The oldest dinosaur types are known from rocks in
Argentina and Brazil and are about 230 million years
old. The most primitive of these types, Eoraptor,
was a small meat-eating dinosaur. Because
Eoraptor's skeleton shows some advanced skeletal
features, older dinosaurs may yet be found.
No. Dinosaurs are a group of ancient reptiles
that had a set of particular skeletal features. The
hips, hind legs, and ankles were specialized and
allowed the legs to move directly under the body,
rather than extending out from the side of the body
as in modern lizards. This arrangement enabled
dinosaurs to bring their knees and ankles directly
below their hips and provided the necessary
attachments for very strong leg muscles. Dinosaur
skeletons were well designed for supporting a large
body, for standing erect (upright), and for running.
The front legs were adapted for grasping prey, for
supporting weight, or for walking and running. The
skulls of dinosaurs were designed for maximum
strength, for minimum weight, and (in some cases)
for grasping, holding, or tearing at prey. These
skeletal features separated dinosaurs from other
ancient reptiles such as Dimetrodon, the
plesiosaurs, and pterosaurs. Fossil mammals, like
mammoths and "saber-toothed tigers" (e.g.,
Smilodon), are also often incorrectly called
dinosaurs.
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These ancient animals are NOT
dinosaurs! |
No! After the dinosaurs died out, nearly 65
million years passed before people appeared on
Earth. However, small mammals (including shrew-sized
primates) were alive at the time of the
dinosaurs. Many scientists who study dinosaurs
(vertebrate paleontologists) now think that birds
are direct descendants of one line of carnivorous
dinosaurs, and some consider that they in fact
represent modern living dinosaurs. This theory
remains under discussion and shows that there is
still much we don't know about dinosaurs.
Paleontologists now have evidence that dinosaurs
lived on all of the continents. At the beginning of
the age of dinosaurs (during the Triassic Period,
about 230 million years ago) the continents we now
know were arranged together as a single
supercontinent called Pangea. During the 165 million
years of dinosaur existence this supercontinent
slowly broke apart. Its pieces then spread across
the globe into a nearly modern arrangement by a
process called plate tectonics. Volcanoes,
earthquakes, mountain building, and sea-floor
spreading are all part of plate tectonics, and this
process is still changing our modern Earth.
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Relative positions of continents
during the age of dinosaurs |
Dinosaur communities were separated by both time
and geography. The "age of dinosaurs" (the Mesozoic
Era) included three consecutive geologic time
periods (the Triassic, Jurassic, and Cretaceous
Periods). Different dinosaur species lived during
each of these three periods. For example, the
Jurassic dinosaur Stegosaurus already had
been extinct for approximately 80 million years
before the appearance of the Cretaceous dinosaur
Tyrannosaurus. In fact, the time separating
Stegosaurus and Tyrannosaurus is greater
than the time separating Tyrannosaurus and
you. At the beginning of dinosaur history (the
Triassic Period), there was one supercontinent on
Earth (Pangea). Many dinosaur types were widespread
across it. However, as Pangea broke apart, dinosaurs
became scattered across the globe on separate
continents, and new types of dinosaurs evolved
separately in each geographic area.
Dinosaurs generally are named after a
characteristic body feature, after the place where
they were found, or after a person involved in the
discovery. Usually the name consists of two Greek or
Latin words (or combinations); in order, these are
the genus (plural, genera) and the species name. For
example, the Greek and Latin combination (binomen)
Tyrannosaurus rex means "king of the tyrant
lizards." Biologists name modern animals exactly the
same way. Some examples include humans (Homo
sapiens), domestic dogs (Canis familiaris),
golden eagles (Aquila chrysaetos), box
turtles (Terrapene carolina), and
rattlesnakes (Crotalus horridus).
The largest complete dinosaur we know of was
Brachiosaurus ("arm lizard"); it reached 23 m in
length and 12 m in height (about the length of two
large school buses and the height of a four-story
building). Fragmentary leg bones and vertebrae of
even larger dinosaur species are known, but these
skeletal remains are too incomplete to determine
their exact size. Several of these (Argentinasaurus
and Amphicoelias) might have been one and a
half to two times larger than Brachiosaurus.
The smallest dinosaurs were just slightly larger
than a chicken; Compsognathus ("pretty jaw")
was 1 m (3 ft) long and probably weighed about 2.5
kg (about 6.5 lb). These three dinosaur types all
lived during the Jurassic Period. Mussaurus
("mouse lizard") was claimed as the smallest
dinosaur, but it is now known to be the hatchling of
a dinosaur type that was much larger than
Compsognathus when fully grown. If birds are
advanced dinosaurs, then the smallest dinosaur would
be the hummingbird!
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Brachiosaurus
(1) and Compsognathus (2)
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Approximately 700 species have been named.
However, a recent scientific review suggests that
only about half of these are based on fairly
complete specimens that can be shown to be unique
and separate species. These species are placed in
about 300 valid dinosaur genera (Stegosaurus,
Diplodocus, etc.), although about 540 have
been named. Recent estimates suggest that about 700
to 900 more dinosaur genera may remain to be
discovered.
Most dinosaur genera presently contain only one
species (for example, Deinonychus) but some
have more (for example, Iguanodon). Even if
all of the roughly 700 published species are valid,
their number is still less than one-tenth the number
of currently known living bird species, less than
one-fifth the number of currently known mammal
species, and less than one-third the number of
currently known spider species.
Scientists have conflicting opinions on this
subject. Some paleontologists think that all
dinosaurs were "warm-blooded" in the same sense that
modern birds and mammals are: that is, they had
rapid metabolic rates. Other scientists think it
unlikely that any dinosaur could have had a rapid
metabolic rate. Some scientists think that very big
dinosaurs could have had warm bodies because of
their large body size, just as some sea turtles do
today. It may be that some dinosaurs were
warm-blooded. The problem is that it is hard to find
evidence that unquestionably shows what dinosaur
metabolisms were like.
Animal lifespans relate in part to their body
size and in part to their type of metabolism.
Dinosaur lifespans probably varied in length from
tens of years to hundreds of years. Their possible
maximum age can be estimated from the maximum
lifespans of modern reptiles, such as the 66-year
lifespan of the common alligator (Alligator
mississippiensis) and the impressive lifespan of
a Black Seychelles Tortoise (Geochelone
(Aldabrachelys) sumeirei). One specimen of this
now-extinct species, which was an adult when
captured, lived a record 152 years in captivity
(1766-1918) and had an accidental death. These
estimates, based on lifespans of cold-blooded
animals, would be too long if dinosaurs had
metabolisms more similar to modern birds and
mammals.
Some dinosaurs ate lizards, turtles, eggs, or
early mammals. Some hunted other dinosaurs or
scavenged dead animals. Most, however, ate plants
(but not grass, which hadn't evolved yet). Rocks
that contains dinosaur bones also contain fossil
pollen and spores that indicate hundreds to
thousands of types of plants existed during the
Mesozoic Era. Many of these plants had edible
leaves, including evergreen conifers (pine trees,
redwoods, and their relatives), ferns, mosses,
horsetail rushes, cycads, ginkos, and in the latter
part of the dinosaur age flowering (fruiting)
plants. Although the exact time of origin for
flowering plants is still uncertain, the last of the
dinosaurs certainly had fruit available to eat.
Estimates of dinosaur speeds vary because several
different methods are used to calculate them. One
recent estimate suggests that an average person
might have been able to to outrun an adult
Tyrannosaurus (although you probably would not
volunteer to try). The two basic approaches for
estimating dinosaur speed are comparing to recorded
speeds of modern animals of similar body size and
build, and measuring distances between fossil
footprints in a trackway and using these distances
to calculate estimated speed. Walking-speed
estimates for medium-sized bipedal (two-legged)
dinosaurs vary from 4 kph to 6 kph, and peak
running-speed estimates vary from 37 kph to 88 kph.
The highest figure (88.6 kph) is the same as the
peak speed of the currently fastest land animals,
such as the North American pronghorn "antelope" (Antilocapra
americana), and very probably is too high.
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Speed comparison of some ancient and modern
animals |
Dinosaurs probably communicated both vocally and
visually. The chambered headcrests on some dinosaurs
such as Corythosaurus and Parasaurolophus
might haved been used to amplify grunts or bellows.
Defensive posturing, courtship behavior, and
territory fights probably involved both vocal and
visual displays. An angry Triceratops bull
shaking his head at you, even silently, would have
made himself very clearly understood!
Paleontologists don't know for certain, but
perhaps a large body size protected them from most
predators, helped to regulate internal body
temperature, or let them reach new sources of food
(some probably browsed treetops, as giraffes do
today). No modern animals except whales are even
close in size to the largest dinosaurs; therefore,
paleontologists think that the dinosaurs' world was
much different from the world today and that climate
and food supplies must have been favorable for
reaching great size.
Although there is no direct way to measure a
dinosaur's intelligence, one of the few possible
measures of intelligence might be a large brain in a
small body. The genus that perhaps fits this
description best was the Cretaceous bird-like
dinosaur Troodon, which also may have had
binocular vision (depth perception) and excellent
eyesight and was built for speed. Even so, this
dinosaur was probably not as "intelligent" as most
modern birds and mammals.
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Ceratosaurus nasicornis, from Marsh
(1896)
Orginal specimen on display at U.S. National
Museum
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Direct fossil evidence for dinosaur skin color is
unknown. Paleontologists think that some dinosaurs
likely had protective coloration, such as pale
undersides to reduce shadows, irregular color
patterns ("camouflage") to make them less visible in
vegetation, and so on. Those dinosaurs that had
enough armor, such as the stegosaurs and
ceratopsians, may not have needed protective
coloration but may have been brightly colored as a
warning to predators or as a display for finding a
mate. Most dinosaurs probably were as brightly
colored as modern lizards, snakes, or birds.
Some dinosaurs were social creatures. Recently
discovered evidence indicates that they travelled
together and that some may even have migrated
(because dinosaur fossils have been found above the
Arctic Circle, where food supply would have been
seasonal). Grouped hadrosaur nest sites have been
found with badly crushed eggshells and skeletons of
baby dinosaurs (with slightly worn teeth) still in
the nests, suggesting that some babies stayed in
their nests after hatching and probably were fed by
parents.
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Group behavior in the Triassic early
dinosaur Coelophysis |
Dinosaurs went extinct about 65 million years ago
(at the end of the Cretaceous Period), after living
on Earth for about 165 million years. If all of
Earth time from the very beginning of the dinosaurs
to today were compressed into 365 days (1 calendar
year), the dinosaurs appeared January 1 and became
extinct the third week of September. (Using this
same time scale, the Earth would have formed
approximately 18.5 years earlier.) By comparison,
people (Homo sapiens) have been on earth only
since December 31 (New Year's eve). The dinosaurs'
long period of dominance certainly makes them
unqualified successes in the history of life on
Earth.
There are dozens of theories to explain a
probable cause or causes. Throughout the Mesozoic
Era, individual dinosaur species were evolving and
becoming extinct for various reasons. The unusually
massive extinction at the end of the Cretaceous
exterminated the last of the dinosaurs, the flying
reptiles, and the large swimming reptiles, as well
as many other marine animals. There is now
widespread evidence that a meteorite impact was at
least the partial cause for this extinction. Impact
craters are visible on most planets in our solar
system. A spectacular example of this was witnessed
in 1994, when Jupiter was struck by a series of
cometary fragments. Some of these impact blasts were
larger than the Earth's diameter. Other factors such
as extensive release of volcanic gases, climatic
cooling (with related changes in ocean currents and
weather patterns), sea-level change, low
reproduction rates, poison gases from a comet, or
changes in the Earth's orbit or magnetic field may
have contributed to this extinction event.
Your local museums, public libraries, and
bookstores are good places to start. Some national
monuments (Dinosaur National Monument, UT and CO),
national parks (Petrified Forest National Park, AZ),
and state parks (for example, Dinosaur Valley State
Park, TX) have outstanding displays. State
geological surveys also have or can provide
information on nearby dinosaur exhibits.
The references below should be helpful to you get
started - In fact, they provided some of the
information above:
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Dodson, P., and Dawson, S.D.,
1991, Making the fossil record of dinosaurs:
Modern Geology, vol. 16, p. 3-15.
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Farlow, J.O., 1993, The dinosaurs
of Dinosaur Valley State Park -- Somervell County,
Texas: Texas Parks and Wildlife Department, Austin,
31 p.
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Glut, D.F., 1982, The New Dinosaur Dictionary:
Secaucus. Citadel Press, 288 p.
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Lambert, D., and the Diagram Group, 1990,
Dinosaur Data Book: New York, Avon Books, 320 p.
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Marsh, O.C., 1896, The dinosaurs of North
America: U.S. Geological Survey, Sixteenth
Annual Report, part I, p. 131-414.
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Norman, D., 1985, Illustrated Encyclopedia of
Dinosaurs: New York, Crescent Books, 208 p.
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Russell, D.A., 1989, An Odyssey in Time, the
Dinosaurs of North America: Minocqua, North Word
Press, 220 p.
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Thulborn, T., 1990, Dinosaus Tracks: London,
Chapman and Hall, 410 p.
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Weishampel, D.B., Dodson, P., and Osmolska, H.,
1990, The Dinosauria: Berkeley, University of
California Press, 733 p.
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