The Cretaceous–Paleogene boundary (K–Pg) crops out on Seymour Island in the upper levels of the López 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 just outside the Antarctic polar circle at around ~64°S latitude. Studies on oxygen-18 isotopes found in belemnites and benthic foraminifera have calculated intermediate-deep-shelf water temperatures at an average of 6 °C (43 °F). This paper also suggested annual temperature variability of 5 °C (41 °F) based on Belemnite growth bands, perhaps in agreement with another study which has suggested that sea surface temperatures may have possibly dropped below freezing and formed sea ice at times. Alternatively, a study using data acquired from ancient bacterial membrane lipids yielded a slightly warmer temperature of 12 ± 5 °C (54 ± 9 °F); further research in other high latitude localities have suggested these methods may be biased towards summer temperatures. More recently a paper has used oxygen-18 isotopes from bony fish fossils to estimate an average water temperature of 6.8 °C (44.2 °F), overall supporting a subpolar climate regime perhaps similar to the modern Magallanes Region.
Southern Chilean forests are a modern analogue for Maastrichtian Antarctica
Fossil content
The López 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.
MLP 98-I-10-47, MLP 98-I-10-50, MLP 98-I-10-52, MLP 98-I-10-76: tarsometatarsus;MLP 98-I-10-59: diaphysis of left tibiotarsus; MLP 98-I-10-51: proximal end of left femur; MLP 98-I-10-48: distal end of left tibiotarsus; MLP 98-I-10-60 and MLP 98-I-10-61: distal end and partial corpus of pedal phalanges.
Initially identified as a fossil of a member of Cariamae, but subsequently reinterpreted as a fossil of an unnamed large-bodied member of the genus Vegavis.
MLP 82-I-28-1, an incomplete skeleton comprising 15 cervical, three pectorals, 21 dorsal, three sacral, and 22 caudal vertebrae, an almost complete left hind limb, some dorsal ribs, incomplete coracoids and fragments of the scapulae
Indeterminate
SW corner, Seymour Island
ZPAL R.8, pectoral, dorsal, and caudal vertebral centra, femur, tibia, and fragments of the humerus, scapula, and ischia
Indeterminate
Seymour Island
TTU P 9240; dorsal, sacral and caudal vertebrae, limbs and paddle fragments
Indeterminate
Seymour Island
SGO.PV.6523, postcranial remains of a single adult individual, including remains of 9 mid-to-posterior cervical vertebrae (6 of them preserving parts of their centra), the right scapula, several fragments of ribs and gastralia, and one phalanx.
Indeterminate
Seymour Island
TTU P 9238; part of cervicals, rib fragments, isolated paddles, and gastroliths
Indeterminate
Seymour Island
TTU P 9239; isolated vertebrae, limb bones, paddle elements, and ribs
Indeterminate
Seymour Island
IAA Pv 443, an incomplete skeleton comprising the mandibular symphysis and part of right and left mandibular rami, cervical and dorsal centra, an incomplete humerus, radius, ulna, ulnare, intermedium, radiale and distal carpal 1, 2 þ 3 and 4, other fragmentary postcranial bones and associated gastroliths
Partially articulated incomplete skeleton including cranial material, many vertebrae, ribs, an ilium, limb bones (right humerus and ulna, a femur), and gastroliths (IAA-Pv 752)
MLP 15-I-7-48, right humerus, ulna, ulnare, intermedium, distal carpal I, distal carpal II+III, pisiform, phalanges and one rib
Indeterminate
Seymour Island
MLP 14-I-20-16, 12 cervical vertebrae, three pectoral vertebrae, 11 dorsal vertebrae, one sacral vertebra, 11 caudal vertebrae, right femur, tibia, fibula and mesopodial elements, fragments of pectoral and pelvic girdles and gastroliths
Mosasaurs
Mosasaurs recorded from López de Bertodano Formation
MLP 80-I-1-1, a cervical vertebra; MLP 80-I-1-2, a mandibular fragment; MLP 80-I-1-3, a cranial fragment; MLP 82-I-28-2, a vertebra; MLP 82-I-3-1/4, four caudal centers
Indeterminate
Seymour Island
DJ.957.133, 18 partially articulated caudal vertebrae, four of which possess transverse process, and DJ.957.505 a caudal vertebra
MLP 92-XII-30, skull fragments including one tooth and a relatively short, and medially constricted suprastapedial process of the quadrate
M. sp.
Seymour Island
DJ.1020.2-A, DJ.1020.2-B and DJ.1053.14- A, teeth; MLP 83-X-12-2, a caudal vertebra
M. sp.
Seymour Island
MLP 15-I-24-41, a partial skull including, partial frontal, right postorbital, parietal, right quadrate, right posterior end of basisphenoid, right coronoid, right angular, splenial and right surangular, a broken marginal tooth and several pterygoid teeth have been associated to this specimen.
Two left upper lateral teeth preserving one root branch and lateral denticle, MLP 13-XI-29-35, MLP 13-XI-29-37; one right upper lateral tooth preserving one root branch and lateral denticle, MLP 13-XI-29-36; several fragmentary teeth, MLP 13-XI29-4, MLP13-XI-29-44 to46, MLP13-XI-29-16, MLP 13-XI-29-13 to 14.
The Maastrichtian represented a period of coolhouse conditions following the gradual global cooling from the Cretaceous Thermal Maximum. Coincident with this trend, an overturning of the Antarctic floral composition occurred during this time, particularly with the diversification of Nothofagaceae and the disappearance of some more archaic angiosperm forms. Other important constituents of the Antarctic floral communities include; Araucariaceae, Podocarpaceae, Atherospermataceae, Myrtaceae, Proteaceae, and Cunoniaceae. Fossil wood and sparse leaves indicates a canopy dominated by Nothofagus, whose wood anatomy suggested a rainforest climate as well as a transition towards deciduousness. Tree rings in Maastrichtian fossil wood are significantly narrower and more distinct than the preceding Coniacian-Campanian periods, indicating less productive growing conditions, and among fossil forests recorded in Antarctica, the Maastrichtian recorded the highest frequency of deferred optimum vessel diameter tree rings which occur when growth commences due to ample moisture availability but temperatures are below the required threshold for peak transpiration. This scenario is common among Nothofagus growing at ca. 55°S today. One study using fossil wood characters calculated mean annual temperatures between 7.3–9.94 °C (45.14–49.89 °F), overall supporting a cool temperate climate for the Maastrichtian Antarctic Peninsula.
Legendre, Lucas J.; Rubilar Rogers, David; Musser, Grace M.; Davis, Sarah N.; Otero, Rodrigo A.; Vargas, Alexander O.; Clarke, Julia A. (2020), , Nature, 583 (7816): 411–414, Bibcode:, doi:, PMID
Further reading
Poole, I.; Mennega, A. M. W.; Cantrill, D. J. (2003). "Valdivian ecosystems in the Late Cretaceous and Early Tertiary of Antarctica: further evidence from myrtaceous and eucryphiaceous fossil wood". Review of Palaeobotany and Palynology. 124 (1–2): 9–27. Bibcode:. doi:. hdl:. S2CID .
