Not all ancient reptiles were dinosaurs, even if they looked or lived like them
This
Jurassic landscape is the color-enhanced update of an 1830 painting. It
depicts what life might have been like 200 million to 146 million years
ago.
Richard Bizley / Science Source
The first of a two-part series
Dinosaurs
died out about 66 million years ago, yet they live on in our
imagination. They populate books and toy shelves. They also star on the
big screen in blockbuster movies like 1993’s
Jurassic Park and this summer’s
Jurassic World. Such efforts have made us all familiar with creatures like the fearsome
Tyrannosaurus rex, the three-horned
Triceratops and the long-necked
Brachiosaurus.
But
not all dinosaurs were immense beasts. Some were only the size of
chickens. And not every prehistoric reptile was a dinosaur.
Dinosaurs
are but one small group of reptiles. Ancient ecosystems also hosted the
ancestors of today’s crocodiles, snakes, lizards and turtles. And the
landscape teemed with many other types of reptiles that are now long
gone. Some lived in the sea. Others took to the air and soared like
birds. Some lived on land and, at first glance, resembled dinosaurs. But
the detailed features of their fossils, or ancient remains, clearly
show they were not dinos.
"In many cases, these animals played
very important roles in their ecosystems," says Thomas Holtz, Jr. He
works at the University of Maryland in College Park. As a vertebrate
paleontologist, he studies the fossils of creatures with backbones.
Many
of these ancient non-dinos died out at the same time as dinosaurs.
Others went extinct millions of years earlier. Scientists still aren't
sure why. Studying the fossil record can provide clues. It also can show
which groups of ancient creatures are most closely related, when they
first appeared and how they evolved — changed over time.
It’s all in the hips
For
millions of years, dinosaurs dominated many of Earth’s ecosystems. The
first dinos emerged in the fossil record about 235 million years ago.
They disappeared some 169 million years later. Some scientists think a
massive comet or asteroid struck Earth and wiped them out.
Some
dinosaurs were big. Others were small. Some ate animals. Others dined
solely on plants. Some ran on two legs; others walked on all fours. But
all had at least one thing in common, notes Sterling Nesbitt. He’s a
vertebrate paleontologist at Virginia Tech in Blacksburg. Every dinosaur
had holes where its thighbones, or femurs, attached to the pelvis. “All
dinosaurs had this open hip socket,” he says. “It’s not found in any
other reptiles.” Simply put, reptiles lacking this hip structure were
not dinosaurs.
Scientists
look at traits like hip structure to distinguish species. Each species,
whether living or long extinct, has its own special set of
characteristics. Birds have feathers, and mammals have hair. Today’s
reptiles have scales. Fossils suggest that most dinosaurs had scales
too. So do fish (but their scales are different). The more traits that
two species share, the more closely related they tend to be. “By looking
at shared characteristics, you can tell what creatures are related,”
says James Clark. He’s a vertebrate paleontologist at George Washington
University in Washington, D.C.
Two
examples of how scientists draw a cladogram, or family tree, for
related species. In both, species A and B are more closely related to
each other than either is to C.
Alexei Kouprianov/Wikimedia Commons (CC-BY-SA 3.0)
For
living creatures, Clark notes, scientists often go beyond anatomy and
consider similarities or differences in genetic material such as DNA.
But for ancient animals, scientists typically have only fossils.
Researchers can use those various characteristics to build a family
tree. This method of mapping family trees is called cladistics
(Klah-DISS-tiks).Cladistics reveals that many different types of
reptiles lived during ancient times. In fact, the Mesozoic Era, which
lasted from 252 million to about 66 million years ago, is often referred
to as “the Age of Reptiles.” Among animals during that lengthy era,
reptiles dominated. But among them, dinosaurs were only one group.
Dino doppelgängers
Even
if something looked and behaved like a dinosaur, it wasn’t a dinosaur
unless it had the right hip structure. Consider a ferocious meat-eater
just described by Nesbitt and his teammates in the November 4, 2014
Journal of Vertebrate Paleontology. The team named the creature
Nundasuchus (NOON-dah-SOO-kuss). In the local Swahili language,
nunda means “predator.” The suffix
suchus is from a Greek word for “crocodile.”
Nundasuchus was
probably about 2.75 meters (9 feet) long. Some of its teeth had edges
that were serrated, like steak knives. But this ancient meat-eater was
not a dinosaur. It didn’t have the right hip structure. And the
reptile’s ankle bones were similar to those of animals in a different
group of reptiles, called Pseudosuchia (SOO-doh-SOO-kee-ah). That name
is a combination of Latin and Greek words that means “false crocodiles.”
(
story continues below image)
Nundasuchus,
which lived about 242 million years ago, looked very much like a
dinosaur, yet was more closely related to crocodiles. (White bones are
the ones recovered by paleontologists.).
Virginia Tech
The
researchers found the animal’s fossil bones in Tanzania, in
southeastern Africa. Those bones had been entombed around 242 million
years ago. They didn’t form a complete skeleton, but they did include
bones, or bits of bones, from almost all parts of the animal. Those
remains let the team measure hundreds of the creature’s anatomical
details. Importantly, the scientists found a left rear leg that was very
nearly complete. “I could tell this was a new species almost
immediately,” Nesbitt says.
It would have belonged to a group of
species that includes true crocodiles and their living and extinct
relatives. It also includes other groups of ancient and now-extinct
reptiles. One is called the rauisuchids (RAOW- ee-SOO-kidz). These
meat-eaters, some of which reached lengths of 6 meters (20 feet), are
often mistaken for dinosaurs because they look so similar. But their
hips show they aren’t dinosaurs at all.
Up in the air!
Birds
weren’t the first vertebrates to take to the skies. One group of
reptiles beat them to it by a good 75 million years or more. Called
pterosaurs (TAIR-oh-saurs), these creatures take their name from the
Greek words for “wing lizard.”
Pterosaurs were close relatives of
dinosaurs. Their hip structures were roughly similar. Pterosaurs and
dinosaurs also are the only groups of reptiles that have one particular
arrangement of bones in their ankles. “It’s very distinctive,” says
Clark. Yet these similarities don’t mean that pterosaurs evolved from
dinosaurs, he explains. It simply means that they both shared some
common ancestor. So, on the reptile family tree, they were only cousins.
This
image depicts a giant azhdarchid pterosaur, named Quetzalcoatlus
northropi. With a 10 meter (almost 33-foot) wingspan, this reptile stood
as tall as a giraffe. These pterosaurs lived alongside tyrant dinosaurs
and likely preyed on smaller reptiles, including this baby
Tyrannosaurus rex.
The
oldest known pterosaurs were well adapted for flying. Like birds, their
bones were hollow and filled with air. That helped make their bodies
light and easier to get off of the ground. But unlike birds, pterosaurs
had no feathers. Instead, their wings were covered in thin membranes
made of skin and other tissues. Those membranes stretched between their
wings’ bones (which included a very long finger) and their bodies.
Sometimes the rear edge of the wings stretched back and connected to a
pterosaur’s ankles. (This past April, scientists reported finding
Yi qi,
a species of small dinosaur with a pterosaur-like wing membrane. But
it’s not yet clear if that creature was truly able to flap and fly like
birds and pterosaurs, the researchers say. Perhaps it could only glide.)
For
the first 70 million years, pterosaurs remained pretty much the same
size, says Roger Benson. He’s a vertebrate paleontologist at the
University of Oxford in England. All pterosaur species during that time
had an average wingspan of about 1.2 meters (4 feet). Then, starting
about 150 million years ago, pterosaurs began enlarging. And that trend
continued until the animals became extinct 66 million years ago. Near
the end, the wings of various species spanned from 3 meters (10 feet) to
10 meters (33 feet) across. (For comparison, a U.S. Air Force F-16
fighter jet has a 10-meter wingspan.)
Two things drove that growth
in average wingspan. Some huge species evolved. At the same time, most
small pterosaur species vanished. This might be linked to the appearance
of the first birds, researchers suggested in 2014 in
Nature Communications.
And here’s why: Birds and small pterosaurs probably dined on the same
types of creatures. Early birds might have been more active or more
agile than pterosaurs their size. So the flying reptiles might have lost
out when competing with birds for a meal. In time, these small
pterosaurs largely died out. Those that survived did so by evolving into
such immense sizes that they no longer competed with birds.
Many
people incorrectly call all flying reptiles “pterodactyls.” That name,
which comes from the Greek words for “wing finger,” belongs to only one
group of pterosaurs. They get their name from the long thin finger that
supports the front edge of their wing, explains Clark. Typically, a
pterosaur is considered a pterodactyl if the longest bone in its wing
finger is eight times longer than its diameter.
That’s just weird
Dinosaurs
are well known for their unusual appearance. Many had horns, spikes,
plates or any of plenty of other odd features. Pterosaurs could look
pretty weird too. Some early species had toothy jaws and long tails.
Many later species had no teeth and almost no tail.
But one
pterosaur might have looked much weirder than the rest. It lived about
120 million years ago in what is now northeastern China. Adults probably
had a wingspan of about 1.5 meters (5 feet). In many ways, this flying
reptile looked like other pterosaurs. But its fossils provide
tantalizing clues that this species was special. The front portion of
its lower jaw had a deep, thin, crescent-shaped keel of bone. Near the
point where this semicircle joined the lower jaw, there was a peculiar
hook-shaped projection.
Its fossils were first described in the journal
Scientific Reports in 2014.
(
story continues below image)
Flying
reptiles — such as these Ikrandraco avatars — were pterosaurs, not
dinosaurs. They did live during the same era as dinos. Fossils reveal
many clues about how and when such creatures evolved, as well as how
they lived. These birdlike reptiles may have had a wingspan of 1.5
meters.
Chuang Zhao
No
other pterosaur has this distinctive feature. In fact, its discoverers
say, no other vertebrate, living or long extinct, has such a bony
projection. Because of that, scientists aren’t quite sure what purpose
this hook served. It may have been an anchor for soft tissue. (Unlike
bones, soft tissues rarely get preserved as fossils.) Indeed, based on
where the hook is located, the pterosaur might have had a pelican-like
pouch beneath its lower jaw. This pouch could have held fish that the
pterosaur scooped up as it swooped across the surfaces of lakes or
rivers.
The scientists named the new species
Ikrandraco avatar. “Draco” is the Latin word for dragon. The other two parts of the name come from the name of the movie
Avatar and the fictional flying beasts, “ikran,” that were ridden by warriors in that film’s battles.
The
lesson with this animal, as with many other ancient creatures, is that
just because something outwardly resembled a dinosaur did not mean it
was one.
Next: The ancient reptiles that ruled the seas
Power Words
(for more about Power Words, click here)
anatomy The study of the organs and tissues of animals. Scientists who work in this field are known as
anatomists.
asteroid
A rocky object in orbit around the sun. Most orbit in a region that
falls between the orbits of Mars and Jupiter. Astronomers refer to this
region as the asteroid belt.
cladistics An
approach to biological classification in which organisms are grouped
together based on whether or not they share anatomical characteristics.
comet A
celestial object consisting of a nucleus of ice and dust. When a comet
passes near the sun, gas and dust vaporize off the comet’s surface,
creating its trailing “tail.”
dinosaur A term that means
terrible lizard. These
ancient reptiles lived from about 250 million years ago to roughly 65
million years ago. All descended from egg-laying reptiles known as
archosaurs. Their descendants eventually split into two lines. They are
distinguished by their hips. The lizard-hipped line became saurichians,
such as two-footed theropods like
T. rex and the lumbering four-footed
Apatosaurus
(once known as brontosaurus). A second line of so-called bird-hipped,
or ornithischian dinosaurs, led to a widely differing group of animals
that included the stegosaurs and duckbilled dinosaurs.
DNA (short for
deoxyribonucleic acid)
A long, double-stranded and spiral-shaped molecule inside most living
cells that carries genetic instructions. In all living things, from
plants and animals to microbes, these instructions tell cells which
molecules to make.
doppelgänger A word from the German that means a look-alike — or double — of someone or some thing.
ecosystem A
group of interacting living organisms — including microorganisms,
plants and animals — and their physical environment within a particular
climate. Examples include tropical reefs, rainforests, alpine meadows
and polar tundra.
evolution (v.
to evolve)
A process by which species undergo changes over time, usually through
genetic variation and natural selection. These changes usually result in
a new type of organism better suited for its environment than the
earlier type. The newer type is not necessarily more “advanced,” just
better adapted to the conditions in which it developed.
extinct An adjective that describes a species for which there are no living members.
femur In
humans, the large bone in the upper leg. It is commonly known as the
thighbone. In tetrapods (creatures with four limbs), it’s the large bone
in the upper hind limbs.
fossil Any preserved
remains or traces of ancient life. There are many different types of
fossils: The bones and other body parts of dinosaurs are called “body
fossils.” Things like footprints are called “trace fossils.” Even
specimens of dinosaur poop are fossils. The process of forming fossils
is called
fossilization.
genus A group of closely related species. For example, the genus
Canis
— which is Latin for “dog” — includes all domestic breeds of dog and
their closest wild relatives, including wolves, coyotes, jackals and
dingoes.
keel A structural feature that runs
along the bottom of a boat’s hull from front to back (stem to stern) and
may almost look like a fin extending down into the water. The boat’s
beams or flooring are usually attached to this.
Mesozoic Era An
interval of geologic time from about 252 million to around 66 million
years ago. Often called the Age of Reptiles, this era includes the
Triassic, Jurassic and Cretaceous periods.
paleontologist A scientist who specializes in studying fossils, the remains of ancient organisms.
paleontology The branch of science concerned with ancient, fossilized animals and plants.
pelvis
Bones that make up the hips, connecting the lower spine to leg bones.
There is a gap in the middle of the pelvis that is larger in females
than in males and can be used to tell the sexes apart.
predator (adjective:
predatory) A creature that preys on other animals for most or all of its food.
projection Something feature that extends out (or projects) from the body of a structure.
pterosaur Any
of various extinct flying reptiles of the order Pterosauria. These
animals lived 245 million years ago to 65 million years ago. Although
not true dinosaurs, they lived during the reign of dinosaurs. Among
members of this order were the pterodactyls of the Jurassic and
Cretaceous periods, which were characterized by wings consisting of a
flap of skin supported by the very long fourth digit on each forelimb.
pterodactyl
A group of pterosaurs that get their name from the long thin finger
that supports the front edge of their wing. Typically, a pterosaur is
considered a pterodactyl if the longest bone in its wing finger is eight
times longer than its diameter.
reptile
Cold-blooded vertebrate animals, whose skin is covered with scales or
horny plates. Snakes, turtles, lizards and alligators are all reptiles.
serrated A description for a saw-like edge, usually found on knives meant to cut through tough meat.
trait A characteristic feature of something. (in genetics) A quality or characteristic that can be inherited.
tyrannosaur
A line of meat-eating dinosaurs that began during the late Jurassic
Period, about 150 million years ago. These species persisted into the
late Cretaceous Period, about 65 million years ago. The best known
member of these species: the late Cretaceous’
Tyrannosaurus rex, a 12-meter (40 foot) long top predator of its time.
Tyrannosaurus rex A top-predator dinosaur that roamed Earth during the late Cretaceous period. Adults could be 12 meters (40 feet) long.
vertebra (plural
vertebrae)
One of the bones that make up the neck, spine and tail of vertebrates.
Bones in the neck are called cervical vertebrae. Bones in the tail, for
animals that have them, are called caudal vertebrae.
vertebrate The
group of animals with a brain, two eyes, and a stiff nerve cord or
backbone running down the back. This group includes all fish,
amphibians, reptiles, birds, and mammals.
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