K-T Boundary

The Cretaceous-Paleogene (K-Pg) boundary, dated about 65 million years ago, marks the base of the Paleogene period and consequently of the Cenozoic era. (“K” is traditionally used as an abbreviation for the Cretaceous period (from the German term Kreide) in order to avoid confusion with the abbreviation “C” of the Cenomanian epoch and the abbreviation “C-T” of the Cenomanian-Turonian boundary.) It is more popularly known as Cretaceous-Tertiary (K-T) boundary, but the term Tertiary has become informal and is not used in the geologic timescale. from the chronostrati-graphic point of view, the K-T (or K-Pg) boundary was formally defined at the base of a dark clay bed at the El Kef stratotypic section (Tunisia). This bed is commonly called the K-T boundary clay, and its basal part is characterized by a millimeter-thick reddish layer, called K-T airfall layer, containing meteoritic impact evidence such as anomalous-concentrated iridium, siderophile trace elements in chondritic proportions, microdia-monds, nickel-rich spinels, shocked quartz, and altered microtektites.

The K-Pg or K-T boundary is widely known, because it represents one of the five major mass extinction events recorded in the earth's history, which affected the most famous paleontological group, the dinosaurs. However, the dinosaurs were not the only victims of this biotic catastrophe. It is considered that 75% to 80% of then extant species were extinguished; this included the total extinction of dinosaurs (Tyrannosaurus, Triceratops, and Ankylosaurus among others), plesiosaurs, mosasaurs, ichthyosaurs, primitive birds (Enantiornitbes, Hesperornitbiformes), ammonites, belemnites, rudists, and orbitoid fora-minifers. Many other groups were severely affected, such as mammals (-80% extinguished), osteichthian fishes (-96%), nautiloids (-50%), gastropods (-80%), bivalves (-60%), brachiopods (-70%), scleractinian corals (-80%), thermospheric ostracods (-75%), planktic foraminifers (-96%), radiolarians (-85%) and calcareous nannofossils (-75%). Marsupial mammals became extinct except for those in Australia and South America. Remarkably, insects, amphibians, lepidosaurs (lizards, snakes, etc.), crocodilians, turtles, and insectivore mammals and birds were not very affected by the K-T extinction. Moreover, although their populations were initially decimated, many species of terrestrial plants and of marine phyto-plankton (diatoms or dinoflagellates) also tended to survive, due surely to their capacity to form resistant cysts, spores, or seeds.

The K-T extinction was therefore selective, with a species' survival depending on its position in the food chains. In the open ocean, the food chain is and was based on the microscopic phytoplankton (unicellular algae), such as coccoli-tophorids (calcareous nannoplankton), dinoflagellates, or diatoms. The marine protozoons and animals at successively higher levels in this food chain (phytoplanktivorous and carnivorous) were very strongly affected (planktic foraminifers, ammonites, carnivorous fishes, and marine reptiles such as plesiosaurs, mosasaurs, and ichthyosaurs). However, those animals whose diet was suspensivorous or detritivorousor those who lived on thesetended to survive, at least partially (e.g., benthic foraminifers and many bivalves, bryozoans, brachiopods, and fishes). In the terrestrial environment, the food chain is and was based on plants, so the herbivorous and carnivorous animals directly or indirectly dependent on this vegetationall dinosaurs and many birds and mammalsbecame extinct. On the contrary, those terrestrial animals whose diet was detritivorous (e.g., insects), who were potentially scavengers (e.g., cocodilians, turtles) or insectivorous (ancestral mammals and many birds, amphibians and lepidosaurs) tended to survive.

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