lycopod(LIE-keh-pod) the first ancient trees of Paleozoic forests; today
comprising club mosses and liverworts
Lystrosaurus an ancient, extinct, mammal-like reptile with large down-
turned tusks
magma a molten r ock material generated within Earth and the constituent
of igneous rocks
mantle the part of a planet below the crust and above the core, composed
of dense rocks that might be in convective flow
maria dark plains on the lunar surface produced by massive basalt floods
marsupial(mar-SUE-pee-al) a primitive mammal that weans underdevel-
oped infants in a belly pouch
megaherbivore a large , plant-eating animal such as an elephant or extinct
mastodon
Mesozoic(meh-zeh-ZOE-ik) literally the period of middle life, referring
to a period between 250 and 65 million years ago
metamorphic rock a r ock crystallized from previous igneous, metamor-
phic, or sedimentary rocks created under conditions of intense temper-
atures and pressures without melting
metamorphism (me-teh-MORE-fi-zem) recrystallization of previous
igneous, metamorphic, and sedimentary rocks under extreme tempera-
tures and pressures without melting
metazoan primitive multicellular animal with cells differentiated for spe-
cific functions
meteorite a metallic or stony celestial body that enters Earth’s atmosphere
and impacts on the surface
methane a hydrocarbon gas liberated by decomposing organic matter and
a major constituent of natural gas
microfossil a fossil that must be studied with a microscope; used for dating
drill cuttings
mollusk(MAH-lusk) a large group of invertebrates, including snails, clams,
squids,and extinct ammonites, characterized by an internal and external
shell surrounding the body
monotreme egg-laying mammals including the platypus and echidna
moraine a ridge of erosional debris deposited by the melting margin of a
glacier
nautiloid(NOT-eh-loid) shell-bearing cephalopods abundant in the Pale-
ozoic with only the nautilus surviving
nebula an extended astronomical object with a cloudlike appearance; some
nebulae are galaxies, others are clouds of dust and gas within our galaxy
Neogene the Cenozoic Miocene and Pliocene epochs
nutrient a food substance that nourishes living organisms
oolite(OH-eh-lite) small, rounded grains in limestoneHistorical Geology