For Remora, from the Worldcreate Yahoo! Group
2. Re: Trans-ocean migration (warning: long reply)
Posted by: "nyeshet" nyeshet@yahoo.com nyeshet
Date: Tue Nov 14, 2006 6:03 am ((PST))
Marsupials developed from proto-mammals in the northern Laurasia
super-continent ~140-125 million years ago (although there is some
controversy over it evolving on Laurasia instead of Gondwana).
Placentals evolved from the Gondwana equivalent of the same
proto-mammals ~125-115 million years ago. At some point the two groups
mixed, in that both existed on both super-continents. But following
~65 million years ago the placentals were overwhelmingly dominant on
Laurasia and the Marsupials were dominant (more or less) on Gondwana
(or, more accurately, South America, as they then migrated from there
into Antarctica and following that Australia). As the continents moved
around the Placentals eventually spread to every continent but
Australia - until recently. Some placentals existed in Australia, but
the Marsupials were so strongly entrenched that it took the arrival of
man - with dingos and rats to begin weakening their position. The
recent arrival of sheep and - far more importantly - rabbits, has
further weakened their position.
According to fossil history, there are four lineages of marsupials:
Australian (dasyuroid), Asian (deltatheroidan), South American
(didelphimorphian), and North American (stagodontid). The Asian and
North American lineages died out millions of years ago. The opossum,
in fact, is the only successful marsupial to migrate from South
America into North America (~7-3 million years ago, wiki the 'Great
American Interchange', the monkeys and rodents that arrived ~30
million yrs ago likely came from Africa across the *much* narrower
South Atlantic ocean). Placentals were far more successful in
migrating south (~same time period), as most South American marsupials
have died off following competition with their northern placental kin
(some South American marsupials still exist - more than is found in
North America, in fact, but far less than is found in Australia).
Regarding the opossum and the radiations of marsupials in general, an
answer found online provides some interesting information:
[quote]The marsupials and placentals diverged from a pantotheran stem
stock in the late Cretaceous. The first marsupials appeared in North
America approximately 80 million years ago, e.g. Alphadon (marsupials
can be distinguished from placentals by their dentition - marsupials
have 3 premolars and 4 molars whilst placentals have 3-5 premolars and
3 molars). Towards the end of the Late Cretaceous, marsupials start
appearing in South America (Peru and Bolivia). In the Eocene
marsupials radiated into Europe, North Africa and reached Asia by the
Oligocene. However these groups rapidly became extinct. South America
parted company with North America in the Eocene, effectively blocking
the rapid radiation of placentals in North America at this time from
spreading to South America. During the Eocene, marsupials reached
Antarctica, which was attached to South America and Australia at this
time. Marsupials could follow a belt of Northophagus vegetation all
the way around from southern South America, across Antarctica into
southern Australia. The first marsupials appear in Australia in the
Oligocene via this route. Australia parted company from Antarctica in
the Miocene, effectively isolating the marsupial fauna here.
Although there has been a marsupial fossil found in the Oligocene of
Asia, it closely resembles the European form Peratherium, which is a
didelphid, and has little affinities to Australian forms. Therefore
colonization of Australia from the north is not considered viable.
Australian marsupials (extant and extinct) share many affinities with
South American marsupials and extinct Antarctic forms, indicating a
southern migration route for marsupials and explaining the lack of
placental mammals in Australia. By the time South America re-docked
with North America in the Plio-Pleistocene, South America was already
separated from Antarctica and Antarctica from Australia. So the
reintroduction of placentals into South America couldn't continue on
to Antarctica/Australia. Note: this does not preclude any placentals
from reaching Australia, just significant numbers of them.[/quote]
_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/
[quote="martathiax"]I just have a hard to figuring out why at the same
longitute you can have deserts and lush forests when little difference
in the geography separates the climate zones.[/quote]
Ocean currents might explain this. They might be bringing warmer or
cooler waters off shore - which in turn could warm or cool the air,
thus affecting regional circulation patterns. Also, the distance from
water is a major factor. The Sahara is dry in part due to latitude and
in part due to the fact that most of it is literally hundreds or even
thousands of miles from any source of water. Clouds lose their
moisture long before reaching those central regions. Central America,
around the same latitude, is remarkably narrow, with vast oceans on
either side. Clouds don't really have as much time to lose moisture,
so the regions are notably wetter than the Sahara - although if you
move north into Mexico (which, while not much wider than the rest of
Central America, is higher in elevation) or the southern United States
(wider and thus more distance for clouds to have to carry water) you
again find drier regions.
[quote="martathiax"]Gondwanaland suffered from mass glaciation at its
souther longitutes, and because there was land that spanned from north
to south poles, oceanic currents couldn't properly circulate heat
causing about 100 reactions that made the non-coastal land hot and
dry.... i don't get it... i figure cold and dry, almost like another
global glaciation, but no.... makes little sense to me.[/quote]
The polar ice cap grew and thus captured a fair degree of the water in
the seas and air, lowering sea levels and air humidity. The world as a
whole was a drier place because of this. As for hot or cold, it
depended on latitude. Nearer the poles the continents would have been
Siberian chilly, while nearer the equator they would have baked like
the Sahara. The drier air would have encouraged deserts to form and
grow, until much of the interior of the continent(s) was desert. The
coasts and the major rivers would have been the only locations where
non-arid biomes might have remained. Near the poles it would have been
vast tracks of tundra and other chill arid biomes, while elsewhere
(possibly excluding the equator, where jungle might still have existed
- at least near the coastlines and possibly some way into the
continent) deserts would have ruled supreme due to the lack of water.
Note that during the last ice age the glaciers only moved as far south
as 50N. That still leaves a lot of room before meeting the equator.
[quote="martathiax"]Good luck on your project, seems to be coming
along well.[/quote]
Thank you!
We have gotten a bit side tracked from the original question, however.
Some further research has shown that rats might have moved from Africa
to South America ~36-45 million years ago - when the continents were
still rather notably separate (although far closer than they are
today). The predominant views are that that rodents (and monkeys, for
that matter, around ~26-30 million years ago) either rafted across on
mats of vegetation on oceanic currents or that they island hopped
using islands that are no longer existent (due to erosion reducing
them below sea level, and / or an ice age having exposed them for the
brief few thousand years they were used to hop between continents).
This suggests a maximum distance traversable across an ocean for small
creatures larger than typical insects and not in and of themselves
aquatic in part or whole (such as the extinct form of hippo that once
existed in Madagascar, which is thought to have somehow made it across
the straight between the two landmasses by chance). The distance
between Madagascar and Africa, in fact, could be a limitation for
chance migration of larger creatures - if semi-aquatic - perhaps.
Around 30 Myrs ago the distance between South America and Africa was
~20% less than it is today, based upon measuring in pixels on a few
different maps of the world of that time period. (Well, more akin to
20% +/- ~3-4%.) Using Natal in Brazil and Lagos in Nigeria - which in
the present are ~4500km apart - this suggests that smaller creatures
such as mice, lizards, very small monkeys, etc might survive on a
floating raft of vegetation a trans-ocean voyage of about 3500 km.
As for larger creatures (such as the hippo example above), the
difference between Africa and Madagascar is ~400 miles. Both instances
however - the hippo lineage formerly in Madagascar and the origin of
rodents and monkeys in South America - are examples of extremely rare
chance migrations. The path was not so easy as to allow for common
passage. Only the hippo managed the migration to Madagascar, and its
semi-aquatic nature - combined with islands that potentially allowed
for island hopping (requiring more distance for the overall traversal
but decreasing the maximum time required at sea for any single
specific traversal) likely aided in this migration. For non-aquatic
creatures the distance might perhaps be halved or quartered - to a
mere 200 - 100 km.
Over millions of years only two traversals seemingly occurred between
Africa and South America over the 3500 km distance - monkeys ~26-30
Myrs and rodents ~36-45 Myrs ago. Again this suggests that this
distance may be the maximum possible for such traversals of smaller
creatures.
As my southern and eastern continents are separated by ~5000 km, this
suggests that they are not yet close enough for even smaller creatures
(other than insects, perhaps) to traverse the distances involved -
even by extreme chance.
On the other hand, the distance between the southern and northern
continents is quite less, and so it is possible that some smaller
species might have made such voyages - at least since the northern
most tip of the southern continent moved north of the equator by more
than a degree or two, thus preventing the equatorial currents from
fully isolating it from the northern continent. As it stands, it would
still be difficult for such a journey unless it occurred on the
southeastern shore of the northern continent and then drifted to the
northern shore of the southern continent. It may even require a
certain time of the year for such to occur, as winds tend to move more
north or south of the equator as summer shifts from one side of the
globe to the other.
So it is possible that some small dinosaurs - perhaps the size of a
small cat - might have (within the last ~4-5 Myrs) made the traversal
to the south continent from the northern continent, adding to the
exotic flavor of its jungle covered northern shore. Archaeic
reptilians and amphibians from the eastern continent, however, are
unlikely to have made the journey across the ocean. Even with air
currents in favor of the exchange it seems unlikely that pterosaurs
could have made the journey. Some few species of birds, however, may
make seasonal trips south, and these feathered rarities may even have
some populations on the northern shore of the southern continent.
Perhaps the situation is even reversed, and bats may similarly have
made migrations north and have limited populations on the southern
shores of the northern continent.
Maybe over the last few million years a few have even evolved to lose
their flight - leading to a new form of 'rodent' slowly making
progress in spreading over the south of the northern continent. That
could be interesting. I wonder what a rodent with echolocation would
be like?
_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/
In any case, I think I may have answered my own question (regarding
migrations). Thanks to all that commented and posted ideas or views in
regards to my first post on this topic.
Posted by: "nyeshet" nyeshet@yahoo.com nyeshet
Date: Tue Nov 14, 2006 6:03 am ((PST))
Marsupials developed from proto-mammals in the northern Laurasia
super-continent ~140-125 million years ago (although there is some
controversy over it evolving on Laurasia instead of Gondwana).
Placentals evolved from the Gondwana equivalent of the same
proto-mammals ~125-115 million years ago. At some point the two groups
mixed, in that both existed on both super-continents. But following
~65 million years ago the placentals were overwhelmingly dominant on
Laurasia and the Marsupials were dominant (more or less) on Gondwana
(or, more accurately, South America, as they then migrated from there
into Antarctica and following that Australia). As the continents moved
around the Placentals eventually spread to every continent but
Australia - until recently. Some placentals existed in Australia, but
the Marsupials were so strongly entrenched that it took the arrival of
man - with dingos and rats to begin weakening their position. The
recent arrival of sheep and - far more importantly - rabbits, has
further weakened their position.
According to fossil history, there are four lineages of marsupials:
Australian (dasyuroid), Asian (deltatheroidan), South American
(didelphimorphian), and North American (stagodontid). The Asian and
North American lineages died out millions of years ago. The opossum,
in fact, is the only successful marsupial to migrate from South
America into North America (~7-3 million years ago, wiki the 'Great
American Interchange', the monkeys and rodents that arrived ~30
million yrs ago likely came from Africa across the *much* narrower
South Atlantic ocean). Placentals were far more successful in
migrating south (~same time period), as most South American marsupials
have died off following competition with their northern placental kin
(some South American marsupials still exist - more than is found in
North America, in fact, but far less than is found in Australia).
Regarding the opossum and the radiations of marsupials in general, an
answer found online provides some interesting information:
[quote]The marsupials and placentals diverged from a pantotheran stem
stock in the late Cretaceous. The first marsupials appeared in North
America approximately 80 million years ago, e.g. Alphadon (marsupials
can be distinguished from placentals by their dentition - marsupials
have 3 premolars and 4 molars whilst placentals have 3-5 premolars and
3 molars). Towards the end of the Late Cretaceous, marsupials start
appearing in South America (Peru and Bolivia). In the Eocene
marsupials radiated into Europe, North Africa and reached Asia by the
Oligocene. However these groups rapidly became extinct. South America
parted company with North America in the Eocene, effectively blocking
the rapid radiation of placentals in North America at this time from
spreading to South America. During the Eocene, marsupials reached
Antarctica, which was attached to South America and Australia at this
time. Marsupials could follow a belt of Northophagus vegetation all
the way around from southern South America, across Antarctica into
southern Australia. The first marsupials appear in Australia in the
Oligocene via this route. Australia parted company from Antarctica in
the Miocene, effectively isolating the marsupial fauna here.
Although there has been a marsupial fossil found in the Oligocene of
Asia, it closely resembles the European form Peratherium, which is a
didelphid, and has little affinities to Australian forms. Therefore
colonization of Australia from the north is not considered viable.
Australian marsupials (extant and extinct) share many affinities with
South American marsupials and extinct Antarctic forms, indicating a
southern migration route for marsupials and explaining the lack of
placental mammals in Australia. By the time South America re-docked
with North America in the Plio-Pleistocene, South America was already
separated from Antarctica and Antarctica from Australia. So the
reintroduction of placentals into South America couldn't continue on
to Antarctica/Australia. Note: this does not preclude any placentals
from reaching Australia, just significant numbers of them.[/quote]
_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/
[quote="martathiax"]I just have a hard to figuring out why at the same
longitute you can have deserts and lush forests when little difference
in the geography separates the climate zones.[/quote]
Ocean currents might explain this. They might be bringing warmer or
cooler waters off shore - which in turn could warm or cool the air,
thus affecting regional circulation patterns. Also, the distance from
water is a major factor. The Sahara is dry in part due to latitude and
in part due to the fact that most of it is literally hundreds or even
thousands of miles from any source of water. Clouds lose their
moisture long before reaching those central regions. Central America,
around the same latitude, is remarkably narrow, with vast oceans on
either side. Clouds don't really have as much time to lose moisture,
so the regions are notably wetter than the Sahara - although if you
move north into Mexico (which, while not much wider than the rest of
Central America, is higher in elevation) or the southern United States
(wider and thus more distance for clouds to have to carry water) you
again find drier regions.
[quote="martathiax"]Gondwanaland suffered from mass glaciation at its
souther longitutes, and because there was land that spanned from north
to south poles, oceanic currents couldn't properly circulate heat
causing about 100 reactions that made the non-coastal land hot and
dry.... i don't get it... i figure cold and dry, almost like another
global glaciation, but no.... makes little sense to me.[/quote]
The polar ice cap grew and thus captured a fair degree of the water in
the seas and air, lowering sea levels and air humidity. The world as a
whole was a drier place because of this. As for hot or cold, it
depended on latitude. Nearer the poles the continents would have been
Siberian chilly, while nearer the equator they would have baked like
the Sahara. The drier air would have encouraged deserts to form and
grow, until much of the interior of the continent(s) was desert. The
coasts and the major rivers would have been the only locations where
non-arid biomes might have remained. Near the poles it would have been
vast tracks of tundra and other chill arid biomes, while elsewhere
(possibly excluding the equator, where jungle might still have existed
- at least near the coastlines and possibly some way into the
continent) deserts would have ruled supreme due to the lack of water.
Note that during the last ice age the glaciers only moved as far south
as 50N. That still leaves a lot of room before meeting the equator.
[quote="martathiax"]Good luck on your project, seems to be coming
along well.[/quote]
Thank you!
We have gotten a bit side tracked from the original question, however.
Some further research has shown that rats might have moved from Africa
to South America ~36-45 million years ago - when the continents were
still rather notably separate (although far closer than they are
today). The predominant views are that that rodents (and monkeys, for
that matter, around ~26-30 million years ago) either rafted across on
mats of vegetation on oceanic currents or that they island hopped
using islands that are no longer existent (due to erosion reducing
them below sea level, and / or an ice age having exposed them for the
brief few thousand years they were used to hop between continents).
This suggests a maximum distance traversable across an ocean for small
creatures larger than typical insects and not in and of themselves
aquatic in part or whole (such as the extinct form of hippo that once
existed in Madagascar, which is thought to have somehow made it across
the straight between the two landmasses by chance). The distance
between Madagascar and Africa, in fact, could be a limitation for
chance migration of larger creatures - if semi-aquatic - perhaps.
Around 30 Myrs ago the distance between South America and Africa was
~20% less than it is today, based upon measuring in pixels on a few
different maps of the world of that time period. (Well, more akin to
20% +/- ~3-4%.) Using Natal in Brazil and Lagos in Nigeria - which in
the present are ~4500km apart - this suggests that smaller creatures
such as mice, lizards, very small monkeys, etc might survive on a
floating raft of vegetation a trans-ocean voyage of about 3500 km.
As for larger creatures (such as the hippo example above), the
difference between Africa and Madagascar is ~400 miles. Both instances
however - the hippo lineage formerly in Madagascar and the origin of
rodents and monkeys in South America - are examples of extremely rare
chance migrations. The path was not so easy as to allow for common
passage. Only the hippo managed the migration to Madagascar, and its
semi-aquatic nature - combined with islands that potentially allowed
for island hopping (requiring more distance for the overall traversal
but decreasing the maximum time required at sea for any single
specific traversal) likely aided in this migration. For non-aquatic
creatures the distance might perhaps be halved or quartered - to a
mere 200 - 100 km.
Over millions of years only two traversals seemingly occurred between
Africa and South America over the 3500 km distance - monkeys ~26-30
Myrs and rodents ~36-45 Myrs ago. Again this suggests that this
distance may be the maximum possible for such traversals of smaller
creatures.
As my southern and eastern continents are separated by ~5000 km, this
suggests that they are not yet close enough for even smaller creatures
(other than insects, perhaps) to traverse the distances involved -
even by extreme chance.
On the other hand, the distance between the southern and northern
continents is quite less, and so it is possible that some smaller
species might have made such voyages - at least since the northern
most tip of the southern continent moved north of the equator by more
than a degree or two, thus preventing the equatorial currents from
fully isolating it from the northern continent. As it stands, it would
still be difficult for such a journey unless it occurred on the
southeastern shore of the northern continent and then drifted to the
northern shore of the southern continent. It may even require a
certain time of the year for such to occur, as winds tend to move more
north or south of the equator as summer shifts from one side of the
globe to the other.
So it is possible that some small dinosaurs - perhaps the size of a
small cat - might have (within the last ~4-5 Myrs) made the traversal
to the south continent from the northern continent, adding to the
exotic flavor of its jungle covered northern shore. Archaeic
reptilians and amphibians from the eastern continent, however, are
unlikely to have made the journey across the ocean. Even with air
currents in favor of the exchange it seems unlikely that pterosaurs
could have made the journey. Some few species of birds, however, may
make seasonal trips south, and these feathered rarities may even have
some populations on the northern shore of the southern continent.
Perhaps the situation is even reversed, and bats may similarly have
made migrations north and have limited populations on the southern
shores of the northern continent.
Maybe over the last few million years a few have even evolved to lose
their flight - leading to a new form of 'rodent' slowly making
progress in spreading over the south of the northern continent. That
could be interesting. I wonder what a rodent with echolocation would
be like?
_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/
In any case, I think I may have answered my own question (regarding
migrations). Thanks to all that commented and posted ideas or views in
regards to my first post on this topic.