Bajocian Research Articles

THE LOWER BAJOCIAN GAETANI LEVEL: LITHOSTRATIGRAPHIC MARKER OF A POTENTIAL OCEANIC ANOXIC EVENT

In this paper we document in detail the transition from the Rosso Ammonitico Lombardo to the Radiolarites outcropping at Alpe Turati (Albavilla, Como) in the Lombardy Basin of the Southern Alps. At this location, the Jurassic succession was deposited on a flank of the Corni di Canzo paleohigh: as found on other structural shallow areas, the upper Pliensbachian–Toarcian–Aalenian interval is represented by pseudonodular to nodular marly limestones of the Morbio Limestone and Rosso Ammonitico Lombardo units. In the early Bajocian deposition of biosiliceous sediments is ubiquitously testified by the Radiolarites (Bajocian–Callovian). At Alpe Turati, the interval immediately below the basal green member of the latter unit, consists of a 10 cm thick black shale, pointing to oxygen-depleted bottom waters. Dysoxia persisted during the Bajocian–Callovian as evidenced by dark green stratified cherts. Calcareous nannofossil biostratigraphy indicate an early Bajocian age for the black shale that correlates with the core of a distinct C isotopic negative anomaly also known in various basins outside the Southern Alps. Very dark grey to black lithologies of early Bajocian age have been documented in a few sections from Poland, Corsica and Morocco. We name Gaetani Level the black shale interval in recognition of Maurizio Gaetani’s pioneering and extensive work on Jurassic sedimentary successions of the Lombardy Basin. The association of the Gaetani black shale with a C isotopic anomaly suggests that it could be the sedimentary record of an Oceanic Anoxic Event whose regional to global extension must be ascertained.

EARLY AND MIDDLE JURASSIC (PLIENSBACHIAN TO BAJOCIAN) RADIOLARIA FROM CHERTS OF KISELEVKA-MANOMA ACCRETIONARY COMPLEX (AMUR RIVER, EASTERN RUSSIA)

The lower part of chert sequence of Kiselevka-Manoma accretionary complex, Sikhote-Alin system (Eastern Russia) is characterized by the following radiolarian assemblages (in ascending order of age): 1. Early Pliensbachian to early Toarcian with Parahsuum izeenze , Bagotum maudense and Katroma clara. 2. Early Toarcian with Bistarkum rigidium and Citriduma hexaptera. 3. Middle-late Toarcian to early-middle Aalenian with Paronaella grahamensis, Praeparvicingula gigantocornis and Hsuum exiguum. 4. Early-middle Aalenian with Parahsuum ? grande, Parahsuum cruciferum and Hsuum altile. 5. Late Aalenian with Parahsuum ? hiconocosta, Transhsuum hisuikyoense and Nassellaria gen. et sp. indet. 1. 6. Early-middle Bajocian with Sella beniderkoulensis and Emiluvia splendida. New species Olanda inflata Bragin, n. sp. is described from this assemblage. Studied radiolarian assemblages are Tethyan in affinity, which allows to utilize the low-latitude zonal radiolarian scales for the Jurassic (Baumgartner et al., 1995b; Carter et al., 2010). The total stratigraphic range of chert sequence of the Kiselevka-Manoma accretionary complex can be estimated as Early Jurassic (Pliensbachian-early Toarcian)-Early Cretaceous (Barremian).

Early Bajocian Marine Gastropods from the Neuquén Basin, Argentina

Fil: Ferrari, Silvia Mariel. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Centro Nacional Patagonico; Argentina

AGE AND GEOCHEMISTRY OF BASALT-CHERT ASSOCIATIONS IN THE OPHIOLITES OF THE IZMIR-ANKARA MÉLANGE EAST OF ANKARA, TURKEY: PRELIMINARY DATA

In this paper, we present the preliminary data on the age of the radiolarian cherts deposited on top of basalts belonging to the Eastern Ankara Melange (part of the Izmir-Ankara Melange). Petrological studies on the basalts were carried out in order to constrain the tectonic setting of the studied basalt-chert sequences. Nine sections were sampled East of Ankara and twenty seven samples were collected for biostratigraphic and geochemical analyses. The oldest radiolarian cherts dated in the present paper are referable to the Late Triassic (Section 6: late Norian) and are associated with basaltic rocks of OIB character. OIB type volcanic rocks are also found in other sections, associated with cherts of Late Jurassic (Section 3: middle-late Oxfordian to late Kimmeridgian-early Tithonian) and Early Cretaceous (Section 1: late Valanginian to late Hauterivian) ages. E-MORB type rocks are associated with radiolarian cherts of Cretaceous age (Section 4: middle late Barremian-early early Aptian and Section 7: Valanginian to middle Aptian-early Albian), whereas the oldest N-MORBs were found in a section of Late Jurassic age (Section 5: early-early late Tithonian). Other N-MORBs are associated with radiolarian cherts of Early Cretaceous age (Section 8: late Valanginian-early Barremian). P-MORBs type rocks were found only in a section of Middle Jurassic age (Section 2: early-middle Bajocian to late Bathonian-early Callovian age). In this work, we document the occurrence of Late Triassic OIB-type rocks and of rocks showing different geochemical affinities (N-, E-, P-MORBs and OIB) generated within the same time span (Middle-Late Jurassic - Early Cretaceous). N-MORBs are compatible with composition of melts generated by partial melting of a depleted MORB mantle source. In contrast, OIBs are compatible with partial melting of an enriched-type mantle source. E-MORBs may have derived from mantle source slightly enriched with respect to a DMM source, whereas P-MORBs are compatible with melts generated from a mantle source significantly enriched, compared to DMM.

MIDDLE JURASSIC BELEMNITES AND AMMONITES (CEPHALOPODA) FROM TELMA-DAREH, NORTHERN IRAN

Ammonites and belemnites from a Middle Jurassic section at Telma-Dareh (Alborz Mountains, northern Iran) are described. The lithology of the studied section consists of an alternation of marls and limestones attributed to the Dalichai Fm., overlying the top of the Shemshak Fm. (sandstones) and underlying the Lar Fm. (limestones). Fossils are rather abundant but concentrated in ten scattered levels, ranging from the Aalenian (Scissum-Murchisonae zones) up to the Bajocian. Ammonites ( Tmetoceras scissum , Leioceras cf. comptum , Ludwigia cf. murchisonae , Onychoceras ? sp., and Leptosphinctes sp.), belemnites ( Brevibelus breviformis , Holcobelus cf. munieri ) and bivalves are the most abundant and well-preserved fossils; accessory elements are scarce brachiopods and gastropods. The palaeobiogeographic affinities of the belemnite fauna is Subboreal-Submediterranean, whereas the ammonites have Submediterranean-Tethyan affinities. The Aalenian age of the lower part of the Dalichai Fm. at Telma-Dareh differs from the Bajocian age attributed by other authors to this formation in other localities.

RADIOLARIAN AGES FOR THE SEDIMENTARY COVER OF SEVAN OPHIOLITE (ARMENIA, LESSER CAUCASUS)

Age data of Armenian ophiolites are of great importance for understanding the palaeogeographic and geodynamic evolution of Tethys in the Caucasus area and for lateral correlations of Tethyan ophiolitic belts in the Middle East area. Moderately well-preserved radiolarian assemblages obtained during this study from the Sarinar section of eastern Armenia allow to investigate the sedimentary and volcanic history recorded in a ca. 30 m-thick radiolarite sequence associated with spilitic lavas of the Sevan ophiolitic suture zone. The latest Bajocian-early Bathonian assemblage yielded from a sample situated close to the base of the studied sequence suggests that radiolarian ooze accumulation took place on oceanic crust of late Middle Jurassic age (probably Bajocian). Several tuffite levels are intercalated within the lower to middle part of the radiolarite sequence; a late Bathonian-early Oxfordian assemblage from a sample taken just below the uppermost tuffitic bed, suggests that during this time the oceanic domain of Sevan ophiolites was close to subaerial volcanic activity.

NEW DATA ON THE AGE OF THE SIMONI MELANGE, NORTHERN MIRDITA OPHIOLITE NAPPE, ALBANIA

In the Albanian sector of the Dinaric-Hellenic Chain, the ophiolites have been subdivided into two parallel belts: the Western Belt (WB) and the Eastern Belt, due to their different stratigraphical, petrological and geochemical characteristics: WB represents oceanic lithosphere generated at a mid-oceanic ridge (MOR ophiolites), EB represents an oceanic basin developed over a subduction zone (SSZ ophiolites) (ISPGJ-IGJN, 1990; Beccaluva et al., 1994; Bortolotti et al., 1996, and bibl. therein). EB overthrust westwards WB. The radiolarian cherts (Kalur Cherts) covering the volcanites of both the successions have the same ages, comprised between latest Bajocian-early Bathonian (UAZones 5 of Baumgartner et al., 1995) and middle Callovian-early Oxfordian (UAZones 8) (Chiari et al., 2004, and bibl. therein). Thin levels of radiolarian cherts intercalated in the basalts of EB have late Bajocian to latest Bajocian-early Bathonian ages (UAZones 4-5) (Chiari et al., 1994 and Chiari et al., 2004). A “blocks in matrix-type” sedimentary melange, the Simoni Melange, unconformably covers both the successions, indistinctly lying on the radiolarian cherts or on the upper portions of the underlying volcanites (Bortolotti et al., 1996; Carosi et al., 1996). It grades upwards to the Firza Flysch (latest Tithonian-Valanginian age after Shallo 1991; Gardin et al., 1996), a pelagic sediment with frequent ophiolitebearing polimict pebbly sandstones and mudstone intercalations. The melange includes blocks of different sizes (up to several hundred meters) of continental derived (predominant) and ocean derived rocks, in a shaly matrix. It can be interpreted as “syn-orogenic....deposited after the inception of ophiolite deformation” (Bortolotti et al., 1996). The age of the melange was indirectly determined, being comprised between middle Callovian-early Oxfordian, the more recent age found at the top of the Kalur Cherts (Marcucci and Prela, 1996), and the latest Tithonian-early Berriasian?, found at the base of the Firza Flysch (Gardin et al., 1996). The aim of this note is to present new biostratigraphical data that define a more precise age of the Simoni Melange, dating a cherty-silty level found at the top of the Kalur Cherts near the Lumi i Zi (northern Albania).

Stratigraphy and paleontology of the marine Jurrassic strata in Vietnam

Marine Jurassic strata in Vietnam have yielded characteristic ammonites and occur in: (1) West Quang Binh Province located in the eastern margin of the Nam Theun Basin (the main part of which is located in Middle Laos); (2) the Nong Son Basin situated in the north of the Kon Turn Geoblock; (3) the Da Lat Basin, the largest Jurassic basin in Vietnam, extending from the southern margin of the Kon Turn Geoblock to southeast of Ho Chi Minh City. In the first two, Lower Jurassic marine beds occupy the lower part of the section and grade up into Middle Jurassic red beds. In central Da Lat Basin the section consists completely of marine strata containing many ammonite levels dated from Early Sinemurian to Bajocian, whereas the marginal basin section is like the first two areas. Based particularly on biostratigraphical evidence, a marine transgression is considered to have entered the Indochina Block as early as the Late Hettangian. During the early transgression the fauna was distinctly endemic, showing the separate character of the basins, but since the Late Sinemurian, communication between the Da Lat and other basins was established; the marine regime in Da Lat Basin ended around the Late Bathonian.

THE MID- JURASSIC MARINE TRANSGRESSION IN EAST AFRICA: NEW DATA ON THE DEPOSITIONAL ENVIRONMENT AND AGE OF THE LOWER KAMBE FORMATION (AALENIAN TO BAJOCIAN) IN THE MOMBASA AREA (S.E. KENYA)

The Lower Kambe Formation crops out from Mombasa towards the northeast, along the coastal area of Kenya; it was deposited during the first phases of the Middle Jurassic marine transgression. The lithology of the Lower Kambe Fm. varies passing from the southern areas towards the north: near Mombasa, this formation consists of an alternance of calcareous and marly intervals; the marly levels become thinner and tend to disappear towards the north, where the succession becomes entirely calcareous and shows features of a carbonate platform. According to field observations, in the Mwachi River area near Mombasa, four lithological units crop out, that we consider as informal members, namely from bottom : (a) Calcarenitic member (Cam), well bedded calcarenites and some conglomerates; (b) Lower Shaly member (LSm), marly shales with marly limestone beds; (c) Conglomeratic member (Cgm), alternance of calcarenitic beds and levels of conglomerates; (d) Upper Shaly member (USm), marly shales and siltstones with thin beds of calcilutites and quartz sandstones. The sedimentological features of the Cam point to a shoreline environment, possibly shoreface; those of the LSm to a mid/outer ramp; the onset of tectonically active slopes is evidenced by the occurrence of gravity flow structures in the fine-grained sediments. At the top of LSm a marked shallowing of the deposition environment occurred. The Cgm was deposited on a inner ramp, where polymictic coarse deposits (arenaceous, shaly and carbonate clasts) indicating a strong erosion of the coastal areas, were intercalated to the wave-winnowed calcarenites; also the calcarenitic upper portion of the Cgm was deposited on an high energy inner ramp depositional environment. Upwards, a deepening occurred, as the fine sediments of the USm point to a mid/outer ramp. The age of the Lower Kambe Formation has been determined by several Authors on the basis of ammonites; in this work calcareous nannofossils stratigraphy is provided. The age given by ammonites is Bajocian; the calcareous nannofossil assemblage, found in various localities along the Mwachi River, indicates an Aalenian - Bajocian p.p. age. The discrepancy is possibly due to a complex geological situation (faults ?) in the localities with ammonites.

DOCUMENTATION AND SIGNIFICANCE OF TECTONIC EVENTS IN THE NORTHERN TABAS BLOCK (EAST-CENTRAL IRAN) DURING THE MIDDLE AND LATE JURASSIC

Apart from global sea-level fluctuations, the sedimentary pattern of the northern Tabas Block during the Middle and Late Jurassic was mainly governed by tectonic events of varying intensity and areal extent. These events took place during the Middle Bajocian (mid-Cimmerian tectonic phase), Early Bathonian, Late Bathonian, Early Callovian, Late Oxfordian, and Late Kimmeridgian. The importance and extent of each event and its influence on the facies pattern of the northern Tabas Block is briefly discussed and demostrated by some examples.

MIDDLE JURASSIC NAUTILOIDEA FROM WESTERN FRANCE

The Middle Jurassic shelf margin limestones and marly limestones of Western France yield quite numerous Nautilida. This record is here described for the first time in detail, nine genera and more than forty species, whose range rarely exceeds one or two ammonite biozones, constitute a rather precise tool to date Middle Jurassic beds. Each taxon is placed in an accurate biostratigraphical chart; a new genus, Pictonautilus nov. gen., is described. The stratigraphical range of Cenoceras , Digonioceras and Gen. B n. ? (sensu Chirat 1997) is extended up to the Middle Callovian. In the Jurassic successions of Poitou, Nautilida always represent a minor part of the fossil record, they are more abundant in four levels: Middle Aalenian, base of Upper Bajocian, Lower Bathonian and top of Middle Bathonian. Dwarf Nautilida ( Paracenoceras ) and species showing more folded septa and a subventral siphuncle (Ps eudaganides ) or strongly ribbed specimens ( Cymatonautilus ) are characteristic of stable open-shelf environments (Callovian), whereas large shelled nautilus would be mostly present during deepening episodes. The geographical extension of many taxa indicates a provincialism that fits with that of ammonites of the same period.

JURASSIC HERITAGE OF PORTUGAL: STATE OF THE ART AND OPEN PROBLEMS

The aim of this work is to present a general view on the situation of Jurassic sites outcropping in Portugal, in particular those located in the Lusitanian Basin, which represent geosites with heritage value. The legal instruments on geoconservation are generically presented, and they happen to be designed for Natural Heritage deeply confused with the Biological Heritage. No specific law for geological values exist in Portugal, but the general law on Natural Conservation has allowed the protection of five geosites of national relevance as Natural Monuments. They correspond to a small area when compared to the whole national area of Protected Sites, and they all correspond to dinosaur tracksites of Jurassic and Cretaceous age. Geosites of regional relevance have been also protected using municipal laws and they show important public use, sometimes giving rise to reference museums and to continuous pedagogical actions. On the other hand, geosites of international relevance for its stratigraphic value (the Bajocian GSSP at Cabo Mondego section, the candidate to the Toarcian GSSP at Peniche section) have no formal protection and classification according to the Portuguese laws. They remain safe because they are located on the coast, which is protected due to other environmental laws. Their integration in a supra-national framework supported by UNESCO is obviously recommended and it would be of great impact among the national institutions with formal responsibilities on the Natural Heritage.

JURASSIC PALEONTOLOGICAL HERITAGE OF MURCIA (BETIC CORDILLERA, SOUTH-EASTERN SPAIN)

Jurassic rocks of the External and Internal Zones of the Betic Cordillera are widespread in the province of Murcia. Four areas are considered of special interest for stratigraphical and paleontological analysis: a) Sierra Quipar and b) Sierras Lugar-Corque (External Subbetic), c) Sierra Ricote (Median Subbetic) and d) Sierra Espuna (Malaguide Complex). The first two contain Jurassic sections including Sinemurian-Tithonian deposits, and major stratigraphic discontinuities, containing significant cephalopod concentrations of taphonomic and taxonomic interest, occuring in the Lower-Upper Pliensbachian, Lower/Middle Jurassic and Middle/Upper Jurassic boundaries. These areas are also relevant for biostratigraphical analysis of the Middle-Upper Jurassic interval. In the Sierra de Ricote, the Mahoma section is of especial interest for the study of Lias/Dogger transition. Casa Chimeneas section constitutes the best Subbetic site for the analysis of the Lower/Upper Bajocian boundary. In the La Bermeja-Casas de Vite area, the Bajocian-Tithonian interval is well-represented, including a parastratotype of the Radiolarite Jarropa Formation. Finally, the Malvariche section in Sierra Espuna represents the best Jurassic succession of Internal Zones of the Betic Cordillera and could be considered as a reference section for this Betic Domain. In this paper a heritage evaluation has been carried out for these classical jurassic sections with the object of protecting these sites according to the legal framework prevailing in the province of Murcia.

THE PRE-OROGENIC VOLCANO-SEDIMENTARY COVERS OF THE WESTERN TETHYS OCEANIC BASIN: A REVIEW

The records of the Jurassic Western Tethys Ocean are the ophiolitic rocks now scattered in the Tertiary orogenic belts of the Alps, Apennines and Betic Cordillera. These ophiolites, involved in a convergent margin environment, are affected I) by HP/LT metamorphism derived from a subduction process or II) by very low grade overprint, corresponding to the tectonic prism at the margin of the overriding plate. On the whole, they share common characteristics: a- The MORB geochemical signature. b- The ophiolitic successions often “reduced” and thin. c- The volcano-sedimentary covers often directly overlying the serpentinised peridotites. d- The widespread occurrence of cherts as Jurassic pelagic sediment. In the thickest “complete” ophiolitic successions, basalt flows, generally thin, are preceded and followed by ophiolitic breccias. Only the basal portion of the breccias on top of the serpentinites (Levanto Breccias) has a tectonic origin, all other levels have a sedimentary origin. These breccia-basalt assemblages are overlain by thick sequences of Mt. Alpe Cherts and Calpionella Limestones, followed by Palombini Shales. In the reduced (or incomplete) successions, thin breccias and cherts were directly deposited, above the Levanto Breccias (ophicalcites pro parte), and followed by Palombini Shales. This stratigraphic pattern seems to be widespread in the whole Western Tethys ocean. In some sequences, transitional mid-ocean ridge (T-MOR) basalts are present and the ophiolitic rocks are associated with Variscan continental slices and debris, as in the Err-Platta succession (Central Alps) and in some exotic blocks in the flysch of the External Ligurides (Northern Apennines). In the Balagne (Corsica) T-MOR basalts are associated with quartzarenites. These occurrences show that an unroofed mantle and sections of oceanic crust evolved very near to a continental margin. The different radiolarian ages of the cherts deposited before, within, or on top of the MOR basalts allow to infer a minimum time interval for the Western Tethys oceanisation. This interval can be considered between 16 and 21 Ma (from Late Bajocian to Kimmeridgian/Tithonian). If we assume 1cm/yr spreading rate during this time, the basin would have reached about 150-200 km width. The same ages suggest that the ocean opening was diachronous along the Western Tethys basin. Mainly on the basis of the Northern Apennines and Corsica data, it is possible to reconstruct the following evolutive geodynamic, paleogeographic and sedimentary evolution of the Western Tethys ocean basin: 1- Bajocian/Bathonian stage: opening of the Ligurian Northern Apennines oceanic segment and, perhaps, also of the Ligurian, Western and Central Alps ones. 2- Bathonian/Callovian stage: opening of all the segments of the Western Tethys ocean basin. The volcano-sedimentary covers formed during these two stages are constituted by breccias, basalts and siliceous pelagites (cherts). 3- Tithonian/Berriasian: end of the ocean spreading (Tithonian) and beginning of the quiescent stage in the whole basin, marked by the lack of any tectonic activity and by the sedimentation of the Calpionella Limestones and, locally, of mixed siliceous-calcareous deposits (Nisportino-Murlo Fm.). 4- Hauterivian/Santhonian: this is the longest quiescent stage of the basin, dominated by the sedimentation of the Palombini Shales and Limestones. Some siliciclastic deposits are shed from both passive continental margin sides. During the Early Cretaceous, there is also evidence of a rare intraplate magmatism in Southern Tuscany. The Western Tethys ophiolitic successions are similar to those of present day, slow spreading oceans, in particular to those of the Atlantic Ocean (Mutter and Karson, 1992; Tucholke and Linn, 1994). The Galician North Atlantic margin provides a model for the process of mantle denudation. For the oceanic evolution, the model of Tucholke and Linn (1994) is particularly taken in consideration. According to this model, tectonic extension was one major process in the Western Tethys oceanic development.

THE MIDDLE JURASSIC SILICEOUS SEDIMENTARY COVER AT THE TOP OF THE VOURINOS OPHIOLITE (GREECE)

The age of the Vourinos ophiolitic massif, pertaining to the Jurassic Tethys Ocean, is not yet exactly known. Four sections in the cherts at the top of the basalts yielded well preserved radiolarian assemblages according to which the age of the end of the “ophiolitic activity” (= the end of ocean spreading) was immediately before or within the latest Bajocian interval. A radiometric (Ar/Ar) age of the metamorphic sole at the base of the ophiolite gave 171±4 Ma (late Bajocian), this is, an age slightly older than that of the cherts: this can mean that the metamorphic soles pertain to an intraoceanic hot thrust, doubling the oceanic crust/lithosphere.

Pseudoborings in Ammonite Molds: The Combined Result of Predation and Taphonomic-Reworking (Bajocian, Normandy, N. France)

Pseudoborings in Ammonite Molds: The Combined Result of Predation and Taphonomic-Reworking (Bajocian, Normandy, N. France)

SYNTHESIS OF CALCAREOUS NANNOFOSSIL EVENTS IN TETHYAN LOWER AND MIDDLE JURASSIC SUCCESSIONS

This paper is a synthesis of calcareous nannofossil biostratigraphy for the Lower and Middle Jurassic of the Mediterranean Province based on several sections from Northern and Central Italy. Nannofossil events were calibrated with ammonite biostratigraphy and, when necessary, ammonite-controlled sections in South East France were incorporated. Data derive from previously published biostratigraphies and unpublished data of the authors.The large data-set allowed estimates of reliability and reproducibility of single events. As a result, in the Hettangian-Bathonian interval we propose 47 main events based on diagenesis-resistant and common taxa, 17 events based on rare but ubiquitous taxa and 12 potential events requiring further investigations due to taxonomic problems and sporadic occurrence. A biostratigraphic scheme, consisting of 11 zones and 15 subzones, is proposed for the Tethyan Lower and Middle Jurassic. The proposed biostratigraphy is compared to recent schemes compiled for Portugal, Morocco, Switzerland and the Boreal Realm. Only 27 events are reproducible in various regions, but diachroneity of most events seems to derive from different ammonite biostratigraphies applied in different areas. A very high stratigraphic resolution is achieved in Italy/France for the Pliensbachian to Lower Bajocian interval. The Sinemurian and Bathonian are characterized by the lowest resolution, and very few sections with ammonite control and/or favourable lithologies are available for improvement of nannofossil biostratigraphy. This study confirms the potential of calcareous nannofossil biostratigraphy for dating Lower and Middle Jurassic successions as well as for intra- and inter-regional correlations.

The Upper Bajocian ammonite Strenoceras in Chile: first circum-Pacific record of the subfurcatum Zone

The Upper Bajocian ammonite Strenoceras in Chile: first circum-Pacific record of the subfurcatum Zone

VI.—The Evolution of Stomatopora dichotomoides (d'Orbigny)

Alecto dichotomoides was the name given by d'Orbigny in 1849 to a specimen figured by Michelin in 1841 from the Bajocian of Bayeux, or of Moutiers, Calvados, Normandy, and identified by Michelin as A. dichotoma, Lamouroux.