confirmed, the tracks would be the old
est dinosaur evidence anywhere, and
would suggest that dinosaurs evolved
even earlier than scientists had thought,
hard up against the end of the Permo
Triassic extinction.
Yet not everyone is convinced that
the footprints are definitive evidence of
dinosaurs. At the time the Polish track
ways were formed, there were plenty of
other dinosaurlike creatures roaming
the landscape, points out Max Langer,
a paleontologist at the University of São
Paulo in Brazil.
That left the slate clean for dinosaurs
to arise in the early part of the Triassic.
“You had your status quo basically wiped
out, and ne w groups had the opportunity
to originate and flourish in this post
apocalyptic world,” Brusatte says.
Beyond dinosaurs, those new groups
included the ancestors of creatures
that would look familiar today, such as
lizards, frogs and salamanders. These
animals — in particular, the crocodilelike
creatures known as crurotarsans — were
far more abundant than the first dino
pipsqueaks. “If you were standing in the
Triassic, you would say these crocodile
like animals, not dinosaurs, would expand
and be dominant” in the eons to come,
says Brusatte.
Triassic Park was not exactly Jurassic
Park. But dinosaurs were around, and
new research hints at when they first
appeared and what they looked like.
Fossil footprints from Poland’s Holy
Cross Mountains provide the oldest
clue. Last fall Brusatte and his col
leagues, including University of
Warsaw paleontologist Grzegorz
Niedz'wiedzki, reported finding three
sets of tracks, the oldest dating back
250 million years. That’s right after
the PermoTriassic extinction.
Only several cen
timeters long, these
tracks were made by
a fourlegged crea
ture no bigger than
a house cat. But cer
tain characteristics
of the footprints are
distinctly dinosaur
like, Brusatte says.
Among other
traits, the outer
digits — the first
and fifth toe — are
smaller than the
others, and the long
bones of the foot bunch together more
closely than in nondinosaurs. The
trackways are also narrow, as if they
were made by an uprightwalking crea
ture instead of a sprawledout crurotar
san, the team reported last year in the
Proceedings of the Royal Society B. If
Written in bone
Researchers usually say that the earliest
dinosaurs lived 230 million years ago,
because that’s the approximate age of
the oldest known actual bones. These
fossils come from the high desert of
northwestern Argentina, near the bor
der with Chile. There, a rock formation
known as the Ischigualasto serves as a
veritable treasure trove of the earliest
dinosaurs, where for decades paleon
tologists have dug up and identified a
boatload of new species.
Several of these earliest animals
remain a puzzle. Scientists aren’t quite
sure how to classify some of the most
ancient species.
The dinosaur
family tree includes
three main branches:
theropods, typically
twolegged, meat
eating animals;
sauropodomorphs,
typically the four
legged, longnecked
plant eaters; and
ornithischians, the
“birdhipped” dino
saurs that included
famous groups like
the stegosaurs and
the horned dinosaurs. Sauropodomorphs
and ornithischians went extinct 65 mil
lion years ago; a few theropods survived
the meteorite impact and gave rise to
modern birds.
Problems arise when researchers
try to assign very primitive dinosaurs
Trackways uncovered in a Polish
quarry may be the earliest sign of
dinosaurs, predating known bone
fossils by about 20 million years.
to one of these three branches. For
instance, one of the Argentinean dinos,
Herrerasaurus, was a twolegged meat
eater about the size of a crocodile. Some
researchers call it an early theropod,
but others say it falls even deeper
back in the family tree, somewhere just
outside being either a theropod or a
sauropodomorph.
Not that such classifications were sig
nificant at the time; if you were prey in
the early Triassic, says Langer, “it prob
ably wouldn’t matter if you were being
chased by a basal theropod or a basal
sauropodomorph.”
Another Argentinean creature, the
smaller Eoraptor, has a similar identity
crisis. Upon its discovery in the 1990s,
it was labeled an early theropod. But in
January in Science, it was reclassified
as a sauropodomorph, in part because
researchers hadn’t seen certain diag
nostic features in its skeleton, such as a
twisted thumb, until they reexamined
it recently. Eoraptor may thus belie its
fiercesounding name and be something
of a mellower, occasional plant muncher.
Paleontologists changed their minds
about Eoraptor after they unearthed yet
another close relative: Eodromaeus, a
small, fleet theropod whose name means
“dawn runner” (SN: 2/12/11, p. 10).
Eoraptor and Eodromaeus may belong
to different dinosaur branches, but they
still look a lot like each other, says their
discoverer, Ricardo Martinez of the Uni
versidad Nacional de San Juan in Argen
tina. In turn, Eoraptor resembles yet
another plant eater from Ischigualasto,
reported on by Martinez and a colleague
in 2009 in PLoS ONE.
Clues from cousins
Just as faded, blackandwhite pictures
of ancestors start to look the same, fos
sils of these early dinosaurs sometimes
resemble one another to the point
of indistinction. All of them begin to
look like an unprepossessing chicken.
Still, scientists say that knowing which
creatures appeared when can fill in
vast yawning blanks on the dinosaur
ancestry chart.
Sometimes, information about a more
