Harlan's Ground Sloth

''Paramylodon'' is an extinct genus of ground sloth of the family Mylodontidae endemic to North America during the Pliocene through Pleistocene epochs, living from around ~4.9 Mya–11,000 years ago. It is also known as Harlan's ground sloth.

Due to the partly very good fossil preservation, the body armor typical for the mylodonts has mostly been preserved in the form of osteoderms. In addition, a sex-specific difference in cranial structure can be demonstrated. According to the finds, the representatives of ''Paramylodon'' lived in open landscapes, sometimes also in mountainous locations, and most likely fed on grass-eating or mixed plant diets. Unique trace fossils also provide information about the locomotion of the animals, which was quadrupedal. In addition, due to the design of the forelimbs, a certain burrowing way of life cannot be disputed.

Within the genus only two species are recognized: ''Paramylodon harlani'' and ''Paramylodon garbanii''. The first fossil findings date back to the beginning of the 1830s. They go back to Richard Harlan, in whose honor the species was named. The genus ''Paramylodon'' was introduced by Barnum Brown in the early 20th century. Altogether, this genus looks back on an eventful history of research. In their first 150 years, the finds that are now attributed to ''Paramylodon'', and later the genus itself, were repeatedly associated with other forms, first with '' Mylodon'', but since the 1950s increasingly with ''Glossotherium''. Especially with the latter genus ''Paramylodon'' shares numerous features that argue for a close relationship. Only since the 1990s have both genera been considered distinct, with ''Glossotherium'' restricted to South America, while ''Paramylodon'' inhabited North America.

Taxonomy

''Paramylodon'' is an extinct genus of sloth from the extinct family Mylodontidae. Mylodontidae is grouped together with modern two-toed sloths of the family Choloepodidae and the extinct Scelidotheriidae, in the superfamily Mylodontoidea, with the former family being their closest living relatives.

''Paramylodon'' is closely related to ''Mylodon'' and ''Glossotherium''. In some later phylogenetic work, the results could be reproduced. In contrast, a study presented in 2019 by Luciano Varela and other involved scientists, which includes numerous fossil forms of the entire sloth suborder, sees this more critically. Here, ''Paramylodon'' and ''Glossotherium'' are closely related, ''Mylodon'', on the other hand, forms the basis of the advanced mylodonts and ''Lestodon'' clades with some forms from northern South America.
In the same year, a more-detailed phylogenetic analysis of the mylodonts was published by a research group led by Alberto Boscaini. According to the study, ''Paramylodon'', ''Glossotherium'', and ''Mylodon'' form a closer relationship within the Mylodontinae. This view also finds support from the aforementioned biochemical data, also presented in 2019.Samantha Presslee, Graham J. Slater, François Pujos, Analía M. Forasiepi, Roman Fischer, Kelly Molloy, Meaghan Mackie, Jesper V. Olsen, Alejandro Kramarz, Matías Taglioretti, Fernando Scaglia, Maximiliano Lezcano, José Luis Lanata, John Southon, Robert Feranec, Jonathan Bloch, Adam Hajduk, Fabiana M. Martin, Rodolfo Salas Gismondi, Marcelo Reguero, Christian de Muizon, Alex Greenwood, Brian T. Chait, Kirsty Penkman, Matthew Collins und Ross D. E. MacPhee: ''Palaeoproteomics resolves sloth relationships.'' Nature Ecology & Evolution 3, 2019, S. 1121–1130, doi:10.1038/s41559-019-0909-z Detailed morphological analyses published as early as 2009 by Robert K. McAfee also suggest that ''Paramylodon'' and ''Glossotherium'' are very closely related and share a common ancestor.

Below is a phylogenetic tree of the Mylodontidae, based on the work of Boscaini ''et al''. 2019, showing the position of ''Paramylodon''.

History of research

Discoveries in North and South America

The research history of ''Paramylodon'' is complex and characterized by more than 150 years of confusion and equation with '' Mylodon'' and ''Glossotherium''. However, it begins with the first discoveries of Richard Harlan (1796-1843) at Big Bone Lick in Boone County in the U.S. state of Kentucky in 1831, which included a right mandible and a clavicle. Harlan recognized that they were remains of an extinct sloth and referred them to ''Megalonyx'', which was already known from North America at the time, and within the genus to the species ''Megalonyx laqueatus'' that he had established shortly before.Richard Harlan: ''Description of the jaws, teeth, and clavicle of the Megalonyx laqueatus.'' The Monthly American journal of geology and natural science 1, 1831, S. 74–76Richard Harlan: ''Description of the jaws, teeth, and clavicle of the Megalonyx laqueatus.'' Medical and physical researches, 1835, S. 334–336 The finds were originally preserved in New York, but are now lost.

Between the years 1831 and 1836, Charles Darwin made his pioneering voyage on HMS ''Beagle'' to South America and brought back from there a large number of fossils. These were then studied by Richard Owen, one of the most important explorers of the Victorian era, and the results published. In a first publication on mammalian remains in general in 1840, he introduced the genus ''Mylodon'' with the species ''Mylodon darwinii''. The genus and species were based on a mandible Darwin found in Punta Alta in the Argentina Buenos Aires province. As a special characteristic, a total of four molar-like teeth per tooth row stood out. At the same time, Owen also noted similarities in tooth structure between Harlan's mandible and that of ''Mylodon darwinii''. Inferring this, he discarded the name coined by Harlan, ''Megalonyx laqueatus'', and created a new species, ''Mylodon harlani''.Richard Owen: ''Fossil Mammalia.'' In Charles Darwin (Hrsg.): ''Zoology of the Voyage of H.M.S Beagle, under the command of Captain Fitzroy, during the years 1832 to 1836. Part I. Fossil Mammals.'' London, 1840, S. 12–111 (S. 63–73) The genus name ''Paramylodon'' is composed of the Greek παρα (para meaning "beside" or "near"), ''μύλη'' (''myle'' " molar") and ''ὀδούς'' (''odoús'' "tooth"), thus translates as "molar tooth". Harlan commented two years later on the use of the name because, in his opinion, it did not describe any outstanding characteristic of the animal and could mean any extinct mammal because almost all of them had the posterior molars.Richard Harlan: ''Notice of two new mammals from Brunswick Cana, Georgia; with observations on the fossil quadrupeds of the United States.'' The American journal of science and arts 43, 1842, S. 141–144

In the same year, 1842, Owen presented a comprehensive description of a skeleton of a mylodont that came from the flood plains of the Río de la Plata north of Buenos Aires; he established for it the new species ''Mylodon robustus''.Richard Owen: ''Description of the skeleton of an extinct gigantic Sloth, Mylodon robustus, Owen, with observations on the osteology, natural affinities, and probable habitats of the Megatherioid quadrupeds in general.'' London, 1842, S. 1–176 At this point, then, the genus ''Mylodon'' consisted of three species, two of which occurred in South America and one in North America. Furthermore, it should prove problematic that Owen identified ''Mylodon darwinii'' as the type species of the genus, although, as he admitted, this was the second known and described species after ''Mylodon harlani''. Accordingly, ''Mylodon harlani'' would actually be entitled to the status of type species. Subsequently, different type species were assigned to ''Mylodon'', for example, Johannes Theodor Reinhardt in 1879 considered it to be ''Mylodon robustus'', Richard Lydekker in 1887, on the other hand, considered it to be ''Mylodon harlani''.Richard Lydekker: ''Catalogue of the fossil Mammalia in the British Museum of Natural History. Part V.'' London, 1887, S. 1–345 (S. 106)Juan Carlos Fernicola, Sergio F. Vizcaíno und Gerardo De Iuliis: ''The fossil mammals collected by Charles Darwin in South America during his travels on board the HMS Beagle.'' Revista de la Asociación Geológica Argentina 64 (1), 2009, S. 147–159

''Paramylodon'' and the ''Mylodon''-''Glossotherium'' problem

Within the genus ''Paramylodon'', only one species, ''P. harlani'', is recognized. Another species, ''P. nebrascensis'', was described in 1903 by Barnum Brown on the basis of a partial skeleton from Hay Spring in Nebraska, but was already united with the type species in the 1920s. Only ten years later, Glover Morrill Allen created the species ''Mylodon garmani'' with the help of another partial skeleton from the Niobrara River in Nebraska,Glover M. Allen: ''A new Mylodon.'' Memoirs of the Museum of Comparative Zoology 40 (7), 1913, S. 319–346 but this is also considered a synonym of ''Paramylodon harlani''. The same applies to several species named by Edward Drinker Cope as early as the 1870s and 1890s, such as ''Mylodon sodalis'' and ''Mylodon sulcidens''. The original subdivision into two subspecies, ''P. h. harlani'' for a robust and ''P. h. tenuiceps'' for a graceful form, as suggested by Chester Stock in 1917, is no longer advocated today. However, the species "''Glossotherium''" ''chapadmalense'' is problematic. The species was originally identified in 1925 by Lucas Kraglievich from a 39 cm long, nearly undamaged skull with mandible from Middle Pliocene strata east of Miramar in the Argentina Buenos Aires province.Lucas Kraglievich: ''Cuatro nuevos gravigrados de la fauna Araucana "Chapadmalense".'' Anales del Museo Nacional de Historia Natural de Buenos Aires 33, 1925, S. 215–235 It shows similarities to ''Glossotherium robustum'', but also possesses individual divergences that may justify its own generic status; for example, the name "''Eumylodon''" (which Kraglievich already used for ''Eumylodon chapadmalense'' in 1925) has been suggested. The form could thus be the common ancestor of ''Glossotherium'' and ''Paramylodon''. However, whether this also applies to the North American finds from the Pliocene of Florida and Mexico, first listed under the same species name by Jesse S. Robertson in 1976, or these are closer to ''Paramylodon'' is currently unclear due to lack of comparative studies. Partially, the early mylodont remains are also listed as ''P. garbanii'', a species name given in 1986 to some Pliocene mandible and limb remains from Arroyo EI Tanque in the Mexican State of Guanajuato had been coined (under the scientific name ''Glossotherium garbanii'').Gary S. Morgan: ''Vertebrate fauna and geochronology of the Great American Biotic Interchange in North America.'' New Mexico Museum of Natural History & Science Bulletin 44, 2008, S. 93–140Marisol Montellano-Ballesteros und Oscar Carranza-Castaneda: ''Descripcion de un milodontido del Blancano temprano de la mesa central de Mexico.'' Instituto de Geología, Universidad Nacional Autónoma de México, Revista 6, 1986, S. 193–203 The species is not fully recognized, however, and other authors consider it a synonym of ''Glossotheridium/Glossotherium chapadmalense''.

In 1903, Barnum Brown (1873-1963) introduced the generic name ''Paramylodon''. He used for this purpose a partial skeleton from Hay Spring in Nebraska that had been discovered in 1897 during an expedition of the American Museum of Natural History. To the genus he assigned ''Paramylodon nebrascensis'' as a species. As defining differences from the North American ''Mylodon harlani'', which Brown considered the type species of ''Mylodon'', he gave the missing anterior caniniform teeth in the upper jaw. Thus, at that time, two distinct representatives of Mylodonts were recognized in the Pleistocene of North America.

Later, Chester Stock (1892-1950), based on his studies of the Rancho La Brea find material, pointed out that the feature of missing upper front teeth is highly variably developed in ''Mylodon harlani''. Therefore, synonymized he replaced ''P. nebrascensis'' with ''Mylodon harlani'' in 1917. In 1928, however, Lucas Kraglievich