Two main types of intraspecific variation can be distinguished in ammonoids, which are not mutually exclusive: continuous and discontinuous variation. Although many authors acknowledge or implicitly assume a large intraspecific variability is possible in shell shape, ornamentation and suture line, it has only been rarely studied quantitatively. Several potential biases need to be taken into account when studying intraspecific variation of fossil populations including paleoecological, taphonomic and collection biases. Intraspecific variation might be controlled both by genetic and environmental parameters, although both are difficult to separate in fossil samples. In ammonoids, a large part of intraspecific variation in morphology and size has been attributed to differences in growth rates and development. Taking intraspecific variation properly into account is not only of prime importance for taxonomy, but also for studies on biostratigraphy, paleobiogeography, ecology, paleobiology and evolution of ammonoids.

https://www.zora.uzh.ch/id/eprint/121836/1/chp%253A10.1007%252F978-94-017-9630-9_9.pdf

Ammonoid Intraspecific Variability

Most ammonoids display a distinctly different morphology when they are adults or subadults. Depending on the taxon, these mature modifications may comprise changes in coiling, changes in ornamentation, and conspicuous changes of the terminal aperture. These mature modifications permit, at least when a combination of which occurs, to identify adult specimens. Co-occurrence of adult forms with differences in size, ornament strength, and aperture shape led already half a century ago to the conclusion that many ammonoids display dimorphism. The smaller forms are usually dubbed microconchs and the larger ones macroconchs. The degrees of differences between the antidimorphs vary and are more or less characteristic for certain clades. Such dimorphism is also known from Recent cephalopods. According to actualistic comparisons and considerations concerning reproductive biology of Recent cephalopods, it is commonly assumed that the microconchs represented the males and macroconchs the females. This hypothesis still needs verification from soft-tissue preservation.

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Mature Modifications and Sexual Dimorphism

We describe a new species of an ornithischian dinosaur, Isaberrysaura mollensis gen. et sp. nov. The specimen, consisting in an almost complete skull and incomplete postcranium was collected from the marine-deltaic deposits of the Los Molles Formation (Toarcian-Bajocian), being the first reported dinosaur for this unit, one of the oldest from Neuquen Basin, and the first neornithischian dinosaur known from the Jurassic of South America. Despite showing a general stegosaurian appearance, the extensive phylogenetic analysis carried out depicts Isaberrysaura mollensis gen. et sp. nov. as a basal ornithopod, suggesting that both Thyreophora and neornithischians could have achieved significant convergent features. The specimen was preserved articulated and with some of its gut content place in the middle-posterior part of the thoracic cavity. Such stomach content was identified as seeds, most of them belonging to the Cycadales group. This finding reveals a possible and unexpected role of this ornithischian species as seed-dispersal agent.

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A new primitive Neornithischian dinosaur from the Jurassic of Patagonia with gut contents.

The Bathonian GSSP is defined at the base of limestone
bed RB071 in the Ravin du Bes Section, Bas-Auran
area, southern Subalpine Chains (France). This section
satisfies most of the erquirements recommended by the
International Commission on Stratigraphy. The exposure
extends over 13 m in thickness. No vertical biofacies,
ichnofacies or taphofacies changes, stratigraphic gaps
or hiatuses have been recorded at the Bajocian-Bathonian
transition. Structural complexity, synsedimentary and
tectonic disturbances, or important alterations by
metamorphism are not relevant constraints. There is a
well-preserved, abundant and diverse fossil record, with
key markers (ammonites and nannofossils). The base of
Bathonian Stage and Zigzag Zone corresponds to the first
occurrence level of Gonolkites convergens Buckman that
coincides with the first occurrence of Morphoceras
parvum Wetzel. Calcareous nannofossils are present in
all beds. The GSSP is below the Tethyan calcareous
nannofossil NJT10b/11 zonal boundary (lowest
Watznaueria barnesiae) and above the Bo real nannofossil
NJ10/11 boundary (lowest Pseudoconus enigma).
Regional analyses of sequence stratigraphy and
manganese chemostratigraphy are available. Spectral
gamma-ray data corroborate interpretation of an Early
Bathonian deepening half-cycle of second order. The criteria of accessibility, conservation and protection are
assured by the “Reserve Naturelle Geologique de Haute
Provence”. The Cabo Mondego Section (Portugal) is the
Bathonian auxiliary section and point (ASSP) .T he
proposal was voted by the Bathonian Working Group in
December, 2007, and by the International Subcommission
on Jurassic Stratigraphy in March, 2008, approved by
the ICS in June 2008, and ratified by the IUGS in Ju,ly
2008.

The Global Boundary Stratotype Section and Point (GSSP) for Base of the Bathonian Stage (Middle Jurassic), Ravin du Bes Section, SE France

The Bathonian Global Stratotype Section and Point (GSSP) is proposed at the base of limestone bed RB071 (bed 23 in Sturani 1967) in the Ravin du Bes Section (43° 57′ 38′′ N, 6° 18′ 55′′ E), Bas-Auran area, “Alpes de Haute Provence” French department. The Ravin du Bes Section, as formal candidate GSSP for the base of the Bathonian Stage, satisfies most of the requirements recommended by the International Commission on Stratigraphy: 1) The exposure extends over 13 m in thickness. At the Bajocian-Bathonian transition, no vertical (bio-, ichno- or tapho-) facies changes, condensation, stratigraphic gaps or hiatuses have been recorded. Structural complexity, synsedimentary and tectonic disturbances, or important alterations by metamorphism are not relevant constraints. 2) There is a well-preserved, abundant and diverse fossil record across the boundary interval, with key markers (ammonites and nannofossils) for worldwide correlation. The base of the Bathonian Stage and Zigzag Zone in Bas-Auran corresponds to the first occurrence level of Gonolkites convergens Buckman, which coincides with the first occurrence of Morphoceras parvum Wetzel. Calcareous nannofossils, as secondary global marker, are present in all beds and allow characterizing the Bajocian-Bathonian transition. 3) Regional analyses of sequence stratigraphy and manganese chemostratigraphy are available. Spectral gamma-ray data corroborate an Early Bathonian deepening half-cycle of second order. 4) The criteria of accessibility, conservation and protection are assured by the “Reserve Naturelle Geologique de Haute Provence”. The Cabo Mondego Section (Portugal) is suggested as the Bathonian auxiliary section and point (ASSP) within this GSSP proposal.

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Formal proposal for the Bathonian GSSP (Middle Jurassic) in the Ravin du Bès Section (Bas-Auran, SE France)

We present a modern revision of the dimorphic ammonite pair Strigoceras QUENSTEDT, 1886/Cadomoceras MUNIER-CHALMAS, 1892 from the Middle Jurassic. The stratigraphically oldest hitherto known chronospecies of Strigoceras, S. praenuntium (BUCKMAN) appears in the Middle Aalenian as an extreme rarity. Records of the microconch genus Cadomoceras remain unknown from strata older than Early Bajocian, but otherwise its stratigraphical range corresponds well with that of the macroconchs. The genus probably became extinct in the Early Bathonian (Zigzag Zone). The youngest known Strigoceras is S. callomoni n. sp. described here from the Macrescens Subzone of the Early Bathonian. Another new strigoceratid species, Granulochetoceras oppeliisculptum n. sp., is recorded by a single specimen from the early Garantiana Zone of eastern Spain, representing an interesting phyletic link between Strigoceras QUENSTEDT, 1886 and Granulochetoceras GEYER, 1960. We include a brief review of the latter genus. By including Granulochetoceras in the Strigoceratidae the family would extend to the Late Kimmeridgian of the Late Jurassic. The palaeogeographic distribution of Strigoceras/Cadomoceras is focussed on the western Tethyan Province, but some species also occur in the northern Pacific and its adjacent shelves. A benefit of this large area of distribution is that strigoceratids may be used to correlate distant locations, but this is partly hampered by the relatively wide range of variability within succeeding chronospecies and remarkably large variation in size of adult macroconchs.

Revision of the Middle Jurassic dimorphic ammonite genera Strigoceras/Cadomoceras (Strigoceratidae) and related forms

We here present a synopsis of the litho- and biostratigraphy of the Middle Jurassic Sengenthal Formation (Lower Bajocian, Discites Zone to Lower Oxfordian, Cordatum Zone) in the section Polsingen-Ursheim (Nordlinger Ries, Bavaria). The ammonite biostratigraphy, based on bed-by-bed collections from three excavation pits, is correlated with the microfauna (ostracods, foraminifera) from the nearby drillcore VB Polsingen-Ursheim. Our results are compared with literature data from other regions. The distribution of ostracods confirms the zonation for the Middle Jurassic of Southern Germany (Franz et al. 2014a; Ohmert 2004) when correlated with the ammonite biostratigraphy of the Nordlingen Ries area (Dietze & Dietl 2006; Dietze et al. 2002, 2007). This positive correlation could be underlined by the presence of seven biostratigraphically valuable species of foraminifera. The synoptical analysis of the three fossil groups leads to a more detailed subdivision of the Sengenthal Formation and to a more pr...

/pdf/stratigraphy-of-the-middle-jurassic-sengenthal-formation-of-4wp9a7rtmz.pdf

Stratigraphy of the Middle Jurassic Sengenthal Formation of Polsingen-Ursheim (Nördlinger Ries, Bavaria, Southern Germany)

The lithostratigraphy and ammonite/ostracod biostratigraphy and the accompanying fauna of several sections and outcrops around the Hohenzollern (Zollernalb, SW Germany), ranging from the topmost Opalinuston Formation (uppermost Lower Aalenian) to the basal Wedelsandstein Formation (lowermost Lower Bajocian) are described and analyzed. The study of ostracods from 41 samples from the Aalenian and Lower Bajocian of the Heiligenbach, Hausterberg and Roschbach sections has yielded approximately 4,100 specimens. Significant changes in the ostracod assemblage occur at the base of the Lower Aalenian “Comptum” Subzone, at the Bradfordensis/ Gigantea subzonal boundary, at the Bradfordensis/Concavum zonal boundary and at the Aalenian/Bajocian boundary (Concavum/ Discites zones). A minor change occurs above the Calceola-Bank within the Concavum Zone. The following new ostracod species are described: Cytheropterina crassicostata sp. nov., Eucytherura eberti sp. nov. and Eucytherura foveolata sp. nov. In addition, 15 presumably new species are briefly described; 10 ‘incertae sedis’ taxa are figured, but left in open nomenclature. The ammonite faunas of the Inopernabank and Konglomeratbank beds (Upper Aalenian, Bradfordensis Zone, Gigantea Subzone) as well as the ammonite faunas from the Calceolabank and Rostrote Kalkbank beds (Upper Aalenian, Concavum Zone, Concavum Subzone, cavatum biohorizon) are described and correlated with those of other areas.

New records of ostracods and ammonites from the Aalenian (mainly Concavum Zone) of the Zollernalb (Swabian Alb, SW Germany)

Rare Middle Jurassic ammonites of the families Erycitidae, Otoitidae and Stephanoceratidae from southern Germany

The Middle Jurassic of the area of the Ipf (Eastern Swabian Alb) is described. About 20 Middle Jurassic ammonite faunal horizons are recognized, ranging from the Upper Bajocian to the Lower Callovian. Two new Middle Bathonian faunal horizons are introduced: those of Bullatimorphites (Rugiferites) rugifer in the Subcontractus Zone and of Holzbergia schwandorfense in the Morrisi Zone. The Swabian succession is compared and correlated with the faunal horizons recognized in southern England and France. In this context, the ammonite faunal horizons from Upper Bajocian to Lower Callovian of southern England are reviewed and slightly revised.

Der Mitteljura des Ipf-Gebiets (östliche Schwäbische Alb, Süddeutschland). Korrelation der süddeutschen Ammoniten-Faunenhorizonte vom Ober-Bajocium bis zum Unter-Callovium mit Südengland und Frankreich

Volker Dietze

Papers

Revision of the Middle Jurassic dimorphic ammonite genera Strigoceras/Cadomoceras (Strigoceratidae) and related forms

Stratigraphy of the Middle Jurassic Sengenthal Formation of Polsingen-Ursheim (Nördlinger Ries, Bavaria, Southern Germany)

New records of ostracods and ammonites from the Aalenian (mainly Concavum Zone) of the Zollernalb (Swabian Alb, SW Germany)

Rare Middle Jurassic ammonites of the families Erycitidae, Otoitidae and Stephanoceratidae from southern Germany

Der Mitteljura des Ipf-Gebiets (östliche Schwäbische Alb, Süddeutschland). Korrelation der süddeutschen Ammoniten-Faunenhorizonte vom Ober-Bajocium bis zum Unter-Callovium mit Südengland und Frankreich