1. Introduction
The Late Jurassic opening of the Gulf of Mexico was accompanied by widespread rifting and continental extension related to sea-floor spreading in the Gulf of Mexico, but the area remained at least temporarily isolated from both the European Realm and the Pacific, with the Florida uplift to the east and the Sinaloa Magmatic Arc to the west forming barriers (e.g. , Imlay, 1941; Goldhammer, 1999; Goldhammer and Johnson, 2001; Stinnesbeck and Frey, 2014). The irregular sea bottom topography and heterogeneity of eco-sedimentary conditions in the ancient northwestern margin of the Gulf of Mexico were used to explain differential ammonite records and endemism as well as the low presence of Tethyan taxa (e.g. , Olóriz, 1992; Villaseñor et al., 2012).
Jurassic aptychi were hitherto only sporadically mentioned from northeastern Mexico (Burckhardt, 1930; Verma and Westermann, 1973). From other areas of the country aptychi were illustrated by Laur (1871), Erben (1956), or López-Caballero (2009), but only a few formal descriptions exist to date (Castillo and Aguilera, 1895; Villaseñor and González-Arreola, 1988; Zell et al., 2014), even though marine sedimentary rocks of Jurassic age are widespread in northeastern and eastern Mexico and known to be rich in diverse ammonite assemblages. In consequence, the data base regarding the palaeobiogeographic distribution and stratigraphic occurrence of aptychi is exceedingly limited in Mexico.
Here we present a collection of 18 well-preserved aptychi from the Kimmeridgian (Upper Jurassic)-lowermost Berriasian (lowermost Cretaceous) La Caja Formation of southern Coahuila, southern Nuevo León and northern San Luis Potosí states (Fig. 1). The specimens were collected from different stratigraphic levels within the La Caja Formation and were assigned to Laevaptychus (Latuslaevaptychus ) latus meyrati Trauth, 1931, L. (Hoplisuslaevaptychus ) mexicanus (Castillo and Aguilera, 1895), L. (Autharislaevaptychus ) favrei Trauth, 1931, Lamellaptychus lamellosus (Parkinson, 1811) and Lam. murocostatus Trauth, 1929.
1.1. Stratigraphic and geologic overview
The La Caja Formation and its equivalents show an overall transgressive lithostratigraphic sequence (Jenchen, 2007). Lithologies of the La Caja Formation are siliciclastic (predominantly siltstone, shale and marl) and were deposited under hemipelagic conditions (e.g. , Michalzik, 1988; Zell et al., 2013).
Fossils are locally abundant and occasionally preserved three-dimensionally in calcareous concretions. Diverse ammonite assemblages have been described (e.g. , Burckhardt, 1906, 1930; Imlay, 1938, 1939; Cantú-Chapa, 1963, 2006; Verma and Westermann, 1973; Olóriz et al., 1998, 1999; Villaseñor et al., 2000; Zell and Stinnesbeck, 2015), but belemnites (e.g. , López-Caballero, 2009; Zell et al ., 2013), bivalves (e.g ., Mora et al., 2000; Zell et al ., 2014, 2015) and brachiopods (Castillo and Aguilera, 1895) were also recorded as well as radiolarians and calpionellids (e.g. , Adatte et al., 1994, 1996). Marine vertebrate remains include fishes, ichthyosaurs, crocodiles and pliosaurs (e.g. , Frey et al., 2002; Buchy et al., 2003, 2005; Zell et al ., 2014). Based on the faunal contents (e.g. , ammonites, calpionellids), a lower Kimmeridgian to lowermost Berriasian was assigned to the La Caja/La Casita formations (e.g. , Adatte et al ., 1994, 1996; Villaseñor et al ., 2000, 2012; Olóriz et al ., 2003; Zell et al ., 2013).
Aptychi are rare in the La Caja Formation, as compared to the abundance and diversity of ammonites. Specimens described here were collected during the 1990`s by one of us (Wolfgang Stinnesbeck). For the location of sites, see Figure 1; for stratigraphic position within the La Caja Formation, see Figure 2. Biostratigraphic assignation of strata investigated here is based on ammonite occurrences, of which representative specimens from the El Verde site (San Luis Potosí) are illustrated in Figure 3.
2. Systematic paleontology
The nomenclatural elements of aptychi are illustrated in Figure 4. Their morphological description follows Trauth (1927), Gąsiorowski (1959), Farinacci et al. (1976), Turculeț and Grigore (2006), Měchová et al. (2010), and sources mentioned therein) and Vašíček et al. (2012). For type species concerning mentioned subgenera of the genus Laevaptychus see Turculeț (2000). All specimens described here are housed in the Colección de Paleontología de Coahuila at the Museo del Desierto, Saltillo, Coahuila, Mexico. Abbreviation of this collection is CPC.
Class Cephalopoda Cuvier, 1787
Order Ammonoidea Zittel, 1884
Family uncertain (sensu Vašíček et al., 2012)
Genus Laevaptychus Trauth, 1927
Type species: Aptychus meneghinii de Zigno, 1870
Subgenus Latuslaevaptychus Gąsiorowski, 1960
Laevaptychus (Latuslaevaptychus ) latus meyrati Trauth, 1931
(Figure 5)
1857. Trigonellites acutus Ooster, p. 25, pl. 6, fig. 16.
1864. Aptychus flamandi Thurmann and Etallon,
1872 - Loriol et al ., p. 69.
1876. Aptychus meyrati Favre, p. 61, pl. 6, fig. 7.
1925. Aptychus cf. meyrati Favre, 1876 - Spath, p. 33.
1931. Laevaptychus latus (Parkinson, 1811) var. nov. meyrati - Trauth, p. 75, pl. 1, fig. 4.
1948. Laevaptychus latus (Parkinson, 1811) var. meyrati - Trauth, p. 171.
1960. Laevaptychus latus (Parkinson, 1811) var. meyrati Trauth, 1931 - Răileanu and Năstăseanu, p. 24, pl. 9, fig. 30.
2006. Laevaptychus (Latuslaevaptychus ) latus meyrati Trauth, 1931 - Turculeț and Grigore, p. 30, pl. 1, figs. 1, 2.
Material. One complete left valve (CPC-1179), one complete right valve (CPC-1177), one paired valve (CPC-1452), one fragmented left valve (CPC-1180) and two fragmented right valves (CPC-1181, CPC-1178) from El Verde, State of San Luis Potosí; one complete left valve (CPC-1427) from Gomez Farías and one fragmented right valve (CPC-1428) from Puerto Piñones, State of Coahuila. Dimensions are provided in Table 1 and abbreviations used therein are explained in Figure 4.
Description. Medium to large (L = 21.1 mm to 62.5 mm; W = 16.7 mm to 41.6 mm), medium convex, thick-walled valves (G = 3.1 mm to 5.4 mm) with an elongated, subtriangular outline. The inner edge is straight in small-sized specimens (Fig. 5a-c, f, h) and slightly concave in large individuals (Fig. 5d, e, g). The inner margin is straight to slightly convex. The lateral margin is convex, widely rounded; the outer margin is short and convex. The harmonic margin and the symphysal edge are almost straight, slightly convex. The lateral external surface is convex. The convexity of the valve is strongest at the apical region just above the position of maximum width. The convexity gradually decreases towards the lateral and outer margins. The convex side is sub-irregularly covered with circular, suboval and subpolygonal pores (Fig. 5h3). The valve is internally sponge-like; pores are smaller, more regular sized and more abundant at the lateral external surface. The convex side is covered with concentric rings of unequal widths and distribution, following the outline of the inner, lateral and outer margins. Radial wrinkles are present in three specimens (Fig. 5b1, d1, h1); they originate at the apex and extend to the lateral and outer margins; they are less prominent and less abundant compared to the concentric rings. Pores remain unaffected at the concentric rings and at the position of radial rings and wrinkles. The concave side is covered with fine, concentric growth lines of unequal width and strength, following the outline of the inner, lateral and outer margins. Radial lines (major depressions, minor swellings) originate from the apex and reach the lateral and outer margins without interruption. They gradually increase in width towards the lateral and outer margins; growth lines remain unaffected in size and width at the radial lines. The largest specimen CPC-1177 exhibits 10 radial lines (Fig. 5d2, six depressions, 4 swellings). One specimen (CPC-1425, Fig. 5f) presents paired valves.
Discussion. The values of the morphometric indexes (Table 1) of our specimens of Laevaptychus (Latuslaevaptychus ) latus meyrati Trauth, 1931 stay within the limits of the subgenus Latuslaevaptychus (Gąsiorowski, 1960, table 6) and of the species L. latus (Parkinson, 1811). According to Trauth (1931, p. 71), L. latus is defined by the following indexes: W/L = 0.67 to 0.8, pW/L ≥ 0.62, H/W < 0.4 and S/L > 0.77. Our specimens are medium-sized compared to L. latus , which, according to Trauth (1931), reaches an average width (W) of 48 mm and length (L) of 63 mm. Only our largest specimen (CPC-1177, Fig. 5d) reaches a similar size. Smaller-sized specimens were also described by Spath (1925).
According to Trauth (1931), the subspecies Laevaptychus (Latuslaevaptychus ) latus meyrati is similar in size and dimensions to L. latus with average (av.) W/L = 0.67 to 0.76, Gmax. = 5.5 mm and an average apical angle of between 90° and 100°. The average values defined for the subspecies fit well with the morphometric measurements of our specimens: W/Lav. = 0.74, Gmax. = 5.4 mm and AAav. = 101.6°. Our specimens belong to Laevaptychus group C of Trauth (1931, p. 66, stout Laevaptychi , W/L between 0.67 and 0.8), based on their W/L values. The G/Wav. value of 0.153 in our material fits measurements of Turculeț and Grigore (2006, G/W = 0.16, p. 30) for the subgenus, which are comparatively lower compared to L. latus (Parkinson, 1811; G/W ≈ 0.21, see Turculeț and Grigore, 2006, p. 30). According to Trauth (1931, p. 77), the most characteristic feature for the subspecies L. (Latuslaevaptychus ) latus meyrati is the presence of concentric and/or radial rings of unequal width and distribution on the convex side, which are also visible in our specimens. However, the occurrence of radial lines was interpreted by Měchová et al. (2010) to be related to an exceptionally favourable type of preservation.
Similar concentric rings are also present in L. longus , L. favrei and L. meneghinii var. rugosa (Trauth, 1931). Laevaptychus (Latuslaevaptychus ) latus meyrati can be distinguished from L. longus (W/L < 0.67) and L. favrei (W/L < 0.5) by the larger W/L index and from L. meneghinii var. rugosa by its larger valve thickness (cf. Trauth, 1931, p. 86).
Occurrence. In Europe, most representatives of the genus Laevaptychus occur by the end of the early Tithonian (see Schindewolf, 1958; Gąsiorowski, 1960; Turculeț, 2000). Laevaptychus latus (Parkinson, 1811) is common throughout the Upper Jurassic and is abundant in the Alpine-Mediterranean region (summarized by Trauth, 1931 and Renz, 1972). It was also recorded in southern Arabia (Beydoun, 1966), Tunisia, Somalia (e.g. , Valduga, 1954) and western North Atlantic (Site 105, Leg XI, Deep Sea Drilling Project; Renz, 1972). In Mexico, the first records of Aptychus latus (= Laevaptychus latus sensu Trauth, 1931) are provided by Laur (1871) and Castillo and Aguilera (1895, p. 49) from the Upper Jurassic of the La Caja Formation at Sierra de Catorce, State of San Luis Potosí and by Burckhardt (1930, p. 77) from the upper Kimmeridgian "Couche à Haploceras d. gr. Fialar " of Sierra de Santa Rosa, Mazapil, State of Zacatecas, unfortunately without descriptions and figures.
The subspecies Laevaptychus (Latuslaevaptychus ) latus meyrati Trauth, 1931 is a characteristic faunal element from the Tethyan Realm and is known from the Jurassic of the Swiss alps (Ooster, 1857; Fischer-Ooster, 1865), from the Upper Jurassic of Somalia (Spath, 1925) from the Oxfordian of the Swiss alps (Favre, 1876), from the ?Oxfordian-upper Tithonian of Romania (Răileanu et al., 1960; Turculeț and Grigore, 2006), from the Kimmeridgian-Tithonian of Germany and Austria (Trauth, 1931), from the middle Kimmeridgian-lower Tithonian of France (Loriol et al., 1872) and from the upper Tithonian of Austria (Trauth, 1948).
In northeastern Mexico, Laevaptychus (Latuslaevaptychus ) latus meyrati Trauth, 1931 is present in mixed siliciclastic-limestone lithologies of the La Caja Formation and was identified in the upper Kimmeridgian Beckeri Zone (López-Caballero, 2009; Zell et al., 2014; Zell and Stinnesbeck, 2015) at Puerto Piñones, in the lower upper Tithonian Kossmatia Beds at Gomez Farías and in the middle Tithonian "Virgatosphinctes " Beds, the lower upper Tithonian Kossmatia Beds and the uppermost Tithonian "Durangites " and Substeueroceras Beds at El Verde.
Subgenus Hoplisuslaevaptychus Gąsiorowski, 1960
Laevaptychus (Hoplisuslaevaptychus ) mexicanus (Castillo and Aguilera, 1895)
1895. Aptychus mexicanus Castillo and Aguilera, p.45, pl. 22, fig. 8.
1931. Laevaptychus mexicanus (Castillo and Aguilera, 1895) - Trauth, p. 131, fig. C14.
1978. Laevaptychus mexicanus (Castillo and Aguilera, 1895) - Renz, p. 499, pl. 1, figs. 1a, 1b.
Material. One fragmented, almost complete left valve (CPC-1437) from Rincón del Agua Dulce and one fragmented, large left valve (CPC-1438) from Iturbide, State of Nuevo León. The specimens were collected from marls of the La Caja Formation, a precise origin of layers is not known. Dimensions are provided in Table 2 and abbreviations used therein are explained in Figure 4.
Description. Medium (L = 20.6 mm) to large (L = 66.2 mm), medium convex, very compressed (H/W = 1), subtrigonal valves. The harmonic margin is almost straight, slightly convex. The inner margin is short and concave, the lateral margin is wide and circular and the outer margin is short and convex. The transition between the margins is almost continuous; it can be recognized by the transition from concave (inner margin) to circular convex (lateral margin) and to slightly less convex (outer margin). The maximum shell width is placed about at the center of the valve's length. The convexity of the valve is strongest at the center of the valve at the position of maximum shell width. The convexity gradually decreases towards the lateral and outer margins. The convex side is covered with widely spaced, oval pores (Fig. 6b2) of homogenous size and distribution; rounded pores are minor represented. Shallow concentric rings of unequal width and distribution are present on the convex side of specimen CPC-1437 (Fig. 6a).
Discussion. The S/L and pW/L values of our specimens (Table 2) fit well to measurements for the subgenera Latuslaevaptychus , Meneghiniilaevaptychus and Hoplisuslaevaptychus defined by Gąsiorowski (1960, table 6), of which only the subgenus Hoplisuslaevaptychus presents high H/W values (≥ 0.42). Based on the high H/W value of 1.01, our specimens are therefore assigned to this subgenus, and here to Laevaptychus group C of Trauth (1931, p. 66, stout laevaptychi , W/L between 0.67 and 0.8), based on their W/L values.
The type material of Aptychus mexicanus Castillo and Aguilera (1895) (= Laevaptychus mexicanus sensu Trauth, 1931) was defined by the following parameters: AA = 90°, S/L = 0.97, pW/L = 0.56 and W/L = 0.86, which are comparable to values measured in our specimens (Table 2). In addition to the parameters, Laevaptychus (Hoplisuslaevaptychus ) mexicanus is also defined by an almost straight harmonic margin, a right apical angle, and by a slightly concave inner margin, which merges continuously into a wide, circular lateral margin. The outer margin is short and slightly less convex, meeting the terminal point in an obtuse angle (≈ 95°, sensu Trauth, 1931, p. 117). Specimen CPC-1438 (L = 66.2 mm; Fig. 6b) is relatively large for the species (L = 35 mm in Castillo and Aguilera, 1895); it is here interpreted to represent an aptychus of an adult ammonite.
L. latissimusTrauth, 1931 is similar to L. (H. ) mexicanus but differs in the position of the maximum valve width, which is placed closer to the inner margin (see Trauth, 1931, p.116), with resulting pW/L values of ≥ 0.62. The inner margin of L. longus (v. Meyer, 1831) is more concave and the apical angle is larger than that of L . (H .) mexicanus .
Occurence. The subgenus Hoplisuslaevaptychus is known from the upper Oxfordian-Tithonian of Europe (Gąsiorowski, 1960). Laevaptychus (H .) mexicanus (Castillo and Aguilera, 1895) is known from the ?Oxfordian of the eastern Atlantic (Renz, 1978; Deep Sea Drilling Project site 367) and from the Tithonian of Croatia (Trauth, 1931). The species was first described from the Upper Jurassic (Kimmeridgian or Tithonian) at Sierra de Catorce, San Luis Potosí, central Mexico (Castillo and Aguilera, 1895). In northeastern Mexico, L . (H .) mexicanus is rare.
Subgenus Autharislaevaptychus Gąsiorowski, 1960
Laevaptychus (Autharislaevaptychus ) favrei Trauth, 1931
1876. Aptychus meyrati Favre, p. 61, pl. 6, fig. 8.
1931. Laevaptychus favrei Trauth, p. 65, pl. 1, fig. 2.
1960. Laevaptychus (Autharislaevaptychus ) - Gąsiorowski, pl. 13, fig. 1.
1962. Laevaptychus (Autharislaevaptychus ) favrei - Gąsiorowski, pl. 3, fig. 3.
Material. Two fragmented left valves (CPC-1239, CPC-1394) from El Verde, State of San Luis Potosí and one fragmented right valve (CPC-1350) from La Chona, State of Nuevo León. Dimensions are provided in Table 3 and abbreviations used therein are explained in Figure 4.
Description. Medium sized (L = 31.1 mm), slender (W/L = 0.58), subtrigonal valves of low convexity (H/W = 0.16 and 0.32). The apical angle is acute (62°), the inner margin and the inner edge are short and slightly convex, projecting backwards. They continuously merge into a wide, circular-rounded lateral margin. The outer margin is short, strongly convex; it curves towards the direction of the apex close to the harmonic margin. The thickness of the valve gradually increases from the harmonic margin, respectively symphysal edge, towards the lateral edge and margin. The convex side is irregularly covered with relatively large pores of unequal size (Fig. 6c3). The pores are circular, suboval and subpolygonal-elongated (Fig. 6c1, c3). The elongated pores point towards the outer and lateral margins. Concentric lines and wrinkles of unequal size and distribution are present on the convex side. They follow the outline of the outer and lateral margins and appear more abundant at the apical area (Fig. 6d1, e). Three radial lines (furrows) are present on the convex side of specimen CPC-1394 (Fig. 6d1). They are best identified in the central area of the valve; they point towards the apex and disappear approximately 14 mm before the lateral and outer margins. The concave side is covered with sub-regularly-spaced, fine growth-lines (Fig. 6c2, d2).
Discussion. The G/W and LE/L values of our specimens (Table 3) are within the range of the subgenus Autharislaevaptychus , as defined by Gąsiorowski (1960, table 6; G/W < 0.25, LE/L < 0.2). In addition to morphometric values, the presence of relatively large pores on the convex side is also considered to be characteristic by the author (Gąsiorowski, 1960). Our specimens are assigned to the group b of Trauth (1931, p. 40; B/L between 0.5 and 0.67, slender laevaptychi).
According to Trauth (1931, p. 65), Laevaptychus (Autharislaevaptychus ) favrei Trauth, 1931 is characterized by W/L values of about 0.64, by an acute apical angle and by the backward-projection of the inner edge and margin, which merges into a wide, convex lateral margin without interruption. Similar to L. latus and other related taxa of the genus, concentric lines and radial lines and wrinkles of irregular strength and distribution are also present (Trauth, 1931). However, the occurrence of radial lines is related to an exceptionally favourable type of preservation (Měchová et al., 2010). The outer margin is short, strongly convex; it curves towards the apex close to the harmonic margin. In L . (A .) favrei , the thickness of the shell is characteristically increasing from the harmonic margin towards the lateral margin, similar to L. longus (v. Meyer, 1831; cf. Trauth, 1931).
Our specimens of Laevaptychus (Autharislaevaptychus ) favrei are similar to L. hybonotus (Trauth, 1931, p. 39), but the latter is characterized by smaller W/L ratios (< 0.5).
Occurence. The subgenus Autharislaevaptychus is known from the Callovian-Tithonian of Europe (Gąsiorowski, 1960). Laevaptychus (Autharislaevaptychus ) favrei Trauth, 1931 was reported from the Oxfordian of Switzerland (Favre, 1876), Poland (Gąsiorowski, 1960, 1962) and possibly from England and Germany (Trauth, 1931).
In northeastern Mexico, L . (A .) favrei Trauth, 1931 was identified in the lower upper Tithonian Kossmatia Beds and in the uppermost Tithonian "Durangites " and Substeueroceras Beds of the La Caja Formation at El Verde, State of San Luis Potosí, and from the ?Tithonian of La Chona, State of Nuevo León.
Family Lamellaptychidae Měchová et al., 2010
Genus Lamellaptychus Trauth, 1927
Type species: Trigonellites lamellosus Parkinson, 1811
Lamellaptychus lamellosus (Parkinson, 1811)
1811. Trigonellites lamellosus Parkinson, p. 184, 186, pl. 13, figs. 10, 11.
1829. Aptychus imbricatus Meyer, p. 127, 139, 169, pl. 54, fig. 12.
1829. Aptychus imbricatus profundus Meyer, p. 127, 140, 141, 169, pl. 43, fig. 10.
1831. Aptychus imbricatus profundus Meyer, p. 397. 399.
1835. Münsteria sulcata Deslongchamps, p. 66, pl. 11, figs. 10, 11.
1837. Aptychus profundus Voltz, p. 434.
1846-49. Aptychus lamellosus (Parkinson, 1811) - Quenstedt, p. 312, pl. 22, figs. 20a, b, 23.
1850-51. Aptychus lamellosus (Parkinson, 1811) - Bronn, p. 379, pl. 15, fig. 16a-c.
1854. Aptychus profundus Pictet, p. 558.
1854. Aptychus lamellosus (Parkinson, 1811) - Pictet, p. 558, pl. 47, fig. 16a, b.
1857. Trigonellites ornatus Ooster, p. 22, pl. 6, fig. 5.
1857. Trigonellites imbricatus Ooster, p. 19, pl. 5, fig. 7.
1857. Trigonellites curvatus Ooster, p. p. 20, pl. 5, fig. 16.
1858. Aptychus lamellosus (Parkinson, 1811) - Quenstedt, p. 596, 600, 622, pl. 74, figs. 12, 13.
1863. Aptychus euglyptus Oppel, p. 254, 265, pl. 70, fig. 2.
1863. Aptychus lamellosus (Parkinson, 1811) - Oppel, p. 254.
1887-88. Aptychus lamellosus (Parkinson, 1811) - Quenstedt, p. 917, pl. 99, fig. 20a.
1907. Aptychus aff. crassicauda Toula, p. 82, pl. 12, fig. 3.
1908. Aptychus lamellosus (Parkinson, 1811) - Engel, p. 402, 414.
1927. Aptychus (Lamellaptychus ) lamellosus (Parkinson, 1811) - Trauth, p. 197-199, 223, 234, 237.
1929. Lamellaptychus lamellosus (Parkinson, 1811) - Trauth, p. 76-77, 79.
1938. Lamellaptychus lamellosus (Parkinson, 1811) - Trauth, p. 149, pl. 11, figs. 1-5.
1962. Lamellaptychus lamellosus (Parkinson, 1811) - Gąsiorowski, pl. 3-5.
1965. Lamellaptychus lamellosus (Parkinson, 1811) - Pozzi, p. 869, pl. 86, fig. 7.
1972. Lamellaptychus lamellosus (Parkinson, 1811) - Renz, p. 613, p. 2, fig. 4a-c.
2000. Lamellaptychus lamellosus (Parkinson, 1811) - Turculeț, p. 102, pl. 5, figs. 9-15, pl. 6, fig. 5.
Material. One complete right valve (CPC-1182) from El Verde, State of San Luis Potosí. Dimensions are provided in Table 4 and abbreviations used therein are explained in Figure 4.
Description. Large-sized (L = 24.8 mm), thin-valved (G = 2.9 mm), weakly vaulted, elongated-subtrigonal valve. The shell reaches its maximum thickness at the outer margin (G/W = 0.15). The valve is strongest convex at the apical region, 2 to 4 mm below the inner edge. The inner edge and margin are straight (Fig. 7a4), the symphysal edge and the harmonic margin are slightly convex. The outer margin is short and convex, the lateral margin arch-shaped. The even symphysal surface and the inner surface are pronounced. The convex side is covered with prominent, simple, slightly arch-shaped ribs. They originate from a shallow ridge situated between the apex and the umbilical point. Broad furrows separate the ribs; they gradual increase in width at the terminal region below the point of inflection. The majority of ribs end at the harmonic margin and some at the outer edge, forming angles of 22° to 27°. Ribs are steep, acute, wall-like, leaning against the apex and harmonic margin. Ribs follow the outline of the lateral margin, with the exception of S-shaped ribs (sensu Měchová et al., 2010) near the symphysal area; these bend towards the harmonic margin along an axis which meets the harmonic margin near the apex at an angle of about 20°. The last four adult ribs end at the outer margin. Ribs are most prominent in the terminal and lateral regions. They become gradually less prominent towards the apical area, where they almost disappear. At the keel slope, ribs are negligibly inflected at the apical region and become gradually more inflected towards the outer margin.
Discussion. Our specimens exhibit the ribbing pattern regarded as typical for the genus Lamellaptychus Trauth, 1927 (cf. Měchová et al., 2010, fig. 7D). They belong to lamellaptychid group b of Trauth (1938, p. 159; lamellaptychi without lateral depression).
According to Trauth (1938, p. 150), Lamellaptychus lamellosus (Parkinson, 1811) is characterized by an obtuse apical angle, relatively high S/L values, a straight to concave inner margin, an arch-shaped lateral margin, W/L values between 0.4 and 0.67, pW/L values > 0.62, a subtriangular to subparallel-elipsoidal outline, an even symphysal surface, G/W values between 0.12 to 0.25, by ribs which originate from a shallow ridge situated between the apex and the umbilical point (cf. Meyer, 1829, p. 139), by ribs which are S-shaped and inflated, and by the absence of a keel, keel slope and lateral depression.
Lamellaptychus gillieroniTrauth (1938) and L. sparsilamellosus (Gümbel, 1861) are similar to L. lamellosus but the latter is distinguished by the absence of an adsymphysal ridge (cf. Trauth, 1938, p. 151). Lamellaptychus rectecostatus (Peters, 1854) and L. beyrichi (Oppel, 1865) are similar to the valve shape of L. lamellosus , but the latter differs by the presence of inflated ribs and an increase of accompanying furrow width at the terminal region, leading to an arched-shaped contour and a curved approximation to the margin (Trauth, 1938, p. 151).
Occurrence. Lamellaptychus is well known from the Bajocian-Valanginian of the Tethyan Realm (e.g. , Carpathians, Gąsiorowski, 1962; France, Fischer, 2003; Spain, Calzada, 1985; distribution summarized by Měchová et al., 2010). Two records of the genus (Lamellaptychus didayi and L. angulicostatus ) from the Middle Jurassic to ?Lower Cretaceous Puebla Group of southeast to central Mexico are provided by Erben (1956), but without a detailed description. The genus was also identified from the uppermost Kimmeridgian Beckeri Zone at Gomez Farías, State of Coahuila (Lamellaptychus sp.; Zell et al., 2014), from the Kimmeridgian upper Idoceras Beds of Puerto Piñones (Lamellaptychus sp.; López-Caballero, 2009), and from the upper Kimmeridgian Procraspedites-Taramelliceras interval (see Villaseñor et al., 2012) of Sierra de Palotes, State of Durango (Lamellaptychus murocostatus Trauth, 1929; Villaseñor and González-Arreola, 1988).
Lamellaptychus lamellosus (Parkinson, 1811) is abundant in the upper Oxfordian-Tithonian of the Tethyan Realm as summarized by Trauth (1938), and associated ammonites were there identified as Oppelia . The taxon was also reported from the Oxfordian or Kimmeridgian of the western North Atlantic (Site 105, Leg XI, Deep Sea Drilling Project; Renz, 1972).
In northeastern Mexico, L. lamellosus is present in the middle Tithonian "Virgatosphinctes " Beds and in the lower upper Tithonian lower Kossmatia Beds of the La Caja Formation at El Verde, State of San Luis Potosí. The species is relatively abundant at El Verde (field observation), but only one well-preserved specimen was suitable for excavation.
Lamellaptychus murocostatusTrauth, 1929
1929. Lamellaptychus murocosta Trauth, p. 77.
1938. Lamellaptychus murocosta Trauth, p. 142, pl. 19, figs. 21-22.
1978. Lamellaptychus murocostatus Trauth, 1938 - Renz, p. 502, pl. 1, fig. 10.
1988. Lamellaptychus murocostatus Trauth, 1938 - Villaseñor and González-Arreola, p. 76, fig. 10.
2009. Lamellaptychus sp. - López-Caballero, pl. 10, fig. D4.
Material.Two fragmented, paired valves from La Mesita (CPC-1351), state of San Luis Potosí and from Rincón del Agua Dulce (CPC-1276), State of Nuevo León, and one fragment (CPC-1275) from Pablillo, State of Nuevo León. The specimens were collected from marls of the La Caja Formation, a precise origin of layers is not known. Dimensions are provided in Table 5 and abbreviations used therein are explained in Figure 4.
Description. Medium-sized (L ≈ 32.7 mm), thin -shelled valve with a weak lateral depression, keel and a keel slope. The outer margin is not preserved, but the distribution of ribs indicates that a short, convex outer margin existed. The lateral margin is wide and convex, the harmonic margin (S = 30.7 mm) is straight. A reconstruction of the outline, based on the distribution of ribs, indicates an equal position of the maximum width (pW ≈ 30 mm) and the apex (S ≈ 10.7 mm), with an S/L value of approximately 0.94. Ribs are prominent, wall-like upward-projected, leaning towards the lateral margin (Fig. 7c2). The cross-section of the ribs is rounded to sub-acute. Ribs are slightly higher than wide. Specimen CPC-1351 represents an S-shaped band of ribs between the apical region and at about midway to the lateral margin. A double S-shaped bend of ribs is seen during later ontogenetic stages leaning towards the lateral margin (Fig. 7c2). The smaller-sized specimen CPC-1276 (Fig. 7e) only presents one S-shaped bend of ribs. Broad furrows separate the ribs, but they are less wide when compared to the width of associated ribs. The majority of ribs end at the harmonic margin, but those at the outer edge generate an angle of 24° to 35° with the latter. Between the keel and the keel slope, ribs are inflected and curve towards the harmonic margin.
Discussion. Our specimens exhibit the typical ribbing for the genus Lamellaptychus Trauth, 1927 (cf. Měchová et al., 2010, fig. 7D). Our specimens are only fragmentarily preserved, and the upper part of the lateral margin is mostly missing. However, the ribbing and dimensions of Lamellaptychus sp. (López-Caballero, 2009, pl. 10, fig. D4) of our specimens are characteristic for Lamellaptychus murocostatus Trauth, 1929, that is described in detail by Trauth (1938). According to Trauth (1938, p. 142), the W/L ratio is approximately 0.63 (0.56 in Renz, 1978, p. 502); ribs are slightly higher than wide and accompanying furrows are slightly less wide. The width of these furrows is gradually decreasing towards the inner margin and the values of pW and S are almost equal (pW/S = 0.90 in Renz, 1978, p. 502).
Lamellaptychus beyrichi (Oppel, 1865) is similar to L. murocostatus , but, according to Trauth (1938, p. 142), the latter differs in the presence of wall-like, upturned ribs leaning towards the lateral margin.
Occurrence. Lamellaptychus murocostatus Trauth, 1929 was previously documented from the Oxfordian of the western North Atlantic (Site 105, Leg XI, Deep Sea Drilling Project; Renz, 1978) and from the Tithonian of Austria (Trauth, 1929, 1938). In Mexico, the taxon was reported from the Kimmeridgian upper Idoceras Beds of Puerto Piñones (López-Caballero, 2009, "layer 8E" in fig. 3) and from the upper Kimmeridgian Procraspedites-Taramelliceras interval (see Villaseñor et al., 2012) of Sierra de Palotes, State of Durango (Villaseñor and González-Arreola, 1988).
Lamellaptychus murocostatusis here described from the Kimmeridgian or Tithonian of the La Caja Formation at La Mesita, State of San Luis Potosí, and the Kimmeridgian or Tithonian of Rincón del Agua Dulce and Pablillo, both in the State of Nuevo León.
3. Discussion
The Upper Jurassic aptychus faunal assemblage of northeastern Mexico is low in diversity when compared to time-equivalent aptychus occurrences of the Tethyan Realm. The species reported here from the Kimmeridgian to Tithonian are of limited stratigraphical distribution in Mexico. While Lamellaptychus is restricted to the upper lower Kimmeridgian-lower upper Tithonian, records of the genus Laevaptychus reach from the uppermost Kimmeridgian to uppermost Tithonian.
All identified Kimmeridgian-Tithonian aptychi were previously known from the Tethyan Realm. Laevaptychus (Latuslaevaptychus ) latus meyrati Trauth, 1931, Laevaptychus (Autharislaevaptychus ) favrei Trauth, 1931 and Lamellaptychus lamellosus (Parkinson, 1811) are here described for Mexico for the first time, which thus extends their palaeogeographic distribution from the Tethys into the western hemisphere. Migration may have been via the Hispanic Corridor, a pathway that likely existed since the Early Jurassic, connecting the Tethys with the Gulf of Mexico and the Pacific Ocean (e.g ., Damborenea, 2000; Aberhan, 2001). Less well-known, however, and quite questionable to date is the hypothesis that this seaway also connected with the northwestern Gulf of Mexico (e.g. NE Mexico) (e.g. , Verma and Westermann, 1973; Adatte et al., 1994, 1996). Positive evidence of faunal exchange is given since the earliest Cretaceous by abundant shared occurrences of ammonite, bivalve, belemnite and calpionellid taxa in both northeastern Mexico and Europe (e.g ., Trejo, 1960, 1976, 1980; Stinnesbeck et al., 1993; Adatte et al ., 1994, 1996; Zell et al., 2013, 2014), but earlier, i.e. latest Jurassic faunal migration pathways, are less well understood. It appears that this pathway was only intermittently open for short intervals of the Kimmeridgian and Tithonian stages, as indicated by short intervals with Boreal belemnites (Zell et al ., 2013), the bivalve Anopaea (Zell et al ., 2015) and cold-water-indicative ammonites (Zell and Stinnesbeck, 2015), and now by the occurrence of Tethyan aptychi.
On the other hand, no endemic aptychi were identified during our survey. This result is unexpected considering the rare access to the northwestern Gulf of Mexico, of Late Jurassic European, or Tethyan, faunal elements, including the high portion of endemic ammonite taxa (e.g ., Verma and Westermann, 1973; Villaseñor et al., 2000, 2012). Aptychi do not appear to follow that pattern, which cannot presently be explained.
4. Conclusions
Here we describe a new and relatively complete aptychi assigned to Laevaptychus and Lamellaptychus . These individuals were collected in northeastern Mexico in the Kimmeridgian-Tithonian succession of the La Caja Formation. Lamellaptychus murocostatus Trauth, 1929 and Laevaptychus (Hoplisuslaevaptychus ) mexicanus (Castillo and Aguilera, 1895) were previously reported from Mexico, while Laevaptychus (Latuslaevaptychus ) latus meyrati Trauth, 1931, Laevaptychus (Autharislaevaptychus ) favrei Trauth, 1931 and Lamellaptychus lamellosus (Parkinson, 1811) are here described from Mexico for the first time. All identified taxa are well known from the Tethyan Realm, where they occur in time-equivalent strata, with exception of the Oxfordian Laevaptychus (Autharislaevaptychus ) favrei Trauth, 1931, which extends into the Tithonian of Mexico. The presence of Tethyan aptychi in Mexico provides additional evidence for faunal exchange across the Hispanic Corridor. The absence of endemic aptychi is quite unexpected, as only a few associated Kimmeridgian and Tithonian ammonite taxa are of Tethyan origin.