Conference Agenda

New biostratigraphic and paleobiogeographic data on Valanginian ammonites from the states of Coahuila and Zacatecas have been produced as part of the Ph.D. thesis of one of the authors (JROF), with a significant portion published in 2024. This new information, combined with previously published data, is integrated and re-evaluated alongside ongoing data collection. This comprehensive dataset confirms that the Lower Valanginian ammonoid record of northern Mexico can be biostratigraphically analyzed using the Standard Mediterranean Ammonite Zonation (SMAZ). However, a few endemic species appear in the uppermost part of the Lower Valanginian. Thus, this strong Lower Valanginian Mediterranean affinity shifts abruptly at the onset of the Upper Valanginian, which is characterized by a significant increase in endemic ammonoids and the absence of SMAZ index species, thus requiring the development of a distinct Mexican biozonation. A publication currently in preparation, based on four stratigraphic sections in the Taraises area (Coahuila), will propose two new Upper Valanginian zones: the Saynoceras americanum and the Oosterella gaudryi zones. Two additional stratigraphic sections in the state of Nuevo León, Entronque San Roberto 1 and 2, have yielded an abundant ammonite assemblage from the uppermost Lower and Upper Valanginian and are under active investigation. Although the abundance of endemic ammonites has slowed down research progress, data from these two stratigraphic sections will be crucial for establishing a robust Upper Valanginian zonation for Mexico. The re-evaluation of the Upper Valanginian ammonoid record from Cuencamé, in Durango State, will also contribute to this Upper Valanginian Mexican zonation.

New taxonomic, biostratigraphic, and paleobiogeographic data on ammonites from the San Isidro Formation in Santiago Huauclilla, southern Mexico, spanning from the Hauterivian to the Barremian stages, are presented. Sampling at five new localities yielded 730 specimens, leading to the identification of 21 taxa. Bed-by-bed sampling in four stratigraphic sections from four localities enabled the identification of eight ammonite zones: six correlate with the Standard Mediterranean Ammonite Zonation (SMAZ), and two with the Austral zonation of the Neuquén Basin (Argentina). The Upper Hauterivian exhibits a transitional pattern, with the two lower zones correlating with the Austral scheme and the two upper zones with the Mediterranean scheme. In the Barremian, the four recognized ammonite zones correspond to the SMAZ. The paleobiogeographic turnover between Austral and Mediterranean ammonite assemblages within the middle part of the Upper Hauterivian is initially gradual, with the Mediterranean index species Subsaynella sayni appearing in the lower half of the Upper Hauterivian. However, this transition ends abruptly, as Austral ammonites are entirely absent in the subsequent SMAZ zones. Three-dimensional models of key ammonites are being produced for biostratigraphic purposes as part of this ongoing study, and preliminary paleobiogeographic analyses support the integration of both zonal schemes. During the Barremian, Mexican ammonoids show a broad distribution of Holcodiscidae and Pulchelliidae. Notably, while Holcodiscidae are well represented in the studied area of Mexico, both in terms of specimen abundance and taxonomic diversity, they are extremely scarce in the Pacific domain (e.g., Colombia) and in the Austral province.

A profuse late Albian magmatism and hydrothermalism related to lithospheric hyperextension affected the thick marine sediments in the centre of the Basque-Cantabrian basin.

In the locality of Sopela, the uppermost Albian deep marine 50 m-thick succession shows basaltic pillow and tabular bodies with intervening pyroclastic deposits developed in a siliciclastic muddy depositional area. Ammonoids indicate a Mortoniceras (M.) fallax Zone to M. (M.) rostratum and perinflatum Zones upwards.

Ammonoids in the mudstones are flattened and scarce, but in the pyroclastics they occur abundantly and well preserved. Grain size varies from fine to coarse ash and lapilli. Scoria grains with bubble structures, irregular basalt and recrystallized limestone clasts with calcitic borders, among others, and a common parallel lamination suggest the deposit of hot water-logged dense high-energy flows formed after a submarine volcanic explosion.

Ammonoids show the shell replaced by calcite and chlorite. All the ontogenetic stages are preserved, being juveniles and smaller forms more abundant than large (cm-sized) adults.

Spheroidal structures 0.3 to 0.7 mm diameter with a very thin calcitic wall, are found in association with ammonitellas of similar size; although the shell is not identified inside these spheroids, some of them may correspond to eggs or very early hatchlings.

A rich community of planktic and benthic forams, ostracods, gastropods, bivalves and small crinoids occurs with the ammonoids.

The origin of the flows and the presence of this rich fossil community suggest a likely ammonoid spawning area affected by catastrophic magmatism leading to intermittent mass-mortality events in the sea-water column.

The evolution of soft-bodied squids, which provide a significant part of the biomass in modern oceans globally, is poorly understood due to their patchy fossil record. We provide a comprehensive evolutionary history of squids through “digital fossil-mining” techniques, revealing a new Lagerstätte (Ikegami et al., in press, 2025; the origin and radiation of squids revealed by digital fossil-mining, Science). The more than 250 fossil beaks of 40 species show that squids originated and rapidly radiated by 100 million years ago. Our new data suggest that the radical shift from heavily shelled, slowly moving cephalopods to soft-bodied forms did not result from the end-Cretaceous mass extinction (66 Ma). Early squids had already formed large populations and their biomass exceeded that of ammonites and fishes. They pioneered the modern-type marine ecosystem as intelligent, fast swimmers.

Sepioids are an evolutionarily successful group of modern ten-armed cephalopods (Decapodiformes) of high biodiversity, which provide a large amount of biomass in present-day oceans. They include the internal-shelled order Sepiida (cuttlefish) and the soft-bodied order Sepiolida (bobtail squid). The phylogenetic position and evolutionary history of these two orders are, however, so far poorly understood due to the patchy fossil record of Decapodiformes. Here we report a new genus and species from the upper Campanian to upper Maastrichtian (~74–67 Ma, Upper Cretaceous), South Dakota, which shows an intermediate morphology between Sepiida and Sepiolida. This is the first study in which an AI (artificial intelligence) model led to the discovery of a new fossil taxon. This new species demonstrates a close relationship between the two sepioid orders, which has previously been interpreted controversially. Our findings indicate that sepioids experienced an early phase of radiation in the later part of the Late Cretaceous.

The world’s largest ammonite, Parapuzosia (P.) seppenradensis (Landois, 1895), has fascinated the world ever since the discovery in 1895 of a specimen measuring 1.74 metres (m) in diameter near Seppenrade in Westfalia, Germany. Its co-fauna was describedin detail The species was also found in sections in Mexico and southern England. The high-resolution integrated stratigraphy allows for precise trans-Atlantic correlation of these occurrences, and the Tepeyac section in Mexico has become Associated Stratotype Section and Point for the base of the Campanian (Gale et al. 2023). It yields giant ammonites in original deposition context (Ifrim et al., 2021) together with a rich macrofossil assemblage (Ifrim and Stinnesbeck, 2021) which is correlated to other parts of the world by ammonoid and inoceramid stratigraphy and the stable carbon isotope curve. The ammonoids in the co-fauna of P.(P.) seppenradensis in Germany and Mexico show common and endemic species. They allow for the interpretation of partially unexpected paleobiogeographic relations.

References

Gale, A., Batenburg, S., et al. 2023. The Global Boundary Stratotype Section and Point (GSSP) of the Campanian Stage at Bottaccione (Gubbio, Italy) and its Auxiliary Sections: Seaford Head (UK), Bocieniec (Poland), Postalm (Austria), Smoky Hill, Kansas (U.S.A), Tepayac (Mexico). Episodes. doi: 10.18814/epiiugs/2022/022048.

Ifrim, C., Gale, et al. 2021. Ontogeny, Evolution and palaeobiogeographic distribution of Parapuzosia (P.) seppenradensis, the world's largest ammonite. PLoS ONE 16, e0258510. doi: 10.1371/journal.pone.0258510.

Ifrim, C., Stinnesbeck, W., 2021. Ammonoids and their biozonation across the Santonian-Campanian boundary in north-eastern Coahuila, Mexico. Paleontologia Electronica 24.3.a34. doi: https://doi.org/10.26879/1046.