Christina Ifrim

Fluctuations of the oxygen minimum zone at the end of Oceanic Anoxic Event 2 reflected by benthic and planktic fossils

The latest Cenomanian to early Turonian (Late Cretaceous) section at Vallecillo, Mexico, contains a continuous and highly fossiliferous sedimentary record across the late Oceanic Anoxic Event (OAE) 2, and shows similar distributions of benthic inoceramids and planktic foraminifers during this period of an expanded oxygen minimum zone (OMZ). We suggest that these synchronous abundance changes result from similar environmental vulnerability of planktotrophic larval stages of inoceramids and deeper-dwelling planktic foraminifers, and that these synchronous changes reflect the expansion of the OMZ. The abundances of ammonites and fishes, used as independent control groups, do not correlate and rule out a preservational bias. Ammonites, fishes, and their hatchlings thus populated shallower water depths than inoceramid gametes and planktic foramininifers. The improving conditions at the end of OAE 2 resulted in the dispersal of new inoceramid and foraminiferal morphotypes.

Upper Cretaceous (Cenomanian-Turonian and Turonian-Coniacian) open marine plattenkalk deposits in NE Mexico

In NE Mexico, the Cenomanian-Turonian plattenkalk of Vallecillo (state of Nuevo Leon) and the Turonian-Coniacian plattenkalk of Muzquiz (state of Coahuila) yield abundant fossil vertebrates and invertebrates with an exceptional preservation of soft parts. Inoceramid bivalves are the only benthic organisms in both deposits. They allow for detailed biostratigraphic subdivisions, precise dating and correlation with biozonations based on ammonites and foraminifers in both sections. In contrast to many other plattenkalks, the Mexican sediments were deposited in an open marine shelf environment, in a distance of >200 km from the nearest coast. Both Konservat-Lagerstatten formed near the southern opening of the Western Interior Seaway and contain mixtures of Western Interior Seaway and Tethyan faunal elements.

Paleobiology and paleoecology of the early Turonian (Late Cretaceous) ammonite Pseudaspidoceras flexuosum

Abstract During the early Turonian, Pseudaspidoceras flexuosum was a common ammonite in low and intermediate latitudes. At Vallecillo section, northeastern Mexico, 160 specimens were recovered from the section, allowing for quantitative analysis. The combination of quantitative data with sedimentology and geochemistry allow for erection of a differentiated model for the mode of life of P. flexuosum. This model provides insight into the paleobiology and paleoecology of this ammonite, and ammonites in general. The Vallecillo section contains the longest record of this species worldwide. Changes in abundance and diameter of this ammonite appear to be related to changes of oxygen content in the water column. P. flexuosum dwelled in the well-oxygenated upper surface water and formed part of the open marine pelagic ecosystem; a demersal mode of life is excluded here. Dimorphism is expressed in size and less in ornamentation, but both dimorphs bear long spines, first recorded for this species. The spines were likely used for stabilization in the water column and protection against suction-feeding fishes and comparable predators, but not against pycnodont sharks. A sensory function is also plausible for the last one to three pairs of spines. This differentiated model of the mode of life of P. flexuosum contrasts with the paleoecology reconstructed for other ammonites. Ammonites covered a wide range of ecological niches, and their complex paleoecology will only be solved with more species-based analyses.

Epizoic Stramentid Cirripedes on Ammonites from Late Cretaceous Platy Limestones in Mexico

Abstract The discovery of platy limestone deposits in northeastern Mexico has led to the collection of well-preserved stramentids of early Turonian age from Vallecillo, state of Nuevo León, and of early Coniacian age from El Carranza, state of Coahuila. Stramentum (Stramentum) pulchellum (Sowerby, 1843) colonized the ammonite shells during the lifetime of the animals, occasionally in two subsequent generations. Colonization of the ammonite shell by Stramentum (S.) pulchellum was hindered by strong ornamentation only. The ammonites did not interfere with their epizoans. Colonization during lifetime shows that these ammonites dwelled in well-oxygenated water levels near the surface, and most stramentids were embedded alive. The known paleobiogeographic occurrence of Stramentum (S.) pulchellum and its long stratigraphic occurrence are considerably enlarged by our findings. The pseudoplanktonic mode of life of Stramentum, and attachment to ammonite shells, may have been a response of a once benthic organism to repeated oxygen-deficient conditions on the seafloor of mid-Cretaceous oceans, i.e., to oceanic anoxic events (OAEs).

Paleobiogeography of Early Cretaceous Ammonoids

Ammonoids were dominant elements of marine Cretaceous faunas. Their fossils are important biostratigraphic indicators, in some cases with worldwide distribution. Their paleobiogeographic distributions were influenced by changing continental positions, currents, sea level change, and perhaps by their mode of life as reflected in the evolution of many diverse lineages of heteromorphic, possibly rather vertically migrating than horizontally swimming taxa through the Cretaceous. Here, we review the current paleobiogeographic understanding of Late Cretaceous ammonoid distribution in an effort to complement and add to previous work in this field. Four principal paleobiogeographic areas can be distinguished in the Cretaceous: The Boreal Realm, the Tethyan Realm, the Pacific Realm, and the Austral Realm. A Tethyan Superrealm is repeatedly recognized throughout the Late Cretaceous, although represented by varying taxa. Migration between the realms and provinces appears to be linked to changes of migration routes, triggered by sea level changes. At the end of the Cretaceous, provincialism disappears, preceding the extinction of the Ammonoids.

An Endemic Cephalopod Assemblage from the Lower Campanian (Late Cretaceous) Parras Shale, Western Coahuila, Mexico

Abstract The cephalopods from Union y Progreso represent the first fossil assemblage described from the Parras Shale in Coahuila, Mexico. Pseudoschloenbachia (Pseudoschloenbachia) aff. P. (P.) mexicana (Renz, 1936), P. (P.) mexicana (Renz, 1936), Baculites haresi Reeside, 1927, and Menabites (Delawarella) vanuxemi (Morton, 1830) have a geographically restricted occurrence. Didymoceras juv. sp., Menuites juv. sp., Polyptychoceras juv. sp., Pseudoxybeloceras (Parasolenoceras) juv. sp., and Scaphites sp. ex gr. S. hippocrepis (DeKay, 1828) are represented by juveniles and could not be determined to species level. Desmophyllites diphylloides (Forbes, 1846) is the only long-ranging, cosmopolitan species described from this assemblage. Three new species are described: Eutrephoceras irritilasi n. sp., Hypophylloceras (Neophylloceras) arturoi n. sp., and Tetragonites silencioensis n. sp. The morphotype Baculites n. sp. is also inferred to be distinct. The faunal composition of this assemblage indicates a late early Campanian age. This assemblage shows a high degree of endemism. The causes for this endemism are currently unknown and difficult to assess. Nevertheless, the generic composition of the Union y Progreso ammonite assemblage suggests a short-term early Campanian endemic event.

Inoceramids and biozonation across the Turonian – Coniacian boundary (Upper Cretaceous) at El Rosario, Coahuila, northeastern Mexico

The Rosario section in northern Coahuila, northeastern Mexico, contains a complete record of sediment across the Turonian–Coniacian boundary. Here we describe the inoceramids and the biozonation based on these bivalves. Inoceramus longealatus, Mytiloides herbichi, M. incertus, M. scupini, Didymotis costatus, Cremnoceramus waltersdorfensis waltersdorfensis, C. waltersdorfensis hannovrensis, C. deformis erectus, and C. crassus inconstans, were identified at El Rosario. The uppermost Turonian Mytiloides scupini and C. waltersdorfensis w. zones and the lower Coniacian C. crassus inconstans zones are well represented and are much more expanded than in sections of the US Western Interior and Europe. The index for the base of the Coniacian, C. deformis erectus, is also present, although few data exist to date from the critical Turon ian–Coniacian boundary level. Ammonites are rare, mostly endemic and provide a much lower resolution across the Turonian–Coniacian boundary than inoceramids. We suggest that various acmes observed across the Turonian–Coniacian boundary are driven by local or regional rather than by global causes. The Rosario section provides an expanded sediment and complete inoceramid record across the Turonian–Coniacian boundary.