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Dinosaurs appeared on Earth about 220 million years ago together with
turtles, plesiosaurs, crocodiles, flying reptiles, mammals and, perhaps,
birds. At the same time, during the Late Triassic Period, many amphibians
and reptiles that lived on land became extinct. So this was a time of profound
change on land, one of the most significant in the history of the Earth.
Everyone wonders how the dinosaurs became extinct about 66 million years
ago. But what about the earlier extinctions that coincided with their appearance?
Did the rise of the dinosaurs, and so many other new types of land animals,
cause the extinction of the old denizens of the land? Did a large meteorite
or comet hit the Earth and contribute to the Late Triassic extinctions?
Or were other, more complex environmental changes the cause of these disappearances?

Until recently, researchers had problems tackling these basic queries,
because our Late Triassic ‘clocks’ have been remarkably poor. For decades,
fossils collected from Triassic strata in the western US could not be placed
in their precise geological position. And, in many places, so few researchers
examined the Late Triassic rocks that the sequence of fossils they contained
remained unknown. But now a wealth of new information on the Late Triassic
rocks and fossils from the western US has established a much more precise
timescale for the beginning of the age of dinosaurs.

The answers to the questions about the beginning of the age of dinosaurs
start with timing, through stratigraphy, the study of the order in which
layers of rock formed. We must know precisely when the animals, new and
old, appeared and disappeared. Supporters of the idea that the dinosaurs
took over when older species became exinct have used the fact that these
two events happened at about the same time. Palaeontologists have conceived
two distinct faunas, at most, within the Late Triassic Period. This gave
rise to the idea that there were two extinctions, one within and one at
the end of the Late Triassic. But now more precise stratigraphy reveals
four separate faunas in these rocks, thriving at different times. The earlier
of the two extinctions, in particular, now looks less clear cut, and researchers
must look to other, more complex, reasons for the rise of the dinosaur dynasty.

Jules Marcou, a Swiss geologist who accompanied a US government expedition
in 1853, was the first to recognise Triassic rocks in the American West.
They make the floors of canyons and form extensive badlands areas stretching
from northern Wyoming to central Texas. Since Marcou, numerous geologists
and palaeontologists have studied fossils from these rocks, many focusing
on small areas, usually within one state. The result of this was unfortunate:
Triassic strata received different names across the West, depending on how
they were first described. Rocks formed at the same time and in much the
say way acquired a plethora of different names in different places, making
it especially hard to match up sequences of rock across the continent.

As an example, take a band of sandstone and conglomerate usually found
about in the middle of the Upper Triassic rock sequence. In West Texas,
these strata were named the Trujillo Formation of the Dockum Group, yet
they received the name Cuervo Member of the Chinle Formation in nearby eastern
New Mexico. They are called the Johnson Gap Formation in south-central Colorado,
but in southwestern Colorado, the local name is the Lower Member of the
Dolores Formation. In northeastern Arizona and west-central New Mexico,
these strata are the Sonsela Sandstone Bed of the Petrified Forest Member
of the Chinle Formation. However, in north-central New Mexico, they are
the Poleo Sandstone Member of the Chinle. And, in southeastern Utah, the
term Moss Back Member of the Chinle Formation applies.

In the past two decades, many researchers have started to unravel the
stratigraphic succession of Upper Triassic rocks. With so many names applied
to these rocks, it is small wonder that it took so long to realise that
they represent a simple and consistent pattern from Wyoming to West Texas:
the rocks formed in a set of river systems that ran across the western US.
And when we link this pattern with the fossils contained in the rocks, the
picture of life at the start of the Late Triassic becomes much clearer.

The story begins with sandstones and conglomerates, representing large
rivers that mark the onset of Late Triassic deposition in western North
America. Within the conglomerates, there are pebbles of quartzite and chert,
as well as larger pieces of limestone that contain fossils of sea creatures,
usually from Permian times, about 250 million years ago. These pebbles indicate
that the rivers were eroding older rocks, presumably from highlands scattered
across the western US.

After this, the rivers built up vast floodplains along their edges,
depositing mud on top of the sandstones and conglomerates. Most of the mudstone
contains the mineral bentonite, which is formed in volcanic eruptions. The
volcanic ash must have fallen onto the mud and soil of the floodplains,
which slowly solidified to form mudstone. In some areas, the water did not
drain to the sea, but remained in lakes and swamps where thin seams of coal
formed. The oldest Late Triassic vertebrae fossils in western North America
come from the lowermost portion of these rocks and the sandstones and conglomerates
beneath. The upper portion of this mudstone contains a vertebrate fauna
that is somewhat younger and distinct.

Above these mudstones there lies more sandstone and conglomerate. As
at the beginning of the Late Triassic sequence, rivers dominated the landscape.
However, the pebbles in these conglomerates are mudstone, siltstone and
limestone fragments eroded from Upper Triassic rocks, rather than the older
Permian strata. By this time, the highlands that first fed the Late Triassic
river systems must have been buried by Upper Triassic sediments.

More rock dominated by mudstone lies above this sandstone and conglomerate,
similar in origin to the earlier mudstone. Here, we also find a diverse
vertebrate fauna in the sedimentary rocks formed in the river channels and
floodplain. Above this, we find the youngest extensive sequence, unlike
those below in being dominated by very fine-grained sandstone and siltstone
(made of even finer particles). The little mudstone present differs from
older mudstones: it does not contain bentonite, suggesting that there was
no longer active volcanoes nearby. Another significant difference is that
the climate had become distinctly drier than before. Large rivers gave way
to closed drainage basins occupied by shallow lakes. The sand and silt formed
dunes, and soils cemented with the mineral calcite formed. Today, such calcareous
soils are characteristic of a semiarid climate. The youngest Triassic fossils
in the western United States comes from these rocks.

The sequence of different types of rocks in different places match well
across the western US. But the fossils of two groups of lare archosaurs
(or ruling reptiles, referring to dinosaurs and allied forms), the phytosaurs
and aetosaurs, are crucial for refining this correlation. Phytosaurs looked
like modern crocodiles and had a similar lifestyle; they could grow as long
as 10 metres. Body shape and stomach contents indicate that they fed in
the water on fishes and reptiles much as modern gavials do in India.

Palaeontologists distinguish Late Triassic phytosaurs through their
skulls. A succession of phytosaurs can be recognised in western North America,
from relatively primitive types, in which the nostrils were well forward
of the orbits, to more advanced phytosaurs in which the nostrils were just
in front of the orbits. But to identify a fossil precisely needs most of
a skull. This makes it difficult to interpret the age of most phytosaur
fossils – fragments of skulls and jaws, isolated teeth, vertebrae and armour
plates.

Aetosaurs, in contrast, can be identified precisely just on the basis
of their armour plates. Aetosaurs could be 5 metres long and had tiny teeth
shaped like leaves, suggesting they lived off plants. Their heads were small
relative to their bodies, which were covered with four-sided plates along
the back and extending down the sides, tail and abdomen. The oldest aetosaur
in North America has a long, narrow carapace and large shoulder horns. The
younger aetosaurs Typothorax and Paratypothorax had carapaces that were
roughly disc-shaped and lacked the large shoulder horns.

The combination of rock- and fossil-based correlations allows us to
identify four successive faunas dominated by archosaurs. And it is within
this sequence that the first traces of the dinosaurs appear. The oldest
fauna is based on fossils from the lowest sandstones, conglomerates and
mudstones of Late Triassic age, when the river system was evolving. The
next fossil fauna comes from the mudstones beneath the sandstone and conglomerate
interval near the middle of the Upper Triassic sediments. On top of the
rocks, the mudstones also contain a third group of fossils. The youngest
layers bear fossils of the fourth vertebrate fauna.

The first traces of dinosaurs

The oldest fauna comes from Triassic strata in Wyoming and West Texas.
No dinosaurs have been found in these rocks, and the phytosaurs are the
primimtive types Paleorhinus and Angistorhinus. The oldest aetosaur and
the dinosaur-like archosaur Poposaurus are also there. This fauna also includes
the youngest North American rhynchosaurs, a group of plant-eating reptiles,
similar to lizards but with beaks like turtles. There are plenty of metoposaurids,
fish-eating amphibians, like 2-metre long salamanders with heavily armoured
skulls. And Placerias makes it appearance. This is the last North American
representative of the dicynodonts, a very successful group of plant-eating
reptiles rather like the mammals that arrived later.

The oldest North American dinosaurs turn up as fragments in the next
youngest fauna, in the mudstones formed as the rivers developed flood plains.
The first traces of creatures who would dominate the world for the following
150 million years are isolated vertebrae and teeth that would fit comfortably
into a child’s palm. This fauna is characterised by the phytosaur Rutidon
and the aetosaurs Stagonolepis and Desmatosuchus. The large (4- to 6-metre-long),
meat-eating archosaur Postosuchus had serrated teeth, each 8 centimetres
long, and must have been the top predator on land at this time. The large
metoposaurids are still present as fossils in these rocks.

The first good look at a diverse group of Late Triassic dinosaurs comes
from the next youngest fauna. Here, the plant-eating dinosaurs are the ‘fabrosaurs’
Revueltosaurus and Technosaurus; Coelophysis and a large staurikosaurid
are the dinosaurs that ate meat. The large amphibians have virtually gone,
being superseded by new and much smaller types. Postosuchus, the successful
predator, is still present.

The youngest fauna includes fossils from the famous Coelophysis quarry
at Ghost Ranch, New Mexico as well as an extensive record of dinosaur footprints
in eastern New Mexico. Most of the characteristic archosaurs are new taxa
that include a phytosaur and an aetosaur, distinct from the older types.

From this record, dinosaurs appear to have become relatively large and
diverse soon after they first appeared. Their appearance long predates the
extinction of some animals (metoposaurs, phytosaurs, aetosaurs) but coincides
with the extinction of others (dicynodonts, rhynchosaurs). So the appearance
of the dinosaurs cannot be a direct result of all these extinctions. The
dinosaurs did not succeed solely by filling gaps left when other creatures
died out; there must be other factors.

Some workers have linked the extinctions to a drying of climate at the
end of the Triassic. But the first convincing evidence of such drying in
the American West is in the uppermost inteval, when sand a silt dominated
the rocks. This stage not only postdates many extinctions, but several amphibious
animals – notably the phytosaurs – continue right into the drier environments
and apparently became extinct long after the change in climate. So climate
change alone cannot have been responsible.

A third cause suggested for the Late Triassic extinctions is the impact
of a large meteorite or comet, as has been suggested for the end of the
dinosaurs’ reign, at the Cretaceuous Period. Advocates of this mechanism
point to the 70-kilometre-wide Manicouagan crater in Quebec, Canada, as
evidence of such a direct hit. Estimates for the age of the crater give
an average of 211 million years, a time in the Late Triassic when we know
some of the extinctions took place. But the estimates are not precise, ranging
from about 200 to 220 million years ago.

Our understanding of the numerical chronology of the Late Triassic of
the American West is also imprecise. There are no good estimates for the
age of these rocks, and the best can only place them between 200 and 220
million years old. The age of the Manicouagan impact falls within this interval,
but with the new stratigraphy, there seems to have been simple mass extinction
of vertebrates. The link between this impact structure and any Late Triassic
vertebrate extinction in the American West begins to look tenuous. The fossil
record during the Late Triassic does not fit the effects of a major impact
followed by a time of cold and darkness (similar to that suggested for a
nuclear winter), as has been invoked to explain the extinction of the dinosaurs
66 million years ago. Indeed the Manicouagan impact may have occurred after
the Upper Triassic rocks of the American West were deposited, if we accept
the youngest possible age for the impact and an older age for the Triassic
rocks.

Refining our understanding of the timing of events has enabled us to
see More clearly that what seemed a synchronous extinction and appearance
of many Late Triassic animals was much more complex. Yesterday’s perceived
mass extinction has become today’s complex series of extinctions and originations,
in which competition for living space, changes in the climate, and even
wandering meteorites may all have played a part. But finding out exactly
what these parts were will have to wait for an even more detailed stratigraphy
for rocks from the end of the Triassic.

Spencer Lucas is Curator of Palaeontology and Chief Curator at the New
Mexico Museum of Natural History.