Simoceras, Villaseñor & Olóriz & González-Arreola, 2011

Villaseñor, Ana B., Olóriz, Federico & González-Arreola, Celestina, 2011, Lower Tithonian microconchiate simoceratins from eastern Mexico: Taxonomy, biostratigraphy, and palaeobiogeography, Acta Palaeontologica Polonica 56 (1), pp. 133-158 : 138-141

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 lytoceroid−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 semiforme / 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 / S. 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 doi:10.4202/app.2010.0030

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 S. 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 adoral 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).