#notoungulates

South America Before GABI

Classic artwork of the litoptern Theosodon and the sparassodont Borhyaena, by Charles Knight (Public Domain)

To begin with, it would help to establish the environmental situation that was North and South America prior to the formation of the Isthmus of Panama. Following the K-Pg Mass Extinction Event, the subsequent 66 million years of Cenozoic history in the Western Hemisphere have been marked by continental rifting & colliding, rising & falling sea levels, and a gradually cooling climate.

South America at the beginning of the Paleocene Epoch was actually connected to both North America and Antarctica, and in turn North America was connected to Eurasia and Antarctica was connected to Australia. In the absence of large ocean, glacial, or mountainous barriers, animals and plants could travel freely between these landmasses (Wen, et al. 2016; Croft, 2016). In fact, paleontologists recognize a similar event to GABI taking place around this time, known as the First American Biotic Interchange or FABI. This event marks the arrival of both metatherian (marsupials & kin) and placental mammals into South America from the north, while evidence shows that boas spread to North America from the south (Patterson, et al. 2012; Croft, 2016). Caimans - relatives of alligators - seem to have made the journey south but remained in the tropical waters between the continents, where they filtered to both North and South America across the Paleogene Period (Hastings, et al. 2013).

Xenarthrans - the lineage of placentals that includes sloths, anteaters, and armadillos - have a poor early fossil record but seem to be present in South America by the early Eocene Epoch at the site of Itaboraí in Brazil (Gaudin & Croft, 2015). Given that recent molecular phylogenies show that xenarthrans ally most closely with afrotherians (e.g. elephants, sea cows, tenrecs, and other ancestally African forms) in a clade called Atlantogenata, it has been proposed that the earliest xenarthrans dispersed across the Southern Atlantic Ocean between 97-84 MYA when it was completely opened but far thinner in span than at present (Tarver, et al. 2016).

Regarding the new southward mammals, we find the first appearance of the South American Native Ungulates or SANUs, which represent at least five lineages of placental mammals, including the litopterns and notoungulates which will be of particular importance here. The oldest fossils date to the earliest Paleocene and recent studies from ancientDNA of the latest surviving species suggest a close evolutionary relationship with the Odd-toed Ungulates or perissodactyls, which include horses and rhinos. A North American origin for the entire group - whose common ancestry still lacks definitive evidence - aligns well with this finding, and potential fossil relations with several incompletely-known forms on both continents may provide key evidence of their early evolution (Croft, et al. 2020). By the Eocene Epoch, the SANUs had radiated into all their major lineages. The metatherians would also come to be the most widespread and speciose mammals in early South America, diversifying into several niches. Sparassodonts in particular would go on to become the primary mammalian predators in South America, developing into cat, hyena, dog, and weasel like forms (Patterson, et al. 2012).

Sebecus, a sort of "land croc" (FunkMonk, CC BY-SA 3.0)

But it wasn't just mammals that enjoyed South America. The last descendants of a major Jurassic lineage of crocodylomorphs were the sebecids. Unlike living crocodiles and alligators, sebecids were primarily terrestrial with ziphodont teeth like knives for slicing through flesh and long limbs which held the body erect (Pochat-Cottilloux, et al. 2023). Fossils of sebecids have been found across South America - some of which reaching several meters in length - and suggest that they were one of the two major reptilian apex predators here. As for the second group, that honor goes to the phorusrhacids, popularly known as "terror birds". In fact, terror birds were just one of a number of lineages belonging to a larger clade of birds known as Cariamiformes, which is today represented by the two species of seriemas. The living and extinct members are all generally ground-living birds which mostly run down their prey and dispatch them with their raptorial feet and beak. In the larger phorusrhacids, in particular, the beak and neck functioned like a hatchet (Degrange, 2021). The oldest caramiform birds in South America date to middle Eocene and, like the sebecids, were confined to South America (Hospitaleche & Jones, 2024).

Phorusrhacos, a "terror bird" (ДиБгд, CC BY 4.0)

Conditions in South America during the Paleocene and Eocene were primarily tropical save for the Andean & Patagonian regions to the west which were warm-temperate due to their elevation. Evidence from pollen shows that modern Neotropical rainforests typical of the Amazon originated at this time (Carvalho, et al. 2021). This environmental shift between the the temperate highlands and tropical lowlands is also reflected in the fossil record (Patterson, et al. 2012). As long as sea-levels remained low, the earliest-established continental connections allowed South America to play a key role in global dispersals. Metatherians in the form of true marsupials evolved here and spread through the warmer and vegetated Antarctica to reach Australia, a finding corroborated by genetic studies and fossil data on the obscure but very adorable monito del monte Dromiciops of the Andes (Nilsson, et al. 2010; Goin, et al. 2007). There is evidence that even the terror birds reached Antarctica and assumed a major apex-predator niche there (Hospitaleche & Jones, 2024). Such crucial connections had led some researchers to propose a new recognition and model of South America's significant role in global mammalian biodiversity, "Entente Cordiale" (Agnolin, 2024).

Faunal and floral connections between the Americas had become severed by the time of the Eocene-Oligocene boundary 33.9 MYA. South America and Antarctica rifted apart, creating the Drake Passage and the Antarctic Circumpolar Current. Evidence from oxygen isotopes, deep-sea cores, and extinctions within planktonic groups supports a change in global climate towards progressively cooler conditions, and sizable glaciers began to grow in Antarctica in a series of pulses (Prothero, 2006). It is from this point that significant environmental changes seem to have affected animal and plant evolution on South America to varying degrees. Some of the older metatherian lines suffered losses due to the cooling environments, but the sparassodonts evolved into larger predators and the modern marsupial lineages (including the first opossums or didelphids) became widely established by the Miocene of 20-19 MYA (Patterson, et al. 2012). Early grassland ecosystems - the very first, in fact - had emerged in South America by 40 MYA, but because open grasslands prefer cooler conditions with enough moisture to discourage the growth of forests, they remained minor elements of the continent's biomes until about 18 MYA (Strömberg, et al. 2013). In the meantime, some of the SANUs developed hypsodont teeth (with high crowns) and diversified into new forms while the older brachydont/bunodont (low crowned & low cusped) forms shifted their ranges to the lower latitudes where broad-leafed forests remained (Patterson, et al. 2012). The sloth, anteater, and armadillo lineages had all become established by the early Miocene, with the descendants of the early sloths becoming folivorous or leaf-eating, perhaps through an omnivorous stage (Gaudin & Croft, 2015).

During this time, there were new mammalian arrivals from east that became established for the first time. First were the rodents of the clade Caviomorpha, related to the African mole rats, cane rats, and dassie rats, who show up in the fossil record by 40 MYA; followed by two lineages of primates, the parapithecids and the platyrrhines or New World monkeys by 35-30 MYA (Croft, 2016; Seiffert, et al. 2020). These mammals (save for the parapithecids) would become very large and diverse lineages of our present South American fauna, a point which paleontologists have honored by coining this period of dispersals as the Trans-Atlantic Dispersal Interval or TADI. If the arrival of xenarthrans to South America seemed simple, making a journey across ~870 miles/1400 kilometers of the Southern Atlantic 40-30 MYA would be even more challenging. The current model for how such animals made the crossing is that, at various points, storms washed out enormous raft-islands of vegetation and debris which carried straggling species over the ocean currents to the new continent, a phenomena well-documented in nature (Croft, 2016). Some of the travelers during TADI, however, seem to have gotten along without needing a raft: molecular studies show that most of South America's tortoises evolved from African ancestors that floated across the Atlantic, a feat that tortoises have no trouble with (Le, et al. 2006).

By the eve of GABI, around 16-14 MYA in the mid Miocene, the environment South America had cooled and dried significantly since the Eocene. Nonetheless, this period represented a particularly rich point in biodiversity known as the Mid-Miocene Climatic Optimum: ocean temperatures rose, much of the high latitudes were briefly freed of large ice sheets, and climates ranged from subtropical to warm-temperate (Prothero, 2006). Open grasslands grew substantially across South America and encouraged the evolution of many plains-adapted herbivores, who turned their hypsodont teeth towards the silica and grit of grass (Strömberg, et al. 2013). Across all the native South American groups was a burst in species evolution and richness alongside that of the newly-established rodents and primates, and many species of SANUs, metatherians, and xenarthrans reached greater sizes (Croft, 2016). Phorusrhacids and sebecids, too, grew to enormous sizes: Kelenken guillermoi had a skull that rivaled a horse's head and stood tall enough to look down on an adult human (Bertelli, et al. 2007); while Barinasuchus is estimated to have reached up to 13 feet/3.9 meters long (Martin, et al. 2023).

The Creation of the Isthmus

The Isthmus of Panama at 6 MYA (Woodburne, 2010)

Today the region connecting North and South America is ringed by five tectonic plates, reflecting millions of years of divergent and convergent boundaries. The South American Plate had been moving northward towards the North American Plate; the Nazca Plate - primarily basaltic oceanic crust - had been getting subducted underneath the South American Plate, actions which are responsible for the uplift of the Andes mountains; likewise the Cocos Plate - also oceanic - has been getting subducted underneath the southern tip of North America; and the status of the Caribbean Plate and its origins remains mysterious, having either emerged from the Pacific or the Atlantic side of the hemisphere (Meschede & Frisch, 2002). These complex tectonic forces have given the region its mountainous, earthquake-prone terrain, and are vital in the story of GABI.

Around 12 million years ago, an arc of volcanic islands began to form between the shrinking Central American seaway due to the collision of the Caribbean and South American Plates (Woodburne, 2010). This uplift likely coincides with the early formation of the Lesser Antilles, which, along with the neighboring Greater Antilles, marks the formation of the modern Caribbean islands and their subsequent role in GABI (Cornée, et al. 2023).

Between 8.6-7.1 MYA, the Central American Isthmus started uplifting, and by 6 MYA the region was characterized by a series of islands, some of which would become flooded from time to time. From 7.1 MYA, marine conditions began to significantly change into shallow water coastal seas, until by 2.8 MYA the Isthmus of Panama had formed as a dry land bridge (Woodburne, 2010). This change is reflected in geologic samples and fossil remains, which show that ocean chemistry and marine fossils between the Caribbean Sea and the Pacific Ocean were the same, but by the time the Panamanian Isthmus fully formed both regions developed their own distinct conditions (Prothero, 2006).

The terrestrial environment of the region was effected by the onset of increasingly cold and dry conditions since the beginning of the Oligocene that led to the full glaciation of Antarctica and the closure of the Tethys Sea and the redirection of ocean currents. Despite a brief period of warming at the start of the Pliocene Epoch, the general trend around the world was cooler (Prothero, 2006). Data from pollen samples suggest that conditions in Central America and South America bordering the Isthmus from the mid Miocene was tropical, consisting of a mosaic of dry rainforests, montane wet forests, and open grassland, the latter habitat having increased steadily by the early Pliocene. Typical floras included acacias, coquitos, cedros, and physic nuts. Such a shift in foliage may be a result of the rain-shadow effect caused by rising elevation, providing the basis for the modern "wet Atlantic/dry Pacific" climatic system of Central America (Burnham & Graham, 1999). The onset of the Quaternary Ice Age 2.58 MYA brought even colder and drier conditions around the world as the northern and southern ice sheets expanded. Tropical forest habitats around Central America shrunk during glacial periods as summer rainfalls declined, fostering the spread of dry grasslands and the subsuming of tropical dry forests, a direct opposite of conditions during the mid Miocene (Bacon, et al. 2016). Alders, oaks, willows, sweetgums, and walnuts were growing in Central America by the end of the Pleistocene (Burnham & Graham, 1999).

It is interesting to note the similarities in plant lineages across the Western Hemisphere: a 2010 analysis based on molecular divergence rates suggests that plants had been intersecting across North and South America for at least 50 million years (Cody, et al. 2010). It leads one to speculate that floral conditions on opposite sides of the Central American Seaway seemed primed for any incoming animals adapted to the habitats those species provide. One recent study seems to corroborate this: around 10 MYA a sort of "holding pen" emerged from Mexico to Panama in which northern mammals established themselves and reached a peak in local biodiversity, correlated with a spread in grassland conditions from temperate North America (Tseng, et al. 2026). Such findings will be important for later.

Waves of Expansion

Simplified diagrams of dispersals to North & South America (by the author, using a map by Martin23230, CC BY-SA 3.0)

Before we establish a timescale for the dispersals across the Isthmus of Panama during GABI, it would help to clarify that such an event was not definitive for all the organisms of the Americas. As stated before, plants seemed to have been easily prone to dispersals throughout the Cenozoic and were not necessarily restricted by the presence of a land bridge (Cody, et al. 2010). Flying birds were found to have been more restricted if they were specialized for tropical rainforests, but if they were generalists then the presence or absence of the isthmus was not a deal-breaker; swallows and wrens flew from North to South America as early as 15-13 MYA, while tanagers and flycatchers crossed South to North around 12-8 MYA, all while the Central American Seaway was still in place (Weir, et al. 2009). The same principle holds true for bees and the aforementioned caimans, for example (Wilson, et al. 2014; Hastings, et al. 2013).

Among mammals, the very first crossings also predate the formation of the land bridge. As early as 35 MYA, ground sloths crossed from South America to the Greater Antilles (Delsuc, et al. 2019), and sizable genera called Thinobadistes & Pliometanastes made the crossing to North America between 9-8.5 MYA (Woodburne, 2010). Sloths are, in fact, very capable swimmers when they need to be, and there is evidence to suggest that peculiarities in the skeleton made some sloths preadapted for aquatic life, giving rise to the amphibious Thalassocnus by the late Miocene (Amson, et al. 2014).

The next dispersals occurred around 7 MYA, this time from the north; the dog-sized Cyonasua, a member of the raccoon lineage Procyonidae, was the first predatory mammal to make the crossing (Woodburne, 2010). It has also been claimed that the sigmodontine mice and rats moved into South America by this time, though phylogenetic work suggests an even earlier expansion as early as 12.4 MYA (Salazar-Bravo, et al. 2013).

Phorusrhacids of the almost human-sized genus Titanis are found in Texas 5 MYA, and just a few hundred thousand years later more ground sloths made the crossing from South America (Woodburne, 2010). Glossotherium was widespread in the southern continent, while its relative Paramylodon made it far to the north, where it thrived on open grassland (McDonald, 2012). Between 3.9-3.3 MYA, glyptodonts & pampatheres (lineages of giant armadillos) and capybaras traveled north while peccaries, camelids, and dogs traveled south (Woodburne, 2010). The camel lineage actually evolved in North America since the Eocene, and their appearance in South American fossil record is marked by the genus Hemiauchenia, which may have given rise to the modern llamas and their relatives (Gasparini, et al. 2017). Dogs of the clade Cerdocyonina, likewise, underwent an extraordinary radiation once they crossed into South America, ranging from fox-like forms to the bush dog and maned wolf (Chavez, et al. 2022).

A selection of Brazilian faunas, with emphasis on immigrants from North America above and native South Americans below (Julio Lacerda, Dantas, et al. 2022)

Population expansions and migrations begin to significantly pick up speed once the Isthmus of Panama had fully formed 2.8 MYA. In fact, for many paleontologists, this is when GABI truly begins to take shape, for it is due to the drying of the climate and the expansion of grasslands that many of the animals of North and South America were able to make their crossings. Between 2.6-1.8 MYA, huge pulses of placental mammals crossed from Central America into South America, including Hippidion horses, deer, tapirs, and gomphotheres (relatives of elephants) like Stegomastodon and Cuvieronius, as well as continued dispersals of peccaries and camels like Palaeolama. Placental carnivores made serious advances too, including cats, bears, and mustelids (Woodburne, 2010). Neotropical deer belong to the clade Odocoileini and are widespread today from rainforests to wetlands; DNA evidence suggests that deer crossed into South America at least eight times during GABI, with the genus Odocoileus (white-tailed & mule deer) even crossing back into North America (Duarte, et al. 2008). The arrival of gomphotheres in South America marks the last continental region that the proboscideans (elephant lineage) reached, having adapted to a wide range of habitats, including the Andes region (Alberdi, et al. 2022). Smilodon and Homotherium, the most familiar of the saber-toothed and scimitar-toothed cats, respectively, settled in South America by this time, thriving in both forests and grasslands as they hunted the largest mammals (Rincón, et al. 2011).

It was only by 1.5-1.3 MYA that SANUs reached North America in the form of the notoungulate Mixotoxodon, which spread as far as Texas (Lundelius Jr, et al. 2013). Sloths, armadillos, and (eventually) anteaters had by now established themselves widely across southern North America. The glyptodonts spread as far north as Oklahoma (Zurita, et al. 2011), while the nine-banded armadillo Dasypus has diversified into four species (Barthe, et al. 2024).

The last million years have consisted of sporadic but continuous dispersals between North and South America. Many marsupials had crossed into North America, including the opossum Didelphis by 800 KYA, which had spread in pre-colonial times as far north as the Mid-Atlantic states. Coatis and cottontail rabbits (Sylvilagus) crossed into North America 125 KYA, as did the modern horse genus Equus. Gray foxes crossed into South America as recently as 25 KYA (Woodburne, 2010). And, of course, Homo sapiens and their domestic dogs were definitively present in Central and South America by 15 KYA, though there are contested claims of earlier arrivals by tens of thousands of years (Raff, 2022).

So What Happened to the South Americans?

Chile during the Late Pleistocene (Jorge González, CC BY-SA 4.0)

Having examined the timing of arrivals for immigrant species across GABI, we have to ask if they correlate with the displacement or extinction of native South American faunas of interest, like the SANUs, phorusrhacids, etc.

By examining pre-GABI (prior to 2.8 MYA) faunas, it seems that manynative South American species had little trouble settling on the North American continent. Xenarthrans like ground sloths and armadillos settled widely across tropical and temperate North America early on and have remained mainstays of the biota right on through the Pleistocene (Patterson, et al. 2012). In fact, a sizable number of the migrants to the north were xenarthrans, which continuously moved into the continent throughout the entire span of GABI.

A key factor in properly analyzing GABI is the recognition that Mexico and Central America belong to the North American continent, a aspect of natural history often forgotten by specialists and non-specialists alike (consider, for example, how many "field guides to North American birds" exclude Mexico). When surveying the diversity of South American-derived faunas in Central America from the dawn of GABI to the present day - with a myriad of species of marsupials, rodents, monkeys, and xenarthrans existing right into Mexico and beyond - an argument can be made that the interchange of faunas was much more even than is generally understood. This is one crack in the model that South Americans were negatively affected by GABI.

As well, when examining the extinction rates of native South Americans across time, one finds that many of the endemic animals were already on the decline long before the emergence of the Isthmus of Panama. Sebecids were long members of the predator guild on mainland South America, but by the early Late Miocene (~11 MYA) they had all but died out, having gradually been restricted to tropical regions in the lower latitudes with the cooling climate. Only in the Greater Antilles did they hold out until 4.5 MYA (López, et al. 2025). Similarly, phorusrhacids and the other giant cariamiform birds were part of the predator guild. There is evidence that changing climates affected terror birds, with some of the ambush-hunting species dying out with the spread of open woodlands, which encouraged other pursuit-hunting species to radiate and grow in size (LaBarge, et al. 2024). As previously mentioned, one lineage of phorusrhacid participated in GABI, Titanis, which established itself in Texas by 5-4.7 MYA (Woodburne, 2010). Having subsequently spread as far east as Florida, it coexisted with saber-toothed cats and other predators into the early Pleistocene, and records vanish by 1.8 MYA. Nonetheless, recent evidence suggests that terror birds may have survived as recently as 96 KYA in Uruguay and 25 KYA in Brazil (Jones, et al. 2017; Machado, et al. 2026). Thus, terror birds seem to have done well enough as their ecosystems incorporated more placental mammals, even predators, and were in fact just one of many giant predatory birds which inhabited the Americas during the Pleistocene, and adapted to changing conditions until the last species petered out.

The sparassodont metatherians - the other great predators of South America - had been in decline since the middle Miocene, and the last genera died out between 3-2.5 MYA, including the saber-toothed Thylacomilus atrox (López-Aguirre, et al. 2016). This was right at the earliest beginnings of placental carnivore dispersals, but again the evidence of prior extinctions in the Miocene do not show that these animals competed. In fact, Thylacosmilus, while resembling a saber-toothed cat like Smilodon, seems not to have behaved very closely to one: a 2021 study of functional morphology suggest this animal killed prey in a similar manner but preferred internal organs to muscle tissue and bone, and may have avoided scavenging kills by other predators (Melchionna, et al. 2021). Sparassodonts appear to have been major predators of the SANUs, with a lock-step peaks in diversity across their history, which means that they were vulnerable to declines in SANU diversity. These periods of low diversity and extinctions were punctuated by the immigration of African rodents and primates, as well as the native evolution of marsupials like opossums, and as these animals took over vacant niches they may have prevented sparassodonts from exploiting them, thus contributing to their overall extinction, what is known as "non-competitive ecological interaction" (López-Aguirre, et al. 2016).

Comparison between the skulls of a saber-toothed cat and the sparassodont Thylacosmilus (Michael Long, CC BY 4.0)

The case for the SANUs is particularly fascinating, because historically it has been assumed that the great diversity of this group converged on body plans and behaviors of other placental mammals in, say, North America and Africa. It is true that many of these animals would have resembled elephants, rhinos, horses, deer, camels, aardvarks, and hares, but upon closer examination the uniqueness of the SANUs is revealed. Some of the litopterns which adapted to running in open environments developed similar digitigrade hooves and knee-locking mechanisms to horses and antelope, but had evolved their long limbs in a different direction than them; meanwhile some of the notoungulates like Toxodon have been compared to hippos and rhinos, yet they seem to have fed broadly on both brush and grasses, with tall curved teeth more akin to rodents. Even if they were only slightly similar to other placental herbivores, nonetheless they too faced a gradual extinction long before GABI. Some lineages, like the pyrotheres, were gone by the Oligocene, while the litopterns and notoungulates retained their levels of diversity right on through the Miocene despite a 45% reduction in clades. Even through GABI, the number of genera remained generally the same, and several SANUs were components of the Late Pleistocene fauna of South America, including Toxodon and Macrauchenia (Croft, et al. 2020).

So, clearly, what we see is a long-term cooling of the climate and changing habitat compositions during the late Paleogene and on through the Neogene which had been chipping away at the diversity of native South Americans without any real contribution from GABI, and even then it was unevenly selective of clades (e.g. the longterm survival and spread of xenarthrans). The dispersals between North and South seem to have been even, and the diversity of mammalian faunas in South America actually increased as a result of GABI, as seen in the composition of ecosystems there today. It seems the the apparent loss of ground sloths, glyptodonts, notoungulates, litopterns, and terror birds had more to do with the end of the Pleistocene than the interchange. Despite the role of climate changes affecting the South Americans, a wide review of the evidence points the blame mainly on Paleolithic Homo sapiens rather than the end of the last glacial maximum ~11 KYA. Around the world, wherever humans traveled, settled, and hunted, sharp declines in megafauna are noted: across both North and South America, all of the ground sloths, glyptodonts, proboscideans (gomphotheres, mastodons, and mammoths alike), horses, SANUs, saber-toothed cats, and giant tortoises were wiped out. There is even evidence that members of surviving species bigger than 22 lbs experienced declines in population and body-size (Svenning, et al. 2024).

Even so, the late Pleistocene extinctions were still not even the final death-nail for the native South Americans. There is growing evidence of mainland survival for many of the native South Americans: the giant ground sloths and notoungulates may have clung on in Brazil alongside saber-toothed cats, camels, and gomphotheres between 7.8-3.5 KYA (Faria, et al. 2025). And, on the Caribbean islands, ground sloths like Megalocnus survived until ~4.2 KYA (MacPhee, et al. 2007).

So GABI did not wipe out the native South American animals; the affected taxa were either already on their way out, or they were driven to extinction by human activities.

Lastly, a word needs to be said about competition in nature. It is often imagined that competition is one of the driving forces of evolution by natural selection: that when two species meet up in an environment and they share the same resource, the one with the best advantages (whatever they may be) will outcompete the other, leaving it to the mercy of extinction. This "competitive exclusion principle" is usually ascribed to Russian biologist Georgy Gause but has been known in scientific circles since the time of Darwin (Hardin, 1960). As a theoretical model, it makes sense, when actually applied to observations in nature it rarely holds up. It turns out that, when given a situation where species might be forced to compete for resources, what ends up happening is that one competitor soon adapts to a different resource, adapts to a different use of that resource (what is known as "resource partitioning"), or disperses to a different environment where their competitor species isn't found (Williams, et al. 2025). In a famous example, many similar species of warblers with similar diets can coexist in North American forests because they access their food at different parts of the trees. As exciting as images of dueling terror birds and saber-toothed cats are, they do not accurately reflect the way organisms evolve in their environment: these great predators found ways to coexist on both continents without needing to fight each other for control of their niches.

One may attempt to use GABI as a lesson about invasive species, but as I've shown extensively in this article, the forces of geologic and environmental change at work in the Americas that allowed different organisms to disperse north and south is not correlated with species extinctions or increases in competition in the way we see during the Anthropocene Event. Labeling such range shifts and migrations as "invasions" is thus inaccurate; species today, as a result of anthropogenic climate change, are expanding their ranges northward as their home ranges become drier and hotter (Bolotnikova, 2021). This is no different than what happened during GABI: as environments changed, organisms followed shifts in their habitats or adapted to new ones when they became available. It's very likely that we'll see new dispersals north and south, similar to those that occurred during GABI, but whether these species can take root is questionable given the massive scale and rate of habitat loss, urban development, and agricultural growth, which limits species movements. Humans wiped out many of the successors of GABI, and unless strong action is taken, the rest are sure to follow.

Book References

Federico Agnolin - History of Cenozoic Mammals of South America: A New Model (Springer Earth Systems Sciences, 2024)

Darin A. Croft - Horned Armadillos & Rafting Monkeys: The Fascinating Fossil Mammals of South America (Indiana University Press, 2016)

Bruce D. Patterson (editor), et al. - Bones, Clones, & Biomes: The History and Geography of Recent Neotropical Mammals (University of Chicago Press, 2012)

Donald Prothero - After the Dinosaurs: The Age of Mammals (Indiana University Press, 2006)

Jennifer Raff - Origin: A Genetic History of the Americas (Grand Central Publishing, 2022)

George Gaylord Simpson - Splendid Isolation: The Curious History of South American Mammals (Yale University Press, 1980)

Jessica Williams (consultant), et al. - The Ecology Book: 2nd Edition ("Big Ideas Simply Explained" series, Dorling Kindersley, 2025)

Paper & Article Citations

María T. Alberdi, et al. 2022. Diversity of the fossil gomphotheres from South America (Historical Biology)

Eli Amson, et al. 2014. Gradual adaptation of bone structure to aquatic lifestyle in extinct sloths from Peru (Proc Biol Sci)

Christine D. Bacon, et al. 2016. Quaternary glaciation and the Great American Biotic Interchange (The Geological Society of America)

Mathilde Barthe, et al. 2024. Exon Capture Museomics Deciphers the Nine-Banded Armadillo Species Complex and Identifies a New Species Endemic to the Guiana Shield (Systematic Biology)

Sara Bertelli, et al. 2007. A new phorusrhacid (Aves: Cariamae) from the Middle Miocene of Patagonia, Argentina (Journal of Vertebrate Paleontology)

Marina Bolotnikova, 2021. It's time to stop demonizing "invasive" species (Vox)

Robyn Burnham & Alan Graham, 1999. The History of Neotropical Vegetation: New Developments and Status (Annals of the Missouri Botanical Garden)

Mónica R. Carvalho, et al. 2021. Extinction at the end-Cretaceous and the origin of modern Neotropical rainforests (Science)

Daniel E. Chavez, et al. 2022. Comparative genomics uncovers the evolutionary history, demography, and molecular adaptations of South American canids (PNAS)

Sarah Cody, et al. 2010. The Great American Biotic Interchange revisited (Ecography)

Jean-Jacques Cornée, et al. 2023. Paleogeographic evolution and vertical motion of the central Lesser Antilles forearc since the Early Miocene: A potential driver for land fauna dispersals between the Americas (Marine and Petroleum Geology)

Darin A. Croft, et al. 2020. Splendid Innovation: The Extinct South American Native Ungulates (Annual Review of Earth and Planetary Sciences)

Mário André Trindade Dantas, et al. 2022. Isotopic paleoecology (δ13C, δ18O) of a late Pleistocene vertebrate community from the Brazilian Intertropical Region (Revista Brasileira de Paleontologia)

Federico J. Degrange, 2021. A revision of skull morphology in Phorusrhacidae (Aves, Cariamiformes) (Journal of Vertebrate Paleontology)

Frederic Delsuc, et al. 2019. Ancient Mitogenomes Reveal the Evolutionary History and Biogeography of Sloths (Current Biology)

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