Australia: The Land Where Time Began |
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The evolution of dinosaurs and their environment
Triassic
As the Mesozoic opened the
terrestrial vertebrates had been severely depleted but there was soon a
bout of explosive radiation that saw the few therapsids that had
survived the Permian Mass Extinction flourish once again, again becoming
important parts of the terrestrial fauna, though now they had
competition from the archosaurs that underwent an explosive radiation of
their own. The first phase of the archosaur radiation saw a large number
of thecodont forms, some reaching large size, possibly a tonne or more.
One group of these thecodonts evolved into crocodile-like forms, while
others evolved into armoured land herbivores. Many were forms carnivores
with erect legs that had achieved their erect posture along a different
path from the dinosaurs. Instead of the femur head turning inwards the
hip socket expanded to envelop the femur head so that the femur shaft
was directed downward. Of these erect-legged archosaurs some were almost
bipedal, though others became herbivores lacking teeth. It seems the
thecodonts of the Triassic filled the niches that would later be held by
dinosaurs.
The crocodilians, the only group
surviving to the present, arose at this time, giving a glimpse of the
appearance of archosaurs from Triassic. The earliest crocodilians from
the Triassic were small, digitigrade, land runners with long legs. The
author3 suggests the evolution of their sophisticated
liver-pump lung system may have evolved to power a highly energetic
lifestyle.
Like many of the thecodonts the
crocodilians had a feature that is considered very un-dinosaurian, in
that their ankles were complex joints that have been compared to a door
hinge, with the muscles on the foot having help with leverage from a
tuber the projected from one of the ankle bones, in a manner similar to
that of mammals. It was in the Triassic that the pterosaurs evolved. It
has been suggested that the pterosaurs were related to dinosaurs because
they had the same type of simple hinge-ankle that was present in the
dinosaurs. The pterosaurs, being energetic animals, were insulated,
though it is not yet known if other nondinosaurian archosaurs were also
covered with thermal fibres.
A number of small predatory
archosaurs appeared in the fossil record in the last stage of the Middle
Triassic, the Landian that exhibited many of the features of dinosaurs.
In these forms the head of the femur had been turned inwards, though the
hip sock was still not open internally, that allowed the legs to
function in a vertical plane. The had a simple hinge ankle, a skull that
was lightly constructed. The lagosuchian protodinosaurs first appear in
South America. It is not yet known if the group originated in South
America or spread to there from elsewhere. By the Norian they had spread
to at least North America. They were extinct by the Norian.
Herrerasaur theropods, that had
fairly large bodies, small hips and 4 toes had arisen. The terrestrial
faunas were dominated at that time by the archosaurs and they had gone
by the early Norian, possibly because they lacked the aerobic capacity
to compete with the competition.
The bird-footed avepod theropods
appeared in the Norian, with large hips and the beginnings of a
respiratory system that was of the avian type, imply an improved aerobic
performance and thermoregulation. At about this time the herbivorous
small-hipped prosauropods that were semibipedal made its first
appearance on the fossil record. They were followed almost immediately
by the quadrupedal sauropods that had bigger hips. The rapid expansion
in diversity and size of these new forms gave the thecodonts increasing
competition. Within 15-20 million years of their evolution of the first
protodinosaurs there were prosauropods and sauropods up to 2 tonnes in
mass had developed, and in another 10 million years there were sauropods
as big as elephants, the first very large land animals. The first
animals that could browse at high levels were these first long-necked
herbivores. At this time dinosaurs were showing their potential for
developing very large bulk, as well as dimensions, on land, the
attributes that were also seen in the mammals. The first of the beaked
herbivorous ornithischians arose in the Carnian. It has been suggested3
that these small semibipedal animals and the small prosauropods may
possibly have dug burrows as protection from the many carnivores. The
saurischians were becoming the dominant land animals by the last stage
of the Triassic, though there were still thecodonts and some therapsids.
The first mammals arose from these therapsids at this time, the mammals
and dinosaurs have lived together for more than 200 million years, the
mammals remaining small for the first 140 million years.
At this time, when the continents
were fused together to form Pangaea, there was no part of the
supercontinent that animals couldn’t roam to. The result was a tendency
for faunas to vary little from one region to another. While the
continents were all part of Pangaea the climate remained harsh over most
of the supercontinent, described by the author3 as a
greenhouse world, the prevailed throughout the Mesozoic.
Despite the Sun being slightly
cooler than at present the temperatures were high enough to prevent even
the poles were considered to have been fairly warm in winter, kept high
by the atmospheric CO2 levels that were 2-10 higher than at
present.
As with Australia at the present
there were not many tall mountain ranges, as a result of low levels of
tectonic activity, to capture rain from any passing clouds, or seaways
in the interior to provide moisture. During this time there were large
expanses of desert and most vegetated lands were seasonally semiarid.
Forests did exist in the few regions that received heavy rainfall and
groundwater created by climatic zones and rising uplands. In many ways
the flora was fairly modern. Rushes and horsetails grew along
watercourses, and some ferns grew in wet areas and on shaded situations
on the floors of forests. Some
ferns grew in open areas that were seasonally arid. The author3
suggests that fern prairies may have existed over large parts of the
world that was comparable to the grassland and shrubland of the present.
In wetter areas it was common for tree ferns to be abundant, and the
cycadeoids, similar to the extant cycads of the tropics, were even more
abundant. The ginkoids, that prefer wetter areas, were among the taller
trees. The maidenhair tree is the only survivor of this line. Conifers
were the dominant plant type of the this time, most having broad leaves
than the needles of the modern species. The conifers could grow into
very large trees, and it was these trees that formed the petrified
forest of Arizona. Angiosperms were completely absent.
There was another mass extinction
about 200 Ma at the close of the Triassic. Millions of years before the
extinction a giant impact occurred in south-eastern Canada. During this
mass extinction of the land vertebrates the thecodonts and the
therapsids were most affected, the thecodonts being exterminated and few
of the therapsids survived along with their mammal relatives.
By the end of this mass
extinction event crocodilians, pterosaurs and especially the dinosaurs
survived very well, carrying on into the Jurassic with few problems.
Jurassic
The prosauropods appear to have
been outcompeted by the related sauropods that were more sophisticated,
and were extinct by the end of the Early Jurassic. According to the
author3 the sauropods had some superior features, such as hip
muscles that were larger and had begun developing a respiratory system
that was of the bird type, 2 features that allowed them to evolve the
high pressure circulatory system and the more efficient respiration that
was necessary before they could grow to very large size. At this time
the gigantic sauropods were not affected by the theropods that were only
getting to moderate sizes. Ornithischians were still uncommon at this
time, and one group was the first dinosaurs to develop armour as
protection. Another group of ornithischians were small heterodontosaurs
with chisel teeth that were semibipedal, are believed to probably be the
first dinosaurs to have evolved fibre coverings to maintain a constant
body temperature. Crocodilians remained small and were either fully or
partially terrestrial at this time, though other groups evolved into
marine giants.
The Tethys tropical ocean
partially split Pangaea into Laurasia in the north and Gondwana in the
south. The Mediterranean Sea is all that remains of this once huge
ocean. African style rift valleys began forming further west along the
east coast of what is now the eastern seaboard of North America as
Pangaea began breaking up, the end result was the opening of the
Atlantic Ocean. The increased tectonic activity that was occurring in
various parts of Pangaea at this time resulted in a rising of the ocean
floor in a number of places to such an extent that shallow
epicontinental seaways formed in a number of places on the various
constituent continents of Pangaea and this greatly affected the
dinosaurs in the various regions that were that were being isolated by
the marine incursions, the dinosaurs on each isolated region evolving
and diversifying indifferent directions as a result of their isolation
from each other.
As the surface area of water on
the continents expanded it had another effect, the humidity levels of
the atmosphere over the continents increased leading to increased
rainfall, though in most habitats would still have a seasonal climate
with yearly dry seasons. The rainfall was also increased by the
formation of more mountain ranges by the tectonic activity.
The Middle Jurassic began about
175 Ma and was a time when the sauropods flourished, as their
increasingly sophisticated circulatory and respiratory systems allowed
them to grow even bigger. The author3 suggests they thrived
in dry habitats by feeding on the forests that lined the river courses,
and in wetter areas, by feeding on the fern prairies.
Some sauropods in China that was
partially isolated by seaways evolved long, slender necks that allowed
them feed 10 m (more than 30 ft.) above the ground. Some developed clubs
and spikes on the tails for defence from the predators. The small
armoured stegosaur ornithischians with tail spikes made their appearance
at this time. The author3 suggests that a group of small
ornithopods that were the beginnings of a group of ornithischians that
had a respiratory system that possible paralleled that of mammals, and
they had dental batteries that increased their evolutionary potential.
Increasingly sophisticated theropods such as tetranurans, avetheropods
and coelurosaurs evolved that had an avian type of respiratory system
that was highly developed they didn’t produce any giant species at this
time.
About 160 Ma, the Late Jurassic
is suggested by the author3 to be the apogee of 2 herbivorous
dinosaur groups, the sauropods and the stegosaurs. It was a time of the
maximum diversity of the sauropods, with genera such as haplocanthosaurs,
mamenchisaurs, dicraeosaurs, diplodocines, apatosaurines, camarasaurs
and the first of the titanosaurs. Some neosauropods grew as large as
50-75 tonnes, with some reaching much more than 100 tonnes, around the
size of the biggest of the baleen whales. The tallest of the sauropods
could feed more than 20 m (70 ft.) into the trees.
By this time theropods first
reached the size of hippos in the form of allosaurs and yangcuanosaurs
that were the size of hippos. It was also a time when some of the
sauropods that had become isolated on islands had been reduced to the
size of rhinos as a result of dwarfing, a process that animals undergo
when they are isolated with limited food resources. The anklylosaurs
were diversifying. Large ornithopods with thumb spikes entered the
fossil record for the first time. In Asia small semibipedal ceratopsians
evolved. It was at this time that the stegosaurs were at their most
diverse, with forms that were from rhino-sized to elephant-sized.
The ancestors of tyrannosaurs
were still small are believed to have been developing at this time, as
well as a number of maniraptor coelurosaurs that were gracile that were
numerous. One theropod,
Scansoriopteryx had a long
thin finger similar to the aye-aye lemur indicating that some theropods
were climbers. By the Late Jurassic the Alvarezsaurs had evolved. Their
thick, short arms and hands are believed to have specialised for
breaking into insect nests. A major occurrence at this time is the first
appearance in the fossil record of the avepectorans that were probably
at least partly arboreal.
Archiornis is a Chinese dinosaur that is the first known
dinosaur to have had large feathers on both its arms and legs. It has
been suggested that this dinosaur, apparently a climber, could possibly
the first example known of an animal that had lost an ability, in this
case flying, as the feathers that were moderately long and symmetrical
were not proper aerofoils in spite of the long arms. A few million years
later what has been thought of as the first bird,
Archaeopteryx, a
deinonychosaur, evolved. Its status as the first bird has been
challenged by a paper in
Nature
and some think it should be classified as a feathered dinosaur rather
than an actual bird. See
Xiaotingia zhengi.
The deposits in which it was found were laid down in a lagoon that was
at the time the northwest edge of the Tethys Ocean. The author3
suggests it was part of the process involved in the development of
powered flight, as indicated by the combination of arms that were very
large and wing feathers that were long and asymmetrical.
The first major increase in the
mental powers of dinosaurs occurred as the brain size and complexity
increased to the lower avian level, with the appearance of the small
avepectorans. The pterosaurs retained the small brains and were of small
size, most also retaining long tails. The crocodilians of types that
were similar to the highly amphibious living members of the group were
evolving at this time, though there were still some small terrestrial
running types. The amphibious types readapted the liver-pump lung system
to be buoyancy control devices. In the Jurassic the still small mammals
were diversifying extensively. There were climbers that were
insectivorous or herbivorous, some were burrowers, others were
freshwater swimmers that reached a maximum of a few kilograms.
The CO2 were very high
during the Middle and Late Jurassic, reaching as high as -5-10 % of the
atmosphere. At the close of the Jurassic the incipient Atlantic was of a
similar size as the Mediterranean Sea of the present. As for the
vegetation, it was much the same as it had been in the Triassic.
According to the author3 there is a common impression that
the monkey-puzzle tree, an umbrella-shaped member of the Araucariacae
conifers from South America was a source of food for sauropods in the
Jurassic, but he says this is wrong. There were conifers similar to
cypress that dominated the wetter areas. He suggests the sauropods could
be expected to have had a profound impact on the floral landscape by
browsing heavily on the trees and probably also broke them as elephants
do at the present, but to a much greater extent. There is disagreement
about what happened to the fauna at the close of the Jurassic, some
proposing a major extinction event, while others disagree with this
scenario.
Cretaceous
At the start of the Cretaceous
about 145 Ma the continents continued the breakup, the South Atlantic
began opening and on the continents marine incursions were snaking their
way across their land surfaces. At this time the dinosaurs underwent
their greatest burst of evolution. The very high CO2 levels
were dropping, but not to the levels of the present. In the early part
of the Cretaceous the arctic regions were balmy even in winter, kept
warm by the warm Arctic Ocean.
The continental climate of the
South Pole had winters that were cold enough for permafrost to form. The
general climate of the globe tended to be slightly wetter than earlier
in the Mesozoic, though seasonal aridity continued to be the rule in
most places, with true rainforest continuing to be scarce at best.
The sauropods, that were often
very large, continued to be abundant though their diversity had
declined. Some diplodocoids, that were small-bodied and with short
necks, some having evolved broad mouths that were square-ended as an
adaptation for grazing. The predominant types were the tall brachiosaurs
and the titanosaurs that were broad-bellied.
The Ornithischians came into
their own in the Cretaceous, all sizes of ornithopods flourishing, but
especially the large ones. In the Northern Hemisphere the iguanodonts,
that had thumb spikes, became common among the herbivores. The author3
suggests that a key to their success may have been their well-developed
batteries of teeth. In a few their vertebral spines grew long and formed
a tall sail on their backs. He also says that it was only recently that
it was realised that the heterodontosaur clade had not gone extinct in
the Jurassic, surviving with little change until the Early Cretaceous in
Asia. Among the ceratopsians the small psittacosaurs, that were
chisel-toothed, from Asia proliferated. The related protoceratopsids,
that had big heads, arose in the same region, as did the first
pachycephalosaurs that had domed heads. The stegosaurs, that soon went
extinct, the last of the major groups of dinosaurs to go extinct since
the prosauropods. The author3 suggests that the dinosaurs
continued increasing their diversity through the Mesozoic by their
tendency to evolve new groups without losing the older groups. The
ankylosaurs soon replaced the stegosaurs becoming a major portion of the
global fauna. The ankylosaurs were extremely fat-bellied armoured
dinosaurs with bodies that were low-slung. The plates and spikes would
have protected them from the large predators of Laurasia, large
allosauroids and in Gondwana, the abelisaurs that were snub-nosed and
had short arms. The spinosaurs were another group of giant theropods
that had crocodile-like snout that appeared to be adapted for catching
fish that probably formed at least part of their diet. Based on bone
isotopes it seem the spinosaurs were semiaquatic as are hippos of the
present, though they don’t appear to have any adaptations for swimming.
Some evolved large sails on their backs.
In the Early Cretaceous
theropods, especially the smaller ones, underwent a rapid
diversification. The first of the ornithomimids, ostrich-mimics,
appeared, as well as the first tyrannosaurs that were still small and
with long running legs and arms that were reduced. Based on the lake
deposits from north-eastern China, the author3 suggests the
focus was on the avepectorans, such as deinonychosaurs, that evolved
into a variety of flying and flightless forms, the flightless forms
possibly being secondary flightless forms that descended from the flying
forms. He also suggests that small aerialists with 2 pairs of wings, the
hind legs and front legs forming equally sized pairs of wings, may have
evolved into the dromaeosaurs that were famous for the sickle-shaped
claws. The troodonts, another deinonychosaur group, that were more
lightly built and are believed to have been able to run faster, also
appeared at this time.
The author3 suggests
the birds had descended from the deinonychosaurs, and as is shown by the
deposits in China, by 125 Ma they had already undergone a spectacular
evolutionary radiation. Some of these birds had teeth, though others had
lost all their teeth. All the birds were small. Among the early beaked
birds in the same formation were the omnivoropterygids that are very
similar to caudipterygid and protoarchaeopterygid oviraptorosaurs.
Another group of possible dinosaur-birds that were secondarily
flightless may have been the short-tailed oviraptorosaurs that were more
advanced than the archaeopterygians and the dromaeosaurs. The
therizinosurs, pot-bellied theropods that were herbivorous, described as
enigmatic by the author3, had appeared by the Early
Cetaceous.
By the Cretaceous most of the
pterosaurs were short-tailed so were flying more dynamically, and the
author3 suggests that they increased in size by competing
with the increasing numbers of birds. The crocodilians of fresh water
were also increasing in size, making them more of a threat to dinosaurs
that came to the water to drink or entered the water. There were some
semi-terrestrial crocodilians that could attack large dinosaurs both in
the water and on the land, though there were also some small running
crocodilians. The smallest of the dinosaurs and their young were also in
danger from some of the bigger mammals that had reached a size of about
12 kg. There also gliding mammals at this time.
A major evolutionary event
occurred in the latest part of the Early Cretaceous that the author
believes aided the dinosaurs in their rapid evolution. Angiosperms
evolved in the late Early Cretaceous. The first of these are believed to
have been pioneer-type species that were small shrubs growing along the
water courses, situations where they capitalised on their pioneer
abilities to invade new territories when they became available. Other
plant types, such as water lilies, the flowers of which were small and
simple, were more fully aquatic. The author3 suggests that
the rapid growth and the potential for strong recovery of angiosperms
may possibly have been a factor in the development of ankylosaurs and
ornithopods that were low browsing animals. He also suggests that it may
have been the browsing pressure of the dinosaurs that was the driving
force involved in the evolution of the angiosperms that were
fast-spreading and fast growing new plants. In South America the
conifers with monkey-puzzle foliage were also appearing.
About 100 Ma the Late Cretaceous
began. At this time continental break was well under way and vast tracts
of epicontinental shallow seas covered the interiors or land of the
continents. As the atmospheric CO2 levels continued declining
the temperatures during the dark winters in the Arctic dropped to
temperatures as low as in the high forests in the north, with glaciers
creeping down high latitude mountains. The mammals were increasing
similar to the modern types and were small. Both marine and terrestrial
pterosaurs grew to gigantic size. The wings of oceanic pteranodonts had
wings 8 m (25 ft) across. Towards the end of the Cretaceous he
azhdarchids grew to even larger sizes with a wingspan of 11 m (more than
35 ft, and are estimated to have been heavier than ostriches. At this
time there were still some small running crocodilians, a few of which
had become herbivores. Of the freshwater crocodilians that continued to
be carnivorous some grew to 12 m long and are estimated to have weighed
up to 10 tonnes. This put them in the size range of the largest
carnivorous theropods. It is believed these very large crocodilians fed
mostly on fish and smaller tetrapods, the largest dinosaurs would appear
to have been the only animals of the time that didn’t need to fear them.
These giants don’t appear to have been very numerous in many places and
they didn’t reach higher latitudes. The author3 suggests
their presence in the water may have prevented the dinosaurs from
evolving forms that were highly aquatic.
The sauropods became limited to
the titanosaurs, but they diversified across most of the globe, greatly
proliferating, though it was the Southern Hemisphere where they became
especially diverse. After 150 million years they had become the most
successful herbivore group of all time. For part of the Late Cretaceous
the sauropods are not found in the fossil record of North America,
though in drier regions that are again present in the fossil record
towards the end of the Cretaceous.
There were some armoured forms of
sauropods, that the author3 suggests may have been to protect
the young of the group from the increasing diversity of predators. A few
of the titanosaurs apparently adapted for grazing, having short necks
and square, broad mouths. Some remained very large, growing to more than
50 tonnes, and possibly even more than 100 tonnes up to the end of the
Cretaceous. The author3 suggests they may have been attacked
by the largest fo the predators such as abelisaurs that sometimes
reached the bulk of bull elephants. In the Late Cretaceous the
spinosaurs, sail-backed dinosaurs from the Late Cretaceous of Africa,
may possibly have been even larger. This group died out before the end
of the Cretaceous. Big though these predators were it must still have
been a hazardous venture tackling 100 plus tonne sauropods.
Especially in the Northern
Hemisphere the ankylosaurs continued to thrive. One group developed tail
clubs, though it is not known if they were mostly used for intraspecies
battles or for defence. As the iguanodonts gradually disappeared from
the fossil record their descendants, the hadrosaurs flourished, evolving
the most complex grinding dental batteries seen
in the dinosaurs. Many species had elaborate head crests that
differed between species. In much of the Northern Hemisphere the
hadrosaurs were the most common herbivores of their time. The author3
suggests they possibly were adapted to browse on the shrubs and ground
cover that was replacing the fern prairies as well as to invade forest
floors. Over large parts of the globe small ornithopods, that were not
much different from the bipedal ornithischians that evolved in the early
days of the Mesozoic, still survived.
Protoceratopsids with small
bodies and large heads were common in many places in the Northern
Hemisphere. They gave rise to the ceratopsids of rhino size and elephant
size, that are some of the most spectacular dinosaurs that had horns on
the oversized heads, as well as neck frills and great beaks that were
parrot-like and slicing dental batteries. They flourished for the last
15 million years of the Mesozoic, largely on the part of North America
that was on the western side of the interior seaway.
Some birds still had teeth, and
both the toothed birds and the toothless birds continued to thrive. One
bird group had evolved into marine divers that had lost the power of
flight completely. The classic coelurosaurs with short arms had
disappeared Of the small predatory theropods there were, according to
the author3, troodonts, that were intelligent and sickle-clawed, and the
leaping dromaeosaurs, some of which he suggests were still capable of
flying. Other successful groups included the avepectorans with short
tails that were non-predatory and oviraptorosaurs that were deep-headed,
and the therizinosaurs that had small heads and large claws. Some
species of both groups became large. The author3 suggests the
ornithomimids with long, slender legs were possibly the fastest dinosaur
to have evolved, though they are believed to have been closely matched
by the alvarezsaurs.
The tyrannosaurids were the
culmination of the 150 million years of theropod evolution, suggested by
the author3 to be ‘the most sophisticated and powerful of the
gigantic predators’. The classic tyrannosaurids evolved about 15 million
years before the close of the Mesozoic and are found only in Asia and
North America. The author3 suggests they appear to wave
wandered, together with other theropods, hadrosaurs and ankylosaurs,
across the subpolar Bering land bridge to North America. There was
apparently a size-race in North America, tyrannosaurids, ceratopsids,
ankylosaurids and pachycephalosaurids, in the last few million years of
the Mesozoic all reaching the largest size they had reached since their
first appearance in the fossil record. The author3 speculates
that this may have been the result of an arms-race between the predators
and their prey or as a result of the expansion of food resources that
resulted from the gradual regression of the epicontinental seaway that
allowed access to the full area of the continent of North America, or
possibly a combination of both. The hadrosaurs, on the other hand,
didn’t increase in size, in fact some of the earlier edmontosaurs were
possibly larger than those near the end of the Mesozoic. Some of the
hadrosaurs that were present at the close of the Mesozoic had adapted
for grazing. The armoured nodosaurids didn’t enlarge at this time.
The author3 suggests
that the ‘the pattern indicates that the enormous size and fire-power of
the Merican
Tyrannosaurus was a
specialisation for hunting the equally oversized contemporary horned
dinosaurs rather than just dispatching the easier-to-kill edmontosaurs’.
A
Triceratops fossil has
been found that had a horn bitten off, the tooth marks indicating a
tyrannosaur was responsible, but it had healed, so it seems the
Triceratops won this
encounter. Life must have been hard tyrannosaurs if they did indeed
specialise in hunting such dangerous prey.
The pattern of continent spread
around the world was beginning to appear similar to the modern
configuration. A round of uplift and mountain building that occurred at
the close of the period had helped drain the much of the epicontinental
seaways. The angiosperms in the flora were becoming increasingly
important. The first hardwood trees, among which is the plane tree,
evolved near the close of the period, and were becoming the first large
hardwood trees when the Period ended. The dominant trees were still
conifers, such as the dawn redwood that barely survived until the
present. The very tall classic redwoods were growing to great heights
then as now. Classic rainforests still hadn’t come into existence.
Grasses that tended to be water-loving forms had arisen, though they
didn’t yet form dry grassland prairies as at present.
Everything was going so well then
it suddenly all ended in a devastating
mass extinction event.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |