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.

/pdf/mature-modifications-and-sexual-dimorphism-27ozuc37y1.pdf

Mature Modifications and Sexual Dimorphism

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.

/pdf/formal-proposal-for-the-bathonian-gssp-middle-jurassic-in-28jn4leqie.pdf

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

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)

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.

Vasilii Mitta

Papers

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