A Post-Flood Guide to Sloths

slow mammals, rapid diversification

Chad Arment (2023)

Brown-throated sloth, Bradypus variegatus (Edu Aguilera, CC BY-NC 2.0)

Just as with kangaroos and other Australian marsupials, South American sloths were a biogeographic challenge that interested early creationists. At the 2023 ICC, Warren H. Johns offered a copy of a paper he is working on, which noted a number of historical commentaries on post-Flood issues, including this one by George Bugg in Scriptural Geology (1826).

“Suppose we adopt the notion that these animals have been transported from the plains about Ararat into New Holland [Australia] and America; how shall we account for their passage over the sea? Was the period of their departure from Asia early after the Flood, or modernly? If early, how were they conveyed? If after their families became numerous, how are we to account for the whole tribe emigrating so as not to leave one pair behind to perpetuate the species? Were the sons of Noah navigators, and did they convey these animals to their respective situations on board a ship? This last suggestion would be ruinous to the philosophy of our Theorists. They would then be obliged to acknowledge that the art of Navigation was known and practiced by the ancients, and for many generations again became extinct.

“If animals are supposed to migrate by their own natural instinct, what shall we say of the sloth, which is peculiar to America? and how, with its present habits did it get from Asia thither? The Northern Straits are on all hands admitted to have been there as long ago as the Deluge. And if not, with the idle habits which naturalists attribute to that animal, it would cost it a journey of many thousand years to range from Ararat to America, half way round the globe. Yet all are said to be extinct in Asia, and peculiar to the new world!!!—M. Cuvier speaks of a fossil animal found in America, allied to the sloth, which is big as a rhinoceros. Such an animal with the habits of a modern sloth, at the assigned rate of ‘fifty yards a week,’ would scarcely arrive in America, even if it could travel without interruption by land, in ten thousand years!!!—The notion, therefore, as expressing a fact, is impracticable, and impossible.

“Even on M. Cuvier’s principle respecting the last catastrophe, as ‘sudden’, and the sea being instantly forced over the land, it is quite impossible that such animals as the ‘sloth’ tribe could escape total destruction. Besides, it is essential to this ‘Theory’ that the animals should be successive, and ‘change with the strata’, and not be transmitted from one catastrophe to another. That would be destructive to the system altogether; for its ‘successive revolutions’ are suspended entirely upon the ‘different’ animals which the ‘successive strata’ exhibit. Therefore the difficulty presses equally hard on them upon every supposition; and these animals must have become extinct in Asia,—must have been transported into America,—or, they must have been newly created, and that since the Deluge!!!—When our geologists shall, upon their own principles, satisfactorily account for such facts and circumstances as the above, it will then be soon enough for us to pay a grave attention to their hypotheses respecting the extinct fossil bones found in England and on the Continent.”

Fortunately, creation biology has come a long way since the 1800s. We know that in fact, there can be significant morphological change within the lineages of created kinds. We know that there were many different mechanisms of post-Flood dispersal, so Noah’s sons were not required to transport animals around the world. And we know that not all fossils are Flood fossils—some, like Cenozoic sloth fossils, are post-Flood fossils.

The Classification of Sloths

Two-toed sloth, Choloepus (Arwen CZ, CC BY 2.0)

Sloths make up the Suborder Folivora, which refers to the ‘leaf-eating’ habits of sloths. They and anteaters (Suborder Vermilingua) are classified in the Order Pilosa (‘hairy’). The Order Pilosa and the Order Cingulata (armadillos, glyptodonts, and pampatheres) make up the Superorder Xenarthra.

Xenarthrans share a number of morphological characteristics, skeletal as well as in soft anatomy (particularly the reproductive organs). Shared traits like strong, curved claws and lack of cone photoreceptors has led evolutionists to speculate that xenarthrans had a fossorial ancestor (Nyakatura 2012; Emerling and Springer 2015). Living xenarthrans have lower metabolic rates than most other placental mammals. If this metabolic condition was also found in fossil xenarthrans, that would likely account for the high diversity of very large xenarthrans in ancient South American landscapes. Up to 19 xenarthran megaherbivore species larger than a ton have been found in Lujanian age (middle Pleistocene to early Holocene) deposits at a single locality (Vizcaíno and Bargo 2014). A lower metabolism would require lower food intake, with less stress on local flora.

Dental anomalies are considered a shared trait of xenarthrans, though there are significant differences between groups. Anteaters lack teeth completely, adult sloths and chlamyphorid armadillos lack enamel, and dasypodid armadillos have very thin enamel (Emerling et al. 2022). A number of different pseudogenes associated with inactivating mutations are found in the various lineages. Xenarthran teeth are rootless, homodont (essentially very similar to each other), and simply crowned, unlike the tribosphenic teeth found in other mammal groups (Vizcaíno 2009). Because the two living sloth genera are not closely related, their dental ontogeny is not strictly homologous (Hautier et al. 2016).

For creationists, it seems unlikely that sloths and anteaters share a common ancestor (let alone a common ancestor with the armadillos and other cingulates), but it is interesting that all xenarthrans are South American. There is a very early (Eocene) indeterminate xenarthran fossil (Davis et al. 2020) from Seymour Island, Antarctica, which may have acted as a gateway from South America to the Antarctic land bridge early after the Flood. Xenarthrans don’t appear in North America until after the Great American Biotic Interchange (outside of some Caribbean genera to be discussed later). How did xenarthrans get to South America in the first place? Rafting from the Old World is the most likely explanation.

There are approximately 116 genera of identified sloths (only two are still living), making up about 8 families in 2-3 superfamilies. Sloth phylogenetics has been investigated several times in recent years, and while the families themselves are relatively stable, the relational placement of those families in higher-level phylogenies can differ by study (Buckley et al. 2015; Slater et al. 2016; Boscaini et al. 2019; Delsuc et al. 2018; Delsuc et al. 2019; Presslee et al. 2019; Casali et al. 2022). One issue is that the living sloths have no fossil record, which evolutionists recognize is problematic when a computer model suggests early divergence of an extant family.

The most parsimonious understanding of sloth dispersal and diversification is that one ancestral pair of ‘sloths’ (whether in or encompassing the Suborder Folivora) was on the Ark, with some of their descendants shortly finding a transatlantic route to the New World where they diversified and spread over time.

We do need to be careful about designating hardline baraminic boundaries, especially for very early post-Flood fossils. The genus Pseudoglyptodon from Eocene-Oligocene deposits “exhibits several diagnostic features previously seen only in sloths,” but also shows a more ‘primitive’ xenarthran trait suggesting it ‘diverged’ prior to “the common ancestor that gave rise to Bradypus and Choloepus plus all its descendants” (McKenna et al. 2006).

Giant ground sloths, Mylodon (Scott 1962)

Tree Sloths

Bradypus (Paco, CC BY 2.0)

It is important to understand that sloths make up a baraminic lineage that is now depauperate, but was exceedingly diverse in the centuries after the Flood. Today we only have tree sloths that engage in suspensory feeding, but the sloth lineage included giant ground sloths, medium-sized sloths that were semi-arboreal or clambered over rocky terrains, and even aquatic sloths. Most sloths have been herbivorous, and have taken advantage of a wide range of plant species for food.

The living sloths today are the tree sloths. Arboreal herbivores are rare among mammals, because being small enough in size to be adequately supported by the canopy limits their capacity for ingesting enough plant material, which is low in digestible nutrients (Pauli et al. 2014). A low metabolism probably set the stage for arboreality, but a number of other adaptations were necessary for a suspensory lifestyle.

There are six species of three-toed sloths (Bradypus) and two species of two-toed sloths (Choloepus). Two-toed sloths actually have two ‘fingers’ on their forelimbs, and three ‘toes’ on their hindlimbs, while three-toed sloths have three digits on each of their limbs. The two genera are remarkably similar in appearance, both engaging in suspensory behavior and slow locomotion while in the treetops. But, they are not closely related—they derive from completely different families.

Their peculiar similarities are considered convergence of morphological and behavioral adaptations (Nyakatura 2012). For example, while almost all other mammals have seven neck (cervical) vertebrae, both genera have broken that constant, allowing greater support of head while inverted. Bradypus has an elongated neck, with 8-10 cervical vertebrae, while Choloepus has a shorter neck, with 5-8 vertebrae (Buchholtz and Stepien 2009). No fossil sloths have morphological adaptations for engaging in suspensory behavior, so exactly how or when either trajectory of living sloths derived is unknown. Evolutionists examining the fossil record speculate that the divergence of tree sloths must have occurred after the Miocene, despite molecular studies suggesting an earlier divergence (Pujos et al. 2017).

Both genera have hair that is adapted to harboring algae colonies, though are not exactly the same in structure (Aiello 1985). Mummified ground sloth remains have been examined, with Mylodon hairs notably lacking any such capability. Sloths are considered mobile ecosystems, with a number of epibionts (organisms living on the surface of other organisms) living in their hair. This includes algae, bacteria, fungi, and arthropods (Suutari et al. 2010; Kaup et al. 2021). These symbiotic relationships can be commensal (with one benefiting while the other suffers no harm), mutualist (with both benefiting from the relationship), or parasitic. Sloth moths are a particularly interesting epibiont: three-toed sloths harbor algae (which the sloths consume, adding to their nutrient input) and pyralid moths (which increase inorganic nitrogen levels). When they descend from a tree to defecate at the base, the moths are able to oviposit in the dung pile and continue the life cycle. Newly emerging moths fly up into the canopy in search of new sloth homes. By increasing the moth biomass, the inorganic nitrogen levels rise in the fur, which promotes algal growth, benefiting the sloth (Pauli et al. 2014).

Choloepus (Dave Pape, CC BY 2.0)

The Ground Sloths

Megatherium (Scott 1962)

Popular conception of ground sloths likely resides with the giant ground sloths like the South American Megatherium (up to 6 meters in length and 4 tons in weight) or the North American Megalonyx. Giant ground sloth skeletons make popular exhibits at natural history museums. Besides fossil bones and skeletons, fossil trackways attributed to ground sloths have been found, mostly in South America, but also Nevada, from Miocene to Pleistocene deposits (Melchor et al. 2015).

The earliest recognizable ground sloths show up in Oligocene deposits, including the mylodontid sloth Octodontotherium (Pujos et al. 2021), indeterminate mygalonychids, and the peculiar Pseudoglyptodon (Pujos and De Iuliis 2007). More recently, Late Quaternary ground sloths have been carbon dated around 10,000 to 12,000 years at various North American sites, 10,000 to 25,000 years at South American sites, and 4,500 to 6,250 years in the West Indies (Steadman et al. 2005). This puts ground sloths as barely surviving the Ice Age in certain parts of the New World. As the climate was changing, humans arrived on the scene, who are known to have hunted and butchered ground sloths (Politis et al. 2019). These pressures likely led to the demise of these fascinating creatures. Not surprisingly, there has been speculation that ground sloths might survive somewhere in the Amazon (Oren 2001), but confirmation is currently lacking.

Ground sloths were diverse, and did not fit a single mold. They varied in size, small to very large. Some were quadrupeds, while others were graviportal (very slow) bipeds. Some could climb, some could swim, and some had adaptations for digging, possibly for roots and tubers (Bargo et al. 2000; Pujos et al. 2007; Shockey and Anaya 2011). Most were herbivores, but some were ground-level grazers (Shockey and Anaya 2011), while others had dentition for feeding on tough items (Pujos et al. 2011). Some sloths were bulk-feeding generalists, while others had narrow muzzles and prehensile lips suggesting they were selective feeders (Bargo et al 2006). Some researchers speculate that the dentition of Megatherium, a specialized feeder, may have allowed it an omnivorous diet (Bargo and Vizcaíno 2008), while isotopic analysis of Mylodon darwinii coprolites concluded that it was omnivorous (Tejada et al. 2021).

Five genera of mylodonts (and possibly two genera of megatheres) are known to have had osteoderms in their skin. Osteoderms make up the bony armor of the armadillos and other cingulates, so evolutionists in the past have suggested that this was a trait originally carried by all xenarthrans that was lost in most other sloths. More recently, McDonald (2018) argued that it would be more likely for the sloths’ osteoderms to have independently ‘evolved’ either once or twice. Sloth osteoderms are not articulated as in cingulates, but are separately embedded in the skin (as has been found in the mummified remains of some ground sloth skins). The osteoderms aren’t covered in keratin and are variable in size. For creationists, if osteoderms were not present in the Ark Kind ‘ancestor to sloths’, they would be a post-Flood adaptation derived from the genomic ‘toolkit’ that provided organisms with greater ability to ‘fill the Earth’.

Hapalops, with glyptodons (Scott 1962)

the aquatic sloths

When researchers discovered a large number of well-preserved sloth fossils in the marine Pliocene deposits of Peru, they were puzzled, given the virtual absence of other land mammals from the site. At this time (just prior to the Ice Age, for creationists), the Peruvian coastline was a desert, so where did all the sloths come from? Close examination of the fossil sloths determined a surprising difference: these sloths were clearly adapted to an aquatic (or semi-aquatic) lifestyle (De Muizon and McDonald 1995). Thalassocnus is a member of the otherwise land-bound Family Nothrotheriidae, with at least five species (De Muizon et al. 2003; De Muizon et al. 2004a; De Muizon et al. 2004b). It is believed to have grazed on nearshore sea-grasses and seaweed.

These swimming sloths have now been found from Late Miocene to Late Pliocene deposits, in Peru as well as in Chile (De Los Arcos et al. 2017), with ‘younger’ species showing greater specialization in grazing adaptations. How would terrestrial sloths move so rapidly into a semi-aquatic lifestyle? Amson et al. (2014) suggested that the naturally compact bones of ground sloths may have facilitated the adaptation (which here would be an exaptation), as denser bones help provide ballast to compensate for other adaptations that lead to buoyancy. This makes as much sense from a creationist perspective as it would for an evolutionist. As the coastline became drier in climate, and vegetation disappeared from land, this trajectory of sloths found opportunity for feeding in marine plants.

sloths and the flood boundary debate

Just as with marsupials, sloths support a Lower Cenozoic Flood/Post-Flood Boundary. All 6 ground sloth families (Megalocnidae, Megalonychidae, Megatheriidae, Mylodontidae, Nothrotheriidae, and Scelidotheriidae) have representative genera on both sides of the Pliocene-Pleistocene boundary. All 6 can be found in Miocene deposits and all 6 in Pleistocene deposits. 5 can even be found in Holocene deposits. 9 genera cross the Pliocene-Pleistocene boundary: Megalocnus (Megalocnidae); Megalonyx (Megalonychidae); Erasmotherium and Megatherium (Megatheriidae); Bolivartherium, Lestodon, Glossotherium, and Paramylodon (Mylodontidae); and Catonyx (Scelidotheriidae). About 34 genera can then be found in Pleistocene deposits.

Someone who holds to an Upper Cenozoic boundary would be forced to accept a minimum of 9 ground sloth pairs on the Ark, somehow all migrating at the same time to the other side of the world, where their fossil relatives were buried, and immediately diversifying into 30+ genera and numerous more species. Those 9 ground sloth pairs on the Ark would include multiple genera from within the same family, which directly contradicts Genesis 7:2, if we understand an unclean kind to represent something more than a vague generality.

The situation becomes even more untenable for the Upper Cenozoic boundary when we look closely at the Family Megalocnidae. There are 6 genera, and as a family ranges from the Oligocene to the Holocene. The Oligocene record is indeterminate to genus, but is diagnostically recognizable as part of this family. Amazingly, these genera are only found in the Greater Antilles in the Caribbean, particularly Cuba and Hispaniola. Two genera are found in Miocene deposits (Imagocnus and Megalocnus), and five are found in both Pleistocene and Holocene deposits. Megalocnus is found in Miocene, Pleistocene, and Holocene deposits (MacPhee et al. 2007; Viñola-Lopez et al. 2022)

the challenge of post-flood biogeography

So how did sloths establish themselves and diversify after the Flood? They appear to have been among several very early opportunists in rafting to the New World. The continents were still moving into their current positions, so the transatlantic crossing was not quite as long. Many other animals appear to have taken advantage of this, not settling in Africa or moving into Asia. Xenarthran groups had a few advantages, such as lower metabolism, which may have contributed to their rafting success. It is likely that, in those first few centuries, the post-Flood devastation created far more opportunities for rafting than we see today.

As with certain other groups, like the lemurs, sloths appear to be a multi-familial baraminic lineage. Sloths do not appear to have been specifically investigated using baraminological techniques, though a few other xenarthrans have been discussed (Thompson and Wood 2018). While I am not addressing intrafamilial diversification, I believe this would be a good subject for future examination. The sloths show a pattern of appearance in South American, then North American, deposits that demonstrate early post-Flood arrival, then rapid and extensive diversification throughout the continents as they took advantage of new habitats, enmeshed themselves in ecosystems, and adapted over time to new challenges with climate and competition. Today, the giants have vanished, and we are left with only a few small, canopy-dwelling species that give only a glimpse of a baramin that survived the Flood.

Bradypus (Dick Culbert, CC BY 2.0)


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Bradypus infuscatus (Guilherme Jofili, CC BY 2.0)