At various times during the last few million years there have been glacial events that lowered sea level enough for a land bridge to exist from the Asian mainland to Borneo, Java, Sumatra, and similarly between Australia and New Guinea and the Aru Islands. To get from Asia to Australia you would still need to cross water, and at maximum glaciation there are paths you could take that would require no single water crossing to be more than 57 km.

Humans got past these barriers in prehistoric times but large terrestrial mammals could not. The Wallace line roughly marks the maximum walkable edge of the Sunda Shelf: big apes, big terrestrial carnivores, big ungulates did not make it to the east of the line until humans brought them. The Lydekker line roughly marks the edge of the Australiasian continent: large terrestrial marsupials did not make it to the west of this line. Between these lines are islands that a pedestrian could not reach from either Australia or Asia even at max glac: Timor, Sulawesi, Halmahera, Seram. That middle area is sometimes called Wallacea.

The reason I specify “large terrestrial” is that small and/or arboreal mammals did cross the lines. It is usually thought that this happened because of animals floating on fallen branches and whatnot. Aside from bats, the only kind of placental mammal that crossed from Asia, past the Wallace and Lydekker lines, into Australasia is the rodents. There are scores of rodents native to Australia: all rats and mice that are descended from critters that have made their way over here during the past 10 million years or so. There are no Australiasian-derived mammals that made it to the west of the Wallace line.

There are certain other mammals that made it past one of these lines but not the other.
—-
From Asia to Wallacea:

People deliberately brought buffalo, dogs and pigs etc to Wallacea thousands of years ago, and introduced civets there in the 19th century for some reason. The following groups of non-rodent placental mammals reached Wallacea without human help:

The shrews. About a half-dozen species of shrew are found throughout Wallacea. Shrews are small and arboreal and would be a prime candidate for marine dispersal on foliage.

The tarsiers. These startled-looking primates are only about 100 grams a piece, so again, easy for them to logride.

The macaques. The crab-eating macaque made it to Timor, and there are several different species of macaque endemic to Sulawesi. It’s interesting, because macaques are typically 5 to 10 kg as adults, so you’d imagine it would be a pretty serious chunk of tree that would carry them across the straits. “Origin of the Sulawesi macaques as suggested by mitochondrial DNA phylogeny” (Evans et al 1999) found that there were likely only two founding events, each of which could have involved very few individuals (in extreme, I suppose, a single female carrying a male fetus). Over millions of years, very rare things might happen occasionally.

—-
From Australasia to Wallacea

What about traffic in the other direction?
About 20 million years ago, before Australiasia got close to Asia, its only mammals were monotremes and marsupials. (At that time the monotremes had gone extinct in the Americas, though the marsupials lived on.) Some 15 million years ago, the first bats arrived from Asia: 10 million years ago, the first rats and mice.
There are echidnas on Salawati, an island narrowly separated from New Guinea’s western shore, but this island would have been connected to New Guinea in near-historical times. No monotremes exist to the west of Lydekker’s line.

Quite a few marsupials made it to Wallacea, however, and as expected most are small and/or arboreal.

Cuscus: some 7 species of cuscus live in various parts of Wallacea, including Timor, Sulawesi and Halmahera, including some only found there. Cuscus usually weigh a few kg. They are tree-dwelling.

Bandicoots: Long-nosed spiny bandicoot exists on the Kai islands. Seram bandicoot is endemic to, wait for it, Seram. They are both around 1 kg. Not tree-dwelling.

Sugar gliders: exist on Halmahera, smol (140 grams), treeish.

Pademelons: the dusky pademelon exists on the Kai islands. Unusual for this group, as it is on the large size (5 kg) and not tree-dwelling.

—-

It does make me wonder why the shrews and tarsiers, with their ratty size and habits, didn’t spread to Australasia like the rodents: and for that matter, why the little sugar gliders never lucked from Sulawesi to Borneo. Perhaps they did, but were unable to compete. Or perhaps the tree just never fell that way.

^{map broadly based on work of Teresa Zubi}

dv said:

At various times during the last few million years there have been glacial events that lowered sea level enough for a land bridge to exist from the Asian mainland to Borneo, Java, Sumatra, and similarly between Australia and New Guinea and the Aru Islands. To get from Asia to Australia you would still need to cross water, and at maximum glaciation there are paths you could take that would require no single water crossing to be more than 57 km.

Humans got past these barriers in prehistoric times but large terrestrial mammals could not. The Wallace line roughly marks the maximum walkable edge of the Sunda Shelf: big apes, big terrestrial carnivores, big ungulates did not make it to the east of the line until humans brought them. The Lydekker line roughly marks the edge of the Australiasian continent: large terrestrial marsupials did not make it to the west of this line. Between these lines are islands that a pedestrian could not reach from either Australia or Asia even at max glac: Timor, Sulawesi, Halmahera, Seram. That middle area is sometimes called Wallacea.

The reason I specify “large terrestrial” is that small and/or arboreal mammals did cross the lines. It is usually thought that this happened because of animals floating on fallen branches and whatnot. Aside from bats, the only kind of placental mammal that crossed from Asia, past the Wallace and Lydekker lines, into Australasia is the rodents. There are scores of rodents native to Australia: all rats and mice that are descended from critters that have made their way over here during the past 10 million years or so. There are no Australiasian-derived mammals that made it to the west of the Wallace line.

There are certain other mammals that made it past one of these lines but not the other.
—-
From Asia to Wallacea:

People deliberately brought buffalo, dogs and pigs etc to Wallacea thousands of years ago, and introduced civets there in the 19th century for some reason. The following groups of non-rodent placental mammals reached Wallacea without human help:

The shrews. About a half-dozen species of shrew are found throughout Wallacea. Shrews are small and arboreal and would be a prime candidate for marine dispersal on foliage.

The tarsiers. These startled-looking primates are only about 100 grams a piece, so again, easy for them to logride.

The macaques. The crab-eating macaque made it to Timor, and there are several different species of macaque endemic to Sulawesi. It’s interesting, because macaques are typically 5 to 10 kg as adults, so you’d imagine it would be a pretty serious chunk of tree that would carry them across the straits. “Origin of the Sulawesi macaques as suggested by mitochondrial DNA phylogeny” (Evans et al 1999) found that there were likely only two founding events, each of which could have involved very few individuals (in extreme, I suppose, a single female carrying a male fetus). Over millions of years, very rare things might happen occasionally.

—-
From Australasia to Wallacea

What about traffic in the other direction?
About 20 million years ago, before Australiasia got close to Asia, its only mammals were monotremes and marsupials. (At that time the monotremes had gone extinct in the Americas, though the marsupials lived on.) Some 15 million years ago, the first bats arrived from Asia: 10 million years ago, the first rats and mice.
There are echidnas on Salawati, an island narrowly separated from New Guinea’s western shore, but this island would have been connected to New Guinea in near-historical times. No monotremes exist to the west of Lydekker’s line.

Quite a few marsupials made it to Wallacea, however, and as expected most are small and/or arboreal.

Cuscus: some 7 species of cuscus live in various parts of Wallacea, including Timor, Sulawesi and Halmahera, including some only found there. Cuscus usually weigh a few kg. They are tree-dwelling.

Bandicoots: Long-nosed spiny bandicoot exists on the Kai islands. Seram bandicoot is endemic to, wait for it, Seram. They are both around 1 kg. Not tree-dwelling.

Sugar gliders: exist on Halmahera, smol (140 grams), treeish.

Pademelons: the dusky pademelon exists on the Kai islands. Unusual for this group, as it is on the large size (5 kg) and not tree-dwelling.

—-

It does make me wonder why the shrews and tarsiers, with their ratty size and habits, didn’t spread to Australasia like the rodents: and for that matter, why the little sugar gliders never lucked from Sulawesi to Borneo. Perhaps they did, but were unable to compete. Or perhaps the tree just never fell that way.

^{map broadly based on work of Teresa Zubi}

Apparently there are very strong ocean currents along the Wallace Line that would have stopped most animals from crossing. Perhaps rodents generally being very common animals, a few might have just struck it lucky after many attempts.

dv said:

…

It does make me wonder why the shrews and tarsiers, with their ratty size and habits, didn’t spread to Australasia like the rodents: and for that matter, why the little sugar gliders never lucked from Sulawesi to Borneo. Perhaps they did, but were unable to compete. Or perhaps the tree just never fell that way.

^{map broadly based on work of Teresa Zubi}

> The Lydekker line roughly marks the edge of the Australiasian continent: large terrestrial marsupials did not make it to the west of this line. Between these lines are islands that a pedestrian could not reach from either Australia or Asia even at max glac: Timor, Sulawesi, Halmahera, Seram. That middle area is sometimes called Wallacea.

Thanks for that clarification. I hadn’t known.

The Wallace line marks the western edge of the Australasian fauna. It is not based on the Eurasian fauna.

It seems to me that whenever Australasian and Eurasian fauna met, the Eurasian fauna tended to dominate. This seems to be a general rule. When fauna from a smaller geographical area meets that from a larger geographical area, the fauna from the larger geographical area tends to dominate. And the fauna from the smaller geographical area tends to die out.

Eurasian fauna made it to Australia several times. There are two orders of bats in Australia, with fossils dating back 15 million years. Riversleigh has a good collection of microbat fossils, along with its megafauna marsupials and monotremes. About 5 to 10 million years ago, the old endemic rodents came to Australia and diversified into 14 species, including the rakali. About a million years ago, the new endemics arrived, diversifying into 7 species of rat. The dingo arrived here about 3,500 years ago. All told, there are more placental mammal species in Australia than marsupials. That count is cheating a bit because it includes marine mammals but, even without counting the cetaceans, the numbers of placental mammal species and marsupial mammal species are similar.

I have to profess almost complete ignorance when it comes to the early migration of Eurasian reptiles into Australia. Ditto insects and plants. Is there a good web link for this?

What is the lizard population of Wallacia, other than the obvious komodo dragon?

> It does make me wonder why the shrews and tarsiers, with their ratty size and habits, didn’t spread to Australasia like the rodents: and for that matter, why the little sugar gliders never lucked from Sulawesi to Borneo. Perhaps they did, but were unable to compete. Or perhaps the tree just never fell that way.

Wasn’t there always a seaway open between Eurasia and Australasia? The Wallace line was never a land bridge, so anything that did make it across would have to either fly, be a good swimmer, or travel across the ocean some other way (eg. on a raft of vegetation or attached to birds). The bats made it because they could fly. The rodents made it because they are strong swimmers. Shrews, tarsiers and sugar gliders are not known for being strong swimmers.

https://www.theguardian.com/science/2022/jul/20/scientists-picking-over-ice-age-bones-discover-vultures-once-soared-in-australias-skies

Link

mollwollfumble said:

.

Wasn’t there always a seaway open between Eurasia and Australasia? The Wallace line was never a land bridge, so anything that did make it across would have to either fly, be a good swimmer, or travel across the ocean some other way (eg. on a raft of vegetation or attached to birds). The bats made it because they could fly. The rodents made it because they are strong swimmers. Shrews, tarsiers and sugar gliders are not known for being strong swimmers.

I mention all that in the text

> I mention all that in the text.

Then you may find this interesting.

The Sailfin dragons (Hydrosaurus) have a sail on the top of their tail. They appear to be true Wallacean fauna. Although their occurrence crosses both the Wallace and Lydecker lines, their heartland is Wallacea.

Hydrosaurus amboinensis is found in Seram and New Guinea.
Hydrosaurus celebensis & Hydrosaurus microlopus are found in Sulawesi
Hydrosaurus weberi is found on Halmahera
Hydrosaurus pustulatus is found in the Philippines