A Short Discussion

 Enjoy this video today from the Natural History Museum of Los Angeles County. This is a short tour of the Cryolophosaurus reconstruction that was at the museum late last year. Dr. Nate Smith talks about our friendly crested theropod. If you need more fun videos, check this out too (you know you want to).

Antarctic Dinosaurs

Over the years this site has covered a lot of different dinosaurs and fossil animals from a lot of different areas, regions, and continents (and of course some animals that were living between continents at one point or another). I have been digging back in the oldest of entries lately to try to re-cover some dinosaurs and fossil animals that we may not have discussed. Interspersed with new animals, I think this is useful for everyone interested in reading, and it lets us all revisit some old friends. One of the past favorites is a large theropod dinosaur from Antarctica with a rather interesting bony headpiece: Cryolophosaurus ellioti (Hammer and Hickerson 1994).

Cryolophosaurus roughly translates to "cold/frozen crest lizard" and for good reason. Being found in Antarctica explains the first part of the name, but the second part of this genus is attributed to the large crest on the top and back of this animal's head. An estimated size of 6.5 m (21.3 ft) and 465 kg (1,025 lb) make Cryolophosaurus the largest theropod of the Early Jurassic. It has been noted that the known remains are not even those of an adult, which means that these size estimates probably are not for a full grown animal. Molina-Pérez and Larramendi (2016) presented estimates of a fully grown adult at approximately 7.7 m (25.3 ft) and 780 kg (1,720 lbs). Regardless of its size, what people notice first is the crest. The dinosaur has been given the nickname "Elvisaurus" (I very much dislike Elvis, but to each their own) because the crest is oriented from one side to the other, making it appear, apparently, like Elvis' pompadour hairstyle. You be the judge on this one.

References

Hammer, W. R., and Hickerson, W. J., (1994). A Crested Theropod Dinosaur from Antarctica. Science. 264 (5160): 828–830. doi:10.1126/science.264.5160.828

Molina-Pérez, R. and Larramendi, A. (2016). Récords y curiosidades de los dinosaurios Terópodos y otros dinosauromorfos. Barcelona, Spain: Larousse. p. 254.

Smith, N. D.; Makovicky, P. J.; Hammer, W. R.; Currie, P. J. (2007). Osteology of Cryolophosaurus ellioti (Dinosauria: Theropoda) from the Early Jurassic of Antarctica and implications for early theropod evolution. Zoological Journal of the Linnean Society. 151 (2): 377–421. doi:10.1111/j.1096-3642.2007.00325.x

Life at the End

It is not too often that we are able to say that we have a fossil that comes from an area of the fossil record that basically butts up against the K-Pg boundary (Cretaceous - Paleogene Boundary; the site in time of the extinction event that claimed the non-avian dinosaurs). However, Alamosaurus, is one of those animals that appears to have been one of the very last of the non-avian dinosaurs that roamed the earth. Known largely from the Ojo Alamo Formation (as previously noted) and the Javelina Formation, many of the fossil remains of Alamosaurus are between 69 and 66.5 million years old. There is at least one juvenile skeleton known from the Black Peaks Formation, an area of the fossil record that envelops (contains) the K-Pg boundary. We can infer from this that Alamosaurus was alive (and well, probably) near, if not right up to, the extinction of the non-avian dinosaurs.  

In its fossil range, Alamosaurus is one of the most commonly found fossils, along with Quetzalcoatlus, the enormous pterosaur. It has been hypothesized that Alamosaurus represents a reintroduction of enormous sauropod dinosaurs to North America. Opposing hypotheses point to ancestral sauropods originating further inland in western North America (and ultimately from Asia) and that Alamosaurus was essentially endemic (restricted in range to a certain area) to the North American inland plains. Lucas et al. (2016) noted that the dispersal of Alamosaurus fossil across the western inland plains provides evidence that Alamosaurus was actually quite mobile and not endemic, having a large demonstrated range. Their review did concede that Alamosaurus has only been found within inland plain environments and that no fossils of Alamosaurus have yet been found that would indicate that the animal made its way to the coastline or coastal plains of the Western Interior Seaway.

References

Lehman, T. M., (2001), Late Cretaceous dinosaur provinciality: In: Mesozoic Vertebrate Life, edited by Tanke, D. H., and Carpenter, K., Indiana University Press, pp. 310–328.

Lucas, S., Sullivan, R, Lichtig, A, Dalman, S., and Jasinski, S. (2016). Late Cretaceous dinosaur biogeography and endemism in the Western Interior basin, North America: A critical re-evaluation. New Mexico Museum of Natural History and Science Bulletin. 71. 195-213. 

Sullivan, R.M., and Lucas, S.G. (2006). "The Kirtlandian land-vertebrate "age" – faunal composition, temporal position and biostratigraphic correlation in the nonmarine Upper Cretaceous of western North America." New Mexico Museum of Natural History and Science, Bulletin 35:7-29.

The Alamo

Reconstructed and restored skeletons of
Tyrannosaurus rex harassing an
Alamosaurus sanjuanensis. CC BY 2.0

Alamosaurus sanjuanensis is estimated to have grown to approximately 30 m (98 ft) long and to have weighed up to 80 tons. That is a just a little more than 4 fully loaded school busses (we can call it 4 and a half just to be safe). If one were curious about length, that is about 10.6 m (8 ft) longer than two school busses; we can make that 2.2 school busses in length. Living immediately prior to the K-Pg (Cretaceous - Paleogene) boundary, notable because of the extinction event that eradicated the non-avian dinosaurs, Alamosaurus was one of the last of the sauropods and, more generically, the non-avian dinosaurs to have evolved and to have lived. Remains from multiple specimens represent differently aged animals. This means that we have, we think, a good representation of this dinosaur species across a large portion of its lifespan from juvenile to adult (also known as ontogeny). Named in 1922 by Charles Whitney Gilmore, a paleontologist at the United States National Museum, now the National Museum of Natural History (commonly referred to at large as "The Smithsonian"), Alamosaurus was originally discovered in New Mexico during 1921 in the Ojo Alamo Formation and not, as the name immediately may appear to suggest, in Texas. It was named by Gilmore for the formation, not for the more widely famous Alamo in San Antonio, TX. The specific name refers to the New Mexico county (San Juan County) where the holotype remains were originally discovered. Remains have been found, since that time, all across the southwestern United States of America and are housed in different institutions from California to Washington D.C. When considering its estimated size, we can say that Alamosaurus is a titanosaur (a group of large sauropod dinosaurs) that is nearly the same weight as the titanosaurs Argentinasaurus and Puertasaurus; both are large sauropods known mainly from Argentina.

Pack or Loner?

 It is unclear exactly how most dinosaurs hunted prey. We have hypotheses, some more well supported and considered than others, but unfortunately we do not have any trail cameras or candid videos documenting behaviors of dinosaur hunting. As many of us may have heard, at least once in our lifetime, many dromaeosaurs are thought to have been pack hunters; tracking, ambushing, and taking down prey that was far larger than a single animal to feed a large group of predators. Not all of the species within this group of animals is thought to have exhibited this complex social system. Often, wolves are used in analogy to describe these pack hunting behaviors, and for good reason. Utahraptor, as the largest of the dromaeosaurs, is one of those dinosaurs thought to have hunted this way and is often compared to wolves. Recently Frederickson et al. (2020) analyzed the teeth of Deinonychus and concluded that pack hunting was not likely to have been a characteristic social interaction in these smaller dromaeosaurs, but no such study has been conducted with Utahraptor. For the moment at least the idea of Utahraptor as a pack hunter still exists. This behavior was central to the theme of the novel discussed the other day. Additionally, this behavior is hinted at by a number of studies of Utahraptor including one that looks at a hypothesized quicksand death trap that may have enveloped an entire pack including juvenile animals (Kirkland et al. 2016). It is possible that, in addition to a complex social system wherein pack mentalities developed, that Utahraptor may have been simply too large to chase fast prey and too small to take on large prey alone. The describing paper, Kirkland et al. (2001), concluded that Utahraptor was likely slower than its cousins Velociraptor and Deinonychus. 

Rather than post an image directly in here today, I refer you to Julius Csotonyi's interpretation of the quicksand event that may have led to the discovery of a pack of fossilized Utahraptors as described in Kirkland et al. (2016).

Literature Cited

Frederickson, J. A.; Engel, M. H.; Cifelli, R. L. (2020). "Ontogenetic dietary shifts in Deinonychus antirrhopus (Theropoda; Dromaeosauridae): Insights into the ecology and social behavior of raptorial dinosaurs through stable isotope analysis". Palaeogeography, Palaeoclimatology, Palaeoecology: 109780. doi:10.1016/j.palaeo.2020.109780

Kirkland, J.I.; Simpson, E.L.; DeBlieux, D.D.; Madsen, S.K.; Bogner, E.; Tibert, N.E. (2016). "Depositional constraints on the Lower Cretaceous stikes quarry dinosaur site: Upper yellow cat member, cedar mountain formation, Utah". PALAIOS. 31 (9): 421–439. doi:10.2110/palo.2016.041

Popular Raptor

Proof that the author did once like a lot
of sports all at once, but more importantly, the
pretty great 1995 Toronto Raptors logo, mostly.

Utahraptor once inspired an arguably wonderful digital mashup on my part, more because I saw an opportunity than the fact that I like basketball. Despite my less-than-excited (as an adult; see photo) attitude toward basketball, Utahraptor was actually the inspiration, it is said, for one basketball team's mascot (and it was not the Utah Jazz, all of my artwork aside). Their logo is not nearly as cool as it used to be of course (dinosaurs playing basketball > basketball with claw marks), but the Toronto Raptors still exist. This has not been the Utahraptor's only foray into the sphere of popular culture. As we witnessed the other day there are many video game incarnations of this large and deadly dromaeosaur and, if we count the "velociraptors" of the Jurassic Park franchise as misnamed Utahraptors, they have appeared in 5 full length films; being highlighted in pretty much every film at some point (unlike the barely seen Dilophosaurus or Triceratops of Jurassic Park). Aside from reading what science I could as a pre-teen and teenager, my major introduction to Utahraptor was through a short novel titled Raptor Red. Whether you like the story or the writing or the science within Bakker's imagined dinosaur tragedy or not, it was, for me in 1995 (along with the Toronto Raptors t-shirt; see photo) one of the coolest dinosaur things I got my hands on. 

The story centered around a Utahraptor that lost her mate, found and joined her sister, was wooed, and, through many more trials and tribulations founds a large and successful pack of Utahraptors (and yes I skipped a lot of storyline). Some of the story may have even inspired later Jurassic World scenes, or are so highly similar that they could be accused of being questionable (we are all looking at you death of "Indominus rex" scene). The point is that this was one of the first books, post-Jurassic Park, that I really enjoyed that was about dinosaurs and it reignited my interest in the animals. Utahraptor, specifically, reignited my want to read more about dinosaurs. Jurassic Park had done a good job of that as well, but I have always been an imaginative person, and seeing that it was okay to speculate and drive a narrative like this resonated with and inspired me in a way that scientific literature never did and still does not. Of course scientific literature is extremely important and I value it. I do not find inspiration in it the way I do fiction though. Of course I have not read it in years and so I cannot say how I would feel about it today. Nor would I say that I recommend it, because I do not know how my adult self would view it. However, 13 year old me thinks it was a wonderful book and that it is worth reading. Maybe it has been long enough that I do not want to destroy the good memory of it, like when you watch a cartoon from your childhood and wonder what made you think that that show was so great. But I cannot say that because I do not know.

Revisiting Utah

On this blog we have covered a lot of different dinosaurs over time. Nine years ago we discussed one of the largest of the dromaeosaurs, Utahraptor ostrommaysi (Kirkland, Gaston and Burge, 1993). I think it might be time to revisit this medium sized carnivorous dinosaur. It was discovered, originally, in 1975, and again in 1991 and named around the time of the release of Jurassic Park in 1993. This quieted (but of course did not silence) some discontent surrounding the "Velociraptors" in that movie. Their scientific name in the movie and the book was Velociraptor antirrhopus, which also appears in a Gregory S. Paul book (Predatory Dinosaurs of the World, 1988) as a synonym for Deinonychus antirrhopus. It appears as though Michael Crichton may have referenced this book in writing Jurassic Park. However, I digress from our main point, Utahraptor. 

Standing at a similar height to the movie 'raptors, Utahraptor, was approximately 7 m (23 ft) long and is estimated to have weighed somewhere between 230 and 500 kg (500 - 1100 lb). It is the largest dromaeosaur known at this time. Like other dromaeosaurs, Utahraptor did possess a large sickle-shaped "Killing Claw" on each foot. The claws known from Utahraptor are estimated to have been approximately 24 cm (9.4 in); actual fossilized unguals, or two claws, are slightly smaller than this.

Toothless in Death

It is not weird to find fossil teeth. It is not weird to find fossil jaws (mandibles, lower jaws, or maxillae, upper jaws). It is not even strange to find fossil jaws with few teeth or many broken teeth. It may be a little alarming, then, to see the skull of Qianzhousaurus and notice it has no fossil teeth. The teeth The team describing the fossil describes long narrow and thin teeth. They based their conclusions off of comparisons of relatives of Qianzhousaurus (both Alioramus species). Some inferences may have been drawn from the shape of the alveoli (the "sockets" of the teeth in the bone) but this is not expressly stated in the paper that describes Qianzhousaurus. The number of alveoli, and therefore teeth, is described in the paper and constitutes some of the differences that the authors use to make the case for this dinosaur being a unique genus and species as opposed to a larger member of the Alioramus genus. There are 18 teeth in the mandible (Tyrannosaurus rex, the author's common comparison species, possesses up to 17 teeth here), 15 in the maxilla, and 4 in the premaxilla. That is a total of 37 teeth. But we have to remember there are two sides to the head, which means there are 74 teeth in the skull of Qianzhousaurus (providing my math checks out).

Standing Tall

 Possessing a femur approximately 70 cm (27.6 in) long and a tibia approximately 76 cm (30 in) long, Qianzhousaurus was a long-legged, though not overall a very tall, dinosaur. These measurements come from the holotype leg that was discovered and may not fully represent the adult height of Qianzhousaurus. Characteristics of the vertebrae indicate that this was an immature animal and, because we do not have an adult specimen, the full length of these legs is not currently known. However, as an immature specimen, possessing a hip probably a little above 1.46 m (57.5 in; this assumes hip height at femoral contact with the pelvis, not at the iliac blade as is often reported and therefore represents a shorter estimation of hip height) above the ground any additional height (from the foot, erectness of posture, and additional growth into adulthood) may allow for Qianzhousaurus to be a taller dinosaur than the current estimate. Estimated to be a little over 7 m (23 ft) from nose to tail, Qianzhousaurus is around 3 m (9.8 ft) shorter than Tarbosaurus bataar, a Mongolian tyrannosaur that appears to have been a contemporary neighbor. However, Qianzhousaurus is longer than either described Alioramus species, sister taxa within the Alioramini tribe, as they are both approximately (A. altai) or just over (A. remotus) 5 m (16.4 ft) in length. Depending on the juvenile status of the Alioramus discoveries, these estimates may be low end estimates as well.

Literature Cited

Brusatte, S. L., Carr, T. D., Erickson, G. M, Bever, G. S., Norell, M. A. (2009). A long-snouted, multihorned tyrannosaurid from the Late Cretaceous of Mongolia. Proc Natl Acad Sci U S A 106:17261–66.

Lü, J., Yi, L., Brusatte, S. L., Yang, L., Li, H., Chen, L. (2014). A new clade of Asian late Cretaceous long-snouted tyrannosaurids. Nat Commun 5:3788.

The Qianzhou lizard

 A young (in terms of how long we have known it) alioramid tyrannosaur from China was first described in 2014 and was immediately given a rather interesting nickname. Due to the narrow and elongate snout, rather different from most other tyrannosaurs that possess dorsoventrally deep (i.e. tall from the bottom of the lower jaw to the the top of the skull) skulls, this dinosaur was given the nickname "Pinocchio rex". The real name of this slender-faced giant is Qianzhousaurus sinensis (Lu et al. 2014). Found in the southern Chinese province of Jianxi near the city of Ganzhou in 2010 (in the middle of a construction project), the holotype fossil consisted of a full skull (minus the teeth; more on this later), 30 vertebrae from along the entirety of the spinal column, parts of the shoulder and pelvic regions, and the majority of the left leg (some elements of the foot are missing). These remains were given the museum designation GM F10004, but the name Qianzhousaurus honors the city of Ganzhou (it is an alternate and older spelling of the city's name). the specific name, sinensis, is a derivative of the Greek word meaning "of/from China" and is used as a species name in many dinosaurs from China. 

Size comparison of the three species in the tribe Alioramini, a group of tyrannosaurid theropod dinosaurs from the Upper Cretaceous of Asia. Composite image by User:Slate Weasel (public domain release) Theropods based on the following references:

• Qianzhousaurus sinensis (blue): Skeletal by PLASTOSPLEEN
• Alioramus remotus (gold): Skeletal by Steveoc 86
• Alioramus altai (yellow): Skeletal by randomdinos

Animatronic restoration of Qianzhousaurus sinensis at Jurassic Quest drive-thru attraction. Photo by the author.

Megatherium Mysteries

As a disclaimer: The story below is simply me relaying the facts. I do not have any stake in how these facts are interpreted or the validity of the published or reported facts.

The Natural History Museum in London has a page on the internet dedicated to Megatherium and it asks, quite simply, at the top "What was Megatherium?" Rather than starting with an outright answer, the museum explains why it would even ask that question by telling you the connection between London and a South American fossil sloth: two of Darwin's fossils (the last two digitized in the NMH collection) are halves of a Megatherium skull. Megatherium was actually well known before Darwin took the fossil skull from its home continent. The first fossils had been unearthed in 1788 by Manuel Torres, who sent them from the Lujan area west of Buenos Aires near the Lujan River of Argentina to the Museo de Ciencias Naturales of Madrid. Modern day Lujan appears to be rather far from the river, though changes in the urban landscape around Buenos Aires may account for this (I have found no record of the river changing course in my, admittedly, short search). These actually made up, as seen here, what is considered the first mounted fossil skeleton in a museum (circa 1795).

Despite having been discovered in 1788 in Argentina, official description of this fossil mammal was not produced until 1796. This publication resulted from an interpretation by Georges Cuvier, a French naturalist, of a series of drawings and descriptions of the bones by Juan Bautista Bru, a museum employee and also a dissector of the Royal Laboratory, who had helped to assemble and mount the skeleton. It is argued by Lopez Piñero (1988) that Bru's work with Megatherium and the description for the museum should count as the original description. Lopez Piñero goes on to state that an agent of the French government, a man named Roume, obtained copies of Bru's plates while in Madrid, sent them back to France, and the Institut de France asked Cuvier to write a description of what Cuvier would later state were "a bad copy of the... whole skeleton." As insulting as this may have been (not to mention that a South American animal was sent to Spain to be described in the first place), when the description was sent to Spain to be translated into Spanish, it was discovered that Bru's highly detailed description already existed. It was published almost immediately, but in the years after, Cuvier incorporated a French translation of Bru's description into his 1812 opus (Recherches sur les ossements fossil) and Bru was all but forgotten in the history of Megatherium. 

Argot (2008) reports that Cuvier changed his hypotheses about the lifestyle of the sloth a number of times. First he stated that the animal, like modern sloths, must have climbed trees. Later he changed his assertions, claiming the animal was a fossorial, or underground, animal somewhat like a mole. Darwin sated the British museum's desire for some Megatherium fossil material in later years, but he never mentioned Bru. Even Tomas Villanova, a professor of zoology at the museum in Madrid, refused to refute Cuvier's description, or his hypotheses, and would not give Bru credit. Megatherium has had, as many fossils did, a very interesting history.

Literature Cited

Argot, C. (2008). "Changing Views in Paleontology: The Story of a Giant (Megatherium, Xenarthra)". In Sargis, E. J.; Dagosto, M. (eds.). Mammalian Evolutionary Morphology: A Tribute to Frederick S. Szalay. Springer. pp. 37–50. 

Cuvier, G. (1812). Recherches sur les ossements fossils, oú l'on rétablit les charactères de plusiers animaux dont les révolutions du globe ont dé-truit. Tome quatrieme. Troisiéme partie. Sur les ossemens fossils de ruminans. Chez G. Dufour et E. D'Ocagne, Libraries, Paris, France.

Lopez Piñero, J. M. (1988). Juan Bautista Bru (1740–1799) and the description of the genus Megatherium. J Hist Biol 21:147–63.

Giant's Diet

 Despite discussing the bipedal implications of biomechanical analyses yesterday, it is important to note that many of the foods Megatherium was thought to have consumed are on the ground. Unlike present day sloths, Megatherium was more of a grasslands and edge of the forest dweller. The open ground of the entirety of the genus' range was different depending on where in the range we discuss. However, the generally accepted diet, aside from trees, consists of lower growing plants like yucca, agave, and grass. As we probably all recognize, herbivores are often large bodied animals. There are a number of reasons for this. The explanation in its simplest form is that plant matter needs to be consumed in large quantities and the digestive anatomy needed to extract the most nutritional value from this material needs to be extensive in these large herbivores (e.g. Westoby 1974; Danell et al. 2006, and Felton et al. 2016). 

Understanding the guts of extinct animals like Megatherium that relied on the leaves of trees and low-lying plants like those mentioned above can be accomplished using living examples that we know well to draw conclusions. The understanding of how Megatherium processed, and even ate, all of that material can also be inferred from living animals, but thankfully for us, we have the capability to model feeding and the technology to do so. This has led to biomechanical and morphometric analyses of Megatherium skulls. Bargo (2001) is one such study that analyzes both the biomechanical capabilities of these sloths and takes into account the morphometrics of the skull as well as dietary limits. What we learned from Bargo (2001) is that Megatherium teeth and jaws were well suited to biting and cutting medium to soft vegetation and do not appear to have been used for grinding tough and fibrous plant materials. 

It has also been suggested that Megatherium was an opportunistic omnivore, and not a strict herbivore. These sorts of occasional carnivory have been documented, anecdotally, at least, in deer that have been reported eating mice. Hummingbirds have been known to feed arthropods to their offspring and augment their nectar diets with fruit flies (Young 1971, Brice 1992, Yanega and Rubega 2004). Occasional carnivory possibly (probably) occurs in other animals as well. Fariña and Blanco (1996) originally put forth the hypothesis that Megatherium may have engaged in scavenging of carcasses based on the muscle attachments of the elbow. This view remains controversial, but hinges (elbow pun intended) on the idea that the anatomical configuration of the Megatherium elbow is like that in other carnivores and accentuates speed of movement rather than force. The authors contended that this permitted Megatherium to use their large curved claws like daggers to stab prey or lash out at carnivores that they were attempting to wrest carcasses away from. Isotope data (and other studies) appears to completely refute this hypothesis, however (e.g. Bargo 2001, Bocherens et al. 2017)

Literature Cited

Bargo, M. S. (2001). "The ground sloth Megatherium americanum: Skull shape, bite forces, and diet". Acta Palaeontologica Polonica. 46 (2): 173–192.

Bocherens, H.; Cotte, M.; Bonini, R.A.; Straccia, P.; Scian, D.; Soibelzon, L.; Prevosti, F.J. (2017). "Isotopic insight on paleodiet of extinct Pleistocene megafaunal Xenarthrans from Argentina". Gondwana Research. 48: 7–14. doi:10.1016/j.gr.2017.04.003

Brice, A. T. (1992). The essentiality of nectar and arthropods in the diet of the Anna's hummingbird (Calypte anna). Comparative Biochemistry and Physiology Part A: Physiology, 101(1), 151-155.

Danell, K., Bergström, R., Duncan, P., & Pastor, J. (Eds.). (2006). Large herbivore ecology, ecosystem dynamics and conservation (Vol. 11). Cambridge University Press.

Fariña, R. A.; R. E. Blanco (1996). Megatherium, the stabber. Proceedings of the Royal Society of London. 263. pp. 1725–1729. doi:10.1098/rspb.1996.0252

Felton, A. M., Felton, A., Raubenheimer, D., Simpson, S. J., Krizsan, S. J., Hedwall, P. O., & Stolter, C. (2016). The nutritional balancing act of a large herbivore: an experiment with captive moose (Alces alces L). PloS one, 11(3), e0150870.

Westoby, M. (1974). An analysis of diet selection by large generalist herbivores. The American Naturalist, 108(961), 290-304.

Yanega, G. M., & Rubega, M. A. (2004). Hummingbird jaw bends to aid insect capture. Nature, 428(6983), 615-615.

Young, A. M. (1971). Foraging for insects by a tropical hummingbird. The Condor, 73(1), 36-45.

Tail for Balance

 In the more distant past, we are thinking about 100 years ago possibly, Megatherium was not considered a bipedal animal. It may have been described as facultatively bipedal, meaning it could occasionally walk on two feet, at most. The most often that Megatherium was thought to have engaged in such behavior was during its feeding forays. It was also hypothesized, in the past, that the tail was used as peg upon which Megatherium could balance its weight while foraging in trees on two legs. This impression was captured in the restoration of Robert Bruce Horsfall published in 1913. This tripod of legs and tail remains relatively popular. However, Casinos (1996) did conduct biomechanical analyses that suggested that the vertebral column and the femur were constructed and situated such that they would resist bending that they would be subjected to if Megatherium was an obligate quadruped, that is, if they were only able to walk on four legs. The analyses appear to indicate that Megatherium was adapted to a bipedal lifestyle and that the strong muscular tail, while it may have acted as an element in a tripod feeding stance, was not only strong and robust to resist this tripod feeding stance.

Literature Cited

Casinos, A. (1996). "Bipedalism and quadrupedalism in Megatherium: An attempt at biomechanical reconstruction". Lethaia. 29: 87–96. doi:10.1111/j.1502-3931.1996.tb01842.x.

Wrinkly Sloth

 In 2002 Richard Fariña published a paper in Ameghina that, while it did not forever change the view of Megatherium, did cast an interesting perspective on what the animal may have looked like. The popular outlets for prehistoric animal shows may not have adopted the look since that paper was released, but some artists, like Marcus Burkhardt (http://natural-and-history-art.de/en/home/) have at least given the idea some thought an produced hypothetical reconstructions incorporating Fariña's hypothesis. What is that hypothesis, one might be asking at this point. The hypothesis contends that, due to their size, it may have been possible that Megatherium was not covered in fur, as it is often depicted, but may have been covered by loose wrinkly skin, more like a modern elephant than the fur covered "ice age" animals we think of. The premise is that, at their size, fur would have potentially maintained their body temperature such that it would have caused overheating that would have been potentially lethal. Taking away the heat retention of fur is thought to have made this less of a problem. However, as the artist makes clear in his description, this idea and the graphic representation of this idea are speculative at best because we do not have any preserved Megatherium integument and fur (or lack thereof). The interpretation is interesting either way.