue of prevailing existing conditions associated with deposition of the ammonite–rich rock units of this age. The lack of integration of biostratigraphic and sedimentological data makes the delimitation of biostratigraphic units a difficult task, and as a consequence, diminished the biochronological value of the Aptian ammonite biozones of northeastern Mexico. This problem was equally heightened by the absence of broad regional correlation between the Mexican ammonite biozones, and their relationship to taxonomic provinciality at that time. Such factors must have certainly played a key role in the specific character of the northeastern Mexican Aptian ammonite assemblages, as progressive westward opening of the Tethys sea resulted in the development of open marine connections over a broad area, allowing the dispersal of the Tethyan ammonite biota into the proto–Gulf of Mexico by Early Cretaceous time (Frakes, 1979). In fact, Johnson (1974) argued that the migrant fauna was influenced by the newly developed environments because of their specific ecological constraints, as previous studies based on ammonite distribution versus facies relationships (Scott 1940b; Wiedmann, 1973) established abathymetric distribution of the ammonite populations of that time. Their studies concluded that Lower Cretaceous ammonites lived only in the environments represented by the outer and inner shelves, and were excluded from the deeper basinal environments.

This is supported by recent findings of Tethyan Valanginian ammonites in northeastern Mexico (González–Arreóla and Barragán, 2007), which are conform to other coeval assemblages of western Europe. The influence of bathymetry on ammonite distribution thus explained why their larvae were unable to travel long distances (Scott, 1940b), and the deep oceans formed actual natural barriers that prevented further migration as they were unable to cross a widening Tethys. The ammonite fauna does show, indeed, that main centers of endemism within the Tethyan province appear to have developed from the Hauterivian onward, when the Lower Cretaceous seas became widely separated (Pozaryska and Brochwicz–Lewinski, 1975; Smith et al., 1981) and vast sedimentary basins developed in northeastern Mexico.

Thus, one of the main problems of broad regional correlations of pioneering Aptian biozonations of northern Mexico stemmed from apparent taxonomic provinciality during this age. This endemism apparently prevented the presence in Mexico of the index ammonites species that conform to the standard schemes for the Mediterranean or eastern Tethyan Realm (Barragán, 2000). To alleviate this limitation, Barragán–Manzo and Méndez–Franco (2005) proposed a new ammonite biozonation for the Aptian of northern Mexico, intended for regional use and constructed on the basis of the stratigraphic ranges of local index taxa. This Mexican scheme can be further correlated to the biozonal scheme of northeastern Europe (Casey, 1961), as well as to the former ammonite zonal standard of the Mediterranean area [Hoedemaeker and Reboulet (reporters), 2003] (Barragán–Manzo and Méndez–Franco, 2005, p. 45).

The present high–resolution analysis of the La Huasteca canyon strata provides further precision in local Aptian ammonite bio stratigraphy that can be correlated with the latest comprehensive Tethyan standard ammonite zonal scheme now in use [Reboulet and Hoedemaeker (reporters), 2006].

On the basis of its record of occurrences (Wright et al., 1996), Dufrenoyia is considered as essentially a Tethyan genus that stratigraphically spans throughout the Aptian. However, the absence of ammonite taxa in the infrajacent biocalcirudite layers of the Cupido Formation limits our true assessment of its chronostratigraphic range in northeastern Mexico. The presence of other faunal elements associated with the Mexican specimens of the species Dufrenoyia justinae, such as Pseudohaploceras resideei, Burckhardtites nazasensis, B. ehlersi, Colombiceras spathi, Kazanskyella aff. arizonica, and Penaceras rursiradiatus, indicate that the interval represented by the basal strata of the La Peña Formation at the La Huasteca canyon section of this study clearly belongs to the upper part of the local early Aptian Dufrenoyia justinae Taxon Range Zone (Barragán–Manzo and Méndez–Franco, 2005). On account of its distinctive ammonite assemblage, the Mexican Dufrenoyia justinae Zone can be further correlated to both: the uppermost early Aptian Dufrenoyia furcata Zone of the Mediterranean standard zonation [biozonation sense Reboulet and Hoedemaeker (reporters), 2006] concurrent with the tripartite scheme of the stage; and to the uppermost early Aptian Tropaeum bowerbankii Zone of the northeastern European standard zonation (biozonation sensu Casey, 1961) associated with the classic bipartite scheme of the stage (Figure 4). Thus, the basal strata of the La Peña Formation are of late early Aptian age regardless of the scheme of subdivision of the stage used.

As pointed out earlier, it is important to emphasize the lack of the lowermost ammonite biozones for the Bedoulian (lower Aptian) in northern Mexico. The absence of a record for the Deshayesites oglanlensis through the Deshayesites deshayesi zones in northern Mexico is best explained by the dominance of extensive shallow carbonate platforms in the area through most of the Bedoulian, as in other parts of the world. A transgressive event, which drowned most of these carbonate platforms at the end of this substage, is characterized locally by the interruption of deposition of facies of the Cupido platform and the onset of deposition of the La Peña Formation. Nonetheless, the uppermost beds of the underlying Cupido Formation include brief intermittent marly levels impoverished inbenthic foraminifera, and with few ammonite shell fragments heralding the development of conditions that favored the incursion of the ammonoid faunas into these areas of Mexico (Barragán, 2001). Thus, the first stratigraphic record of identifiable Aptian ammonites in this part of Mexico is represented by a rich assemblage of species belonging mainly to the genus Dufrenoyia deposited in the basal strata of the La Peña Formation.

CONCLUSIONS

The results of the present study provide evidence that the basal strata of the La Peña Formation of northern Mexico include index ammonites representative of the uppermost early Aptian Dufrenoyia justinae Taxon Range Zone. The chronostratigraphic position of this Mexican Biozone can be correlated elsewhere with well established lower Aptian ammonite zonations (Figure 4) for both the Mediterranean region [Dufrenoyia furcata Zone, sensu Reboulet and Hoedemaeker (reporters), 2006], and northeastern Europe (Tropaeum bowerbankii Zone sensu Casey, 1961). Thus, in Mexico, the shift of deposition from the predominately biocalcarenites and biocalcirudites with rudists, distinctive of the shallow carbonate Cupido platform, to the marly inorganic rich beds of the deeper facies, La Peña Formation, took place sometime earlier– i.e. late early Aptian– instead of the late Aptian as previously assigned.

ACKNOWLEDGEMENTS

This work was financially supported by the Project DGAPA PAPIIT IN–107106 and private funds. The authors express their gratitude to Antonio Altamira for the excellent photographs of the ammonites. Heartfelt thanks to Gilberto Nieto, Rüdiger Marten, Fabian Duque–Botero and Marco Antonio Argáez for their assistance in the field. The authors also express their gratitude to Dr. Jaap Klein and Dr. Alexander Lukeneder who suggested many useful improvements to the manuscipt.

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