Introduction

Upper Jurassic deposits from East-Central Mexico have long been investigated due to their importance for oil exploration. Cantú-Chapa (1967) gave valuable preliminary information about Middle Tithonian to Berriasian ammonites gathered from outcrops in the Mazatepec area, Puebla, Mexico. CantúChapa (1989) proposed a type-section for the Tithonian—Berriasian boundary in the Mazatepec area, and Adatte et al. (1992) provided a precise biostratigraphy for calpionellids, indicating the Jurassic—Cretaceous boundary in the area. On the basis of calpionellid and ammonite biostratigraphy, Stinnesbeck et al. (1993) questioned the previous proposal for the Jurassic/Cretaceous boundary type-section made by CantúChapa (1989). A further approach to sedimentology and diagenesis in Kimmeridgian deposits of the “San Andrés” Formation at Jonotla (Puebla) and Tlacolula (Veracruz), a region which includes the Mazatepec area, was made by Hernández De La Fuente (1996).

Institutional abbreviation.—BSPG, Bayerische Staatssammlung für Paläontologie und Geologie, Munich, Germany; CCS, Material housed in the Museo di Paleontologia del Servizio Geologico d'ltalia, Roma, Italy; GPIBO, Goldfuss-Museum und Paläontologisches Institut der Universität Bonn, Germany; IGM, National Paleontological Collection (Museo Maria Carmen Perrilliat Montoya), Institute of Geology, Mexico City, Mexico; NS3Col, Museo di Paleontologia dell'Università di Roma, Italy; Rin, Collection housed in the Comune di Piobicco, Pesaro, Italy; SMNS, Staatlisches Museum für Naturkunde, Stuttgart, Germany; UNAM, Universidad Nacional Autónoma de México, Mexico; USNM, United States Natural Museum, New York, USA.

Other abbreviations.—Ac, A; Cantú-Chapa collection; C, number of constrictions; CT, Colle Tordina (Monti della Rossa, Appennino Marchigiano), Italy; Dm, measured shell diameter; M, macroconchs; m, microconchs; FAD, first occurrence datum; MTQ, Mazatepec Quesos section in the Apulco River area in Fig. 2; PS, periumbilical sculpture; PS/2, number of periumbilical sculptural elements, ribs, bullae or tubercles, per half-a-whorl; U, umbilical size; U/D, size ratio between the umbilicus and shell diameter; Wh, whorl height; Wh/D, size ratio between whorl height and shell diameter; specimen number a, b, letters refer to mould and cast of the same specimen.

Fig. 1.

Location of the studied sections. A. Regional location for Puebla state. B. Inlet for the precise location of the sections investigated on the banks of the Apulco River in the Mazatepec area.

Geological setting

The geological locality Apulco River section MT-2 lies within the Apulco Valley, Sierra Norte de Puebla, near the village of Mazatepec, Puebla (Figs. 1, 2). This mountainous region belongs to the geologic subprovince of Eastern Sierra Madre (López Ramos 1979a: 291, figs. 6-1, 6–8) and the Mexican Fold-and-Thrust Belt (Suter 1990; a geologic province according to Ortega-Gutiérrez et al. 1992) where NNW—SSE trending folds are interrupted southward by rocks of the Transmexican Volcanic Axis or Belt.

Upper Jurassic outcrops above the Apulco River banks are difficult to access due to dense forest coverage in the area near Mazatepec. The Upper Jurassic section shows a lower, coarse bedded part containing mid-shelf to lower-ramp carbonates (San Andres Formation; Hernández De La Fuente 1996). Overlying it are thinner bedded, brownish to grey-black siltstones with silty interbeds and occasional calcareous horizons showing common, mainly parallel, fine cross lamination (upper Lower Kimmeridgian and Tithonian Tamán Formation; see Cantú-Chapa 1971 for regional biostratigraphy and correlation). Regional deepening is 108° to 308° NE, and common faulting impedes precise lateral observation and correlation among outcrops. Evidence of synsedimentary sliding (slumps) and horizons containing more calcareous concretions are common in the Lower Tithonian succession. Coverage by alluvium (severe flooding during 1999) affects the Tithonian deposits. Cherty intercalations in grayish to darkish and more calcareous clayey limestones characterise the Pimienta Formation (López Ramos 1979b: 315).

The new Apulco section MTQ crops out along the west bank of the Apulco River, close to the hydroelectric station (Figs. 1, 2) and shows a 22 m thick succession made of brownish, more or less calcareous siltstones and silty limestones (Taman Formation) containing decimetre to metre size concretions in the lower and middle part of the section.

Historical background

An overview of Upper Jurassic rocks and palaeoenvironments in the Apulco Valley, located in Sierra Norte de Puebla between Jonotla (Puebla) and Tlacolula (Veracruz), was given by Hernández De La Fuente (1996). In studying fossiliferous sections of the area, this author included some outcrops mentioned by Cantú-Chapa (1971: fig. 1). In his geological locality Apulco River, Hernández De La Fuente (1996) collected Idoceras sp., Mazapilites sp., and Uhligites sp. (without illustration), and mentioned the common occurrence of ammonites within the Tamán Formation.

Over the past ten years, two of us (ABV and FO) have studied outcrops around the Apulco River as part of a research programme focusing on sections of the San Andrés, Tamán, and Pimienta Formations, to provide a precise biostratigraphy based on bed-by-bed sampling (e.g., Villaseñor et al. 2000a, 2003; Villaseñor and Olóriz 2001, 2009; López-Caballero 2006; López-Caballero et al. 2007).

This paper presents the palaeontological analysis carried out on the first specimens of simoceratins collected bed-bybed in Mexico from Lower Tithonian siltstones of the Tamán Formation cropping out along the Apulco River, Mazatepec, Puebla (Figs. 1, 2). The studied material was collected from Apulco section MT-2 (Villaseñor et al. 2003; López-Caballero et al. 2007; Villaseñor and Olóriz 2009) and the new MTQ section.

Fig. 2.

Stratigraphic sections with indication of precise records of Pseudovolanoceras aesinense (Meneghini, 1885) and Pseudovolanoceras aesinense chignahuapense (Cantú-Chapa, 1990) in the Apulco sections MT-2 and MTQ. Note the occurrence of other ammonites significant for biostratigraphy and correlation.

Records of simoceratins from the Americas.—Precise information about the record of Lower Tithonian simoceratins from the Americas is rare, and to date only four specimens have been reported with illustration, aside from some text citations. Krantz (1928) identified the southernmost record, from Argentina, while the other three records were reported by Imlay (1942) from Cuba, and Verma and Westermann (1973), and Cantú-Chapa (1990) from Mexico. Among citations without illustrations, Leanza and Hugo (1977) alluded to Krantz's specimen and reported Simoceras aff. S. volanense Krantz, 1928 non Oppel (1863) from the same section or area studied by Krantz (1928); Leanza and Zeiss (1990, 1992, 1994) and Parent and Capello (1999) cited the occurrence of Simoceras in the Neuquen Basin, Argentina; Leanza (in Leanza and Zeiss 1990) cited Volanoceras from the same locality, and Parent (2001) and Parent et al. (2006) mentioned Volanoceras krantzense Cantú-Chapa, 1990. The mention without illustration made by Krantz (1928) of a geographically intermediate record of Simoceras in Peru (Welter 1913 in Krantz 1928: 13) was not taken into account in later revisions of simoceratins, and was unavailable for control. Based on the descriptions provided by all these authors, the following additional comments concern the interpretations made below.

Krantz (1928: pl. 3: 7) described Simoceras aff. volanense Oppel, 1863 from his locality 30 (surroundings of Barda Blanca, Argentina). The specimen was collected from clayey marls of the Middle Tithonian, together with Aspidoceras (Pseudhimalayites) steinmanni Haupt, 1907 and Pseudolissoceras zitteli Burckhardt, 1903, among other ammonites. Basically, Krantz (1928: 13–14) described his specimen of 85 mm in diameter as an extremely evolute and strongly sculptured form. In the outer whorl, radial periumbilical tubercles are connected by ribs with peripheral ones elongated towards the aperture. The beginning of the outer row of tubercles in the third whorl precedes that of the periumbilical tubercles. Krantz (1928) recognised differences between the Argentinean specimen and the European Simoceras volanense Oppel, 1863 (in Zittel 1870), namely the absence of the constrictions described by Zittel (1870), although he noted some more excavated inter-rib spaces in the Argentinean specimen. Krantz (1928) distinguished his specimen from Simoceras schwertschlageri Schneid, 1915 on the basis of its stronger, more radial sculpture. Later interpretations of this specimen include those of Santantonio (1986), Cecca (1999: 22) and Carlo Sarti in personal communication to Parent and Capello (1999), who reinterpret the Argentinean specimen as Simoceras aesinense (Meneghini, 1885); Fözy (1988) raises doubts about the species- and even genus-level meaning; Cantú-Chapa (1990) put forth the species Volanoceras krantzense CantúChapa, 1990, a proposal followed by Parent (2001), Cecca (2002a), who considered Simoceras (Pseudovolanoceras) krantzense Cantú-Chapa, 1990 a genuine species after examination of a gypsum cast, Schweigert et al. (2002) and Parent et al. (2006); and Villaseñor et al. (2000b) foresee the possibility of revealing a local subspecies of S. aesinense (Meneghini, 1885) (see below).

Imlay (1942: pl. 3: 2–3) described Simoceras sp. juv. cf. volanense (Oppel, 1863) from the Viñales Limestone in the northeastern slope of Loma Sabanilla, Santa Clara Province, north-central Cuba, locality 268 of the Atlantic Refining Company of Cuba. Imlay (1942: pl. 3: 2–3) identified a very loose coiling, subquadrate whorl section with maximal thickness near the middle of gently convex flanks, and a coarse sculpture in the specimen 11 mm in size (note three-time magnification indicated by Imlay). This author also observed prominent and slightly prorsiradiate ribs terminating in coarse tubercles on the shoulders. Imlay (1942: 1446) noted the resemblance of the Cuban specimen with both the European Ammonites volanense (Oppel 1863: pl. 58: 2a, b), which was included in Simoceras, and the South American Simoceras aff. volanense Oppel in Krantz (1928: pl. 3: 7). Later, Cecca (1999: 22) interpreted the Cuban specimen as S. aesinense (Meneghini, 1885) while Schweigert et al. (2002: 8) reinterpreted this species as belonging to Volanoceras, a younger synonym of genus Simoceras according to Cecca (1999: 11).

Verma and Westermann (1973: 196, pl. 37: 2) described Simoceras cf. S. volanense (Oppel, 1863) from the Virgatosphinctinae Beds, upper part of the El Pastor Member, La Caja Formation, in locality 40C: a creek close to the small village of El Pastor in the southern part of Sierra de Catorce, San Luis Potosí, Mexican Altiplano. These authors recognised that the specimen was somewhat distorted, and they describe loose coiling, a subquadrate to depressed whorl section, and blunt ribs between two rows of tubercles in the specimen of 72 mm under study. They also observed a constriction at the end of the preserved inner mould, which they interpreted as a probable indication of complete growth, and the possible existence of constrictions in the inner whorls. Verma and Westermann (1973: 196–197) assumed a close affinity with Simoceras aff. volanense Oppel in Krantz (1928), which was considered synonymous with Oppel's (1863) original, and denser ribbing in Simoceras sp. juv. cf. volanense Oppel in Imlay (1942). Later interpretations considered the specimen described by Verma and Westermann (1973) as: Simoceras (Volanoceras) cf. volanense (Oppel, 1863) by Callomon (1992), who envisaged an age corresponding to the mid-to-upper Lower Tithonian in Europe; Simoceras close to S. aesinense (Meneghini, 1885) (Olóriz et al. 1999: 478); S. aesinense (Meneghini, 1885) (synonym Simoceras schwertschlageri Schneid, 1915) (Cecca 1999), Simoceras of the group of S. aesinense (Meneghini, 1885) (Villaseñor et al. 2000b), S. aesinense (Meneghini, 1885) (Villaseñor et al. 2000a: A-469) with the local subspecies S. aesinense chignahuapense (Cantú-Chapa, 1990) for eastern and north-central Mexico and doubts as to how many subspecies of S. aesinense (Meneghini, 1885) were in the Americas; and Volanoceras krantzense Cantú-Chapa, 1990 (Schweigert et al. 2002: 9).

Cantú-Chapa (1990: 41–45, fig. 2a–d) described Volanoceras chignahuapense sp. nov. from Lower Tithonian deposits that crop out in Chignahuapan, Puebla, central-eastern Mexico. The specimen of 40.13 mm in shell size was gathered from a calcareous concretion and magnified at 1.4 and 2.2 in Cantú-Chapa (1990: fig. 2a, b and 2c, d, respectively). This author noted a serpenticone shell with a rectangular whorl section, two to three constrictions per complete whorl, and simple, flattened ribs as wide as the inter-rib spaces. He also described periumbilical swelling of ribs and periumbilical tubercles on the inner whorls (the latter unidentifiable in his illustrations), and ventrolateral termination of ribs in spiny, tangentially elongated tubercles. Cantú-Chapa (1990) indicated Volanoceras (Geyssant, 1985) as the most appropriate genus-level interpretation, and therefore made a detailed examination and comparative analysis of supposedly conspecific forms showing simple or bifurcate ribs on the inner whorls (as discussed in point d below). This author recognised a particular affinity with Simoceras aff. volanense Oppel in Krantz (1928) through ventrolateral tuberculation, and rightly remarked that the Argentinean specimen is distinguished by an absence of constrictions, a feature that he also used to differentiate Ammonites volanense in Oppel (1863) from the specimen described by Krantz (1928). Cantú-Chapa (1990) likewise proposed the interpretation of the specimen described by Krantz (1928) as Volanoceras krantzense sp. nov., and thus a separation from the European “Simoceras cfr. volanense Oppel, 1863 sp. var. aesinense” in Meneghini (1885), (= Simoceras, Volanoceras or Pseudovolanoceras aesinense Meneghini, 1885 of several authors). Cantú's (1990) proposal (Cantú-Chapa 1990: 43) assumed the occurrence of bifurcate ribs in Volanoceras aesinense (Meneghini, 1885); but they are absent from both the corresponding type specimens. CantúChapa (1990: 43) rejected the conspecific relationships interpreted by Geyssant (1988) for the Italian species V. aesinense (Meneghini, 1885) with all simoceratin specimens referred to Tithonian Simoceras in the Americas. Concerning the small Cuban specimen (Imlay 1942), Cantú-Chapa (1990) stated that because it was a juvenile it would not be used for comparative analyses (but see point e below). Moreover, according to Cantú-Chapa, the Mexican specimen described by Verma and Westermann (1973) could not be interpreted at the species level because of poor preservation. Cantú-Chapa (1990: 43) rightly separated his Mexican specimen from Simoceras volanense Oppel in Zittel (1870) and Simoceras schwertschlageri in Schneid (1915). Finally, Cantú-Chapa (1990: 43, 45) outlined similarities and differences between Volanoceras chignahuapense sp. nov. and the closest species V. aesinense (Meneghini, 1885), indicating similarity in tuberculation and constrictions but absence of bifurcate ribs in the Mexican specimen. As commented above, the type of “Simoceras cfr. volanense Oppel, 1863 sp. var. aesinense” in Meneghini (1885: 376) does not show bifurcations, and both these forms furthermore show an exclusive character typical of the Meneghini' species, namely the longitudinal furrow between tubercles on the shoulders in moulds. This feature was not analysed by Cantú-Chapa (1990), but it is seen clearly in his fig. 2a, b. Later interpretations of Cantú's species Volanoceras chignahuapense Cantú-Chapa, 1990, refer to Cecca (1999: 26) and Schweigert et al. (2002), who assumed Volanoceras chignahuapense Cantú-Chapa, 1990 as conspecific with Simoceras aesinense (Meneghini, 1885) but belonging to different genera/subgenera (Simoceras and Volanoceras, respectively), whereas Villaseñor et al. (2000a) considered it to be the geographic subspecies Simoceras aesinense chignahuapense (Cantú-Chapa, 1990). Cecca (2002a) did not change his interpretation at the species level (e.g., Cecca 1999), but included it in his new subgenus Pseudovolanoceras.

With regard to the hypotheses given below, this revision of interpretations of Lower Tithonian simoceratids from the Americas features some significant points:

(i) The early separation of the Argentinean specimen from the European species S. volanense Oppel, 1863 made by Krantz (1928), who compared two supposedly adult specimens.

(ii) The similarity Imlay (1942) found between his Cuban nucleus, assumed to be juvenile, the European species S. volanense Oppel, 1863, and the Argentinean specimen described by Krantz (1928). This does not indicate defective analysis, since the very nucleus (precisely the part that is barely observable in the Cuban specimen but crucial for interpretation) is not well preserved in the European Ammonites volanense in Oppel (1863) and S. volanense Oppel in Zittel (1870). The outer whorls in the latter were considered to belong to other species V. aesinense (Meneghini, 1885) by Geyssant (1985), a reinterpretation that could apply to the complete illustration (e.g., Santantonio 1986). In fact, true A. volanense Oppel, 1863 nuclei have been characterised by the occurrence of dominant bifurcates below ca. 20 mm, which was considered by Santantonio (1986) to be crucial for differentiation of the morphologically close species S. aesinense (Meneghini, 1885) and S. volanense (Oppel, 1863). The nucleus studied by Imlay (1942) basically coincides with that of the Argentinean specimen described by Krantz (1928). As mentioned above, the latter specimen was interpreted as Volanoceras krantzense Cantú-Chapa, 1990 by Schweigert et al. (2002) following Cantú-Chapa (1990), admitting the occurrence “pseudo-bifurcations” in nuclei and an intermediate, evolutionary place between V. aesinense (Meneghini, 1885) and V. schwertschlageri (Schneid, 1915). “Pseudo-bifurcations” for these authors (Schweigert et al. 2002: fig. 5a) refer to the sculptural structure resulting from the incidence of growth increments on ribbing orientation on the flanks. Connected to growth increments, these structures must show congruent orientation with adjacent, later simple ribs. However, it is not the sole case among interpreted Volanoceras illustrated by Schweigert et al. (2002; e.g., V. krantzense Cantú-Chapa, 1990 in pl. 2: 2 showing a specimen from Spain with an incorrect reference in the figure caption) and real bifurcate ribs occur (e.g., in the same illustration and in a later ontogenetic phase in Volanoceras schwertschlageri (Schneid, 1915, illustrated in their pl. 3: 1a)—i.e., in contrast to “pseudo-bifurcations”, real bifurcate ribs show neither a congruence of rib projection on the flanks nor necessarily imply relation with more or less accentuated constrictions.

(iii) The interpreted affinity proposed by Verma and Westermann (1973) between their specimen collected from Sierra de Catorce, Mexico, and those described from Cuba (Imlay 1942) and Argentina (Krantz 1928), the latter being considered synonymous by these authors in contrast to some later interpretations (see above).

(iv) The artificial, compound character that Geyssant (1985: 679) interpreted for the illustration of S. volanense Oppel in Zittel (1870: pl. 32: 7), who interpreted the inner whorls as resembling A. volanense in Oppel (1863: pl. 58: 2a, b) while the outer evoked “Simoceras cfr. volanense var. aesinense” in Meneghini [1885 = Volanoceras aesinense (Meneghini, 1885) in Geyssant's (1985) interpretation]. Even if coinciding with Geyssant's (1985) observation (but see Santantonio 1986), assuming no difference in ribbing on the inner whorls among species of his new genus Volanoceras, the reference to Zittel's (1870) illustration does not help to clarify a crucial difference: the occurrence of bifurcate ribs on the inner whorls in Ammonites volanense Oppel, 1863 and the occurrence of simple ribs in Simoceras aesinense (Meneghini, 1885) (see above comments on “pseudo-bifurcations” sensu Schweigert et al. 2002). This distinction, unappreciated in interpretations subsequent to Oppel (1863: 232, pl. 58: 2a), was highlighted by Santantonio (1986), who described bifurcate ribs on the inner whorls of Simoceras volanense Oppel, 1863 (at least up to 18 mm) as he interpreted this taxon. In fact, close analysis of the nucleus in Oppel's illustration (1863: pl. 58: 2a) at less than 20 mm (see also Schweigert et al. 2002: pl. 4: 1 under magnification ×300) appears to reveal convergence of ribs close to the whorl overlap. Unfortunately, sculptural details in Zittel's (1870) illustration are unappreciable, most likely due to unfavourable preservation of the original specimen.

(v) Comments by Cantú-Chapa (1990) are relevant in two senses. Firstly, the close affinity between his Mexican species, the Argentinean specimen described by Krantz (1928) and S. aesinense (Meneghini, 1885) was soundly based on tuberculation, but he over-emphasised the sole difference in the development of constrictions with respect to the former, and did not observe the crucial shallow furrow recognisable in moulds of inner whorls of the species S. aesinense (Meneghini, 1885) as well as in the specimen he described (e.g., Cantú-Chapa 1990: fig. 2d). Other differences are minor or do not exist (e.g., bifurcate ribs in S. aesinense Meneghini, 1885). Secondly, he rightly interpreted that a sole nucleus (in reference to the Cuban specimen described by Imlay 1942) must not be used for comparative analysis at the species level (and, by extension, at the genus level); nevertheless, he worked with a potential juvenile and/or incomplete specimen showing probably less than one-fourth of the body chamber at 40.13 mm, and described his new species as included in the genus Volanoceras Geyssant, 1985. Villaseñor et al. (2000a) gave a preliminary interpretation as S. aesinense chignahuapense (Cantú-Chapa, 1990) for this incomplete specimen described by Cantú-Chapa (1990), while Cecca (2002a) proposed it as a junior synonym of the Meneghini (1885) species.

In addition to the above, we agree with the guidelines given by Santantonio (1986) for the precise differentiation of Tithonian simoceratin species, i.e., the occurrence of dominant bifurcate ribs vs. simple ribs in their nuclei. Hence, we assume a well established fact in the species group of S. aesinense (Meneghini, 1885) and A. volanense Oppel, 1863—i.e., different nuclei with simple and bifurcate ribbing, respectively. Genus Volanoceras, as proposed by Geyssant (1985), is therefore polyphyletic, as it has been the common interpretation for Simoceras, even when the use of subgenera is restricted (e.g., Fözy 1988).

Material and methods

In both of the sections investigated bed-by-bed, macrofossils —mainly bivalves and ammonites (1200 specimens and fragments)—were found in discontinuous horizons within the stratigraphic interval containing simoceratins. In general, macrofossil preservation is moderate to poor (imprints and leaf preservation sensu Seilacher et al. 1976), and preservation in volume as inner moulds is rare. Haploceratidae, perisphinctids and simoceratins predominate in ammonite assemblages where most of the specimens can be identified only at the genus level.

The simoceratins described were found in the lowermost 3.5 m of the Apulco section MT-2, and in the upper 10 to 12 m of the new Apulco section MTQ (Fig. 2). The total number of identified specimens was twenty-one. Among stratigraphically collected specimens, fifteen belong to section MT-2, and six specimens were gathered from bed MTQ-11 in section MTQ.

Biochronostratigraphy was conducted according to the Secondary Standard of reference for the Tithonian (e.g., Geyssant 1997).

Preparation of the material and shell/mould measurement (in millimetres; see Appendix 1) was mainly performed in the Laboratory for Invertebrates of the UNAM, with complementary work carried out at the University of Granada, Spain.

Notes on Simoceratinae: systematics, taxonomy, and evolutionary patterns

Several proposals have been put forth over the past 30 years regarding systematics, taxonomy, and evolution in Lower Tithonian simoceratins above the species level (e.g., Olóriz 1978; Geyssant 1982, 1985, 1988; Santatonio 1986; Fözy 1988; Cecca 2002a; Schweigert et al. 2002). An abridged review is given below.

Olóriz (1978) interpreted a comprehensive genus Simoceras differentiated into morphologic, evolutionary subgenera according to basic phenotype differences through ontogeny, which starts with an initial perisphinctoid-stage followed by: (i) tuberculate or simoceroid-stage (subgenus Simoceras); (ii) shell smoothing or Iytoceroid-stage (Lytogyroceras); or their combination (subgenus Simolytoceras). Olóriz (1978: 237) envisaged three potential interpretations of evolution at the subgenus level, which can be simplified in terms of: Iterative evolution from a perisphinctoid nucleus (then considered as evidence of phylogenetic connection with Idoceratinae); and anagenetic and/or cladogenetic evolution from the basic simoceroid-stage. This author acknowledged difficulties with stratigraphic condensation in obtaining fine stratigraphy to clarify the relative FADs of Simolytoceras and Lytogyroceras, and did not study the species S. aesinense (Meneghini, 1885) among the rare (then considered Simoceras volanense Oppel, 1863 group) simoceratins collected from Haploceras verruciferum Biozone deposits in ammonitico rosso facies (Semiformiceras semiforme/Haploceras verruciferum Chronozone in the Secondary Standard for Tethyan areas; e.g., Geyssant 1997). In addition, this author identified a “simoceratin-like” morphology in one of the two groups interpreted as Virgatosimoceras (Olóriz 1978: 204), overemphasising their perisphinctoid nuclei resembling those shown by the older Nebrodites as well as the occurrence of Idoceras-like rib furcations.

Santantonio (1986) analysed abundant, well-preserved material, rightly typified traits' recognition for identification of Lower Tithonian simoceratin species, and considered common, defective preservation in ammonitico rosso facies as a major difficulty in separating Simoceras aesinense (Meneghini, 1885) from the S. volanense Oppel, 1863 group (genus level interpretation according to Santantonio 1986). He put forth Virgatosimoceras as a key-branch for evolution in Lower Tithonian simoceratins, potential polyphyly for the S. volanense Oppel (1863) group, as well as an obscure origin for S. aesinense (Meneghini, 1885), and recommended future separation of the latter at the genus or subgenus level. S. aesinense (Meneghini, 1885) was interpreted as a separate branch within the polyphyletic genus Simoceras.

Geyssant (1982, 1985, 1988) interpreted the evolutionary pattern in Lower Tithonian simoceratins and proposed punctuated equilibrium and palaeogeographic dynamics as the appropriate template for their interpretation. Geyssant (1982) interpreted genus Simoceras in a broad sense, as usual at the time, and identified “Simoceras n. sp. gr. volanense Oppel, 1863” from the Semiformiceras semiforme Biozone (equivalent to the Haploceras verruciferum Zone = Semiformiceras semiformel Haploceras verruciferum Chronozone in the Secondary Standard for Tethyan areas; e.g., Geyssant 1997), envisaging a widespread geographic distribution (southern Europe, Cuba, Mexico, and Argentina). This author interpreted discontinuous in-situ evolution, allopatric speciation and recolonisation by species belonging to genus Simoceras in southern Europe and nearby northern areas (Franconia). Based on ICZN rules, Geyssant (1985) restricted the use of genus Simoceras to the Simoceras admirandum Zittel, 1869/ Simoceras biruncinatum Quenstedt, 1847 group and erected the new genus Volanoceras for Lower Tithonian simoceratins such as V. aesinense (Meneghini, 1885), V. schwertschlageri (Schneid, 1915), and V. volanense (Oppel, 1863). Implicit in Geyssant's (1985) interpretation is the polyphyletic character of her new genus Volanoceras, while Simoceras turned to be monophyletic. No particular comments about V. aesinense (Meneghini, 1885) were provided. Geyssant (1988) revisited her punctuated equilibrium hypothesis, now applied to Volanoceras that she envisaged as a single monophyletic linage.

Fözy (1988) approached the interpretation of Simoceratinae promoting the use of assumed monophyletic taxa, and followed Geyssant (1985, 1988) when analysing Volanoceras, thus anticipating hypotheses later detailed by Schweigert et al. (2002). However, this author did not provide an evolutionary pattern at the subfamily level, and the degree of monophyly resulted heterogeneous in the taxa he identified in simoceratins. In his study of the material gathered from ammonitico rosso and related facies containing envisaged juveniles and adults, Fözy (1988) approached dimorphic couples at the subgenus level following Callomon (1969).

In 2002 the most recent reinterpretations of Tithonian simoceratins were published, in parallel, by Fabrizio Cecca and Günter Schweigert and collaborators. Schweigert et al. (2002) revisited Lower Tithonian simoceratins and reinterpreted Volanoceras' species, their stratigraphy, palaeobiogeography and evolution, partially based on bibliographic re-evaluations; they proposed invalidation of the oldest species related to the S. aesinense (Meneghini, 1885) group (S. praecursor in Santantonio 1986) and promoted recuperation of an old species name (Ammonites perarmatiforme Schauroth, 1865) for the youngest specimens reported. These authors envisaged a chronocline for Volanoceras and, following Geyssant (1985), did not recognise relevant differences in inner whorl sculpture between the V. aesinense (Meneghini, 1885) and V. volanense (Oppel, 1863) groups. Hence, the potential polyphyly derived from Santantonio's (1986) model was overlooked. Schweigert et al. (2002) interpreted V. aesinense (Meneghini, 1885) including European, Cuban, and east-Mexican records, and confirmed species-level status and conspecifity for the Argentinean and central Mexico records. On the other hand, Cecca (2002a) basically followed Santantonio (1986) and proposed the new subgenus Pseudovolanoceras for S. aesinense (Meneghini, 1885) and related species from the Americas. This author recognised difficulty with Volanoceras' evolution as proposed by Geyssant (1982), due to limitations forced by both the defective preservation of inner whorls and the precise biostratigraphic interpretation of some species crucial for Geyssant's (1982) interpretation. He therefore proposed species reorganisation among Lower Tithonian simoceratins, which were allocated in three morpho-evolutionary subgenera within genus Simoceras: S. (Simoceras) restricted to the S. admirandum Zittel, 1869/5. biruncinatum Quenstedt, 1847 group, S. (Volanoceras) for the S. volanense Oppel, 1863/S. vicentinum group, and S. (Pseudovolanoceras) for S. aesinense (Meneghini, 1885) and related species.

On the basis of all the above, we consider two alternative options for the present interpretation of Lower Tithonian simoceratins above the species level: (i) interpreting a comprehensive genus Simoceras subdivided in subgenera with evolutionary, not merely morphologic meaning (e.g., Olóriz 1978; Cecca 2002a); or (ii) interpreting evolutionary relevance at the genus level, meaning that no subgenera apply. In option (i) the genus level is strictly nominal and provides the closest allusion to the immediately higher taxonomic level of reference (e.g., subfamily/family), and thus a higher level of polyphyly is assumed; whereas subgenera will provide phylogenetic information revealing relatively major phenotype breaks (innovations) within an assumed cohesive grouping of ammonites such as Simoceratinae. Alternatively, in option (ii) the genus level directly identifies these breaks in phenotype evolution (innovations forcing relative monophyly), with no direct information about the immediately higher level of taxonomic reference (family/subfamily level). At present, we favour option (ii), interpreting genus-level clade identification (through recognition of innovations) reinforcing taxonomy with evolutionary significance at this level—i.e., based on inner whorl sculpture as the crucial phenotype stage for the interpretation of higher-level taxonomy and evolution, and later morphological breaks (innovations) for clade-level, phylogenetic identification at the genus level.

This proposal is supported by (i) available information about precise biostratigraphy; (ii) iterative evolution of simoceratins from a “perisphinctoid”, Virgatosimoceras-like or closely related branch as the main evolutionary pattern; (iii) difficulty in approaching genus-level monophyly sensu stricto some other way; (iv) the polyphyletic character of the latter genus-level taxon created for Simoceratinae (Volanoceras as interpreted by Geyssant 1985, 1988 and Schweigert et al. 2002); and (v) avoiding maintenance of genera as large, heterogeneous and presumably polyphyletic species complexes. Accordingly, and based on the available information, the genus-level taxonomy proposed points to approaching the least inclusive taxonomic unit at this level.

At the species level, our interpretation accords with postulates derived from: (vi) phenotype cohesion (e.g., Templeton 1989); (vii) relevance of dynamic biogeography, at least for determining subspecies (e.g., Nelson and Platnick 1981); and (viii) recognition of species-flock, species complex and metapopulation-metacommunity concepts and their enlightening potential for interpreting cephalopod diversity, biogeography and underlying dynamics at the population and species levels (e.g., Yacobucci 1999; Norman 2003; Yoshida et al. 2006; Bolstad 2009; Gillanders et al. 2009; Olóriz and Villaseñor 2010, and references therein). Thus, the species level is approached within the conceptual framework favouring the relevance of environmental forcing, of biogeographic range processes, and autapomorphy (e.g., Van Valen 1976 and Ridley 1989 to complement citations above).

Remarks on the simoceratin species Simoceras aesinense (Meneghini, 1885) sensu Santantonio (1986).—Some observations concerning the ontogenetic course of Simoceras cfr. volanense Oppel, 1863 sp. var. aesinense in Meneghini (1885) deserve mention here.

Meneghini (1885: 376–378) described a specimen of 62 mm in size (which accords with his illustration in pl. 20: 4a, b), identified the last whorl as the last one preserved, and recognised both difficulty in identifying constrictions on the inner whorls and their deepness on the outer whorl. In addition, Meneghini (1885) noted depressed tubercles on shell periphery (shoulders), pointed ones on the umbilical edge, ribs between these two rows that changed around the middle of the penultimate whorl (29–30 mm in his illustration) to become ventrally enlarged towards the outer whorl while showing a small, and delicate tuberculation on the inner whorls; he likewise reported suture line details from the third whorl. In comparison with the Lower Tithonian species Ammonites volanense Oppel, 1863, Meneghini (1885) found differences in coiling degree and rib density on the inner whorls, differential crowding of ribs and morphology of the peripheral tubercles with respect to Simoceras volanense (Oppel in Zittel 1870: 95, pl. 32: 7–9), and lesser similarity in both the looser coiling and gradual change in sculpture with respect to A. volanensis in Oppel (1863: pl. 58: 2a, b). On this basis, Meneghini (1885) formalised the recognition of his “S. cfr. volanense Oppel, 1863 sp. var. aesinense” as the most frequent among the Simoceras that he compared from Tithonian deposits in the Marche (central Italy), indicating that specimens larger than 45 mm show spiny periumbilical and peripheral tubercles on the outer whorl preserved. In this whorl, the author described two constrictions towards the end, the posterior one deep and bordered by an aboral simple, slightly finer rib concave forwards and terminating in the corresponding spiny tubercle on the shoulder. A similar constriction was identified by Meneghini (1885) closer to the end of the preserved outer whorl, as well as another two early in the same whorl, four in the penultimate whorl, and a few more irregularly spaced ones on the innermost whorls that show ribs with spiny tubercles on the shoulders. Finally, Meneghini (1885) stressed the absence of suture lines adorally from the outermost constriction described.

Santantonio (1986) indicated that Meneghini's (1885) type was lost during the Second World War, but was able to analyse a cast, and promoted the name S. aesinense (Meneghini, 1885) as a valid species. On the basis of abundant and well preserved material, this author accurately described the ontogeny in the S. aesinense (Meneghini, 1885) species and interpreted its morphologic variability. He recognised dimorphism on the exclusive basis of shell size within a range of small to medium sized individuals (up to 65 and 94 mm, respectively), and identified the beginning of the body chamber at around 60 mm in macroconchs (between 20 and 35 mm in microconchs). In addition, he observed subquadrate whorl sections, loose coiling that decreased throughout the ontogeny, and no difference in sculpture between sexual dimorphs other than those related to shell size. Santantonio (1986) did not describe the type of peristome in microconchs (see previous allusion to his comments about defective preservation in rosso ammonitico facies), but interpreted crowded constrictions at the end of preserved shells and moulds as an indication of complete individuals. This author established the following ontogenetic phases in the sculpture of the species S. aesinense (Meneghini, 1885): (i) below 15 mm, ribs are simple, wider than inter-rib spaces, slightly concave adorally and terminating in small ventrolateral clavi that elongate tangentially during the ontogeny; (ii) at 30 mm, the periumbilical part of the ribs may have a comparatively higher relief, which coincides with their relative depression at the mid-flank, while ventrolateral clavi are prominent; (iii) around 40–45 mm the typical combination of progressive rib swelling, decreasing sculpture at the mid-flank, and the tangential expansion of clavi occurs, and typical individuals show radial-periumbilical bullae, a mid-flank depression with possible development of geminate ribs, and ventrolateral clavi elongated longitudinally. This third stage is accentuated in macroconchs larger than 60 mm, especially in development of bullae and the mid-flank germination of ribs. Santantonio (1986) gave fewer details about constrictions, but accurately noted their decreasing number (from around two in the inner whorls), their increasing excavation toward the outer whorls, and the persistent occurrence of relative sharp edges. In addition, he described the significant relief of clavi, which could be connected by subtle, wide folds of the ventral region. Santantonio (1986) placed special emphasis on the occurrence of a “cordone spirale” observable between clavi of well preserved shells, which determines an interclavi groove identifiable in phragmocones and body chambers preserved as inner moulds. Moreover, he mentioned small ribs (i.e., riblets) in the clavi walls of well-preserved specimens. In Santantonio's (1986) comparative analysis of the Lower Tithonian species S. aesinense (Meneghini, 1885) and S. volanense (Oppel, 1863) he stressed high variability in the change of bullae to periumbilical tubercles in specimens of greater size, and in their onset during the ontogeny (from 25–30 mm onwards), and gave the precise stratigraphic range for species S. aesinense (Meneghini, 1885) in coincidence with the Lower Tithonian Haploceras verruciferum/ Semiformiceras semiforme Chronozone in the Mediterranean Tethys.

Among the data provided by Meneghini (1885) and Santantonio (1986), the following points are of relevance for the interpretation of Lower Tithonian simoceratins from the Americas:

(i) Species S. aesinense (Meneghini, 1885) has small macroconchs (body chamber beginning as small as 60–70 mm). Recognition of the outer whorl in Meneghini's type as the last preserved (Meneghini 1885), and the clarification about a small final part without suture lines in the type (Meneghini 1985), indicate the possible existence of larger individuals (later confirmed by Santantonio 1986).

(ii) The noteworthy difference in recognition of constrictions indicates intra-species variability for this character.

(iii) Ribbing changes at ca. 30 mm, described by Meneghini (1885), coincide with the second ontogenetic stage described by Santantonio (1986), i.e., the beginning of periumbilical tuberculation together with mid-flank depression of ribbing. These two traits seem to be in co-variance.

(iv) Typical bituberculation comprising spiny-perpendicular and spiny-tangential elements above 45 mm (Meneghini 1885) agrees with the third ontogenetic stage of Santantonio (1986), who added comments that support co-variation in shell features.

(v) Ontogenetic stages and variability affecting sculpture expression (timing for change from bullae to periumbilical tubercles included) as described by Santantonio (1986).

(vi) Occurrence of subtly ribbed clavi and “cordone spirale” in relation with good preservation, in particular the appearance of the latter as a groove on inner moulds of phragmocones and body chambers (Santantonio 1986).

(vii) Identification of the precise stratigraphic range of 5. aesinense (Meneghini, 1885) within the Lower Tithonian (Santantonio 1986; genus-level interpretation according to this author).

(viii) Phenotypic expression of dimorphism. Interpreted on the basis of shell size without significant incidence in sculpture (Santantonio 1986), as commonly assumed for Lower Tithonian simoceratins (e.g., Geyssant 1988). No apertural structures are accurately known for macroconchs, and they are unknown in microconchs (in coincidence with Geyssant 1988). The sole reference to peristome in Tithonian simoceratins was made by Geyssant (1979: 15–16, text-fig. 15, pl. 2: 3), who envisaged a microconchiate peristome in correspondence with a small, pointed adorai projection separating dorsal and ventral concavities at the end of the shell in her Upper Tithonian genus Baeticoceras Geyssant, 1979, a younger synonym of Cordubiceras Olóriz and Tavera, 1979, as recently recognised by Benzaggagh et al. (2010).

Extensive reference to interpretations of Meneghini's (1885) species at the species level and higher ones is given above. At present, we interpret that “S. cfr. volanense Oppel, 1863 sp. var. aesinense” Meneghini, 1885 represents a widespread species, or species complex, inhabiting epioceanic environments in western Tethys but also, less commonly, epicontinental shelves in the area and the Americas, where geographical differentiation existed (see below).

Systematic palaeontology

Fig. 3.

Simoceratin ammonoid Pseudovolanoceras aesinense chignahuapense (Cantú-Chapa, 1990) [M]. A. Left-side view of IGM 9555 from MTQ section, bed 11; arrows showing body chamber, and incipient longitudinal furrow between tubercles (see text for allusion to “cordone spirale”). B–E from the Apulco section (MT-2). B. Left-side view of IGM 9547a, bed 22b. C. IGM 9545a, bed 22b. D. IGM 9545b, bed 22b. E. IGM 9548a, bed 21b. F. Simoceras aff. volanense Oppel, 1863 [type of Krantz 1928: pl. 3: 7; GPIBO 1 = Pseudovolanoceras aesinense krantzense (Cantú-Chapa, 1990)]; slightly oblique, right-side view of plastic mold; arrows for longitudinal furrow between tubercles (see text for allusion to “cordone spirale”). G. Simoceras cf. S. volanense (Oppel, 1863) [type of Verma and Westermann 1973: pl. 37: 2; IGM 2764 = Pseudovolanoceras aesinense (Meneghini, 1885) transitional form between Pseudovolanoceras aesinense chignahuapense (Cantú-Chapa, 1990) and Pseudovolanoceras aesinense krantzense (Cantú-Chapa, 1990)], close up view; arrows for longitudinal furrow between tubercles (see text for allusion to “cordone spirale”).

Fig. 4.

Simoceratin ammonoid Pseudovolanoceras aesinense chignahuapense (Cantú-Chapa, 1990) [microconch]. A–F. From the Apulco section (MT-2). A. Right-side view of IGM 6085-2a showing lappet, bed 21b. B. Right-side view of IGM 9541; eroded and slightly distorted specimen, bed 21b. C. IGM 9543a, close-up view showing incipient longitudinal furrow between tubercles (C₁) (see text for allusion to “cordone spirale”); right-side view (C₂), bed 25. D, E. IGM 9546a and b, both from the same flattened inner mould. D. Imprint of the left side. E. Left-side view; arrows for lappet and tubercles showing riblets, bed 25. F. Left-side view of IGM 9542a, bed 21b. G. IGM 9544; G₁, right-side view, arrow showing the last suture line preserved; G₂, close-up view from the body chamber showing tubercles with riblets (arrows); G₃, ventral view showing longitudinal furrow between tubercles, arrows indicate the trace of the “cordone spirale” (see text for allusion to “cordone spirale”); G₄, close-up view of inner whorls showing tubercles with riblets (arrows); from MTQ section, bed 11.

Fig. 5.

Bivariate plots for shell features of simoceratin ammonoid Pseudovolanoceras aesinense chignahuapense (Cantú-Chapa, 1990) against shell diameter of Mexican, Cuban and Argentinean specimens. A. Umbilicus size (U). B. Size ratio between the umbilicus and shell diameter (U/D). C. Size ratio between whorl height and shell diameter (Wh/D). D. Periumbilical sculpture per half-a-whorl (PS/2). Shell diameter (D).

Fig. 6.

Bivariate plots for shell features of simoceratin ammonoid Pseudovolanoceras aesinense chignahuapense (Cantú-Chapa, 1990) and Pseudovolanoceras aesinense (Meneghini, 1885)against shell diameter in American and European assemblages. A. Umbilicus size (U). B. Size ratio between the umbilicus and shell diameter (U/D). C. Size ratio between whorl height and shell diameter (Wh/D). Shell diameter (D).

Fig. 7.

Periumbilical sculpture per half-a-whorl against shell diameter (PS/D) (shadow area indicates tubercles as inner sculpture) in simoceratin ammonoid Pseudovolanoceras aesinense chignahuapense (Cantú-Chapa, 1990) and Pseudovolanoceras aesinense (Meneghini, 1885).

Fig. 8.

Simoceratin ammonoid Pseudovolanoceras aesinense chiagnahuapense (Cantú-Chapa, 1990), suture lines. A. IGM 9555 [M], last incomplete suture at 70 mm of shell diameter. B. IGM 9544 [m], incomplete suture at 26.19 mm of shell diameter.

Remarks on palaeobiogeography

The occurrence of Lower Tithonian simoceratins in the Americas has been known since the early 20th century, and their relation with European (Tethyan) species has been progressively accepted. On the basis of present information, the occurrence of simoceratin species in the Americas is interpreted as relating the arrival of Tethyan (ubiquitous) taxa to available shelf environments in these areas, a phenomenon that largely involved ammonites throughout Jurassic times. This scenario is compatible with a general model for interpreting patterns of ammonoid biogeography (Olóriz 1985, 1990), identified in the Americas (Mexican areas included) under a rather variable degree of understanding (see Olóriz 1985, 1987, 1990, 1992; Leanza and Olóriz 1987; Olóriz et al. 1988, 1990; 1999; Leanza and Zeiss 1992; Cecca 1999; Parent and Capello 1999; Villaseñor et al. 2000a, 2003; Parent 2001; Villaseñor and Olóriz 2001, 2004, 2009, Parent et al. 2006, among others, for precise appliance of the model and/or compatible interpretations). A conceptual complement to this template is provided by assuming biogeographic dynamics as recognised from present cephalopods (e.g., Norman 2003; Yoshida et al. 2006; Bolstad 2009; Gillanders et al. 2009) and the potential application to past cephalopod records (e.g., Olóriz et al. 2006; Olóriz and Villaseñor 2010; and references therein)—see Notes on Simoceratinae chapter above.

The occurrence of simoceratins in the Americas is associated with the most significant flooding recorded during the Early Tithonian (e.g., Olóriz et al. 1997, 1999; Villaseñor et al. 2003; and see Parent 2001 for biogeographic expansion of genus Pseudolissoceras). Support for this interpretation in Argentina are the records of Pseudovolanoceras aesinense krantzense (Cantú-Chapa, 1990) (Krantz 1928) together with Pseudhimalayites steinmanni (Haupt, 1907) (Krantz 1928) or with Pseudolissoceras zitteli Burckhardt, 1903 (Parent 2001), and of Simocosmoceras adversum andinum (Leanza and Olóriz, 1987), with Pseudhimalayites steinmanni (Haupt, 1907) (Leanza and Olóriz 1987), the latter reinterpreted as Pseudhimalayites subpretiousus (Uhlig, 1878) by Parent (2001). In Cuba, the only support available is the occurrence of P. aesinense (Meneghini, 1885) (nucleus illustrated by Imlay 1942) and Simocosmoceras sp. = Simocosmoceras pszczolkowskii Myczyński, 1989 (Myczyński 1989, 1990) according to the biochronostratigraphic interpretation made by Villaseñor and Olóriz (2001; see also Parent 2001). Villaseñor and Olóriz (2001) first described the occurrence of this genus in Mexico, later identified as Simocosmoceras pszczolkowskii apulcoensis Villaseñor, Olóriz, González-Arreola, 2003 (Villaseñor et al. 2003).

In Mexico, there are data and interpretations in accordance with the hypotheses related to the model mentioned above (Olóriz 1985, 1990), compatible as well with interpretations in the palaeobiogeographical overview provided by Cecca (1999). Of special relevance are: (i) the single specimen of Simoceras cf. S. volanense (Oppel, 1863) reported by Verma and Westermann (1973) from Sierra de Catorce; (ii) the biochronostratigraphic interpretation of the encasing Virgatosphinctinae Beds in Olóriz et al. (1999); (iii) the interpretation of these deposits as related to the Early Tithonian flooding mentioned above (Olóriz 1992; Olóriz et al. 1996, 1999; Villaseñor et al. 2000b); (iv) the interpretation of the specimen of Sierra de Catorce as evidence of the occurrence of S. aesinense (Meneghini, 1885) in Mexico (e.g., Geyssant 1988; Cecca 1999; Oláriz et al. 1999; Villaseñor et al. 2000b), here reinterpreted as P. aesinense (Meneghini, 1885), an intermediate phenotype between subspecies P. aesinense chignahuapense (Cantú-Chapa, 1990) and P. aesinense krantzense (Cantú-Chapa, 1990); (v) the reinterpretation of the record of a microconchiate simoceratin in Puebla (Cantú-Chapa 1990) as P. aesinense chignahuapense (Cantú-Chapa, 1990) and its close relationships to earlier interpretations concluding in close P. aesinense (Meneghini, 1885) affinity or even belonging to the latter species (e.g., Cecca 1999, 2002a; Villaseñor et al. 2000a, b; Schweigert et al. 2002); (vi) the combined record of Simocosmoceras and simoceratins in the Mazatepec area (Villaseñor and Olóriz 2001; Villaseñor et al. 2003); and (vii) the record of Pseudhimalayites steinmanni (Haupt, 1907) and the simoceratin species P. aesinense (Meneghini, 1885) (Villaseñor and Olóriz 2004; see above for their combined record in Argentina). All this information evidences the inter-related occurrence of Early Tithonian floodings in the Americas and the occurrence of Tethyan simoceratin species, here interpreted at the genus level as belonging to Pseudovolanoceras.

Whereas Mexico-Caribbean records of simoceratins (Pseudovolanoceras) and Simocosmoceras, assembled or not with Pseudhimalayites, accord with the hypothesis of faunal influence via the Hispanic Corridor, the record of Pseudovolanoceras aesinense krantzense (Cantú-Chapa, 1990) in Argentina, together with records of Simocosmoceras, Pseudhimalayites, and Pseudolissoceras within the Pseudolissoceras zitteli Biozone requires a more intricate configuration supporting the known palaeobiogeographic pattern of distribution of the species, or species complex, Pseudovolanoceras aesinense (Meneghini, 1885), in accordance with the assumed pattern of marine currents (e.g., Leanza and Olóriz 1987 for Simocosmoceras).

The palaeobiogeographical pattern known for American simoceratins accords with the model proposed by Olóriz (1985, 1990), which was later acknowledged by Cecca (1999) and is compatible with more recent overviews on palaeobiogeographical interpretations (e.g., Cecca 2002b). Accordingly, incursions of peripheral populations of widespread cephalopods species, here the case of Pseudovolanoceras aesinense (Meneghini, 1885), into American neritic shelves would have occurred when and where these were available during relative sea-level rises, regardless of their palaeogeographic location within the range of marine influence of Tethyan water-masses, but according to the pattern of marine currents (e.g., Olóriz et al. 1997, 1999, 2000; Villaseñor et al. 2003; Olóriz and Villaseñor 2006). Widespread species, or species complexes without relevant phenotype differences, would be subjected to metacommunity and metapopulation dynamics (formally first proposed for ammonites by Olóriz et al. 2006, and treated in depth by Olóriz and Villaseñor 2010), and fragmentation of bioeographic ranges would depend on environmental dynamics. Aside from expatriation, rare relevant post-mortem transportation, and their combination, (perhaps the case of the single specimen registered of species V. schwertschlageri [Schneid, 1915], which is allocated inside giant Aspidoceras, as noted by Schweigert et al. 2002), vicariancy (fragmentation) and/or temporary, ancillary, selective vicariance (e.g., Olóriz et al. 2008) better explains local records of low-swimming cephalopods showing multispecies assemblies containing a variable percentage in combining local (i.e., endemics) and Tethyan and/or Tethyan-like taxa (i.e., ubiquitous colonisers with persistent phenotype expression).

In this context, the explanation of Mexico-Caribbean records agrees with the role usually assumed for the Hispanic Corridor, but providing a regional example of the general dynamics, which supports the model proposed by Olóriz (1985, 1990) and its complement derived from information from present cephalopods (e.g., Olóriz and Villaseñor 2010). Hence, faunal expansion during flooding of neritic shelves forced phenotype deviation in simoceratin colonisers of Argentinean shelves (see Parent 2001 for Pseudolissoceras). Faunal capture resulting in subspeciation processes is here favoured to explain the occurrence of comparatively massive shells in Pseudovolanoceras aesinense krantzense (Cantú-Chapa, 1990), and other deviations such as those registered in eastern Mexico P. aesinense chignahuapense (Cantú-Chapa, 1990). Both of these cases reinforce the implications of Olóriz's (1985, 1990) model in terms of palaeobiologic response to a palaeobiogeographic dynamics promoting in situ evolution (e.g., Olóriz and Villaseñor 1999 for potential Early Tithonian example of species flock in neritic northcentral Mexico, and Olóriz et al. 2000 for a lower level phenotype influence in the same area). The Argentinean case reveals extreme phenotype expression, which cannot be related to hypermorphosis but rather to early innovation promoting massive shells without conclusive evidence, at present, of relative gigantism. Mexican records indicate phenotype deviation throughout the combination of heterochronic processes (acceleration) in microconchs, interpreted as resulting in allometric heterochrony (sensu McKinney 1988) since no precise information exists about the growth rate and age in these ammonites, and post-embryonic, late innovation.

The case-study could thus reveal early phases of selective vicariance owing to the coeval co-existence of new and ancestral forms in a restricted part of the biogeographic range of the latter, with possible niche subdivision (ecospace reorganisation) and species fragmentation inside a heterogeneous environmental/palaeogeographical complex.

On the basis of all the above, future research is encouraged to realise bed-by-bed sampling followed by palaeobiological and eco-evolutive interpretations of these ammonites in Mexico and Tethyan areas within the framework of a combined geo-biological approach.