Idoceras de agua fría del Kimmeridgiano (Jurásico Tardío) del sur de Coahuila, noreste de México, asociadas con bivalbos Boreales y belemnites

Patrick Zell* and Wolfgang Stinnesbeck

Institute for Earth Sciences, Heidelberg University, Im Neuenheimer Feld 234, 69120 Heidelberg, Germany. *Patrick.Zell@geow.uni-heidelberg.de

Manuscript received: November 26, 2014 ]]> Corrected manuscript received: January10, 2015
Manuscript accepted: January 14, 2015

ABSTRACT

Here we present two early Kimmeridgian faunal assemblages composed of the ammonite Idoceras (Idoceras pinonense n. sp. and I. inflatum Burckhardt, 1906), Boreal belemnites Cylindroteuthis cuspidata Sachs and Nalnjaeva, 1964 and Cylindroteuthis ex. gr. jacutica Sachs and Nalnjaeva, 1964, as well as the Boreal bivalve Buchia concentrica (J. de C. Sowerby, 1827). The assemblages were discovered in inner- to outer shelf sediments of the lower La Casita Formation at Puerto Piñones, southern Coahuila, and suggest that some taxa of Idoceras inhabited cold-water environments.

Key words: La Casita Formation, Kimmeridgian, idoceratid ammonites, Boreal bivalves, Boreal belemnites.

RESUMEN

Aquí se presentan dos asociaciones de fauna compuestas por los amonites Idoceras (Idoceras pinonense n. sp. e I. inflatum Burckhardt, 1906) con los belemnitas Boreales (Cylindroteuthis cuspidata Sachs y Nalnjaeva, 1964 y C. ex. gr. jacutica Sachs y Nalnjaeva, 1964) y el bivalvo Boreal Buchia concentrica (J. de C. Sowerby, 1827). Los fósiles indican una edad del Kimmeridgiano temprano y han sido descubiertos en sedimentos de plataforma marina en la porción inferior de la formación La Casita en Puerto Piñones, en el Sur de Coahuila. Se sugiere que algunas especies de Idoceras poblaron ambientes de agua fría.

Palabras clave: Formación La Casita, Kimmeridgiano, amonites Idoceras, bivalvos Boreales, belemnitas Boreales.

INTRODUCTION

The Jurassic is frequently described as a period of extended global greenhouse conditions and warm temperatures reaching to the poles (e.g., Frakes et al., 1992; Sellwood et al., 2000; Veizer et al., 2000). Nevertheless, recent palaeontological, geochemical and stable isotope studies provide evidence for major climatic oscillations, at least for the Late Jurassic period (e.g., Föllmi, 1995; Lécuyer et al., 2003; Weissert and Erba, 2004; Cecca et al., 2005; Brigaud et al., 2008). A global warming during the mid-Oxfordian (e.g., Riboulleau et al., 1998; Abbink et al., 2001; Chumakov et al., 2014) was followed by a cool period during the late Oxfordian-early Kimmeridgian (e.g., Jenkyns et al., 2002; Weissert and Erba, 2004) and a long-term gradual warming trend towards the Jurassic-Cretaceous boundary (e.g., Abbink et al., 2001; Lécuyer et al., 2003; Gröcke et al., 2003; Zakharov et al., 2014). Palynological data suggest that the latest Jurassic was also marked by significant fluctuations in paleotemperature and climate (e.g., Abbink et al., 2001).

Upper Jurassic-Lower Cretaceous marine associations containing both Tethyan and Boreal elements [e.g. ammonites, belemnites (Cylindroteuthis) and bivalves (Buchia)], were described from numerous localities of the Western Cordillera belt from Alaska to California (e.g., Jeletzky, 1965), while Boreal (Buchia) and even southern high latitude (Austral) elements (e.g. Anopaea) were reported from the upper Tithonian of Cuba (e.g., Myczyński, 1999). The latter author suggested, that their occurrences in Cuba resulted from changing oceanic currents and upwelling of cold water. In Mexico, the Austral bivalve Anopaea was only reported from the middle Tithonian La Pimienta Formation at Mazatepec, Puebla (Lecolle de Cantú, 1967) and from the subsurface of Nuevo León, also in Tithonian sediments (Cantú-Chapa, 1989). At Mazatepec, Anopaea is associated with the ammonites Kossmatia victoris and Pseudolissoceras zitteli (Cantú-Chapa, 1967). Reports of Boreal faunal elements (Buchia, Cylindroteuthis) are more abundant in the Upper Jurassic of Mexico (e.g., Mora et al., 2000; Zell et al., 2013) but stratigraphic assignation to specific intervals of the sediment sequence and taxonomic documentation of associated ammonites are rare. The first Mexican record of Buchia concentrica (J. de C. Sowerby, 1827) associated with the ammonite Idoceras (Idoceras sp.) was provided by Buitrón (1984, p. 93) from the early to ?middle Kimmeridgian La Caja Formation at Sierrecilla de San Antonio, northern Zacatecas.

Late Jurassic cold intervals in northeastern Mexico

Late Jurassic cold-water intervals were recently identified in northeastern Mexico by Zell et al. (2013) based on the occurrence of the Boreal belemnite Cylindroteuthis in distinctive Kimmeridgian to upper Tithonian units of the Upper Jurassic-lowermost Cretaceous La Caja and La Casita formations. These inner- to outer shelf sediments are widespread in northeastern Mexico and were described by Michalzik (1988), Götte (1990), Goldhammer and Wilson (1991), Adatte et al. (1994), Goldhammer (1999), Goldhammer and Johnson (2001) among other authors. The proximal La Casita and the coeval distal La Caja formations are widely known for their abundant and well-preserved marine vertebrate and invertebrate assemblages. Most taxa are endemic to the ancient Gulf of Mexico but some show affinities with the European Tethys (e.g., Verma and Westermann, 1973; Buchy, 2010; Buchy et al., 2003; Zell et al., 2013, 2014). On the other hand, belemnites assigned to Cylindroteuthis and the bivalve Buchia are of Boreal origin (Seibertz and Spaeth, 1999, 2008; Mora et al., 2000; López-Caballero, 2009; Pessagno et al., 2009; Zell et al., 2013). These Boreal taxa are present in at least two distinct units of the lower Kimmeridgian and upper Tithonian portion of the La Caja and La Casita formations (Mora et al., 2000; López-Caballero, 2009; Zell et al., 2013), occasionally forming monospecific assemblages. Other taxa (e.g. ammonites) are extremely rare in these units.

Cold-water assemblages at Puerto Piñones

Here, we present two species of the genus Idoceras (Idoceras pinonense n. sp. and I. inflatum Burckhardt, 1906) which are associated with Boreal taxa (belemnites, Buchia) in two distinctive lower Kimmeridgian horizons of the La Casita Formation at Puerto Piñones, southern Coahuila. A detailed stratigraphic section is presented in Figure 1. Assemblage I was collected from a dark-grey silt- to sandstone with limestone concretions at approximately 8 m above the base of the La Casita Formation, 0.3 m below phosphatic marls, and consists of Idoceras pinonense n. sp., I. inflatum Burckhardt, 1906 and the Boreal belemnite Cylindroteuthis cuspidata Sachs and Nalnjaeva, 1964. Assemblage II was identified in dark-grey limestone concretions embedded in dark-grey siltstone at approximately 16 m above the formational base, 2 m above phosphatic marls, and consists of Idoceras pinonense n. sp., I. inflatum Burckhardt, 1906, the Boreal bivalve Buchia concentrica (J. de C. Sowerby, 1827) and the Boreal belemnites Cylindroteuthis cuspidata Sachs and Nalnjaeva, 1964 and Cylindroteuthis ex. gr. jacutica Sachs and Nalnjaeva, 1964. Figure 2 illustrates specimens documented here.

SYSTEMATIC PALEONTOLOGY

The systematic description of ammonites follows Arkell et al. (1957), Verma and Westermann (1973) and Wright et al. (1996). Abbreviations used: D, diameter; Wh, whorl height; Ww, whorl width; U, umbilical diameter; U/D, umbilical ratio. Ribs were counted adapertural. Bivalve taxonomy follows Cox et al. (1969), Zakharov (1987), Bieler et al. (2010) and Sha (2012). Abbreviations used for bivalves: H, height; L, length; H/L, height ratio. Synonymy lists are restricted to the most important citations and references used for determinations. Belemnite specimens (CPC-1090, CPC-1082, CPC-1079) were previously described by Zell et al. (2013) but were not illustrated. All specimens described here are housed in the Colección de Paleontología de Coahuila, at the Museo del Desierto, Saltillo, Coahuila, Mexico. The abbreviation of this collection is CPC.

Class Cephalopoda Cuvier, 1795
Order Ammonoidea Zittel, 1884
Suborder Ammonitina Hyatt, 1889
Superfamily Perisphinctoidea Steinmann, 1890
Family Perisphinctidae Steinmann, 1890
Subfamily Idoceratinae Steinmann, 1890 ]]> Genus Idoceras Burckhardt, 1906

Type species. Ammonites balderus (Oppel, 1863).

Idoceras pinonense n. sp. (Figure 2a, 2b)

Diagnosis. Evolute idoceratid with abundant constrictions, which are bounded anteriorly by one or two simple and prominent ribs. A simple rib is always situated half-way between two constrictions. Parabolic ribs are present posteriorly to constrictions on the outermost whorl.

Description. Evolute idoceratid, with slightly convergent and convex flanks on inner whorls and nearly straight and parallel flanks on the outermost preserved whorl. The venter is rounded. The whorl section is widest above the umbilical edge. It is oval, compressed, slightly wider than high and somewhat trapezoidal for the outermost preserved whorl. The umbilical region is wide and shallow; the umbilical wall is short and almost vertical, with a pronounced rounded umbilical edge.

Ornamentation of the outermost whorl consists of fine, sharp and densely spaced ribs and shallow wide constrictions. Ribs and constrictions initiate almost radially on the umbilical wall and innermost flank, but they incline forward at mid-flank and they are prorsiradiate at the outer third of the flank. Ribs and constrictions cross the venter without interruption and without a decrease in strength. Most ribs bifurcate at about mid-flank but about one third (31 %) remains simple. Two sets of simple ribs are present; one is situated at about half-way between two constrictions. They are as prominent as neighboring bifurcating ribs. The second set of simple ribs is always adjacent anteriorly, and occasionally posteriorly, to each constriction. One or two simple ribs, which border the constrictions anteriorly, are the most prominent ribs. At the outermost preserved whorl, specimen CPC-1430 (Figure 2b) exhibits two parabolic ribs with parabolic-node-like swellings situated at the posterior border of the last-preserved two constrictions. One simple rib is present in specimen CPC-1430 (Figure 2b) between a constriction and the simple rib situated in the center between two constrictions. One or two intercalated ribs are present at the posterior side of each constriction. Some ribs initiate between the middle of the flank, others at the ventrolateral shoulder. Ribs are rounded in cross-section, while the majority of ribs is acute at the ventrolateral shoulder and mid-flank of the outermost preserved whorl. Forty six bifurcating ribs (about seven between two constrictions), eight intercalatory ribs, 21 simple ribs and eight constrictions are present on the outermost whorl of specimen CPC-1429 (Figure 2a).

The ornamentation of the inner whorls is similar to that of the outermost whorl, except for slightly wider-spaced ribs. Sutures are only fragmentarily visible.

Etymology. The species name "pinonense" derives from the locality Puerto Piñones, from which both specimens were collected.

Types. Holotype CPC-1429 is a three-dimensionally preserved, almost complete specimen (Figure 2a). Paratype CPC-1430 is a whorl fragment preserved three-dimensionally (Figure 2b). The type material was collected by one of us (WS) in the La Casita Formation at Puerto Piñones, 16 m above the boundary between the Zuloaga and the La Casita formations (Figure 1).

Occurrence. The genus Idoceras is abundant and well known in Mexico from early to early late Kimmeridgian strata and is represented by numerous species (summarized by Pessagno and Martin, 2003, and Villaseñor et al., 2012). Reports of idoceratids are rare from younger stratigraphic intervals (e.g., Schumann, 1988; Zell et al., 2014). The two specimens of Idoceras pinonense n. sp. were collected from lower Kimmeridgian strata of the La Casita Formation at Puerto Piñones, at approximately 8 m and 16 m above the formational base (Figure 1). The species is associated with Idoceras inflatum Burckhardt, 1906 (Figure 2c-2d), with the bivalve Buchia concentrica (J. de C. Sowerby, 1827) (Figure 2h-2i) and with the belemnites Cylindroteuthis cuspidata Sachs and Nalnjaeva, 1964 (Figure 2f-2g) and C. ex. gr. jacutica Sachs and Nalnjaeva, 1964 (Figure 2e).

Discussion. The species is morphologically similar to Idoceras densicostatum Imlay (1939, p. 40, pl. 8, figs. 3-5) from the Kimmeridgian "Idoceras beds" of the La Casita Formation at La Escondida, south of Soledad, Nuevo León. Compared to our specimens, Idoceras densicostatum Imlay, 1939 is less evolute, the whorls are more compressed, the whorl section is widest at the umbilical edge, constrictions are more strongly curved towards anteriorly and the venter is almost smooth. Our specimens resemble "Virgatosphinctes" cf. denseplicatus (Waagen), described by Imlay (1943, p. 535, pl. 89, figs. 1-4) from the Upper Jurassic of the Placer de Guadalupe district, eastern Chihuahua. The latter is less evolute and has a lower portion of bifurcating ribs (about one-third) as compared to simple ribs. In addition, ribbing is denser and ribs incline stronger forward on the flanks; they cross the venter nearly in transverse direction. The two parabolic ribs with parabolic-node-like swellings situated at the outermost preserved whorl of our largest specimen CPC-1430 (Figure 2b) indicate that the final body chamber is preserved. A similar aperture was documented for the Oxfordian Nebrodites (Enayites), in which three parabolic nodes associated with constrictions were interpreted to characterise the final body chamber (Brochwicz-Lewiński and Różak, 1976). Parabolic ribs and parabolic-node-like swellings are not present in specimen CPC-1429 (Figure 2a), which is smaller than CPC-1430.

Idoceras inflatum Burckhardt, 1906
(Figure 2c, 2d)

Idoceras inflatum Burckhardt, 1906, p. 65, pl. 8, figs. 5-8.

Description. Evolute idoceratid; inner whorls with convergent and convexly rounded flanks and outermost preserved whorl with nearly flat and parallel flanks. The venter is rounded. The whorl section is rounded to slightly depressed ovate, slightly wider (Ww=15.5 mm) than high (Wh=14 mm), widest at mid-flank. The umbilical region is wide and shallow, the umbilical wall pronounced and vertical, the umbilical edge is rounded.

Fine to prominent unequally spaced ribs are present on the outermost preserved whorl. Ribs initiate radially on the umbilical wall; they incline forward on the umbilical shoulder and are nearly straight to about mid-flank where they slightly bend forward and pass the outer third of the flanks and the venter prorsiradiately. Ribs cross the venter without interruption or decrease in strength. Most ribs bifurcate slightly above the middle of the flank, while about every fifth (19 %) remains simple. A single trifurcating rib is preserved on the outermost whorl. Intercalated ribs of unequal strength initiate between mid-flank and the ventrolateral shoulder. Ribs are rounded in cross-section. Specimen CPC-1432 (Figure 2d) presents 30 bifurcating ribs, four intercalated ribs and seven simple ribs on the outermost whorl. Ornamentation of inner and outer whorls seems to be identical. Sutures are not visible in the present material.

Material. Two three-dimensionally preserved fragmented specimens (CPC-1431 and CPC-1432).

Occurrence. Idoceras inflatum Burckhardt, 1906 was previously known only from the Kimmeridgian Idoceras Beds of the La Casita Formation at Vereda del Quemado, Mazapil region, Zacatecas (Burckhardt, 1906).

Class Bivalvia Linnaeus, 1758
Order Pectinida Gray, 1854
Superfamily Buchioidea Cox, 1953
Family Buchiidae Cox, 1953
Genus Buchia Rouillier, 1845

Type species. Avicula mosquensis von Buch, 1844.

Buchia concentrica (J. de C. Sowerby, 1827)
(Figure 2h-2k)

Plagiostoma concentrica J. de C. Sowerby, 1827 in: Sowerby and Sowerby, 1812-1846, p. 113, pl. 559, fig. 1.

Buchia concentrica (J. de C. Sowerby, 1827). Zakharov, 1981, p. 64, pl. 3, figs. 1-12.

Buchia (Anaucella) concentrica (J. de C. Sowerby, 1827). Buitrón, 1984, p. 93, pl. 1, figs. 4-9.

Buchia (Anaucella) concentrica (J. de C. Sowerby, 1827). Poulton et al., 1988, p. 108, pl. 5.3, figs. 13-20.

Buchia (Anaucella) concentrica (J. de C. Sowerby, 1827). Mora et al., 2000, fig. 3.

Description. Distinctly inequivalve shell; the left valve is strongly convex, the right valve only slightly convex to almost flat, with strongest convexity in the umbonal region. The posterodorsal margin is long and slightly convex. The anterodorsal margins are shorter and slightly convex. Anterior, posterior and dorsal margins are rounded convex. Opisthogyrous umbones curve inward and are situated anteriorly at about one-third of the shell length. The outline of our specimens is variable; two left valves (Figure 2h, CPC-1433; H=25.4 mm, L=28.5 mm, H/L=0.89) and the right valve (Figure 2k, CPC-1436; H=18.4 mm, L=18.5 mm, H/L=0.99) are inversoid, whereas the internal mould of a left valve in Figure 2i indicates an obliquoid outline. A prominent posterior wing is fragmentarily preserved on the left valve of CPC-1433 (Figure 2h). The ornamentation consists of regularly spaced, distinct and acute concentric ribs crossed by slightly less prominent radial ribs (CPC-1436, Figure 2k). With increasing shell size, ornamentation becomes more distinct. Internal features are not visible.

Material. Two three-dimensionally preserved almost complete left valves (CPC-1433 and CPC-1435), one internal mould of a left valve (CPC-1434), and one almost complete right valve (CPC-1436); all from assemblage II (see Figure 1).

Occurrence. Buchia concentrica (J. de C. Sowerby, 1827) was considered to be a Boreal faunal element by Poulton et al. (1988) and is common in the upper Oxfordian-lower Kimmeridgian of northern Eurasia and northern North America (e.g., Imlay, 1961; Poulton et al., 1988; Århus et al., 1989). Rare upper Kimmeridgian occurrences from the region are summarized by Surlyk and Zakharov (1982). In Mexico, Buchia concentrica (J. de C. Sowerby, 1827) was first described by del Castillo and Aguilera (1895) from the early to ?middle Kimmeridgian of San Luis Potosí and Zacatecas. According to Buitrón (1984) the taxon is a common constituent of the early Kimmeridgian to early late Kimmeridgian "lower shale member" of the La Caja Formation at San Pedro del Gallo, Durango, while Pessagno et al. (1999) reported the taxon from the early late Kimmeridgian "Unit E" of the La Caja Formation at Mazapil, northern Zacatecas. It was further identified by López-Caballero (2009; pl. 8, fig. L) in the latest Kimmeridgian Beckeri Zone at Puerto Piñones and by Mora et al. (2000) in the lower Tithonian La Caja Formation at Cañón San Matías, northern Zacatecas.

Discussion. Buchia concentrica (J. de C. Sowerby, 1827) is distinguished from Buchia tenuistriata (Lahusen, 1888), described by Buitrón (1984, p. 93, pl. 1, figs. 10-14) from the late Kimmeridgian of Zacatecas, by stronger, less acute and wider-spaced concentric ribs. In addition, concentric ribs of B. tenuistriata (Lahusen, 1888) are less regularly spaced.

Age and depositional environment of assemblages described here

The two faunal assemblages described here from Puerto Piñones, southern Coahuila, were deposited in inner- to outer shelf sediments (e.g., Michalzik, 1988; Goldhammer and Johnson, 2001) of the lower La Casita Formation. The lower portion of the La Casita Formation is reported to contain abundant Idoceras between 11.5 m and 18 m above the formational base (López-Caballero, 2009). These taxa were described by López-Caballero (2009) and assigned to Idoceras zacatecanum Burckhardt, 1906, I. cf. densicostatum Imlay, 1939, I. cf. lorioli Burckhardt, 1906, I. cf. tamaulipanum Imlay, 1939 and I. sp. gr. durangensis Burckhardt, 1906. Layers containing these taxa were assigned by the author to range from the early Kimmeridgian Hypselocyclum-Divisum Zones to possibly the earliest late Kimmeridgian lowermost Acanthicum Zone (see Pessagno and Martin, 2003; Villaseñor et al., 2012). Assemblage I is described here from about 8 m above the formational base and represents the lowermost record of idoceratids at Puerto Piñones, whereas assemblage II, collected at 16 m above the formational base, is situated at the upper portion of the Idoceras assemblage described by López-Caballero (2009).

Haploceras transatlanticum Burckhardt, 1906 was identified at Puerto Piñones within a coquina unit accumulating aptychs, at about 17 m above assemblage II described here, or 33 m above the boundary between the Zuloaga and the La Casita formations (Figure 1). This coquina unit clearly reflects sediment condensation. Its red colour, attributed to Fe³⁺ oxides, and the unusual concentration of ammonite shells (e.g. Haploceras transatlanticum Burckhardt, 1906) and aptychs welded to each other, are similar to a 1.5 meter thick coquina unit documented from the La Casita Formation of Gomez Farías, southern Coahuila. At Gomez Farías, situated at six kilometres SE of Puerto Piñones, the coquina unit is located at 19 m above the base of the La Casita Formation within the uppermost Kimmeridgian Beckeri Zone (Zell et al., 2014). The Gomez Farías coquinite is documented to contain abundant marine reptilian remains and was interpreted by Zell et al. (2014) as a condensational unit. Idoceras cf. schucherti (Cragin, 1905) and Haploceras transatlanticum Burckhardt, 1906 are present in this coquinite. Haploceras transatlanticum Burckhardt, 1906 is widespread in northeastern and central Mexico and was reported from the lower Kimmeridgian of Nuevo León (Mullerried, 1946), from the upper Kimmeridgian of Durango (Contreras-Montero et al., 1988; Villaseñor et al., 2000), Tamaulipas (Cantú-Chapa, 1963), Zacatecas, Coahuila and San Luis Potosí (Burckhardt, 1906; Imlay, 1939; Villaseñor et al., 2000; Zell et al., 2014), and from the lower Tithonian of Zacatecas (Burckhardt, 1906; Uhlig, 1911; Imlay, 1939), San Luis Potosí (Verma and Westermann, 1973) and Nuevo León (Imlay, 1939).

According to data summarized by Dzyuba (2004), the belemnite Cylindroteuthis cuspidata Sachs and Nalnjaeva, 1964 ranges from the middle Oxfordian to Kimmeridgian in northern Siberia and north of European Russia. The species was also identified in the early Callovian to early Kimmeridgian of Staffin Bay, Isle of Skye (Nunn, 2007). Cylindroteuthis ex. gr. jacutica Sachs and Nalnjaeva, 1964 was reported from the Tithonian (Volgian) of Siberia (Sachs and Nalnjaeva, 1964; Dzyuba, 2012), but is now known to be common also in distinct lower Kimmeridgian and Tithonian units of the La Caja and La Casita formations (Zell et al., 2013).

The ecologic tolerances of characteristic Boreal Buchia spp. vary and some taxa may not imply cold-water environment (e.g., Århus et al., 1990). However, Buchia concentrica (J. de C. Sowerby, 1827) is common in northern Eurasia and northern America (e.g., Imlay, 1961; Zakharov, 1981; Poulton et al., 1988; Århus et al., 1989) and is here regarded to indicate cold-water, at least for the seafloor. It is majorly known from the upper Oxfordian-lower Kimmeridgian, but rare upper Kimmeridgian occurrences are also documented (summarized by Surlyk and Zakharov, 1982). Recently, the taxon was identified at Puerto Piñones in the latest Kimmeridgian Beckeri Zone by López-Caballero (2009, pl. 8, fig. L), at about 37 m above the base of the La Casita Formation, and in the lower Tithonian La Caja Formation at Cañón San Matías, northern Zacatecas by Mora et al. (2000). The latter authors also identified an early Tithonian horizon containing Buchia, "Posidonia" and Aulacomyella. This horizon is here interpreted to correlate to a level containing Buchia concentrica (J. de C. Sowerby, 1827) and a bivalve resembling Posidonia at Puerto Piñones, at approximately 42 m above the base of the formation. The presence of Buchia concentrica (J. de C. Sowerby, 1827), in assemblage II, indicates that oxygenation of the bottom water was sufficient for epifaunal colonization of the seafloor, even though hydrodynamics were low in energy (cf. Marinov et al., 2006).

Assemblage II correlates to 15 m below the latest Kimmeridgian (Beckeri Zone) vertebrate Lagerstätten coquina unit of Gomez Farías (Zell et al., 2014) (Figure 3) and to approximately 25 m below the early Tithonian Buchia-"Posidonia"-Aulacomyella assemblage at Cañón San Matías, northern Zacatecas (Mora et al., 2000) (Figure 3).

The two cold-water assemblages described here correlate to the early Kimmeridgian "Lithological Unit II" of the basal La Caja Formation of Mora et al. (2000), as documented for Cañón de San Matías, Mazapil region, northern Zacatecas. This unit is approximately 9 m thick and consists of siltstone. Based on ammonites, Mora et al. (2000) assigned this unit to the early Kimmeridgian upper Hypselocyclum to lower Divisum Zones.

Lithostratigraphic correlations and taxa identified by López-Caballero (2009) indicate an early Kimmeridgian age and represent the "Lower Idoceras Assemblage", or Hypselocyclum to Divisum Zones (see Villaseñor et al., 2012).

The epifaunal bivalve Buchia concentrica (J. de C. Sowerby, 1827), identified in assemblage II of Puerto Piñones is here interpreted to represent an in situ record. The mono-specific benthic assemblage and a high organic carbon-content (C_org) in the limestone concretion, indicated by dark-color and foul-smelling when split with a hammer, suggest low-oxygen bottom water conditions. These observations coincide with time-equivalent palaeoenvironmental interpretations for the genus Buchia in the La Caja Formation at Mazapil (Mora et al., 2000).

Idoceratids associated with Boreal faunal elements

A global faunal migration episode is documented for the early Kimmeridgian, known as the "Early Kimmeridgian Boreal spread" (Arkell, 1956; Stevens, 1967; Zakharov and Rogov, 2003). Buchiidae, equally abundant in both the Boreal and the Pacific realms, extended their southern geographic distribution at that time reaching as far south as Mexico (e.g., Imlay, 1940, 1965; Mora et al., 2000; Pessagno et al., 2009). At the same time, ammonites (e.g. Aspidoceras, Epicephalites, Kossmatia) extended their distribution westwards into the Pacific Realm and eastwards into the Mediterranean Tethys (Stevens, 1967, and several sources mentioned therein). The cause of the Kimmeridgian "Boreal spread" is not yet conclusively known, but the bioevent correlates with a worldwide incremental encroachment of seas over continental margins (Hallam, 1978; Rogov and Poulton, 2015). Stable isotope data indicate that the Kimmeridgian was a period of extreme greenhouse conditions (e.g., Rogov et al., 2009; Price and Rogov, 2009; Dera et al., 2011). The Kimmeridgian episode of "Boreal spread" was therefore interpreted to be caused either by a re-organization of water circulation during the break-up of Pangea, or by short-term cooling events (Rogov and Poulton, 2015). We agree with Stevens (1967) and other authors that this "spread" was driven by the expansion of cold-water currents and suggest that these ingressions also included the ancient Gulf of Mexico and adjacent regions.

Here we present the first record of Mexican idoceratids associated with Boreal bivalves and Boreal belemnites. The co-occurrence, in assemblage II, of Boreal bivalves and belemnites, indicates low water temperatures at and near the sea-floor. Idoceras inflatum Burckhardt, 1906 and I. pinonense n. sp., associated with these Boreal faunal elements, may therefore either (1) have been current-transported post-mortem from warm water regions into cold-water environments of the ancient Gulf of Mexico. Alternatively, (2) Idoceras species inhabited warm shallow water areas while Boreal belemnites lived in the deep shelf of the Gulf of Mexico in cold-water. (3) Idoceratids identified here represent opportunistic taxa adapted to both cold and warm water environment, or (4) these idoceratids inhabited low-temperature waters.

Stable isotopic signals of belemnites and ammonites co-occurring in the same layers, indicate that these cephalopods colonized different bathymetric levels. While ammonites may have inhabited the upper part of the water column close to the surface, belemnites were interpreted to be deep-water demersal dwellers (e.g., Anderson et al., 1994; Wierzbowski et al., 2013) although the mode of life of these extinct cephalopods is not conclusively known (cf. Mitchell, 2005).

Idoceras inflatum Burckhardt, 1906 is a rare taxon in Mexico and was previously reported only from the Kimmeridgian La Casita Formation at Mazapil, Zacatecas (Burckhardt, 1906). Idoceras pinonense n. sp., is here described for the first time and thus also rare. Among the abundant and diverse idoceratids from the La Casita Formation only these rare species are presently associated with the Boreal taxa reported here. Preservation of the body chamber and aperture of Idoceras pinonense n. sp. specimen CPC-1430 (Figure 2b) indicates that this shell was not transported long-distance from a warm to a cold-water region. The two assemblages described here from southern Coahuila were deposited in a shelf setting of the La Caja Formation, in water depths not exceeding 200 metres (Michalzik, 1988; Götte, 1990; Goldhammer and Wilson, 1991; Adatte et al., 1994; Michalzik and Schumann, 1994; Goldhammer, 1999; Goldhammer and Johnson, 2001). Vertical temperature differences between the surface and seafloor should therefore be low and insignificant, compared to the water column in deep-marine environments.

The limited stratigraphic distribution of Idoceras inflatum Burckhardt, 1906 and I. pinonense n. sp., the absence of other ammonite taxa in the two assemblages discussed here, as well as their deposition in a shallow marine environment, may therefore indicate that both idoceratids were taxa adapted to cold-water environments. In contrast, other Mexican idoceratids were affected by cold-water ingression. The Tethyan genus Idoceras first appears in the early Kimmeridgian Planula Zone (cf. Wierzbowski, 2010) of Europe and is also known e.g. from New Zealand, Japan and Africa, as summarized by Brochwicz-Lewiński (1973). In Mexico, idoceratids represent the most abundant and diverse ammonite group in early Kimmeridgian strata (e.g., Burckhardt, 1906; Salvador et al., 1993; Pessagno et al., 1999; Villaseñor et al., 2000, 2012), including several endemic species (summarized by Salvador et al., 1993). They are, however, increasingly rare in subsequent cold-water units of the La Casita Formation. This decrease in abundance and diversity may have been a consequence of the "Boreal spread" (e.g., Stevens, 1967).

During the past decades, several Boreal Buchia "assemblages" were documented from Upper Jurassic strata of Mexico and adjacent regions, but frequently without a precise information on layers of origin. For example, Contreras-Montero et al. (1988) identified Buchia, among them Buchia concentrica (J. de C. Sowerby, 1827), in Kimmeridgian strata of the La Casita Formation at San Pedro del Gallo, Durango. Haploceras transatlanticum Burckhardt, 1906, Idoceras neohispanicum Burckhardt, 1912, I. zacatecanum Burckhardt, 1906, and several other ammonite species, were also collected from the same layers. From San Pedro del Gallo, Durango, Idoceras spp., Haploceras cf. fialar (Oppel, 1863) (and other taxa) and the bivalve Aucella ex. gr. pallasii Keyserling (=Buchia mosquensis; see Imlay, 1955, and Surlyk and Zakharov, 1982, and several sources mentioned therein) were described by Burckhardt (1912, p. 205) from Kimmeridgian sediments of the La Casita Formation, while Imlay (1961) reported on Subdichotomoceras sp. and Perisphinctes (Dichotomosphinctes) sp. co-occuring with Buchia concentrica (J. de C. Sowerby, 1827) in the Kimmeridgian of California and Oregon.

Here, we provide the first record in Mexico of idoceratid ammonites associated with Boreal belemnites and Boreal bivalves. Idoceras pinonense n. sp. and I. inflatum Burckhardt, 1906 were collected from limestone concretions, situated in two inner to outer shelf marlstone units of the basal La Casita Formation at Puerto Piñones, southern Coahuila. They co-occur with the belemnites Cylindroteuthis ex. gr. jacutica Sachs and Nalnjaeva, 1964 and C. cuspidata Sachs and Nalnjaeva, 1964, and the bivalve Buchia concentrica (J. de C. Sowerby, 1827). This unusual faunal assemblage is early Kimmeridgian in age and characterises episodes of cold-water ingression into the Gulf of Mexico. The faunal migration, known as the "Early Kimmeridgian Boreal spread", decimated warm water assemblages in the Gulf of Mexico region, which were dominated by idoceratid ammonites other than the taxa described here.

ACKNOWLEDGEMENTS

The authors acknowledge Seija Beckmann for fossil photography. We are grateful to the Co-Editor in Chief Thierry Calmus and to reviewers Mikhail Rogov (Moscow), Viktor Zakharov (Moscow), Simon Schneider (Cambridge) and Alexey P. Ippolitov (Moscow) for many helpful suggestions and constructive comments. Financial support to this research was provided by the Deutsche Forschungsgemeinschaft (DFG STI 128-17; to W.S.) and the Heidelberg University Graduate Academy (LGFG 2012-9; to P.Z.).

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