SEXUAL DIMORPHISM: PLIENSBACHIAN APODEROCERAS

Apodoceras / Stonebarrow Fossils

Apoderoceras is a wonderful example of sexual dimorphism within ammonites as the macroconch (female) shell grew to diameters in excess of 40 cm – many times larger than the diameters of the microconch (male) shell.

Apoderoceras has been found in the Lower Jurassic of Argentina, Hungary, Italy, Portugal, and most of North-West and central Europe, including as this one is, the United Kingdom. This specimen was found on the beaches of Charmouth in West Dorset.

Neither Apoderoceras nor Bifericeras donovani are strictly index fossils for the Taylori subzone, the index being Phricodoceras taylori. Note that Bifericeras is typical of the earlier Oxynotum Zone, and ‘Bifericeras’ donovani is doubtfully attributable to the genus. The International Commission on Stratigraphy (ICS) has assigned the First Appearance Datum of genus Apoderoceras and of Bifericeras donovani the defining biological marker for the start of the Pliensbachian Stage of the Jurassic, 190.8 ± 1.0 million years ago.

Apoderoceras, Family Coeloceratidae, appears out of nowhere in the basal Pliensbachian and dominates the ammonite faunas of NW Europe. It is superficially similar to the earlier Eteoderoceras, Family Eoderoceratidae, of the Raricostatum Zone, but on close inspection can be seen to be quite different. It is therefore an ‘invader’ and its ancestry is cryptic.

The Pacific ammonite Andicoeloceras, known from Chile, appears quite closely related and may be ancestral, but the time correlation of Pacific and NW European ammonite faunas is challenging. 

Even if Andicoeloceras is ancestral to Apoderoceras, no other preceding ammonites attributable to Coeloceratidae are known. We may yet find clues in the Lias of Canada. Apoderoceras remains present in NW Europe throughout the Taylori Subzone, showing endemic evolution. It becomes progressively more inflated during this interval of time, the adult ribs more distant, and there is evidence that the diameter of the macroconch evolved to become larger. 

At the end of the Taylori Subzone, Apoderoceras disappeared as suddenly as it appeared in the region, and ammonite faunas of the remaining Jamesoni Zone are dominated by the Platypleuroceras–Uptonia lineage, generally assigned (though erroneously) to the Family Polymorphitidae.

In the NW European Taylori Subzone, Apoderoceras is accompanied (as well as by the Eoderoceratid, B. donovani, which is only documented from the Yorkshire coast, although there are known examples from Northern Ireland) by the oxycones Radstockiceras (quite common) and Oxynoticeras (very rare), the late Schlotheimid, Phricoderoceras (uncommon) 

Note: P. taylori is a microconch, and P. lamellosum, the macroconch), and the Eoderoceratid, Tetraspidoceras (very rare). The lovely large specimen (macroconch) of Apoderoceras pictured here is likely a female. Her larger body perfected for egg production.

FROM RUSSIA WITH LOVE: CADOCHAMOUSSETIA

From Russia with love. This lovely inflated ammonite is the female macroconch, Cadochamoussetia tschernyschewi (Sokolov, 1912) from the Jurassic, Lower Callovian, Elatmae Zone, Subpatruus Subzone, Stupachenkoi Horizon, Unzha River, Makarev-Manturovo, Kostroma Region, Russia.

This beautiful — fully Бомба — specimen is courtesy of Emil Black and one of the finest in his collection. 

It has a chunkiness that reminds me of the Cadoceras we find in the Pacific Northwest, particularly the macroconch Cadoceras comma from the Callovian Mysterious Creek Formation near Harrison Lake in British Columbia.

In the last decade, the Siberian zonal scale of the Callovian has been considerably revised because of new ammonite collections from the Callovian reference sections in Siberia. Species of Cadoceratinae thought of as exclusively European were recorded for the first time in Siberia. 

Both these newly recovered specimens and recent studies have considerably expanded our knowledge on the taxonomic composition of genera and species of Callovian ammonites and revision of the generic classification and stratigraphic position of genera and species of the family Cardioceratidae. The proposed Lower Callovian ammonite scale largely coincides with the East European scale and correlates with the scales of East Greenland, Arctic Canada, and Alaska (Kniazev et al., 2009, 2010, 2011, 2015; Nikitenko et al., 2013).

Jurassic deposits crop out on the right bank of the

Anabar River between the mouths of the Srednyaya

and Sodiemykha rivers, over a length of about 24 km.

During recent fieldwork at the Middle-Upper Jurassic of the Anabar River basin, a lovely representative ammonite collection was assembled, amongst which was the Early Callovian genus Cadochamoussetia (Mitta, 1996). 

Cadochamoussetia is widespread in East European sections but these beauties were the first recorded specimen of this chunky species from the Anabar.

The genus Cadochamoussetia (Mitta, 1996) was established in European Russian (Gerasimov et al., 1996) and later in England (Navarro et al., 2005).

In the lower Callovian of European Russia, beds with Cadochamoussetia were originally considered part of the Cadochamoussetia subpatruus upper subzone of the Cadoceras elatmae Zone (Mitta, 2000). 

In 2005 and 2009, proposals were made to move these beds from subzone to zone (Gulyaev, 2005, 2009). However, the Unified Regional Stratigraphic Scheme of Jurassic Deposits of the East European Platform (2012), suggested it remained a subzone. The Anabar section contains two species of Сadochamoussetia, which were used as the basis of the Сadochamoussetia tschernyschewi Zone.

In previous papers (Kniazev et al., 2010), considered the composition of the genus Cadoceras as it was interpreted in (Treatise, 1957). 

Several groups of species are now recognized within the genus: Cadoceras elatmae group, including C. frearsi, C. harveyi, C. sublaeve, including species widespread in the Arctic C. tolype, C. emelianzevi, C. septentrionale, C. durum, etc. 

Kniazev et al. proposed assigning a group of Bathonian species Catacadoceras laptievi, C. barnstoni, C. perrarum, C. subcatastoma, and C. nageli.

Photos: Cadochamoussetia tschernyschewi (12 cm) graciously shared by the deeply awesome of Emil Black. He has shared many wonderful specimen photos and stories with me over the years and I am honoured by his generosity in doing so. It is because of him that I am able to share these with all of you! So a collective, Спасибо, мой друг. Spasibo, moy drug. 

I have placed views of this lovely Cadochamoussetia tschernyschewi into a teaching tool that includes the specimen name, length and provenance.

References:

• The Early Callovian genus Сadochamoussetia (Ammonoidea, Cardioceratidae) in the lower reaches of the Anabar River, Northern Central Siberia; Original Russian Text © V.G. Kniazev, S.V. Meledina, A.S. Alifirov, B.L. Nikitenko, 2017, published in Stratigrafiya, Geologicheskaya Korrelyatsiya, 2017, Vol. 25, No. 4, pp. 26–41.
• Kniazev, V.G., Meledina, S.V., Alifirirov, A.S., and Kutygin, R.V., The Middle Callovian stage of evlution of Siberian cardioceratids, in Sovremennye problemy izucheniya golovonogikh mollyuskov. Morfologiya, sistematika, evolyutsiya, ekologiya i biostratigrafiya. Vyp. 4 (Current Problems in Study of Cephalopods: Morphology, Systematics, Evolution, Ecology, and Biostratigraphy. Iss. 4), Moscow: Paleontol. Inst. Ross. Akad. Nauk, 2015, pp. 40–45.
• Meledina, S.V, Correlation of the Bajocian and Bathonian zones in light of new paleontological data, Stratigr. Geol. Correl., 2014, vol. 22, no. 6, pp. 594–605.
• Kniazev, V.G., Meledina, S.V., Alifirirov, A.S., and Kutygin, R.V., The Middle Callovian stage of evlution of Siberian cardioceratids, in Sovremennye problemy izucheniya golovonogikh mollyuskov. Morfologiya, sistematika, evolyutsiya, ekologiya i biostratigrafiya. Vyp. 
• If you do not speak Russian that roughly translates to: Current Problems in Study of Cephalopods: Morphology, Systematics, Evolution, Ecology, and Biostratigraphy. Iss. 4, Moscow: Paleontol. Inst. Ross. Akad. Nauk, 2015, pp. 40–45.
• Meledina, S.V, Correlation of the Bajocian and Bathonian zones in light of new paleontological data, Stratigr. Geol. Correl., 2014, vol. 22, no. 6, pp. 594–605.
• Treatise on Invertebrate Paleontology. Pt. L. Mollusca 4, Cephalopoda, Ammonoidea, N.Y. Lawrence: Geol. Soc. Amer., Univ. Kansas Press, 1957, vol. 4. TSCreatorProvisualization of Enhanced Geologic Time Scale 2004 database (Vers. 6.2, 2014). http://www.tscreator. org, 2014.
• Treatise on Invertebrate Paleontology. Pt. L. Mollusca 4, Cephalopoda, Ammonoidea, N.Y. Lawrence: Geol. Soc. Amer., Univ. Kansas Press, 1957, vol. 4. TSCreatorProvisualization of Enhanced Geologic Time Scale 2004 database (Vers. 6.2, 2014). http://www.tscreator. org, 2014.