The Lopez de Bertodano Formation is a geological formation in the James Ross archipelago of the Antarctic Peninsula. The strata date from the end of the Late Cretaceous (upper-lower Maastrichtian stage ) to the Danian stage of the lower Paleocene, from about 70 to 65.5 million years ago, straddling the Cretaceous-Paleogene boundary.

Cretaceous-Paleogene boundary

The Cretaceous–Paleogene boundary (K–Pg) crops out on Seymour Island in the upper levels of the Lopez de Bertodano Formation. A small (but significant) iridium anomaly occurs at the boundary on Seymour Island, as at lower latitudes, thought to be fallout from the Chicxulub impactor in the Gulf of Mexico. Directly above the boundary a layer of disarticulated fish fossils occurs, victims of a disturbed ecosystem immediately following the impact event. Multiple reports have described evidence for climatic changes in Antarctica prior to the mass extinction, but the extent to which these affected marine biodiversity is debated. Based on extensive marine fossil collections from Seymour Island, recent work has confirmed that a single and severe mass extinction event occurred at this time in Antarctica just as at lower latitudes.

Climate

During the Maastrichtian, Seymour Island was located within the Antarctic polar circle at around ~65°S latitude. Chemical studies on oxygen-18 isotopes found in shells and benthic foraminifera have calculated intermediate-depth and deep-sea ocean temperatures at a mean average of 6 C with fluctuations of 4–12 C throughout the Maastrichtian; one of the same studies has also suggested that sea surface temperatures may have been colder, possibly dropping below freezing and forming sea ice at times. Alternatively, a study using data acquired from ancient bacterial membrane lipids yielded a slightly warmer temperature of 12 ± around 66 Ma. Nevertheless, these estimated climates characterize primarily cool temperate environments with possible subpolar and warm episodes.

Fossil content

The Lopez de Bertodano Formation has provided many fossils of flora, dinosaurs and birds. Also the first fossil egg from Antarctica, Antarcticoolithus, was found in the formation. Dinosaur remains are among the fossils that have been recovered from the formation and include at least two and probably as much as six lineages of indisputably modern birds: one related to waterfowl, a primitive shorebird or related form, 1 to 2 species of possible loons, a large and possibly flightless bird belonging to a lineage extinct today as well as a partial skull that might belong to either of the smaller species or represent yet another one. The formation also contains a rich fossil invertebrate fauna, including bivalves, gastropods, and cephalopods (ammonites and nautiloids). The fish assemblage of the López de Bertodano Formation was dominated by Enchodus and ichthyodectiformes, accounting for 21.95% and 45.6% of local fish diversity respectively. Of the remaining percentages, sand sharks made up 10.5%, the cow shark Notidanodon 6.8%, chimaeras 3.9%, saw sharks 2.7%, various other teleost fish 2.4%, and the remaining 6% were shared between other sharks like Paraorthacodus, frilled sharks, Protosqualus, and Cretalamna.

Vertebrates

Dinosaurs

Ornithischians

Theropods

Fish

Bony Fishes

Chondrichthyes

Reptiles

Elasmosaurs

Mosasaurs

Other fossils