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out the link in the description. In the last episode, we saw how the eastern
and western clades began to diverge from each other as a result of the separation of the
two continents. But as well as these continents, there are
numerous other landmasses across the planet, each one representing a separate instance
of allopatry, isolated from and unaffected by evolutionary developments that take place
elsewhere in the world. Of particular relevance are islands, which
differ from continents in several key aspects that have significant implications for the
species that inhabit them. The defining characteristics of islands are
their size and their isolation, both of which will affect the potential species richness
the island can support: the larger the island is, the greater the number of species it can
accommodate, and the closer it is to the mainland, the easier it will be for new species to migrate
to it. But these same factors also exert unique pressures
on the islandâs inhabitants, often resulting in highly unusual species and ecosystems in
comparison to the mainland. For our alien planet, thereâs no way we
could realistically delineate the ecosystems of every single one of the thousands of islands
across the ocean, but we can focus on a select few of them that best exemplify the effects
of ecological isolation on the native species. Much of the time, islands will have once been
part of the mainland before becoming separated by continental drift or rising sea levels. On our alien planet, many such islands are
likely to form when the eastward and westward movement of the continents eventually grinds
to a halt, at which point, the coastal mountain ranges, no longer being sustained by tectonic
activity, will diminish as theyâre eroded by wind and rain, and as both continents begin
to drift south, the regions of northern tundra will be moved out of the arctic circle. Both of these factors will decrease the area
available for the formation of glaciers, which may ultimately bring the ice age to an end,
returning the world to a greenhouse state. This will have many effects on the flora and
fauna of the mainland, which weâll follow up on in future episodes, but as this happens,
the melting ice caps will cause a considerable rise in sea levels, trapping many species
on the islands that form as the low-lying areas become submerged. Letâs take a closer look at just one of
these islands, a small landmass about 50 kilometres across just off the coast of the eastern continent
that weâll call Isla Proxima. On islands like this, the native species will
initially be more or less identical to those found on the nearby mainland, which in this
case will include clades such as the titanopods, cryptodonts, and camptopods that live along
the northern coast, but soon after they become stranded, these clades may undergo some rapid
developments in response to their newfound isolation. The smaller a landmass is, the less food and
space it will offer, and on an island as small as Isla Proxima, animals that require large
amounts of resources may struggle. In particular, large predators often do poorly
on islands, since they require extensive hunting grounds and sizable prey populations, which
small islands are rarely able to support. On top of this, species that find themselves
on islands habitats have to contend with the founder effect, the reduction in genetic variation
that arises from a new population being established by a small number of individuals, which results
in a high degree of inbreeding and increases the risk of extinction. Predators typically live at much lower population
densities than their prey, and the very few members of a given species that become stranded
on newly formed islands may not be sufficient to form to a stable breeding population. All of these factors mean that large predators
are especially vulnerable to dying out in isolated habitats and so are frequently absent
on small islands, although there are some notable exceptions weâll touch on later. Therefore, Isla Proxima may not be able to
accommodate the cryptodonts and other large onychodonts, so these species may go extinct
on this island soon after the rising sea levels sever its connection to the mainland. While other large animals on Isla Proxima
will likely face similar pressures, things will generally be easier for megafaunal herbivores
like the titanopods, especially with the disappearance of the big predators, letting them browse
on the islandâs vegetation without fear of attack. However, in comparison to the endless stretches
of bountiful steppe and forests on the mainland, the tiny island offers a very limited supply
of food, which the titanopods will have to adapt for if theyâre to survive. One of the most common evolutionary trends
for island-dwelling animals is described by Fosterâs rule, which posits that species
may undergo a change in body size to best suit the availability of resources in their
environment. While this can theoretically apply to any
habitat, the most obvious and dramatic examples of this phenomenon usually occur in isolated
environments like islands, so much so that Fosterâs rule is sometimes simply called
âthe island ruleâ. Since smaller animals need less food to survive,
large animals that end up on islands very often exhibit a significant reduction in body
size in comparison to their mainland counterparts. This manifestation of Fosterâs rule is called
insular dwarfism, and can be seen in bovids like the anoa, or dwarf buffalo, in the dozens
of unrelated species of dwarf elephants, and is even known to have happened among dinosaurs
like Europasaurus and Magyarosaurus. Generally, the smaller the island is, the
fewer resources will be available, and therefore the more extreme the effects of insular dwarfism
will be, so on an island as small as Isla Proxima, the local titanopods may end up shrinking
down to less than a quarter of the size of their relatives on the eastern continent. Since such a small island will only be able
to maintain a very limited level of biodiversity, this clade will contain only a single species,
which, to contrast them with their larger cousins, weâll call Nanopus proximensis. But while large animals like these may respond
to their isolation on Isla Proxima by getting smaller, somewhat paradoxically, Fosterâs
rule may also result in other animals evolving in the opposite direction; living on a small,
secluded island means there wonât be much competition, which, along with the lack of
large predators, will relieve a lot of evolutionary pressure for the inhabitants, particularly
for animals that would otherwise need to remain small to help them hide or escape danger. This means that for small animals, islands
may actually offer greater access to resources than the mainland, and therefore, in accordance
with Fosterâs rule, they may undergo a change in body size, in this case resulting in insular
gigantism, growing much larger than their continental relatives. On earth, this is seen in numerous species
like the Tenerife giant rat, the giant parrots of New Zealand like the kakapo, and the New
Caledonian giant gecko, the largest living gecko species. On our alien planet, some of the most common
and adaptable osteopods are the eurycheirids, which by this point have become a very successful
and widespread group, with their endothermy letting them survive in a variety of climates,
and their generalist diets helping them take advantage of every available food source,
giving rise to clades like the lystrocheirids in the deserts and the related trypanocheirids
in the temperate steppes and woodlands, and being so prolific, its more than likely that
at least a few species will end up on Isla Proxima. The mainland varieties can vary in size from
only about 20 centimetres to almost a meter, and are primarily small scavengers and generalist
omnivores, but on Isla Proxima, once the onychodonts die out, not only will they be free to grow
bigger, but also to move into the vacant predatory niches. Although their maximum size will be constrained
by the limited food and space on the island, they may still grow into some of the largest
of all eurycheirids, going from generalist scavengers to dedicated predators. They may evolve to use their broad digging
claws and strong forelimbs to grab large prey and wrestle it to the ground, while their
serrated pedipalps, once used for stripping meat from inside carcasses, may adapt into
sharp saw-like appendages to pierce the preyâs hide and tear off chunks of flesh. Once again, due to the limited species richness
the island can support, this clade will only include a handful of species, all of which
will be united in a single genus that weâll call Teratocheirus. But these wonât be the only creatures to
take advantage of the demise of the onychodonts. Ever since the onychodonts first evolved,
theyâve dominated the niches of terrestrial hypercarnivores thanks to adaptations like
their thermoregulation, centaurism, and cursoriality. Meanwhile, the other dromaeopods that lack
these specializations have been on the decline for the last few tens of millions of years,
and have been largely outcompeted in most open habitats, but a few of these more basal
forms will still survive as small mesocarnivores in tropical latitudes. However, if any of these species end up on
Isla Proxima, then once the onychodonts disappear, theyâll experience a huge decrease in competitive
pressures, and will now be free to exploit the macropredatory niches that theyâve been
precluded from on the mainland, which may once again result in an instance of insular
gigantism. This will likely bring them into competition
with the teratocheirids, so the two clades may need to undergo some form of niche partitioning,
which may involve specializing for different styles of hunting. Like most other basal dromaeopods, these forms
may use their powerful back legs and great speed to chase down prey over short distances,
making them much more able to tackle fast-running prey, while the eurycheirids donât have
the same adaptations for pursuit-hunting, but will still be able to employ ambush tactics
to hunt smaller or slower prey, and are adaptable enough to subsist on carrion and mixed vegetation
if need be. Along with nanopus and teratocheirus, these
dromaeopods, which weâll place in the genus Rhomaleopus, will be some of the many clades
on Isla Proxima whose evolution will be shaped by Fosterâs rule, resulting in substantially
different ecosystems from those on the mainland in only a few million years following its
separation. However, the longer an island remains isolated,
the more divergent its ecosystems will become. On earth, Madagascar has been separated from
Africa for over 88 million years, and as a result, about 90% of all Madagascan plant
and animal species are found nowhere else in the world. Our alien planet offers some even more extreme
examples, with some landmasses, like many of the islands dotting the southern ocean,
having been isolated since the invasion of land over 150 million years ago, and with
such a long period of isolation, theyâll be totally unlike any other environment on
the planet. A key factor relating to this is that, unlike
Isla Proxima, none of the mainland clades will have ever had any presence on these landmasses,
since they only evolved after these islands became separated, so everything that comes
to live there will have had to disperse there from elsewhere. In the case of plants, most chemophytes reproduce
by releasing their diaspores into the wind, so while the early chemophytes were spreading
across the mainland, some of these diaspores may have been swept up by the winds of the
massive storm systems that were common during this time, and, by sheer luck, may have been
blown across the sea to some of these islands. Dispersing across such huge distances by accident
will obviously be an extremely rare event, so these islands will initially be home to
only a handful of plant species, but some chemophytes may be more likely to make the
journey than others. Once the xylophytes evolve, most of them will
still make use of wind dispersal like other plants, but in the swamps along the coasts
of the old supercontinent, some plants may have evolved to take advantage of the plentiful
rivers and floodwaters for dispersal. Once fertilized, their female gametangia may
grow into bulbous pods that detach from the parent plant and drop into the water below,
floating downstream and eventually settling on a new stretch of riverbank. This same strategy is used by plants on earth
like coconut palms, knickernuts, and the looking glass tree, which produce buoyant structures
called drift seeds, or drift fruit, that are adapted for being dispersed by water currents. Although these xylophytes will have initially
evolved in the coastal swamps, these drift fruit may serve as a preadaptation for colonizing
islands, since theyâll be able to cross stretches of water that would be virtually
impossible to bridge using wind dispersal, with some drift seeds on earth known ride
the ocean currents for thousands of kilometres, and to remain adrift for months at a time. If some of these xylophytes evolve similar
specializations for oceanic dispersal, they may become some of the most common and successful
plants in coastal and island habitats. On account of their buoyant drift fruit, weâll
call these plants coelophytes. And fortunately for these coelophytes, the
islands that they arrive on will not only be devoid of competition from other plant
clades, but also of any of the mainland herbivores, and so act as a safe haven for them to diversify
and flourish, but it will only be a matter of time before other colonists find their
way here. Before the evolution of flight and semiaquatic
lifestyles, most animals will be confined to the supercontinent, but eventually, a very
small minority of species might happen upon another form of oceanic dispersal: rafting
events occur when organisms are carried from one landmass to another by floating mats of
vegetation and other debris. As improbable as this may seem, this has been
directly, albeit very rarely, observed on earth, such as how green iguanas have been
recorded rafting over 100 kilometres between islands in the Caribbean, and the biogeography
of some clades suggests that rafting events like these can even take place over distances
of thousands of kilometres, which is thought to be how lemurs originally arrived on Madagascar
and how the ancestors of new world monkeys and caviomorph rodents travelled from Africa
to South America. Even though these events are extremely rare,
over timespans of millions of years, theyâre bound to happen at some point, and even a
single breeding pair reaching a new landmass may be enough to establish a population. The most likely animals to undergo rafting
events are species that are small enough to have their weight supported by floating mats
of plant matter, and are hardy enough to survive a journey that might take anywhere from several
days to a few weeks. This being the case, the malacoforms seem
like prime candidates for rafting, since theyâre tiny enough to stay afloat on even the smallest
piece of debris, including, for instance, the drift fruit of the coelophytes. Any malacoforms that infest the tissues of
these fruit may be carried with them once they detach from the parent plant, serving
as a perfect vessel to ferry them between islands and letting them spread relatively
quickly among all the landmasses that the coelophytes reach. On the other hand, rafting events involving
larger animals will happen much less frequently, since theyâll require much sturdier rafts
such as mats of fallen branches, logs, and other driftwood, which arenât likely to
form very often. Relating to what we mentioned at the beginning
of this episode, the rate at which new species will raft to a given island will be contingent
on its size and its distance to the mainland, so landmasses like those off the southwest
coast of the supercontinent, which are relatively large and only a few hundred kilometres away,
are likely to experience a relatively high rate of rafting events, and so share many
clades in common with the mainland, while more remote landmasses may see only see one
or two rafting events throughout their existence, which will have some crucial implications
for their biogeography. Long before the first osteopods, the lophostomes
were already a well-established group, even including various megafaunal clades among
their ranks. Along with the ubiquitous malacoforms, a few
of these larger species might have also been rafted across the sea during this early period. Perhaps in one chance event, a few members
of a small, adaptable species of the ancestral desmostracans were sent on a voyage of over
2000 kilometres to make landfall on the island off the supercontinentâs southeastern coast,
a landmass which weâll hereafter call Crescentia. If enough of them survive to overcome the
founder effect and form a stable breeding population, then theyâll find themselves
in an ideal sanctuary, lacking any other animals beside the malacoforms they feed on. This will be especially fortunate for them
considering that a few million years after this rafting event occurs, competition from
the newly evolved osteopod clades will drive most of the older lophostome species to extinction,
with the only lineage of desmostracans that survive on the mainland descending from a
clade of small malacovores. But on Crescentia, theyâll be safely secluded
from the developments taking place on the supercontinent, and thus theyâll survive
as the only representatives of an ancient branch of lophostomes that wonât exist anywhere
else in the world. These surviving species will represent whatâs
known as a relict, the remnant of a population or clade that was once widespread, but now
retains only a fraction of its former range and diversity. An area or habitat that supports a relict
is called a refugium, which islands frequently serve as, such how the thylacine survived
on the island of Tasmania up until the last century despite having died off on the Australian
mainland thousands of years earlier, and how New Zealand is home to the tuatara, which,
although it closely resembles a lizard, is actually the only surviving species of rhynchocephalian,
an entirely separate clade of reptile that once had a global distribution before being
largely wiped out along with the non-avian dinosaurs. Likewise, Crescentia will serve as a refugium
for these relict desmostracans, which will continue to thrive on the island even as the
megafaunal lophostomes are replaced by the osteopods elsewhere in the world. Weâll call their unique lineage the notoforms. Once theyâve fully colonized the island,
the lack of competition will have several immediate effects: first of all, although
their lack of an internal skeleton and the diminishing levels of atmospheric oxygen will
still restrict them to only a dozen or so kilograms at the most, the abundance of food
and the absence of any osteopods to compete will once again result in insular gigantism,
with many species growing much larger than the vast majority of the mainland lophostomes,
much like how the lack of mammals in New Zealand has allowed the giant wÄtÄ to evolve into
one of the largest insects on earth. Another common occurrence among animals that
migrate to isolated or uncolonized habitats is for them to undergo an adaptive radiation,
rapidly diversifying into a multitude of different forms to fill all the unoccupied niches. An especially famous example of this is Darwinâs
finches, which all descend from a recent common ancestor that arrived on the Galapagos islands
within the last 2-3 million years that then promptly evolved into over a dozen different
species. This effect will be particularly extreme on
an island like Crescentia, since all the niches for animals larger than the malacoforms will
be completely vacant when the notoforms first arrive, leaving almost the entire food web
up for grabs. The original notoform species that first rafted
to the island was a small, generalist omnivore, a lifestyle that many subsequent notoforms
species may retain. But soon after they arrive, one branch may
specialize for feeding on vegetation, evolving a large, powerful gut supported by bulky,
weight-bearing legs and strong, compact mouthparts to masticate plant matter. Being indiscriminate herbivores, they wonât
need the antennae-like sensory tentacles of the early notoforms to sift through the leaf-litter
for food, so these appendages may shrink to not interfere with feeding, and their banded
shell may acquire a distinct saddle shape to let them angle their heads upward to reach
for food. Weâll call these herbivores thyreostracans. Meanwhile, another group may specialize for
carnivory, evolving into the islandâs first large predators. Like most lophostomes, the notoforms wonât
be very nimble animals, with their stubby, boneless limbs only letting them manage an
awkward waddling gait. However, the desmostracans are more manoeuvrable
than most lophostomes thanks to their flexible banded shell, and reducing the shell even
further will allow the muscles along their flank to contribute to locomotion, increasing
their stride length by bending the body into an alternating S-shape. This will make them some of the fastest of
all land-dwelling lophotomes, which in addition to other adaptations such as camouflage patterns
to conceal them amid the undergrowth and sharp toothy mouthparts to latch onto prey, will
let them fill the majority of the carnivorous niches on Crescentia, from small worm-like
mesocarnivores to meter-long macropredators. Both the thyreostracans and these predators,
which weâll call campylospondyls, will continue to proliferate over tens of millions of years
to exploit all the niches Crescentia has to offer, resulting in ecosystems completely
unlike anything on the mainland. However, over the 50 million years following
their arrival on the island, the conditions may eventually come into place for another
rafting event to occur. Perhaps this time, a handful of small, arboreal
platydonts will be cast out to sea within a tangle of fallen branches, brought by the
currents to the shores of Crescentia. Even though by this point, the island will
have been thoroughly colonized and its ecosystems filled, the arrival of these platydonts will
pose a significant threat for the notoforms. Because islands generally have lower levels
of competition and predation than the mainland, newly introduced species can quickly become
invasive and cause considerable ecological disruption, as seen in species like the brown
tree snake in Guam, the common brushtail possum in New Zealand, and the water hyacinth in
Madagascar. In this case, these platydonts will present
a new source of competition that the notoforms will have no way of preparing for, and will
have a huge competitive advantage due to their internal skeletons and active respiration,
which will let them exploit niches that are inaccessible to the notoforms and allow them
to undergo an adaptive radiation of their own, giving rise to a new lineage that weâll
call the xenodonts. Since theyâre already predominantly arboreal,
the xenodonts may quickly come to monopolise the niches in the tree tops, feeding on leaves
that the bulky thyreostracans on the ground canât reach, and within the relative safety
of the canopy, they can proliferate and evolve into larger forms. And following the changes in climate and atmospheric
composition taking place during this period, the megafaunal notoforms will gradually begin
losing ground, giving the xenodonts the opportunity to take their place as the Crescentiaâs
dominant fauna. One of the xenodontsâ most distinct advantages
will be their speed, as their skeleton and limb configuration will give them more efficient
locomotion than even the campylospondyls, which one clade may take advantage of to become
fast, nimble predators. These forms may retain some arboreal behaviours,
but may also exploit the relative lack of competition on the forest floor and spend
time hunting at ground-level as well, with the hooked claws that originally evolved to
help them climb trees now letting them to latch onto and disembowel their prey. However, such large claws will be at risk
of being worn down from abrasion against the ground, so theyâll need to evolve some way
of keeping them sharp. One solution is to keep the claw raised off
the ground, like the foot claws of dromaeosaurids, or the retractable claws seen in most varieties
of cats, the Madagascan fossa, and the grey fox, all of which rely on their claws to climb
trees and catch prey, and so keep them retracted when not in use to prevent unnecessary wear
and tear. Alternatively, another option is knuckle-walking,
a fairly rare condition wherein the animal supports its weight on its knuckles, keeping
the tips of the digits off the ground so they can be used for other purposes. Knuckle-walking can be seen in semi-arboreal
animals like chimpanzees and gorillas, which use their dextrous fingers for handling food
and climbing, but itâs also seen in animals that need to maintain long, sharp claws to
help them feed, such as the giant anteater, which uses its huge claws to break open insect
mounds, while the extinct chalicotheres, which used their claws to reach for and pull down
tree branches, even saw the parallel evolution of both knuckle-walking and retractable claws. While the anatomy of the osteopod foot isnât
exactly equivalent to that of vertebrates, these xenodonts will have inherited flexible
ankle joints from their arboreal ancestors to wrap around and grip tree branches, so
when they come down to the ground, they may walk on their wrists with their feet bent
backwards, keeping the points of their claws facing upward and away from the ground, a
trait for which weâll call them streptotarsans. Meanwhile, the relative lack of competition
and bounty of vegetation may encourage some of the other xenodonts to independently evolve
a ground-dwelling lifestyle and adapt into dedicated herbivores. No longer needing to remain light enough to
be supported by the tree branches, these herbivores can now afford to grow much larger, evolving
into a clade of heavy-set browsers that fill similar niches to the megalobrachids on the
mainland, and convergently evolving strong, robust mandibles with flattened teeth to crush
and grind down plant material, and a large, multi-chambered foregut to maximize digestive
efficiency. Unlike the megalobrachids though, their front
walking legs, used by their arboreal ancestors to reach out for and hook onto branches, may
retain their large-sickle shaped claws to assist the pedipalps in reaching upward and
pulling down foliage towards the mouth. Once again, these claws will need to be kept
long for them to be effective in feeding, so these animals will also need to a way of
preventing them from becoming worn down. But unlike the streptotarsans, which need
to keep all of their claws sharp so they can be used for climbing trees, these exclusively
ground-dwelling herbivores will only need to keep their feeding claws long, which provides
an additional option beyond retractable claws or knuckle-walking. In other episodes, weâve seen centaurism
evolve in both the onychodonts and the allobrachids, in both instances evolving as a means to allow
the animal to run faster and more efficiently, but in this clade, centaurism may occur as
a natural consequence of evolving a ground-dwelling lifestyle, as only the hind three pairs of
legs evolve to bear the animals weight, with the claws on these limbs becoming blunt and
hoof-like, while the front limbs may become specialized for feeding by being held off
the ground to stop their claws from scraping against the soil. This will make them vaguely analogous to animals
like therizinosaurus, which, thanks to its centaurism, could afford evolve enormous rake-like
feeding claws to reach for vegetation. Due to this distinctive feeding arrangement,
weâll call these xenodonts brachiocephalians. The brachiocephalians and streptotarsans will
represent only a fraction of the diversity that the xenodonts will achieve over the tens
of millions of years that they hold dominion over the Crescentian ecosystems, but even
these creatures wonât be able to maintain their supremacy in the face of a rapidly changing
climate. With the onset of the ice age, the landscape
of Crescentia will change drastically, with most of the lush jungles giving way to seasonal
forests and tundra. This will be a huge challenge for the native
species, as most animals on Crescentia wonât have any form of thermoregulation, making
them very sensitive to cold temperatures, and with the scarcity of food brought on by
the retreat of the tropical forests, only a small minority of Crescentiaâs former
diversity will manage to survive. But on top of this, during the glacial periods,
the stretch of sea between Crescentia and the eastern continent will shrink as the sea
levels drop, and if the sea freezes over, it may be possible for animals to travel from
one landmass to the other. Crossing a several hundred-kilometer stretch
of sea ice will be a hazardous and impractical journey for most animals, but a small number
of migratory or wide-ranging species from the mainland might be able to trek across
the ice and find their way to Crescentia. The most likely animals to make a journey
like this are small, adaptable species that are able to withstand the cold, which on earth
include animals like the arctic fox and the Falkland Islands wolf, both of which were
able to cross the frozen seaways that formed during the last ice age to arrive on Iceland
and the Falkland islands respectively, and are both the only land mammals other than
humans to ever inhabit their home islands. In this case, the allodonts living on the
southern coast of the eastern continent are generalist enough to feed on whatever food
they can find on the icy plains, and their endothermy will help them resist the cold
once the ice age arrives, an advantage they may strengthen by evolving a shaggy coat of
fur from the tiny setae that cover their bodies, a near-identical development to what occurred
in the closely related thecopods, making the two clades appear very similar despite existing
on opposite sides of the planet. For these cold-adapted allodonts, which weâll
call eriotheres, migrating south would present an escape from the fierce competition of the
mainland, prompting at least one species to cross the frozen seaway to Crescentia. By the time they arrive, almost all the large
xenodonts will have died out, so theyâll face very little competition as they move
into the islandâs megafaunal niches. And once the ice age ends, theyâll become
stranded on Crescentia as the sea ice melts, leaving them to thrive in allopatric isolation
just as the notoforms and xenodonts did before them, undergoing their own radiation over
the coming tens of millions of years as Crescentia continues to slowly drift south. But Crescentia will be only one of thousands
of islands scattered across the ocean, many of which will be too small and remote to be
reached by crossing sea ice or land bridges, and for which rafting events will be vanishingly
rare. By far the most likely animals to make it
to islands like these will be the flying and semiaquatic clades, especially migratory species
that are adapted for travelling over long distances. The first clade of animals to develop flight
were the opisthopterans, and once long-ranging species like the magnopterans evolve, it wonât
be long before some of them adapt for a sea-going lifestyle, soaring over the ocean and snatching
acanthopods out of the water, much like seabirds and fishing bats on earth. For animals like these, remote islands will
serve as ideal mating grounds, where they can carry out their courtship and lay their
eggs without needing to worry about predators. This degree of safety may even encourage some
animals to spend their entire lives here, which may go hand in hand with another development. Flight provides a number of distinct advantages,
especially when it comes to dispersal and escaping predators, but it comes at a steep
metabolic cost, hugely increasing the amount of food the animal needs to consume and requiring
a number of anatomical specializations to be efficient. This means that if a flying animal finds itself
in an isolated environment with few predators, then flight may no longer be useful enough
to justify its energetic demands, and the species may end up losing its ability to fly
altogether. As such, flightlessness is an extremely common
evolutionary trend on islands, seen in groups like the elephant birds of Madagascar, kiwis
and moa in New Zealand, dodos on Mauritius, and many many others. Becoming flightless may also allow species
to expand into new niches with which flight would be incompatible. Since the majority of seabirds feed on fish,
they obviously need to enter the water in order to find and catch their prey, but swimming
and flying are very different forms of locomotion, mainly due to the drastic difference in density
between water and air, and specializing for moving through one medium will unavoidably
decrease the animalâs efficiency of moving through the other. This is exemplified by seabirds like the pelagic
cormorant, whose short, broad wings help them steer while underwater, but also give them
the lowest flying efficiency of any bird species. This tradeoff has also shaped the evolution
of specialized feeding behaviours: some flying piscivores, like skimmers and greater bulldog
bats, never enter the water but instead fly overhead and snatch up fish that swim near
the surface, while others, like gannets and brown pelicans, dive into the water to pursue
their prey. For species that specialize for piscivory,
the pressure to evolve effective swimming may outweigh the benefits provided by flight,
so becoming flightless will allow them to adapt more completely for aquatic life, and
has consequently occurred in many seabirds, like penguins, great auks, and some varieties
of waterfowl. Likewise, many of the island-dwelling magnopterans
will no longer need to worry about evading predators or dispersing to new areas, so they
can afford to give up flight entirely to let them more efficiently hunt aquatic prey. This flightlessnness may be accompanied by
an increase in body size, not only due to insular gigantism, but also because larger
sizes increase diving efficiency. This will be complemented by their efficient
respiratory system and the internal support provided by their gladius, letting them grow
into some of the largest lophostomes on the planet. To support a heavier body, their wings and
canards, having a much greater degree of musculature than the boneless walking legs, may take over
the role of locomotion on land as well, letting them awkwardly haul themselves along the ground,
unlike most other opisthopterans, which fold their wings backwards when they land. These wings may shorten into a pair of powerful
flippers to provide effective underwater propulsion, while the canards will help them steer and
maintain stability, making their style of swimming less like that of wing-propelled
divers like penguins and auks and more similar to foot-propelled divers like loons and the
extinct Hesperornis. Along with a more streamlined body, they may
evolve valves within their spiracles to let them hold their breath while underwater, and
the regional endothermy that lets their wing muscles function optimally will also allow
them to maintain a stable body temperature in cold water, which will be of great benefit
once the ice age arrives, and will give them an advantage in competing with other semiaquatic
clades like the aktatheres. These creatures, which weâll call dyptopterids,
will be just one of many opisthopteran clades that independently become flightless in response
to their secluded island habitats, and once theyâre sufficiently adapted for a semiaquatic
lifestyle, theyâll be able to swim across the sea and reach other islands or even the
coasts of the mainland, coming ashore to breed and to lay their eggs. However, the evolution of the dyptopterids
will be shortly followed by the appearance of the pleuropterans, which, having also evolved
flight, will share the same long-distance capabilities as the opisthopterans, and so
will likewise be poised to spread across the planet and find their way to habitats that
are unreachable for other clades. Since the opisthopterans evolved over 60 million
years before the pleuropterans, theyâll have had a head-start in colonizing most islands,
but a perfect opportunity for the pleuropterans may arise with one particular geological development. Over millions of years, underwater volcanic
activity fuelled by hot spots on the planetâs mantle can produce plumes of rock large enough
to emerge above the oceanâs surface, effectively creating brand a new island or even chain
of islands as the crust slowly drifts over the top of the hotspot. On earth, islands like Iceland and Hawaii
were formed through this process, and on our alien planet one such hotspot may occur in
a remote region of the ocean, resulting in the formation of an archipelago that weâll
call Pyronesia. When they first form, these islands will be
nothing but barren outcrops of solid rock, completely devoid of any native species, but
even habitats as desolate as this will still attract species to colonize them. However, as this archipelago will be over
4000 kilometers away from the nearest coast, it will be almost impossible to reach by passive
dispersal mechanisms like rafting, so flying animals will once again be the most likely
organisms to find their way here, including not only the opisthopterans but also the newly
evolved pleuropterans. Like the ancestors of the dyptopterids, the
flying species that arrive on Pyronesia will likely be migratory acanthovores that come
to the islands to feed and lay their eggs away from the perils of the mainland. As they migrate to the archipelago, they may
unwittingly bring other clades with them, such as phoretic malacoforms clinging to their
skin and the diaspores of plants adapted for animal-assisted dispersal like the chromatophytes. The populations that these species establish
will slowly transform the archipelago into a stable habitat through a process called
ecological succession, the progressive change in the structure and composition of an ecosystem
over time. Succession can be subdivided into primary
succession, in which an ecosystem forms in a brand new habitat with no pre-existing species,
and secondary succession, when the ecosystem recovers after an ecological disturbance. Pyronesia will be a perfect example of the
former, as the landscape will initially consist of nothing but newly exposed rock. The first stage of succession is initiated
by pioneer species, usually small plants, lichens and fungi that are hardy enough to
grow on bare rock, which, assisted by erosion from the wind, waves, and rain, will start
to break down the rock into mineral-rich soil. In the case of Pyronesia, the pioneer species
may include some of the chromatophytes, which, being epiphytes, are already adapted for growing
in the absence of soil, and without any towering altiphytes around them to block out sunlight,
theyâll have a much easier time growing on these islands than in the rainforest, forming
a carpet of multicolored filaments along the ground. These pioneer species will pave the way for
intermediate species like some of the other brachyphyte clades and various caulophytes,
which will further contribute to soil formation until it ultimately becomes suitable for trees
to take root. In most ecosystems across this planet, the
niches of trees are filled by the xylophytes, but with the exception of the coelophytes,
these plants arenât well-suited for long-distance dispersal, as their diaspores are much smaller
and more prone to desiccation than plants like the chromatophytes, making them comparatively
unlikely to survive a trip all the way across the ocean. This means the xylophytes may not have much
of a presence on Pyronesia, providing an opportunity for other plants on the archipelago to move
into their niches. The xylophytesâ signature adaptation is
their layer of rigid tissues that gives them the structural support to grow enormously
tall trunks to reach upward for light, but although this has allowed them to become the
dominant form of tree across the planet, itâs unlikely to be a unique adaptation. Even on earth, plantâs that we would recognize
as trees have evolved independently across many clades, like for example, how palm trees
are actually more closely related to grasses than to conifer trees. Similarly, on our alien planet, even though
the xylophytes are the most widespread group of tree-like plants, there are likely to be
many other plants that have convergently evolved tough, inflexible stems for greater support,
and on Pyronesia, with very few xylophyte species to compete with, one such clade may
now be able to grow into the archipelagoâs dominant large flora, a clade of plants that
weâll call nothodendrons. And throughout the process of succession,
new niches will also be opened for animals as well, which may incentivize the migratory
opisthopterans and pleuropterans to take up permanent residence here and to integrate
with the local ecology, and with no other diplostomes or osteopods on the islands, the
conditions will be ideal for both clades to undergo an adaptive radiation to spread into
every niche the islands have to offer. Unlike the ancestors of the dyptopterids,
the opisthopterans on these islands will still need to contend with a considerable amount
of competition and predation from the pleuropterans, and so they may still retain their flight
to help them escape danger. Due to their limited size and lack of a true
skeleton, they wonât be able to effectively compete for the niches of apex predators and
megafaunal herbivores, and so will be relegated to the niches of malacovores, scavengers,
and even frugivores and nectarivores, convergently evolving to fill similar niches to the picopterans
and latopterans on the mainland. On the other hand, the pleuropterans will
be at the top of the food chain, so once they begin adapting for ground-level niches, their
flight may be more of a hindrance than an asset, with their wings restricting their
movement through the dense groves of nothodendrons. But if they become flightless, the middle
two pairs of legs and the wing membrane they support can shrink so as to not get in the
way of the forelimbs and hindlimbs, which will be the only legs involved in locomotion,
making these forms the only quadrupedal osteopods to evolve thus far, and thus giving them some
of the most efficient locomotion of any land animal. In reference to the vestiges of the wing membrane
connecting their legs, letâs call these animals chlamypterans. No longer constrained by the demands of flight,
these chlamypterans will now be free to expand into new niches they would otherwise be precluded
from. As mentioned in part 9, leaves are difficult
to digest and donât offer enough nutrition for folivores to maintain a very active lifestyle,
making them a poor food source for flying animals, but once these theropterans become
flightless, they wonât need to maintain such a high-energy diet and will now be better
able to exploit the niches of large herbivores, similar to how the majority of ratite species
took up herbivory after they gave up flight. To cope with a constant input of tough, nutrient-poor
foliage, these theropterans may evolve an expanded, complex foregut like many of the
megafaunal herbivores on the mainland, and an elongated cephalothorax to browse on the
leaves of the nothodendrons. In compliance with fosterâs rule, their
body size will be contingent on the availability of food on their home island, which, while
not enough to let them grow to sizes comparable to mainland megafauna like the titanopods,
will still let these animals, which weâll call hypsirhynchids, evolve into some of the
largest animals on Pyronesia. And with clades like these filling the herbivorous
niches, other varieties of chlamypterans may subsequently adapt to prey on them. Having evolved from the same flying ancestors
as the hypsirhynchids, theyâll inherit the same long-legged gait, chasing down their
prey in a sort of bounding gallop. Their pedipalps, used by their ancestors to
snatch acanthopods out of the water, may evolve scythe-like claws to kill and dismember their
prey, recalling large island-dwelling predators like the giant marabou stork and the enormous
pterosaur hatzegopteryx. With the evolution of these predators, which
weâll call temnorhynchids, all the major niches of the Pyronesian ecosystems will be
filled, and the process of succession will reach a point of equilibrium, culminating
in whatâs sometimes called a âclimaxâ or âsteady stateâ community. Processes like these will occur on each and
every one of the innumerable islands on this planet, each one independently cultivating
its own unique assemblage of species and ecosystems, with the few weâve described here serving
to illustrate a small selection of noteworthy island-dwelling clades to evolve throughout
the planetâs history up to about 20 million years or so after the ice age comes to an
end. But away from isolated habitats like these,
there are other innovations quietly taking place across the planet at large that deserve
special attention. In the next episode, weâll return to the
mainland to take a closer look at the evolution of sociality and cooperative behaviours. Thanks to all the artists over on discord
who contributed artwork for this episode, who save me an enormous amount of time and
effort when making these videos while simultaneously making them look a hell of a lot better in
the process. Links to the main server and the alien biospheres
fan server in the description. And once again, a massive thanks to all the
patrons, whose continued support makes videos like this possible. Thanks for watching, and Iâll see you in
the next video.
Lets GOOOO!!!! It's giant nightmare spider time BAYBEE!!
I hate how every time you think the lophostomes are getting a chance, the osteopods screw every thing up
FINALLY!!!
NEW VIDEO
YAY
It's so fucked up how none of the dwarf elephants are still around.
Hey, did anyone notice that at 17:59, a symbol flashes in the bottom right corner? It's purple and has a white crop circle type pattern in it.