Etymology
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Mesozoic (adj.)

in geology, "of or found in that part of the geological series between the Paleozoic and what was then called the Tertiary," 1840, from Greek mesos "middle" (from PIE root *medhyo- "middle") + zoe "life" (from PIE root *gwei- "to live") + -ic. The name was coined by British geologist John Phillips for the fossil era "between" the Paleozoic and what is now the Cenozoic. An older name for it was Secondary.

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Cenozoic (adj.)

"the third great geological period," 1841, Cainozoic, from Latinized form of Greek kainos "new, fresh, recent, novel" (see recent) + zōon "animal," but here with a sense of "life" (from PIE root *gwei- "to live"). The era that began with the demise of the dinosaurs and the rise of "recent" species and continues to the present; it also is known as the Tertiary. Compare Paleozoic, Mesozoic.

We observe that Lyell, in his geological works, even the most recent, uses the word Cainozoic instead of Coenozoic or Cenozoic. Why the propounder of the terms Eocene, Miocene, etc., should thus spell the word is incomprehensible. If he is right in it, then to be consistent he ought to say Eocain, Miocain, Pliocain, Post-pliocain; for all have the same root καινός. [American Journal of Sciences and Arts, 1873]
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*medhyo- 
Proto-Indo-European root meaning "middle." Perhaps related to PIE root *me- (2) "to measure."

It forms all or part of: amid; intermediate; mean (adj.2) "occupying a middle or intermediate place;" medal; medial; median; mediate; medieval; mediocre; Mediterranean; medium; meridian; mesic; mesial; meso-; meson; Mesopotamia; Mesozoic; mezzanine; mezzo; mezzotint; mid (prep., adj.); middle; Midgard; midriff; midst; midwife; milieu; minge; mizzen; moiety; mullion.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit madhyah, Avestan madiya- "middle," Greek mesos, Latin medius "in the middle, between; from the middle," Gothic midjis, Old English midd "middle," Old Church Slavonic medzu "between," Armenian mej "middle."
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*gwei- 
also *gweie-, Proto-Indo-European root meaning "to live."

It forms all or part of: abiogenesis; aerobic; amphibian; anaerobic; azo-; azoic; azotemia; bio-; biography; biology; biome; bionics; biopsy; biota; biotic; cenobite; Cenozoic; convivial; couch-grass; epizoic; epizoon; epizootic; macrobiotic; Mesozoic; microbe; Protozoa; protozoic; quick; quicken; quicksand; quicksilver; quiver (v.) "to tremble;" revive; survive; symbiosis; viable; viand; viper; vita; vital; vitamin; victuals; viva; vivace; vivacious; vivarium; vivid; vivify; viviparous; vivisection; whiskey; wyvern; zodiac; Zoe; zoetrope; zoic; zoo-; zoolatry; zoology; zoon; zoophilia; zoophobia; zooplankton.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit jivah "alive, living;" Old Persian *jivaka- "alive," Middle Persian zhiwak "alive;" Greek bios "one's life, course or way of living, lifetime," zoe "animal life, organic life;" Old English cwic, cwicu "living, alive;" Latin vivus "living, alive," vita "life;" Old Church Slavonic zivo "to live;" Lithuanian gyvas "living, alive," gyvata "(eternal) life;" Old Irish bethu "life," bith "age;" Welsh byd "world."
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elasmosaurus (n.)

giant sea reptile from the Mesozoic, 1868, from Modern Latin (coined by E.D. Cope), from Greek elasmos "metal plate" (from elan "to strike;" see elastic) + -saurus. So called from the caudal laminae and the great plate-bones.

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Paleozoic (adj.)

in reference to the geological era between the Precambrian and the Mesozoic, a geological series characterized by the earliest record of modern life forms, 1838, coined by Adam Sedgwick (1785-1873) from paleo- "ancient" + Greek zoe "life" (from PIE root *gwei- "to live") + -ic.

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dinosaur (n.)

one of the Dinosauria, a class of extinct Mesozoic reptiles often of enormous size, 1841, coined in Modern Latin by Sir Richard Owen, from Greek deinos "terrible" (see dire) + sauros "lizard" (see -saurus). Figurative sense of "person or institution not adapting to change" is from 1952. Related: Dinosaurian.

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tertiary (adj.)
1650s, "of the third order, rank, degree, etc.," from Latin tertiarius "of or pertaining to a third," from tertius "third, a third," from root of tres "three" (see three). The geological sense (with capital T-) of "era after the Mesozoic" (which formerly was called the Secondary) is attested from 1794, after Italian terziari, used in this sense 1760 by Italian geologist Giovanni Arduino (1714-1795).
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