Bones belonging to the earliest dinosaurs can help researchers build the dino family tree.
Today the tree is divided into three main branches (note that the pterosaurs and crurotarsans
are archosaurs but not dinosaurs).
Sauropodomorpha
Theropoda
Ornithischia
Pterosauria
Herrerasauria
Silesauridae
Crurotarsi
Marasuchus
Dinosauria
Euparkeria
Archosauria
“crown-group”
Ornithischians
After ornithischian tooth remains were
reidenti;ed as coming from crocodile-
like creatures and not dinosaurs,
the evidence for this group in the late
Triassic was whittled down to just
three body fossils from a small area of
southern Gondwana. These creatures,
known as the “bird-hipped” dinosaurs
because of their hip structure, were
probably small, just over a meter
long— at least at ;rst. Size increased
during the early Jurassic, and over
time horned dinos like Triceratops and
Einiosaurus (shown) arose.
distant relative can help flesh out the
family tree. That’s why paleontologists
are also looking at fossils of animals that
weren’t true dinosaurs but were very
closely related to them.
Technically, a dinosaur is defined as
any member of the group whose lineage includes both the horned dinosaur
Triceratops and the modern house sparrow (because of those theropods whose
descendants fly around today as birds).
Today’s crocodiles and lizards are not
descended from the common ancestor of
Triceratops and the sparrow, and hence
are not dinosaurs.
Theropods
Theropods — the t wo-legged, meat-
eating dinosaurs made famous by
Tyrannosaurus rex — probably date to
around 230 million years ago. Though
the name means “beast foot,” most
theropods alive during the Triassic were
small and slender, around 2 meters
in body length. Mightier creatures are
known from the Jurassic, during which
theropods became more common and
varied (Cryolophosaurus shown). Other
dinos went extinct around 65 million
years ago, but some small, non;ying
theropods gave rise to modern birds.
Many other creatures in the Triassic were also reptiles but not dinosaurs.
Among these are a group known as silesaurids: mostly four-legged, mostly plant-eating creatures. For paleontologists,
silesaurids are turning out to be nearly as
exciting as dinosaurs, because silesaurid
bones help illuminate crucial differences
between dinosaurs and their relatives
deep in the reptilian past.
Last year, scientists reported finding
the oldest known silesaurid, a creature
that lived 244 million years ago in what is
now Tanzania. Less than 10 million years
after the Permo-Triassic extinction, this
Sauropodomorphs
Sauropodomorphs came in two
forms —the big bipedal omnivores and
the bigger, four-legged, barrel-chested,
long-necked herbivores (Brachiosaurus
shown). Evidence for these creatures
dates to about 230 million years ago
in South America, and by about 200
million years ago they could be found
across South America, Europe, Africa
and Asia. Sauropodomorphs have the
best fossil record among the dinosaur
branches in terms of diversity and
abundance, but relationships within
this branch remain controversial.
animal had already evolved characteristics of later silesaurids, such as a beak-shaped lower jaw and teeth shaped like
leaves. That evolution shows that the
silesaurid and dinosaurs have not been
closely related for a long, long time, says
team leader Sterling Nesbitt, now at the
University of Washington in Seattle.
The Tanzanian creature is also the
oldest dinosaur relative ever found. During the Triassic, the Earth’s continents
were united as Pangaea, with one southern landmass known as Gondwana and
one northern one known as Laurasia.
Africa was part of Gondwana, which is
