Simomylodon

''Simomylodon'' is an extinct genus of ground sloths from the family Mylodontidae. It lived from the Late Miocene to the Middle Pliocene of what is now Bolivia and Argentina, 5.3 to 2.8 million years ago. The most important find material comes from the central Altiplano in Bolivia and includes several skulls and dentition remains. Thus, the so far documented body skeleton is the best known and most significant of a Miocene representative of the Mylodontidae. On the basis of the remains, it can be concluded that it is a rather small member of the Mylodontidae. The construction of the limbs supports ground-dwelling locomotion, but this does not exclude occasional digging or climbing. The type and only known species is ''Simomylodon uccasamamensis''.

Discovery and naming

The scientific first description of ''Simomylodon'' was made in 2010 by Pierre-Antoine Saint-André and fellow researchers. As a basis served the finding material of the sites Ayo Ayo and Viscachani in the Bolivian departament of La Paz, additionally also that from Pomata-Ayte in the Oruro Department, whereby Viscachani is considered as type locality. The holotype (specimen number GB 078) consists of an anterior partial skull lacking teeth. The genus name ''Simomylodon'' derives on the one hand from the Greek ''σίμός'' for "blunt-nosed" and refers to the partly blunt expression of the median jawbone, on the other hand it refers to the genus '' Mylodon'', which, according to the opinion of the first authors at that time, could be derived from ''Simomylodon''. The only species named by Saint-André and colleagues was ''Simomylodon uccasamamensis''. The species epithet has its origin in Sanskrit (''ucca'' for ''high'' and ''samam'' for ''plain'' or ''vastness'') and is a reference to the region of discovery in the Altiplano ( Spanish for "plateau"). The species name ''uccasamensis'' had already been mentioned by Saint-André in 1994 in a modified form (''uccasamamense'') in his unpublished graduate thesis in connection with "Simotherium".Pierre-Antoine Saint-André, François Pujos, Cástor Cartelle, Gerardo De Iuliis, Timothy J. Gaudin, H. Gregory McDonald und Bernardino Mamani Quispe: ''Nouveaux paresseux terrestres (Mammalia, Xenarthra, Mylodontidae) du Néogène de l'Altiplano bolivien.'' Geodiversitas 32 (2), 2010, S. 255–306

In the same paper that contained the initial description of ''Simomylodon,'' Saint-André and his research team established the new species ''P. dalenzae'' placed in the genus ''Pleurolestodon'' on the basis of a complete skull from Choquecota in the Bolivian department of Oruro. Like ''uccasamamense'', Saint-André had already used the species name ''dalenzae'' in his 1994 doctoral dissertation, but linked it to ''Glossotheriscum''. Due to numerous new finds that did not reveal any differences with ''Simomylodon'', ''Pleurolestodon dalenzae'' was synonymized with ''Simomylodon uccasamamensis'' in 2019 by a team of scientists led by Alberto Boscaini. The latter species is consequently the only recognized one within the genus ''Simomylodon'' so far.Alberto Boscaini, Timothy Gaudin, Bernardino Mamani Quispe, Philippe Münch, Pierre-Olivier Antoine und François Pujos: ''New well-preserved craniodental remains of Simomylodon uccasamamensis (Xenarthra, Mylodontidae) from the Pliocene of the Bolivian Altiplano: phylogenetic, chronostratigraphic, and paleobiogeographic implications.'' Zoological Journal of the Linnean Society 185 (2), 2019, S. 459–486 Other fossil material, such as single mandibles from Inchasi in the department of Oruro, was previously referred to other genera of mylodonts, such as '' Glossotheridium''.Federico Anaya und Bruce J. MacFadden: ''Pliocene mammals from Inchasi, Bolivia: the endemic fauna just before the Great American Interchange.'' Bulletin of the Florida Museum of Natural History 39 (3), 1995, S. 87–140

Description

Skull and dentition features

''Simomylodon'' was a rather small representative of the Mylodontidae, whose body weight was estimated to be around 228kg or 370kg, depending on the measurement methodology.Ascanio D. Rincón, Andrés Solórzano, H. Gregory McDonald und Mónica Núñez Flores: ''Baraguatherium takumara, Gen. et Sp. Nov., the Earliest Mylodontoid Sloth (Early Miocene) from Northern South America.'' Journal of Mammalian Evolution 24 (2), 2017, S. 179–191Alberto Boscaini, Néstor Toledo, Bernardino Mamani Quispe, Rubén Andrade Flores, Marcos Fernández‐Monescillo, Laurent Marivaux, Pierre‐Olivier Antoine, Philippe Münch, Timothy J. Gaudin und François Pujos: ''Postcranial anatomy of the extinct terrestrial sloth Simomylodon uccasamamensis (Xenarthra, Mylodontidae) from the Pliocene of the Bolivian Altiplano, and its evolutionary implications.'' Papers in Palaeontology, 2021, doi:10.1002/spp2.1353 Finds of the genus include several skulls and additional postcranial skeletal elements. The skull was elongate, with the skullcap and skull base nearly parallel in lateral view, but the anterior section was slightly lower than the posterior. In plan view, the posterior section of the skull was strikingly broad compared to the overall length. This is reminiscent of '' Pleurolestodon'', but differs from other mylodonts such as ''Glossotherium'' and '' Paramylodon''. The rostrum was short. The nasal bone showed a sharp constriction just in front of the eye windows and progressively widened both anteriorly and posteriorly. The anterior margin was largely convex. The forward widening snout in plan view is a typical characteristic of mylodonts. In lateral view, the snout was dominated by the maxilla, which fused with the nasal bone along nearly its entire length. The midjaw bone possessed a V-shaped to more curved appearance. forehead and parietal bone were flat. Slight temporal ridges rose on the parietal bones, which ran parallel but did not unite to form a parietal crest. The zygomatic arch was not closed. The anterior arch segment, attached to the zygomatic bone, consisted of three processes, one ascending, one horizontal, and one descending, as is common in sloths. The largest was the ascending process, which began broadly at the base and ended in a rounded tip. The horizontal process had a triangular shape and met with the long-narrow posterior arcuate portion of the temporal bone. The extremely long shape of the posterior zygomatic arch segment can be taken as an atypical feature of mylodonts. The descending process ended partially hook-shaped. The lacrimal bone was broad and formed part of the orbital rim, with the section involved in the ocular window exceeding that of the face. The occipital bone was vertical to sloping backward. The occipital bulge was strongly developed throughout. The joints for articulation with the cervical spine were prominent and clearly separated. At the base of the skull, the wing bones showed marked inflation. The palatine roof widened anteriorly analogous to the rostrum, thus possessing a V-shaped configuration.
The lower jaw was about 24 cm long and about 5.5 cm high below the molar-like teeth. Thus it appeared short and stocky. At the lower edge the horizontal bone body was almost straight, a characteristic of the mylodonts. Anteriorly, the symphysis extended into a process typical of faunal bones, which in ''Simomylodon'' was short and broad and terminated in a straight edge, comparable to ''Glossotherium'' and ''Lestodon'' but divergent from ''Paramylodon''. In lateral profile, the lower edge of the symphysis was irregular due to various depressions and indentations. The variable formation of the foramina mentalia with one to six openings is remarkable, although size and number varied greatly even in single individuals. The foramen mandibulae opened on the inner side of the mandible clearly behind the last tooth and below the crown process. On the ascending branch, all three processes (crown, articular, and angular) occurred distinctly separated from each other. The anterior margin of the ascending branch did not cover the last molar-like tooth. This is consistent with ''Paramylodon'', but differs from ''Pleurolestodon'' and '' Mylodon''. The crown process rose steeply and had a hooked end. The articular process sat at about the level of the masticatory plane of the teeth, and the joint itself was wider than long. The angular process at the posteriormost end of the mandible stood out clearly from the horizontal body of the bone by its bulging lower edge

The dentition of ''Simomylodon'' showed the typical structure of sloths. The respective upper row of teeth consisted of five, the lower of four teeth, thus a total of 18 teeth were formed. All teeth were in line except for the anterior tooth of the lower jaw, which was slightly displaced laterally to the outside. In the upper dentition the rows diverged from each other, in the lower they were more parallel. In both the upper and lower dentition, the foremost tooth in each case had a caniniform shape, all other teeth were molariform. This unites ''Simomylodon'' with most mylodonts except for instance ''Mylodon'', whose foremost tooth in the upper dentition was reduced. A diastema between the caniniform and the molariform teeth was not developed, which is a difference to Lestodon with its extraordinarily large tooth gap. The anterior, canine tooth had a semicircular to triangular cross-section. It was relatively small, roughly consistent with Glossotherium and Pleurolestodon, but clearly different from ''Lestodon'' with its greatly enlarged anterior teeth. The molar-like teeth had the flat occlusal surface characteristic of mylodonts formed from two lobate (bilobate) structures that appeared diamond- or T-shaped in outline. Only the anterior upper molar-like tooth was oval in cross-section. The upper row of teeth was between 9.0 and 10.4 cm long, of which the molariform teeth occupied between 7.5 and 8.6 cm. The largest tooth was the rearmost in each case.

Skeletal characteristics

From the body skeleton of ''Simomylodon'' mainly the elements of the forelimbs and hindlimbs have been preserved. The in plan view triangular shoulder blade was largely similar to that of other mylodonts and possessed a massive shoulder bone. This extended ventrally and merged with the acromion or shoulder level, which in turn joined with the coracoid process. This created a characteristic arch called the "acromiocoracoid arch," a striking characteristic of sloths. The humerus measured 20.8 to 25.8 cm in length and had a hemispherical condyle to which the small protuberance (tuberculum minus) was directly attached. The large protuberance (tuberculum majus) merged into a pronounced deltopectoral groin on the shaft side, which functioned as a muscle attachment site. It was more prominent than in ''Glossotherium'' or ''Lestodon''. The lower end of the joint protruded characteristically, but the external epicondyle was less conspicuously developed than in ''Glossotherium''. The radius reached a length of 18.9 to 20.1 cm and appeared comparatively shorter and more compact than in ''Paramylodon'', for example. The ulna has been preserved only fragmentarily, lacking the significant upper articular process, the olecranon. However, it was comparatively short in shape. The femur was 30.7 to 34.3 cm long. It was board-like flat, as in most ground sloths. Its shaft curved slightly sideways. There was a roughened area on the outer longitudinal edge that indicated the third rolling mound and was more developed than in ''Glossotherium''. The greater trochanter was broad and deep, but not very raised, and thus lay below the condyle. This in turn sat on a short neck. At only 15.4 to 18.8 cm long, the tibia was barely half the length of the femur. This is typical for mylodonts and a clear contrast to, for example, the Megatheriidae with their strikingly longer lower leg sections. The joint ends appeared robust and broad in ''Simomylodon'', the upper exceeding the lower in width, which is typical of ground-dwelling sloths. In addition to the slender fibula, a cyamella was also developed, a sesamoid bone of globular shape. Hands and feet survived with single root bones, metapodials, and phalanges. Reconstructed, the hand consisted of five rays, but only the first three fingers were claw-reinforced, which can be recognized by the corresponding pointed end-limbs. This configuration also commonly occurs in later mylodonts. The metacarpal bones were slender and elongate in construction. The third metacarpal reached up to 7.2 cm in length and in some cases was fused to the capitate bone. The second and first metacarpal bones were 5.5 and 3.8 cm long, respectively. The longest was the fourth with about 8 cm, while the outer one had the dimensions of the third. Only a few metatarsals are documented from the foot, except for the talus and calcaneus and other root bones, which also appeared relatively slender and graceful.

Osteoderms

Mylodonts were the only group of sloths to possess bony skin deposits, or the so-called osteoderms, which today are only found in armadillos. This is known especially from ''Mylodon'', ''Glossotherium'' and ''Paramylodon''. For ''Simomylodon'' also single bone platelets have been preserved, which belong to the earliest evidence within the mylodonts. A larger concretion found together with a skull consists of several osteoderms about 4 mm long. A piece recovered in isolation again measured about 17 mm in diameter. The wide variation in dimensions is remarkable. A position close to the head or neck is presumed for the compact assemblage.Sofía I. Quiñones, Ángel R. Miño-Boilini, Alfredo E. Zurita, Silvina A. Contreras, Carlos A. Luna, Adriana M. Candela, María Camacho, Marcos D. Ercoli, Natalia Solís und Diego Brandoni: ''New records of Neogene Xenarthra (Mammalia) from eastern Puna (Argentina): diversity and biochronology.'' Journal of Paleontology 93 (6), 2019, S. 1258–127, doi:10.1017/jpa.2019.64

Distribution

Most of the fossil remains of ''Simomylodon'' were recovered in the central part of the Altiplano in southwestern Bolivia. The remains are distributed among several find points. The currently oldest is a skull from Choquecota in the Oruro Department about 3.5km southwest of the eponymous locality. It was deposited in the upper part of the largely Miocene Rosa Pata Formation in a depositional sequence of reddish sandstones about 15m below a prominent tuff band. This in turn is referred to as Toba 76 and is common as a marker horizon in the central Altiplano. Radiometric datings give the tuff an age of about 5.3 million years, placing it in the transition from the Upper Miocene to the Lower Pliocene

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