u/Psilopterus

https://c.org/8x54SqDRZF

We have previous few interglacial sites with which to answer this question, though there are a couple in southern Ontario and Indiana, as well as sites that would have had cool-temperate climates during glacials elsewhere in the US. Using these and co-occurrences, I've made a list of 23 large mammals that I think would have been present in the Great Lakes region of the US and Canada during the last interglacial, some with greater levels of certainty than others.

It's an important question to investigate because this region today has a very similar climate to that of central/northern Europe, where it has been demonstrated that during the last interglacial (when the climate was similar to today but when modern humans were absent and the megafauna consequently still diverse and abundant) open and lightly-forested conditions were enforced by large mammals. It is reasonable to think that our own megafauna here may very well have done the same.

While we don't have nearly as much data with which to determine this in North America as there is in Europe, we do have some indications. For example, the vast majority of (note-uncorrected) pollen from the interglacial record at the Don Valley site in Toronto come from three groups - oaks, hickories, and pines. These are all notably light-demanding trees that, in the modern day, require thinning or prescribed/cultural burning to remove competition from more shade-tolerant trees like maple, beech, and hornbeam. The success of oak, hickory, and pine in silvopasture would suggest that megafauna might once have provided this disturbance service instead. Interestingly, the Don Valley site also preserves macrofossils of three "megafaunal fruits" - plants with fruit/seeds that are not effectively dispersed by living species - osage orange, honey-locust, and pawpaw.

Note for the chart - moose and elk are listed separately because they are recent immigrants to North America that were not present during the last interglacial.

Today, the Komodo dragon (Varanus komodoensis) is confined to the islands of Komodo, Rinca, Flores, Gili Dasami, and Gili Motang, all in the Indonesian Lesser Sundas. In prehistory, however, this species and its even larger relatives were much more widespread, occurring on other large islands in the Lesser Sundas like Sumba and Timor, as well as Java (in the Greater Sundas) and mainland Australia.

Although most later fossils of dragons in Australia belong to a much larger species (Varanus prisca) and (in Central Australia) an as-yet unnamed and intermediately-sized species, fossils from the Pliocene and Early-Middle Pleistocene of Australia are attributable to the living species. These earlier dragons would have colonized the Sundas by water, perhaps at a time when low sea levels made the distance shorter. Timor seems to have been colonized separately by a larger form, perhaps similar to those from Central Australia.

It has at various times in the past been suggested that dragons could be reintroduced to Australia. I think this is an interesting idea by virtue of the fact that it would be reintroducing an endangered lineage to a continent that has experienced a lot of extinction, and which retains very few native megafauna. However. I disagree with the oft-suggested premise that it would help reduce numbers of introduced ungulates like pigs and buffalo. It has been demonstrated that dragons have no meaningful effect on the populations of these species where they already overlap due to their low metabolic requirements and preference for scavenging.

Still, it is interesting to imagine the large animals that giant monitors would have interacted with in the past - giant marsupials in Australia, pygmy elephants in the Lesser Sundas, any number of large mammals in Java. There was also over 40,000 years of time where dragons were still extant in Flores and surrounding islands but where modern humans had eradicated all other native megafauna (aside from crocodiles) until pigs, deer, and other species were introduced to Flores by people in the Holocene.

An interesting factoid that I like to throw around sometimes is that camels, much like horses, originated in North America. Six million years ago, camels of the genus Procamelus crossed the Bering strait into Eurasia where they would later produce the genus Camelus, to which all one- and two-humped camels belong. Camels did not disappear from North America as soon as they'd left the continent, however, and by the time humans arrived, there were still North American camels, specifically the "Western" or "Yesterday's" camel Camelops hesternus.

Although originally thought to be a very large llama, Camelops had its DNA sequenced in 2016, which demonstrated that it was in fact a true camel, having split from the ancestors of the living camels about eleven million years ago. This ultimately makes a lot of sense because the extinct species was very similar the Old World camels, being of similar size (400-1000 kg) and, at least according to isotopic studies and other proxies, diet.

Camelops had a very wide distribution occurring from Alaska and the Yukon in the North to Guatemala in the South. The only places they seem notably absent are those areas where precipitation would have been very high, which makes sense given that they were, after all, camels. Their extremely wide temperature tolerance makes me skeptical of climatic explanations for their extinction (as I am for most climate-driven extinction scenarios where megafauna are concerned), and it is notable that this is a species we know to have been hunted by people.

Given how recent the extinction of this species occurred (the date doesn't even register on the timescale below, on which it is accurately plotted), it is relevant to wonder how the extinction of this species would have affected North American ecosystems. Camels preferentially browse succulent and thorny plants over grasses, unlike cattle and horses. Domestic camels imported to the southwestern US in the 1800s were reported to browse on saltbush, pricklypear, mesquite, and creosote, all plants that can easily crowd out rangelands in the south today, reducing biodiversity and creating a fire hazard.

Note for Plot - blue lines represent points where lineages left North America

Macaques have been present in Europe for millions of years, withe the last populations disappearing only during the last glacial period, between 85-40,000 years ago. Continuously present in southern Europe, during warm interglacials much like the current one they would colonize central and northern Europe as well. Interestingly, the last macaques in Europe were not an extinct species, but the same as now occupy small parts of northwestern Africa, i.e. the Barbary macaque Macaca sylvanus. Some other forms have been named from European fossils, such as M. florentina or M. majori, but the former seems to be simply a progenitor of the living species and the latter and island offshoot known only from Early Pleistocene Sardinia.

Exactly why the species disappeared from Europe is unclear. Certainly the last glacial would have pushed them back to southern Europe - Iberia, Greece, Italy, etc., but that had occurred multiple times previously. Perhaps newly arrived modern humans were responsible for their ultimate demise - people do hunt monkeys and there is some archaeological evidence to suggest hunting of monkeys in Greece and Italy. Perhaps the Atlas mountains, their current stronghold, simply offered more protection by restricting access to people while also remaining temperate through glacial periods. I'm not really sure, especially since other North African populations, e.g. in Tunisia, apparently survived well into the Late Holocene.

Vampire bats are an interesting example of a species who are both clearly adapted to prehistoric conditions and very well-suited to the world they now find themselves in. As the name would suggest, vampire bats primarily drink blood, and the common vampire bat (Desmodus rotundus) specifically drinks the blood of mammals. While they will feed from native mammals in the Americas where they occur, such as deer, capybara, peccary, and tapir, they are far more likely to feed on domestic animals that have been introduced to the Americas only in the past 500 years, such as cattle, horses, and pigs. It has been suggested that vampire bats preferentially feed on larger mammals because that was their original niche - to feed on now-extinct large mammals like ground sloths and toxodons.

Two larger species in the same genus also existed in the Late Pleistocene, both about 25% heavier, and are also thought to have fed on large mammals. One of these (D. draculae) was sympatric with the living species and also dispersed over Central and South America. The other (D. stocki) instead ranged from the north of Mexico into what is now the US, with fossils ranging into northern California and West Virginia. The living species was also previously found in Cuba, where they likely fed on dwarf ground sloths like Megalocnus.

Vampire bats have benefited greatly from the introduction of livestock and have even expanded their distribution, including into the former range of their northern counterparts. It is expected that this expansion will continue, with vampire bats set to arrive in the southern US in the very near future.

California condors (Gymnogyps californianus) are one of the greatest conservation success stories on Earth, with over 600 individuals now from a low of 22 in 1982. While the historic near-extinction of this species was mostly due to persecution by collectors and ranchers and lead poisoning, the latter still very much in play, the fossil evidence would suggest that declines have been going on for a very long time.

During the Pleistocene, condors in this genus were much more widespread, but when most terrestrial megafauna went extinct near the end of the Pleistocene, the condor's major food source was eliminated, leading to their widespread extinction. Condors survived only on the Pacific coast, where carcasses from marine mammals like whales and sea lions could support their need for carrion. During the 18-19th centuries they seem to have experienced an expansion out of this range, probably due to the introduction of livestock such as sheep and cattle, before their further reduction from hunting and poisoning.

While all mainland North American condor fossils in this genus have been assigned to either the living species or its larger conspecific/predecessor (G. (c.?) amplus), two additional species are known from the Late Quaternary - one from Peru (G. howardae) and one from Cuba. (Gymnogyps varonai). Exactly when the Cuban species went extinct is unclear, but it is possible that it survived well into the Holocene, disappearing when its likely source of carrion, Caribbean ground sloths, were hunted to extinction.

Even just looking at the living, mainland species, the fossils suggest a very high climatic tolerance. The New York fossil was deposited at a time when the site would have had a boreal climate and supported spruce and jack pine, for example. Fossils are sufficiently widespread, especially given how badly birds generally fossilize, that it is reasonable to assume Gymnogyps condors were fairly ubiquitous across North America, as well as the Antilles, northern South America, and quite possibly Central America, the only limiters being a source of carrion and possibly the availability of nest sites.

Many species that we now associate mostly with grassland habitats in North America occurred much further east at other points in the Late Quaternary. Some of the IUCN ranges included here definitely also include areas of recent expansion due to agriculture and do not necessarily reflect pre-Columbian distributions. I couldn't plot out an equivalent for horned larks (Eremophila alpestris) despite a similar fossil record because the IUCN already considers their post-1800 CE eastward expansion as part of their natural range. The same is true of brown-headed cowbirds (Molothrus ater).

Although the presence of these species almost certainly indicates more open conditions during the Late Pleistocene, the explanation for this is unclear. Several authors when discussing these species records suggest that a drier climate during the last glacial must be responsible. The issue there is that, as far as climatic reconstructions show (e.g. PaleoClim), precipitation during this time was actually higher, with cooler temperatures also lowering potential evaporation. Nevertheless, parkland habitats were widespread in the east at this time, leaving open the possibility that extinct large herbivores might have created much more open conditions.