The late, great armored sloth(s)

If you read my previous blog, you know that armadillos and their extinct relatives weren’t the only armored South American mammals. A few extinct sloths also had small pieces of bone in their skin (osteoderms), though referring to any of them as armored would be a stretch of the imagination. Nevertheless, some paleontologists once thought that something like an armored sloth really did live in South America during the late Oligocene, about 25 million years ago. This is no longer thought to be the case, but is an interesting tale of scrappy specimens, taxonomic tangles, and a bit of imagination.

The idea of a sloth-like mammal with armor traces its roots back to a French paleontologist named Albert Gaudry. Gaudry wrote a series of publications around the turn of the 20th century about the evolution of mammals in South America and came to the conclusion that the fossil remains of several species described by Florentino Ameghino – Palaeopeltis inornatus, Orophodon hapaloides, and Octodontotherium grande – all pertained to the same animal. The latter two species were based on isolated, sloth-like teeth of two different shapes and sizes: small and cylindrical in the case of Orophodon and larger and lobed in the case of Octodontotherium. Palaeopeltis (“ancient small shield”) was based on osteoderms that were large but quite plain, unlike those of most armadillos and glyptodonts. The famous paleontologist G.G. Simpson described them as “in a sense, characterized by [their] lack of characters, … practically devoid of any decoration” (Simpson 1948:94).

Figure 1 Osteoderms Palaeopeltis.jpg
Ameghino’s original illustration of Palaeopeltis inornatus osteoderms (left, from Ameghino 1897:fig. 83), compared to Palaeopeltis specimens collected by Elmer Riggs (middle) and an osteoderm of a typical glyptodont, Propalaehoplophorus (right, from Pampa Castillo, Chile). Images not to scale. Photos by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

It isn’t entirely clear why Gaudry believed that both types of teeth and the osteoderms all pertained to a single species, resulting in an animal somewhere between a sloth and an armadillo. He did think that the size differences between the teeth could be explained if the smaller teeth were premolars and the larger ones molars (premolars are smaller and simpler than molars in most mammals), but their association with the osteoderms seems to have been based more on imagination than informed interpretation. Unfortunately, once this suggestion had been made, a lot of time was spent by subsequent paleontologists debating whether or not that was the case.

The next person to address this issue was the Argentine paleontologist Lucas Kraglievich. In 1931, Kraglievich published the description of a new species of sloth from geologically younger (late Miocene) rocks of Buenos Aires Province that had teeth resembling those of Orophodon and Octodontotherium. As part of his study, he examined some new sloth fossils from Argentina that had been collected by Elmer Riggs of the Field Museum. Kraglievich noticed that a partial skull and other specimens collected by Riggs had figure-eight-shaped teeth matching those of Ameghino’s Octodontotherium (a characteristic referred to by the “octo” in the name Octodontotherium) but no small, simple, Orophodon-like teeth. He concluded that Octodontotherium grande was a species of sloth distinct from Orophodon hapaloides but left open the possibility that the teeth of Orophodon could belong to the same animal as the Palaeopeltis osteoderms.

Figure 2 Orophodon Octodontotherium.jpg
Ameghino’s original illustrations of Orophodon and Octodontotherium (left, from Ameghino 1897:figs 81-82) compared to a partially reconstructed skull of Octodontotherium collected by Elmer Riggs (right, FMNH P13583, photo by D. Croft). Note the size difference between the Orophodon and Octodontotherium teeth (which are sized correctly relative to one another) and the similarity between the Octodontotherium tooth and the tooth sockets of Rigg’s skull (scale bar = 5 cm; not at same scale as the Ameghino illustrations). Reuse permitted under CC BY-NC-SA 2.0.

Two decades later, additional clues about the issue came to light in the form of new late Oligocene fossils from Rio Negro province, Argentina. Among these new specimens were two large, simple osteoderms of Palaeopeltis associated with an unusual upper jaw and facial region unlike that of any other animal. The teeth of this specimen were cylindrical like those of most armadillos and some sloths, but the snout was short and deep like that of a glyptodont rather than long and narrow like an armadillo. Equally unusual was the presence of teeth at the front of the mouth (in the premaxilla) in addition to the sides. Although this is the condition in most mammals, including humans, incisor-like teeth at the front of the mouth are absent in nearly all sloths, armadillos, and glyptodonts. The only exceptions are the peculiar “horned” armadillos known as peltephilids, which have a continuous, semicircular row of teeth that runs from one side to the other. In every other respect, the new Rio Negro specimen bore little resemblance to a peltephilid. Unfortunately, all of the teeth had broken tips, which made detailed comparisons with other types of sloths and armadillos difficult.

The new Rio Negro specimens were described by Jorge Lucas Kraglievich (son of Lucas Kraglievich) and Santiago Rivas. In their view, the Orophodon teeth described by Ameghino were compatible in size and shape with teeth preserved in the new upper jaw. Since that jaw was conclusively associated with Palaeopeltis osteoderms, they concluded that Orophodon and Palaeopeltis were the same animal. But in their mind, the animal was a glyptodont-like animal (and therefore a cingulate) with sloth-like teeth rather than an armored sloth (a pilosan). They decided to erect a new superfamily for this strange armored animal to separate it from both glyptodonts and armadillos (including peltephilids), settling on the name Orophodontoidea. (Since Ameghino had named Orophodon before Palaeopeltis – in the same publication but on an earlier page – Orophodon was the valid name for the animal and any groups based upon it.)

Figure 3 Occlusal View.jpg
Upper jaw of Palaeopeltis from Rio Negro (from Kraglievich and Rivas 1951:fig. 4) compared to those of other armored mammals, including the horned armadillo Peltephilus pumilus (modified from Scott 1903a:pl. XVI.4), a glyptodont (Parapropalaehoplophorus australis, modified from Scott 1903b:pl. XXIII.2), and an extinct armadillo (Proeutatus lagena, modified from Scott 1903a:pl. XIII.2). The Palaeopeltis specimen only includes one side of the upper jaw rather than both, but note that the arrangement of its teeth is more similar to that of Peltephilus than the glyptodont or the armadillo. Images not to same scale. Reuse permitted under CC BY-NC-SA 2.0.

In 1954, a French paleontologist, Robert Hoffstetter, entered this nomenclatural foray and resurrected the idea of an armored sloth (actually two armored sloths) based on previously undescribed specimens he studied at the natural history museum in Paris. These specimens included a partial jaw bearing Orophodon-like teeth and two others with Octodontotherium-like teeth. Since all of them clearly pertained to sloths, Hoffstetter reached the conclusion that Orophodon and Octodontotherium were separate sloth species but that they both likely also had Palaeopeltis-like osteoderms. Because the peculiar upper jaw described by Kraglievich and Rivas came from a different part of Patagonia than Ameghino’s specimens, Hoffstetter thought it represented a different species altogether that simply happened to have similar osteoderms. Therefore, he decided to make the Rio Negro specimens the basis for a new species, Pseudorophodon kraglievichi. The genus name, meaning “false Orophodon” reflected Hoffstetter’s belief that it represented a glyptodont-like or armadillo-like animal – perhaps related to the horned armadillos – that had independently evolved features similar to the armored sloths Orophodon and Octodontotherium.

Figure 4 Hoffstetter.jpg
Lower jaw specimens of Orophodon and Octodontotherium in side (above) and occlusal (below) views, from Hoffstetter (1954:figs. 2-3.) Note that M4 (the fourth molariform tooth) is slightly lobed in Orophodon, whereas both M3 and M4 appear “pinched” and resemble the number eight in Octodontotherium. Images not to same scale. Reuse permitted under CC BY-NC-SA 2.0.

In 1968, Bryan Patterson and Rosendo Pascual reached a different conclusion about these specimens that essentially vindicated Ameghino’s original description of them: Orophodon hapaloides and Octodontotherium grande were two separate but closely-related sloths, and Palaeopeltis inornatus (represented by plain osteoderms and a strange upper jaw) was an unusual xenarthran more closely related to armadillos and glyptodonts than sloths. Patterson and Pascual did not think that Orophodon and Octodontotherium were armored but, somewhat coincidentally, these sloths are now recognized as early members of the family Mylodontidae – the same family in which at least some geologically younger species had small osteoderms in their skin.

The affinities of Palaeopeltis are no clearer now than they were half a century ago when Patterson and Pascual placed it in its own family, Palaeopeltidae. Whatever Palaeopeltis is or was, we do know that it lived throughout much of South America, as its distinctively plain osteoderms have been found in Uruguay and Bolivia in addition to southern Argentina. We also know that its lineage existed in South America for at least 10 million years; Florentino Ameghino referred two species to the genus: Palaeopeltis tesseratum from the late Eocene and Palaeopeltis inornatus from the late Oligocene. It seems to have been a relatively large, glyptodont-like animal – perhaps the size of a giant tortoise – but given the sparse material currently known, one cannot say much more than that.

Palaeopeltis is a quintessential example of an animal whose existence has been known for more than a century but about which we still know very little. Animals such as this are one of the main reasons paleontologists continue to visit the same sites year after year, decade after decade. Sooner or later, someone is going to find the bones that connect the osteoderms of Palaeopeltis with its strange upper jaw, allowing us to finally flesh out another fascinating South American mammal.

References Cited:

  • Ameghino, F. 1895. Premiere contribution à la connaissance de la faune mammalogique des couches à Pyrotherium; pp. 361-445 in A. J. Torcelli (ed.), Obras Completas y Correspondencia Científica de Florentino Ameghino, Vol. XI. Taller de Impresiones Oficiales, La Plata, Argentina.
  • Ameghino, F. 1897. Mammiféres crétacés de l’Argentine. Deuxiéme contribution á la connaissance de la faune mammalogique des couches á Pyrotherium; pp. 299-461 in A. J. Torcelli (ed.), Obras Completas y Correspondencia Científica de Florentino Ameghino, Vol. XII. Taller de Impresiones Oficiales, La Plata, Argentina.
  • Gaudry, A. 1906. Fossiles de Patagonie: Étude sur une portion du monde antarctique. Annales de paléontologie 1:101-143.
  • Hoffstetter, R. 1954. Les gravigrades cuirasses du Deseadien de Patagonie (Note preliminaire). Mammalia 18:159-169.
  • Kraglievich, L. 1931. Cuatro notas paleontológicas (sobre Octomylodon aversus Amegh., Argyrolagus palmeri Amegh., Tetrastylus montanus Amegh. y Munizia paranensis, n. gen., n. sp.). Physis 10:242-266.
  • Kraglievich, J. L., and S. Rivas. 1951. Orophodon Amegh. representante de una nueva superfamilia Orophodontoidea del suborden Xenarthra (nota preliminar). Comunicaciones del Instituto Nacional de Investigación de las Ciencias Naturales “Bernardino Rivadavia,” Ciencias Zoológicas 2:9-28.
  • Patterson, B., and R. Pascual. 1968. The fossil mammal fauna of South America. Quarterly Review of Biology 43:409-451.
  • Scott, W. B. 1903a. Mammalia of the Santa Cruz Beds. Volume V, Paleontology. Part I, Edentata. 1. Dasypoda; pp. 1-106 in W. B. Scott (ed.), Reports of the Princeton University Expeditions to Patagonia, 1896-1899. Princeton University, E. Schweizerbart’sche Verlagshandlung (E. Nägele), Stuttgart.
  • Scott, W. B. 1903b. Mammalia of the Santa Cruz Beds. Volume V, Paleontology. Part I, Edentata. 2. Glyptodonta and Gravigrada; pp. 107-227 in W. B. Scott (ed.), Reports of the Princeton University Expeditions to Patagonia, 1896-1899. Princeton University, E. Schweizerbart’sche Verlagshandlung (E. Nägele), Stuttgart.

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