taxon_rank gondwanatheria
Mammals are any vertebrates within the class Mammalia ( from Latin " mamma " " breast "), a clade of endothermic amniotes distinguished from reptiles and birds by the possession of a neocortex ( a region of the brain ), hair , three middle ear bones and mammary glands . The sister ___MASK56___ of mammals may be the extinct " Haldanodon ." The mammals represent the only living Synapsida , which together with the Sauropsida ___MASK44___ the Amniota clade . The mammals consist of the Yinotheria including monotrema and the Theriiformes including the theria . The Cretaceous is a geologic period and system that spans 79 million years from the end of the Jurassic Period million years ago ( Mya ) to the beginning of the Paleogene Period Mya . It is the last period of the Mesozoic Era . The Cretaceous Period is usually abbreviated K , for its German translation " Kreide " ( chalk ). An ocean ( the sea of classical antiquity ) is a body of saline water that composes much of a planet ' s hydrosphere . On Earth , an ocean is one of the major conventional divisions of the World Ocean , which covers almost 71 % of its surface . These are , in descending ___MASK7___ by area , the Pacific , Atlantic , Indian , Southern ( Antarctic ), and Arctic Oceans . The word " sea " is often used interchangeably with " ocean " in American English but , strictly speaking , a sea is a body of saline water ( generally a division of the world ocean ) partly or fully enclosed by land . Gondwanatheria is an extinct ___MASK56___ of mammals that lived during the Upper Cretaceous through the Miocene in the Southern Hemisphere , including Antarctica . They are known only from isolated teeth , a few lower jaws , two partial skulls and one complete cranium . Because of this fragmentary knowledge their placement is not clear . The Paleogene ( or ; also spelled Palaeogene or Palæogene ; informally Lower Tertiary ) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period million years ago ( Mya ) to the beginning of the Neogene Period Mya . It is the beginning of the Cenozoic Era of the present Phanerozoic Eon . The Paleogene is most notable for being the time during which mammals diversified from relatively small , simple forms into a large ___MASK56___ of diverse animals in the wake of the Cretaceous  Paleogene extinction event that ended the preceding Cretaceous Period . The Miocene is the first geological epoch of the Neogene Period and extends from about ( Ma ). The Miocene was named by Sir Charles Lyell . Its name comes from the Greek words (',  less ) and (',  new ) and means " less recent " because it has 18 % fewer modern sea invertebrates than the Pliocene . The Miocene follows the Oligocene Epoch and is followed by the Pliocene Epoch . The neocortex , also called the neopallium and isocortex , is the part of the mammalian brain involved in higher - ___MASK7___ brain functions such as sensory perception , cognition , generation of motor commands , spatial reasoning and language . The Southern Hemisphere is the half sphere of Earth which is south of the equator . It contains all or parts of five continents ( Antarctica , Australia , about 90 % of South America , the southern third of Africa , and several southern islands off the continental mainland of Asia ), four oceans ( Indian , South Atlantic , Southern , and South Pacific ) and most of the Pacific Islands in Oceania . Its surface is 80 . 9 % water , compared with 60 . 7 % water in the case of the Northern Hemisphere , and it contains 32 . 7 % of Earth ' s land . Invertebrates are animals that neither possess nor develop a vertebral column ( commonly known as a " backbone " or " spine "), derived from the notochord . This includes all animals apart from the ___MASK95___ Vertebrata . Familiar examples of invertebrates include insects ; crabs , lobsters and their kin ; snails , clams , octopuses and their kin ; starfish , sea - urchins and their kin ; and worms . Chalk is a soft , white , porous , sedimentary carbonate rock , a ___MASK44___ of limestone composed of the mineral calcite . Calcite is an ionic salt called calcium carbonate or CaCO . It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite shells ( coccoliths ) shed from micro - organisms called coccolithophores . Flint ( a type of chert unique to chalk ) is very common as bands parallel to the bedding or as nodules embedded in chalk . It is probably derived from sponge spicules or other siliceous organisms as water is expelled upwards during compaction . Flint is often deposited around larger fossils such as Echinoidea which may be silicified ( i . e . replaced molecule by molecule by flint ).