Shasta Ground Sloth (Nothrotheriops shastensis)
Fig 1. An artist’s interpretation of Nothrotheriops shastensis. Painting used with permission from the artist Philip Edwin.
Taxonomy
Nothrotheriops shastensis, often called the Shasta ground sloth, is a species of extinct ground sloth first described in 1905 by William John Sinclair. It is one of two species within the genus Nothrotheriops, and is placed in the family Nothrotheriidae along with a host of other mid-sized sloth genera, such as the contemporaneous Nothrotherium and Nothropus and the aquatic, Miocene-Pliocene Thalassocnus (1).
Like other sloth groups, the phylogenetic position of Nothrotheriidae has been a matter of debate, as the paucity of sloth remains in areas conducive to DNA preservation make molecular analyses inviable. Thus, comparisons of morphology have long been solely relied upon for extinct sloth taxonomy. The modest-sized nothrotheriids have long-thought to have shared a close evolutionary relationship with the largest of all sloths, the giants of Megatheriidae, and the small amount of ancient DNA data recovered has also consistently grouped the living three-toed sloths (Bradypodidae) as relatives to Nothrotheriidae. In 2019, a collagen and mtDNA analysis of all known sloth families placed Nothrotheriidae and Megatheriidae as sister taxa, confirming earlier hypotheses. The analysis recovered this clade as sister to one consisting of Bradypodidae + Megalonychidae, and the two clades make up the superfamily Megatherioidea (2, 3).
Nothrotheriids first appeared in mid-Miocene (13.8mya) South America, with species like Huilabradys magdaleniensis occurring in the La Venta formation of Colombia (1). Like most other sloth families, the nothrotheriids would spread to North America following the formation of the Isthmus of Panama around 2.7mya (4, 5).
Distribution and Age
The earliest occurrences of the genus Nothrotheriops come from Irvingtonian (1.8mya) remains across North America, from Mexico and California to coastal Florida (1). The remains from this period are assigned to N. shastensis and its sister taxon, Nothrotheriops texanus. However, the two species differ only by subtle skeletal characteristics, and intermediate forms between the two are known to exist, making classification difficult (6). Whatever the case, N. texanus appears to have become extinct around 300kya, leaving N. shastensis as the sole member of the genus Late Pleistocene (remains from Argentina have been recently been referred to Nothrotheriops, although the species is yet to be named (5)) until its terminus at the onset of the Holocene around 11,000 years ago with most of the other North American megafauna (6).
N. shastensis is known from northern California to the south as far as Belize and as far east as coastal Florida (1). In some areas, it appears to have undergone range contraction and expansion in response to glacial cycles, likely indicating an intolerance to cold temperature extremes. In fact, the species seems to have disappeared altogether from North America east of the Rocky Mountains by the onset of the Rancholabrean (240kya). This has been proposed to have been caused by intensifying glaciation towards the end of the Pleistocene; in mountainous areas, the sloths could escape the cold via vertical migration to lower, warmer altitudes, or by taking shelter in caves (6, 7). The south-central and southeastern portions of what is now the United States largely lack the topographical features that enable such behavior, resulting in the extirpation of N. shastensis (6).
Despite such climatic sensitivities, it has been described as difficult to explain the extinction of the sloth by climate and dietary change alone, as ample suitable habitat existed at the time of its extinction. Human hunting appears to remain to most plausible explanation (8).
Fig. 2. The estimated Ranchoblanchean palaeorange of Nothrotheriops shastensis based on fossil sites (6, 14)
Morphology and Ecology
Often being cited as similar in size to an American black bear, N. shastensis has been estimated as having had an average mass of 250 kg (551 lbs), making it smaller than sympatric ground sloths such as Megalonyx, Paramylodon, and Eremotherium. It had a similar body plan to other ground sloths, with a stout body, inward-turned limbs, large claws, and a robust tail that could likely be used for support in a tripod stance (9, 10). Like other species of North American sloths, the cranial morphology of this species has been interpreted as more primitive and unspecialized when compared to its South American relatives. The toothless premaxilla is relatively narrow and pointed, but less so than South American nothrotheriids (4).
These cranial characteristics, combined with the relatively-small size of N. shastensis and the fact that ground sloths are thought to have been foregut fermenters like their surviving relatives, point to a generalist low browser feeding strategy, feeding mostly on shrubs and low trees along with occasional consumption of grasses and forbs (4).
Both young and adult specimens have been recovered. Tooth growth patterns and behavioral comparison with extant sloths has led some researchers to believe this species was solitary apart from parents and their dependent offspring (7).
Remarkably, dry fossilized feces recovered from sites such as Rampart Cave, Gypsum Cave, and Shelter Cave in the southwestern US have preserved sugars, amino acids, pollen, and even fragments of plant cuticle and seed pods. The identity of the dung and the remains within it has been confirmed by DNA fingerprinting, granting direct insight into the diet of this species and adding credence to morphological hypotheses surrounding diet (8, 10, 11). The vast majority of food items are xerophytic (arid-adapted) plants, such as Joshua tree (Yucca brevifolia), creosote (Larrea spp.), catclaw acacia (Acacia greggii), desert globemallow (Sphaeralcea ambigua), Mormon-teas (Ephedra spp.), and saltbushes (Atriplex spp). However, more mesic and even riparian plants have been reported as well, including Utah juniper (Juniperus utahensis), three-leaf sumac (Rhus trilobata), reeds (Phragmites spp), and cattails (Typha spp.).
The presence of Joshua tree seed pods in the dung has received significant attention, as this species is well-known for being an especially-poor disperser. Prior to the Holocene, the tree was a prolific fixture of the American west, but has since undergone a significant decline despite new suitable habitat having opened up in the wake of climatic changes. There are scant few historical instances of seedlings becoming established on their own, and none whatsoever of the tree expanding its existing range. While rhizomes and seed-caching rodents are probably responsible for the continued maintenance of Joshua tree populations, these mechanisms alone appear to be insufficient in enabling the species with the ability to expand its range naturally. This has raised concerns that the tree’s decline may only steepen due to modern anthropogenic climate change, with no way to disperse away from increasingly-unsuitable habitat. Discovery of excretion of intact Yucca seed pods has been interpreted as direct evidence of a seed-dispersal role by Nothrotheriops. An early Holocene extinction of the sloth is congruent with the decline of the Joshua tree; without a megafaunal disperser, the tree and its symbiotes seem to have been existing on borrowed time, as evolutionary anachronisms (12).
As the diet of N. shastensis would imply, the species was arid-adapted, more-so than any other North American sloth. Radiocarbon dating of fossilized dung and soft tissue indicate the presence desert and dry shrubland communities at the time of the sloth’s occurrence (10). However, other specimens have been recovered from more mesic habitats, such as the chaparral and coastal sage scrub of Rancho La Brea, which was of a Mediterranean climate not dissimilar from what exists in coastal California today (1, 10). Additionally, the presence of certain plants in the diet would point to at least seasonal occurrence in upland juniper woodlands and even montane conifer forests (8).
In Rancho La Brea, and likely other locales, the species was sympatric with Paramylodon harlani and Megalonyx jeffersoni, implying some system of niche partitioning was present. It is possible the habitat was only marginal for Nothrotheriops and Megalonyx, as Paramylodon was by far the dominant sloth species in the area (1, 10).
As previously mentioned, it is likely N. shastensis was sensitive to cold temperatures; more-so than the aforementioned sympatric sloths. Its distribution contracted in response to glacial periods, and it is thought to have disappeared from the eastern US due to intensifying cold temperatures and a lack of thermal buffers, such as decreasing elevations and caves, to use as refugia (6, 10). Moreover, its relatively small size has led researchers to believe it was a poorer thermoregulator than more northerly sloths like Megalonyx (10).
Due to the wealth of fossilized remains and dung recovered from inside karst environments, it is clear that taking shelter in caves was a common behavior among at least some populations of Nothrotheriops shastensis (6, 7, 8, 10, 11).
N. shastensis remains have been found alongside those of camelids, pronghorn, horses, collared peccary, and a variety of xeric-adapted mesopredators, birds, and reptiles (1). As camelids are also considered to be non-ruminant foregut fermenters which typically incorporate browse into their diet, its possible that there was some competition between these species and Nothrotheriops (4). A potential avenue of niche partitioning in this case is vertical zonation; large camelids such as Camelops and Hemiauchenia would’ve been able to reach higher than the sloth, and perhaps avoided competition by feeding on higher browse. Sympatric predators that may have preyed upon N. shastensis include the dire wolf (Aenocyon dirus), American lion (Panthera atrox), jaguar (Panthera onca), Smilodon fatalis, and Arctodus simus (1).
Analysis of coprolites has also revealed an array of intestinal parasites, including nematodes and coccidia (13).
References
1. Nothrotherium shastense. Fossilworks. (n.d.). Retrieved October 15, 2021 from http://fossilworks.org/bridge.pl?a=taxonInfo&taxon_no=49231
2. Delsuc, Frédéric & Kuch, Melanie & Gibb, Gillian & Karpinski, Emil & Hackenberger, Dirk & Szpak, Paul & Martínez, Jorge & Mead, Jim & McDonald, H. & Macphee, Ross & Billet, Guillaume & Hautier, Lionel & Poinar, Hendrik. (2019). Ancient Mitogenomes Reveal the Evolutionary History and Biogeography of Sloths. Current Biology. 29. 1-12. 10.1016/j.cub.2019.05.043.
3. Presslee, Samantha & Penkman, Kirsty & Collins, Matthew & Macphee, Ross. (2019). Palaeoproteomics resolves sloth relationships. Nature Ecology and Evolution.
4. McDonald, H.G. (2005). The paleoecology of extinct xenarthrans and the Great American Biotic Interchange. Florida Museum of Natural History.
5. Brandoni, Diego & Vezzosi, Raúl. (2019). Nothrotheriops sp. (Mammalia, Xenarthra) from the Late Pleistocene of Argentina: implications for the dispersion of ground sloths during the Great American Biotic Interchange. Boreas. 48. 10.1111/bor.12401.
6. Akersten, William & McDonald, H.. (1991). Nothrotheriops from the Pleistocene of Oklahoma and Paleogeography of the Genus. The Southwestern Naturalist. 36. 178. 10.2307/3671918.
7. Naples, Virginia. (1990). Morphological changes in the facial region and a model of dental growth and wear pattern development in Nothrotheriops shastensis. Journal of Vertebrate Paleontology - J VERTEBRATE PALEONTOL. 10. 372-389. 10.1080/02724634.1990.10011821.
8. Thompson, Robert & Van Devender, Thomas & Martin, Paul & Foppe, Theresa & Long, Austin. (1980). Shasta Ground Sloth ( Nothrotheriops shastense Hoffstetter) at Shelter Cave, New Mexico: Environment, Diet, and Extinction. Quaternary Research. 14. 360-376. 10.1016/0033-5894(80)90017-4.
9. Extinct Ground Sloth, Tardigrada. San Diego Zoo (2009). Retrieved October 15, 2021, from https://web.archive.org/web/20150214074751/http://library.sandiegozoo.org/factsheets/_extinct/sloth_extinct/extinct_sloth.htm#physical
10. McDonald, H.. (2021). Yukon to the Yucatan: Habitat partitioning in North American Late Pleistocene ground sloths (Xenarthra, Pilosa). 70. 237-251.
11. Poinar, Hendrik & Hofreiter, Michael & Spaulding, Walter & Martin, Paul & Stankiewicz, Artur & Bland, Hubert & Evershed, Richard & GR, Possnert & Pääbo, Svante. (1998). Molecular Coproscopy: Dung and Diet of the Extinct Ground Sloth Nothrotheriops shastensis. Science (New York, N.Y.). 281. 402-6. 10.1126/science.281.5375.402.
12. Cole, Kenneth & Ironside, Kirsten & Eischeid, Jon & Garfin, Gregg & Duffy, Phillip & Toney, Chris. (2011). Past and ongoing shifts in Joshua tree distribution support future modeled range contraction. Ecological applications : a publication of the Ecological Society of America. 21. 137-49. 10.2307/29779642.
13. Schmidt, Gerald & Duszynski, Donald & Martin, Paul. (1992). Parasites of the Extinct Shasta Ground Sloth, Nothrotheriops shastensis, in Rampart Cave, Arizona. The Journal of Parasitology. 78. 811. 10.2307/3283310.
14. Palaeobiology Database. (2021). Nothrotherium shastense. https://paleobiodb.org