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1 Named after one of the “seven deadly sins,” sloths are, no doubt, considered one of the laziest animals in the world. In fact, a scientific journal in 1749 labelled them as the “lowest form of existence around.” A rather harsh description of these adorable animals!
Sloths are far from the simple and lazy animals we’ve thought them to be. They are mysterious and complex creatures with an evolutionary history so profound that some people might even have trouble believing it; giant ground sloths were wandering the Earth at one point in time!
The sloths throughout the world today are categorized into six main species. These six species are divided into two genera: the Choloepus (the two-fingered sloths) and the Bradypus (the three-fingered sloths). Spanning over two and four species, respectively, these two genera define the sloths that we all know and love.
However, the current number of sloth species is only a small fraction of what existed during the Cenozoic era. In fact, the six currently extant species result from an elevated rate of speciation spurred on by the extinction of greater terrestrial life forms, which took place toward the end of the Quaternary period. They are indeed one of the most fascinating animals of ice age. Let’s dive into sloth history!
Featured image via Eden, Janine and Jim on Flickr
Giant Sloth Species and Families
More than 100 different sloth genera of the Cenozoic age have been documented, which include larger species of the Pleistocene and Pliocene (commonly known as the giant ground sloths of the Ice Age).
The majority of the extinct Quaternary sloths have become extinct relatively recently, such that there are a lot of remains left behind in the form of skin fragments, teeth, bones, hair, paleofeces, and keratinous sheaths.
With this amount of DNA left behind, sloths are considered the ideal target group for radiation unlocking by using ancient DNA.
Using data methods to conduct phylogenetic analysis (the examination of evolutionary development) of all the xenarthran mitogenomes (Mitochondrial DNA of the Xenarthra superorder, which includes sloths, armadillos, and anteaters), more than ten ancient sloth genomes, which represent the six extinct genera of ancient sloths, were assembled. These include:
- Megalonyx
- Mylodon
- Nothrotheriops
- Megatherium
- Acratocnus
- Parocnus
The phylogenetic analysis formed eight distinct lineages grouped in three perfectly interrelated clads in agreement with the currently widely accepted structural topography.
Morphological vs. Molecular Phylogeny
Molecular phylogeny, which uses molecular structure to gain insight into the evolutionary relationship of an organism, has had major implications on what previous interpretations of morphological sloth biogeography, diversification, and evolution may suggest.
This recent development suggested that the Megalonychidae family is polyphyletic since it had three independent origins, vouched for by its three consecutive members, namely the extant Choloepus, the extinct Caribbean sloths, and the Megalonyx Jeffersonii (the extinct Jefferson’s ground sloth).
Molecular phylogeny states that while the Acratocnus and Parocnus, which are a part of the more recently extinct Caribbean sloths, may have shared a single origin, they consist of two highly disparate lineages that are, for the most part, not directly related to the Choloepus (or the two-fingered sloth). Instead, the Choloepus is more closely related to the Mylodon.
Furthermore, the extant Bradypus, which were previously considered a sister species to the other species of sloths, were reconsidered and nested within a clade of extinct ground sloths, namely the Megalonyx, Nothrotheriops, and Megatherium.
With the use of molecular dating (a section of biological sciences that deals with estimating the age of evolutionary events), it was also revealed that all of the recently discovered sloth families originated between 28-36 million years ago.
While both phylogenies suggest that the currently existing sloth species have had an independent evolution despite sharing the same origin, there is a stark difference between the evolutionary timeline suggested by each model.
A Timeline for Ground Sloth Evolution
Molecular dating has unveiled a new take on sloth evolution and their biogeography. It supports the idea of rapid diversification following soon after the origin of the three main sloth clades at the Eocene/Oligocene boundary.
The Eocene/Oligocene boundary is an excursion of the oxygen isotope that has marked the origin of the sheet of ice in Antarctica. To put it in simpler terms, the three clades are believed to date back to 35 million years ago (mya).
Shortly following this origin was the diversification into eight distinct lineages during the early Oligocene period, somewhere around 28-31 mya.
The period in question conforms to the set-up of the global glacial maximum (which is supported by the origin of the Antarctic ice sheet and the erection of the circum-Antarctic currents).
These changes prompted a decrease in the terrestrial temperature worldwide. A change that would transition the South America of that time from a topography full of humid and dense tropical forests to one with much open and drier habitats.
According to estimates and the overlapping of the appropriate timelines, it is believed that this change in the topography of the South American continent may have triggered the diversification of mammalian herbivore communities, including the sloth families.
However, these findings again go against the timescale marked using a morphological Bayesian clock model (a timescale used to mark the timeline of existence using fossil remains). This is unsurprising due to the already established differences between the two models when it comes to the renewed phylogenetic positions of the two extant sloth species.
In terms of three-fingered sloths or the Bradypus, morphological sources estimate its divergence to have taken place around 40 mya. At the same time, the molecular sources estimate the Bradypus to have separated from its closely related Megalonyx and Nothrotheriops at around 29 mya.
The most noticeable difference, however, comes about when discussing the split of the Caribbean sloth (previously thought to be closely connected to the Choloepus sloths of today).
The morphological estimate times the split between Acratocnus and Parocnus at eight mya and between Parocnus and Choloepus at five mya, whereas the molecular estimate times the split at 29 mya for the two distinct Caribbean sloth genera.
In molecular estimates, the corresponding split between the Choloepus and Mylodon is timed similarly, at 29 mya.
Nevertheless, fossil records imply that most diversified sloth families took root during the early Miocene period, as depicted below.
Image via Current Biology
A Timeline for Ground Sloth Biogeography
From a biogeographical standpoint, the sloth colonization of the Caribbean islands taking place 35 mya is in line with the idea of a rapid radiation of sloth lineages taking place around the same time.
These estimates and their correspondence provide evidence to support the much-debated GAARlandia hypothesis.
The GAARlandia hypothesis suggests the existence of a land bridge that tied the Greater Antilles-Aves Rise magmatic arc and the northernmost part of South America together between 33-35 mya.
The landscape in question in the discussion above is considered the Aves Ridge, or rather its uplift, a paleo-island that has long since been submerged in the water of the Caribbean Sea.
As you might have guessed at this point, the hypothesis was conceived as an attempt to validate the mammal distributions and to also answer the question of how these South American animals might have been able to reach these islands without invoking any measure of overwater dispersal.
With their evidence to support the GAARlandia hypothesis, sloths are the first Caribbean mammals that provide the said support with their molecular dating-based mitogenomics.
Overall, however, the results from molecular dating conclude that neoteric Quaternary extinctions took at least six out of the eight discovered families that originated at least 28 mya, which also includes at least two Caribbean endemic sloth lineages.
What were the Giant Ground Sloths of the Ice Age like?
The giant sloths were unlike any sloth seen today. These herbivores stood tall and proud and were comparable to the larger animals of today in size.
Unlike the scarce options (only around six species) you get to say today, the sloth families in those days were very diverse. In fact, you even had an aquatic sloth roaming around. With that being said, let’s look at the sloth history and the ancient ice age animals that once wandered our world:
Burrowing Sloths
In the early 2000s, in Novo Hamburgo, a small town in Brazil, Heinrick Frank, a Federal University of Rio Grande professor, was touring a construction site and stumbled upon rather unique and massive tunnels.
This was just the beginning. Since then, over 400 such tunnels have been found in the Rio Grande do Sul alone. Most of them stretch hundreds of feet, with the largest being 2,000 feet (610 meters), 6 feet (1.8 meters) tall, and between 3 to 13 feet (0.9 to 4 meters) wide.
To put this in perspective, whoever dug them would have moved 4,000 metric tons of dirt.
Right: Near San Pedro (A); between Mar del Plata and Miramar (B)
Image via Ichnos
These massive tunnels are declared paleoburrows; structures developed before humans invaded those areas and were made by extinct organisms.
To understand them further, these ichnospecies have been divided into two types: Megaichnus minor and Megaichnus major.
Megaichnus Minor
Shape: Subcircular to subelliptical
Dimensions: 2ft to 3ft (0.6m – 0.9m), 1.6ft to 2.3ft (0.5m – 0.7m) high, and at least 98ft (30m) long
4ft to 5ft (1.2m – 1.5m) wide, 3ft (0.9m) high, and at least 98ft (30m) long
Location: Outskirts of the town of Cristal, near San Pedro and on the coast of the Buenos Aires
Megaichnus Major
Shape: Sub horizontal and sub elliptical
Dimensions: Up to 13 ft (4m) wide, up to 6.7ft (2m) high, and at least 164 ft (50m) long
Location: Brazil
So, who dug these massive South American tunnels or paleoburrows?
While there is a lack of body fossils inside the Brazilian paleoburrows, the presence of M. minor (in Cenozoic) indicates that the ones responsible for these massive tunnels were probably mammals (depicted below).
Image via Ichnos
On the other hand, the tunnels found in South America have plenty of mammal fossils to indicate that they were probably the ones to dig these tunnels up.
When it comes to the M. major tunnels, so far, they have only been found in significantly older rocks. However, there is a striking resemblance between the claw marks found in the M. minor and those present in the M. major tunnels.
Therefore, it’d be safe to assume that the Megaichnus major were dug up by mammals too. Once it was established that mammals dug up the tunnels, the next question to answer would be which ones?
Present-day South America has no mammals that could be responsible for creating fresh tunnels with such dimensions. When considering the Cenozoic era, however, the situation changes.
That said, the most appropriate option would be the two most notorious burrowers of the era, namely sloths and armadillos.
Regarding the Megaichnus minor in Argentina, they are primarily believed to be the work of armadillos. This is because the osteoderm imprints, claw impressions, and the general morphology inside these tunnels all point towards those of the standard armadillo, albeit one much bigger in size than those found today.
The only problem is that the largest armadillo ever known to exist, that is, the Priodontes Maximus tends to dig burrows that have a height of 35 centimeters with a 45 centimeters width.
The specimens of the Megaichnus minor tunnels in Argentina are twice as large as the measurements produced by the Priodontes Maximus, which points to the existence of a much larger, unknown, and extinct species of armadillo.
As for the tunnels in Brazil, they’re attributed to Scelidotherium and Glossotherium. However, only the Glossotherium have known fossil localities, and that too in Southern Brazil. Scelidotherium, on the other hand, has yet to be discovered in Brazil.
Therefore, two other sloth species similar in morphology to the Scelidotherium, namely the Valgipes Bucklandii and the Catonyx Cuvieri, take credit for them.
The Megaichnus major, on the other hand, is entirely the work of sloths. These tunnels are by no means compatible with any work done by armadillos, and their dimensions point towards the larger body attributed to some of the ground sloth species.
Some of the large paleoburrows that are similar in size to the M. minor in Argentina are believed to be the works of mylodontid sloths such as Scelidotherium and Glossotherium.
The M. major tunnels of Brazil are bigger than Argentina’s paleoburrows and are also attributed to Scelidotherium and Glossotherium.
At the highest portion of these mega-tunnels, just beside the ceiling, there are smooth surfaces believed to be caused due to the friction produced by the animals rubbing against the wall.
With the ceiling and surrounding areas being at least two meters high, this automatically eliminates the armadillos, the Scelidotherium, and Glossotherium from the conversation.
Image via Ichnos
The only explanation for this surface would be for a larger species of sloth to have taken part in the production of those tunnels. Furthermore, the genera would have poor digging skills, hence the friction and smooth surface.
Enter the megatheriids Eremotherium and Megatherium and the mylodontid Lestodon. All three of these genera were giants larger in size than Scelidotherium and Glossotherium and also possessed poor digging skills.
Lestodon
Image via Wikipedia
Conservation Status: Extinct
Scientific Name: Lestodon
Family: Mylodontidae
Height: Around 4.6 m (15 ft) when standing on two legs
Weight: around 4.0-4.5 tonnes, around 8,000-9,000 lbs
Time: Lestodon – From the Early Pliocene (5 million years ago) to the Early Holocene (10,000 years ago)
Location: South America
The Lestodon is one of the largest ancient sloth species. Their skulls are twice or thrice as large as the skulls of sloths today, and they towered over 4.6 m in height when standing on their hind legs.
Due to its large size, the Lestodon would have had to consume large amounts of plant matter to sustain itself. Furthermore, while they may have relatively well-developed mandible muscles, they still wouldn’t have been enough to support their large jaws and thus are considered to have been very weak.
The Lestodons used to forage through the ground to sustain themselves. Since they’re considered to have been a bulk eater with no regard for precision while eating, Lestodons at the time would have consumed large amounts of soil and dirt while sifting for food.
Their large spade-like and sharp phalanges made it easier to dig through the soil. Lestodons are believed to be unable to gallop, and due to their large size, there were almost no predators that preferred to dine on their flesh.
The Lestodons died out at the start of the Holocene era, some 10,000 years ago, and it is believed to be caused by a mix of climate change and overhunting by Paleo-Indians (ancient humans recorded after the last ice age).
Glossotherium
Image via Wikipedia
Conservation Status: Extinct
Scientific Name: Glossotherium
Family: Mylodontidae
Height: Around 4 m (13 ft 1 in) when standing on two legs
Weight: around 1.5 tonnes (1.7 tons), which is equivalent to 3,500 lbs (3,700 lbs)
Time: Glossotherium – From the Pliocene (4 million years ago) to the late Pleistocene (12,500 years ago)
Location: Argentina
Standing at around 4 m, the Glossotherium was one of the larger sloths around. Not only was it massive, but it also had incredible strength in its body, a feat that made its natural enemies scarce in number.
While there are no dung deposits available to analyze their eating habits, their teeth structure suggests they were primarily herbivores.
They had large ears, which meant they had trouble hearing high frequencies. This, however, also meant that they could hear low-frequency sounds very well.
It is assumed that sloths produced low-frequency sounds to communicate with one another, mainly through their nostrils. The Glossotherium sloths had huge nostrils that were capable of sound emission. In fact, their flared nostrils are assumed to have produced a low-frequency sound of 600 Hz.
Research assumes that the mighty Glossotherium became extinct due to a mix of human hunting and climate change.
Scelidotherium
Image via Wikipedia
Conservation Status: Extinct
Scientific Name: Scelidotherium
Family: Mylodontidae
Height: Around 1.1 m (3 ft 6 in) when standing on two legs
Weight: around 0.8 tonnes (0.85 tons), which is equivalent to 1500 lbs (1,870 lbs)
Time: Scelidotherium – From the Middle Pleistocene (780,000 years ago) to the late Pleistocene (11,000 years ago)
Location: Argentina
The Scelidotherium are depicted with a superficially elongated head, much like the anteaters of today. Standing at around 1.1 m tall (on hind legs), these sloths were medium-sized (according to the era) and lived until at least 11,000 years ago.
The Scelidotherium family lived a short life in comparison to the other families on the list, with a period of only 0.67 million years. A Scelidotherium fossil was discovered by Charles Darwin in 1832, and the name was coined by Owen in 1840.
Much like the Glossotherium, these sloths are considered to have gone extinct due to human hunting (which started somewhere around 12,600 years ago).
Catonyx Cuvieri
Image via Wikipedia
Conservation Status: Extinct
Scientific Name: Catonyx Cuvieri
Family: Mylodontidae
Height: More than 4 m (13 ft 1 in) when standing on two legs
Time: Catonyx Cuvieri – From the Early Pleistocene (2.5 million years ago) to the Late Pleistocene (10,000 years ago)
Location: Brazil
The Catonyx Cuvieri was a sloth of mighty build and strength. It is assumed to have been of at least 4 m (although many believe it had an exponentially larger size). The skull which was retrieved of the Catonyx was around 50 cm in length.
This height and build meant that the Catonyx was not a preying option for many predators. These sloths had triangle-shaped snout tips, large teeth, a defined rostrum bulge, and a palate with a median groove. It also had large claws and strong-boned limbs.
The catonyx Cuvieri was a browser herbivore which meant that it fed on soft shoots, fruits, and leaves. These animals were incapable of adapting to the subtle climate changes and got tired very easily (mainly due to their weight), a quality that made them perfect hunting animals for humans.
Valgipes Bucklandii
Image via Wikipedia
Conservation Status: Extinct
Scientific Name: Valgipes Bucklandii
Family: Mylodontidae
Height: More than 4 m (13 ft 1 in) when standing on two legs
Time: Valgipes Bucklandii– Late Pleistocene (126,000 years ago) to the Early Holocene (10,000 years ago)
Location: Brazil
There is not much to go on when it comes to the Valgipes Bucklandii. Only one species has been discovered so far, and most of the evidence points towards the fact that they were primarily forest dwellers.
These sloths were similar to the Catonyx Cuvieri in size but had a wider skull and much more graceful and longer limb bones. There is a possibility of sexual dimorphism in the species since most of the adult Valgipes Bucklandii skulls have sagittal crests (the ridge of bone along the midline top of the skull).
They had large claws which curved towards their palms along with embedded osteoderms in their skin.
The Valgipes Bucklandii are believed to have gone extinct due to changing climate and human hunting.
Swimming Sloths
Aquatic Sloth Thalassocnus
Image via Wikipedia
Conservation status: Extinct
Scientific name: Thalassocnus
Family: Nothrotheriidae
Height: Greater than 2.5 m (8 ft 2 in) when standing on two legs
Weight: around 50 kg, which is equivalent to 100 lbs (110 lbs)
Time: Thalassocnus – From the Late Miocene (7 million years ago) to the Late Pliocene (3 million years ago)
Locations: Peru, South America
Ever since the 19th century, it has been established that the bone structure of virtually any body can adapt in response to psychological processes such as the magnitude of mechanical loads and their pattern.
It was also established that the alteration could sometimes take place systematically (throughout the entire organism). In simpler words, it points to the evidence of bone structure modification throughout the evolutionary lineage of a being.
The postcranial bone structure of the Thalassocnus aquatic sloth unveiled the gradual increase of BMI from the earliest to the last of its kind. This pronounced increase suggests that over the time the species inhabited Earth, it gradually adopted aquatic habits.
The Thalassocnus is considered the ideal sloth species to document the change brought in bone structure while adapting to aquatic life. Let’s look at a few examples below.
The Early Thalassocnus vs. The Last Thalassocnus
When investigating the earliest species of the Thalassocnus, the compactness of their ribs (which was measured at different intervals) showed little to no difference from those of the sloth genera Nothrotheriops or the Nothrotherium; these ribs were neither osteosclerotic (having an irregular increase in bone density) nor were the ribs amedullar.
Moving the study onto some of the later species, such as the T. carolomartini, an increase in the compactness of rib is observed, which was around 22% higher than its initial value.
Similar values in the increase of bone compactness have been reported in other species, such as the Paleoparadoxia and the Behomotops, who similarly adapted to living their life in the water (supported by their osteosclerotic bones).
Furthermore, the earliest Thalassocnus species (along with other terrestrial sloths) have a pneumatized skull root due to the presence of at least one (or two) frontal sinuses fully developed.
In later species, however, these sinuses are filled at least partially with compact bones, thereby increasing the compactness of the skull roof. It is estimated that this compactness reached an average of at least 96% by the time of the T. carolomartini.
The mid-frontal and bregma regions of the late Thalassocnus were around 15mm, a value five times higher than the Megatherium, a terrestrial sloth the size of the current day elephant.
All of the evidence above supports the Thalassocnus’ gradual metamorphosis into a primarily aquatic sloth species. However, much like the hippopotamus of today, their feeding style involved foraging on the sea bed.
This is important because their feeding style is what ultimately led them to extinction. At the end of the Pliocene era, a cooling trend resulted in the Central American Seaway closing; this killed off most of the sea grass on the coast of Pacific South America.
Not only did the Thalassocnus have their primary food supply cut off, but their dense bones and sparsely layered blubber (which allowed them to walk on sea beds) made it difficult to adapt to the change in temperature of their surrounding water.
Coupled with the low metabolism rate most Xenarthrans (sloth species) are plagued by, the weather itself would have been enough for the Thalassocnus to die out even if their primary source of food had survived.
Megatheriidae
The Megatheriidae include medium-sized sloth lifeforms to giants such as the Pleistocene Eremotherium and Megatherium.
Eremotherium
Image Via Wikipedia
Conservation status: Extinct
Scientific name: Eremotherium and Megatherium
Family: Megatheriidae
Height: More than 3.5 m (11 ft 6 in) when standing on two legs
Weight: around 4 tonnes (4.4 tons), which is equivalent to 8000 lbs (8800 lbs)
Time: Eremotherium – From the Pliocene (5.3 million years ago) to the late Pleistocene (10,000 years ago)
Locations: North and South America
The Eremotherium was one of the largest mammals in the Americas and lived in the subtropical and tropical lowlands. They were fond of partially close and open landscapes and were staunch herbivores.
These sloths were considered to be generalists, which meant that they could shift their dietary habits to adapt to the climate and region of their surrounding areas.
They were also relative in size to the elephants of today and had a robust physique equipped with vigorous and long limbs. These sloths had high-crowned uniform teeth (like the sloths of today) with a relatively gracile skull, and it’s believed that the Eremotherium may have had semi-aquatic habits (just like the current-day hippopotamus).
Only two species of the Eremotherium are known and considered valid, the E. eomigrans and the Eremotherium laurillardi, the latter of which was named in 1842 by Peter Lund, a Danish paleontologist after the evaluation of a tooth recovered from Pleistocene deposits in Brazil.
Initially, the Eremotherium was considered to be a species of Megatherium. And that was the case until paleontologist Franz Spillman noticed its distinctness from other species of the Megatheriids and thereby created the genus Eremotherium.
The Eremotherium went extinct around 10,000 years ago during the late Pleistocene era and is highly linked with the arrival of humans in the area. There is a lot of evidence to suggest that the establishment of human settlements in the area may have led the species to extinction.
Megatherium
Image via Wikipedia
Conservation status: Extinct
Scientific name: Megatherium
Family: Megatheriidae
Height: More than 3.5 m (11 ft 6 in) when standing on two legs
Weight: around 4 tonnes (4.4 tons), which is equivalent to 8000 lbs (8800 lbs)
Time: Megatherium – Middle Pleistocene (400,000 years ago) to the start of the Holocene (8,000 years ago)
Locations: North and South America
Also known as the Megathere, these giant ground sloths weighed up to 4 tons and towered over 3.5 m in size when standing on two legs. These sloths had a very robust skeleton and a broad tail.
They had a large pelvic girdle, and some of these sloths could even reach up to 6 m in size. During their time on Earth, these herbivores were only exceeded in size by a few species of mammoths.
Due to their large size, the megatherium sloths could reach plants and leaves at heights other herbivores couldn’t hope to match. They also had very large claws, which prevented them from walking normally.
Thus, much like the anteaters of today, they walked on the sides of their feet. These sloths are believed to be hairless for the most part (just like elephants) since fur on such a large surface area would have made the animal susceptible to overheating.
The Megatherium sloths went extinct around 8,000 years ago during the start of the Holocene. Their extinction is largely connected to human hunting since the time of their disappearance coincides with the Americas settlements and also because multiple kill sites with M. Americanum remains have since been uncovered.
Conclusion
The sloths today form six main species in two genera, the two-fingered and the three-fingered sloth. These sloths make up a tiny fraction of their ancestor sloths that originated at least 28 million years ago.
Right after they originated, these sloths diversified in a very short period into eight minor lineages, which eventually led to the evolution of the sloths we know today.
Out of these eight lineages, however, six went extinct towards the end of the late quaternary period, including the giant ground sloths, some of which were as big as modern elephants.
While these sloths went extinct around 10,000 years ago, they left plenty of sites (like the Megaichnus Minor and Major or the paleoburrows in Argentina and Brazil) that allude to them and allow scientists to delve deeper into ancient secrets.
