Megathericulus is an extinct genus of ground sloths in the Megatheriidae family. It lived during the Middle Miocene, 11-16 Ma in what is now South America. Fossils have been found principally in Argentina, Bolivia, and Peru. It is a smaller representative of the megatheres. Despite being one of the earliest-known members of the family, its dentition structure is associated with homodont teeth belonging to the more modern line of evolution. The genus was scientifically named in 1904. Only one species is currently recognized, Megathericulus patagonicus.
Description Megathericulus is a small member of the Megatheriidae. The known fossil material of the genus consists of individual skull, remains of teeth, and parts of the musculoskeletal system. The skull measured up to 33 cm in length, it was elongated and narrow with the greatest width in the area of the anterior zygomatic arch and the mastoid process and slight constrictions near the orbit. In its general shape it closely resembled the skulls of Planops or Pyramiodontherium, while that of Megatherium was significantly more robust. When viewed from the side, it had a low height. The forehead line was arched and sloping towards the rostrum. The latter was extremely elongated, with the front toothless area corresponding in extent to the tooth-bearing area. In later megatheriids, the anterior, toothless section was mostly shorter in proportion. The temporal fossae were prominent and converged toward the midline of the skull, but did not form a prominent crest. The infraorbital foramen opened approximately 2 cm above the alveolar plane. The anterior arch of the zygomatic arch attached between the second and third molar-like teeth, which is set somewhat further back than in Anisodontherium and Pyramiodontherium. The occiput was vertical, the joints for articulation with the cervical spine were slightly above the chewing plane of the maxillary teeth, approximately halfway up the skull. They were higher than in Anisodontherium and Pyramiodontherium but lower than in Megatherium. The palate extended through the extended snout like a spatula. It was very narrow and, with a width of 2.5 cm, did not exceed the dimensions of the tooth alveoli. When viewed from the side, the lower jaw had a lower edge that clearly bulged downwards, which is considered a typical feature of megatheres. The deepest point and thus the greatest height of the horizontal bone body was reached under the third to fourth molar-like teeth. The position was significantly further back than in phylogenetically younger Megatheriums. The alveolar surface was slightly dented, which roughly corresponded to the ratio in Anisodontherium or Promegatherium, but differed from the straight course in Pyramiodontherium. The symphysis ended just short of the foremost molar-like tooth as in Anisodontherium, while in Pyramiodontherium and Pliomegatherium reached further back. The anterior edge of the ascending branch inserted at an obtuse angle to the alveolar plane of the horizontal body of bone. Below the anterior end of the ascending branch was the posterior opening of the mandibular canal. The dentition of Megathericulus corresponded to that of the typical sloth with five teeth per upper jaw and four teeth per lower jaw. A total of 18 teeth were thus formed. The teeth of the upper jaw stood closed in parallel rows, but the outer edge made a slight curve. In the lower jaw, too, the teeth formed a closed row, and there was no lateral displacement of the first tooth, as is often observed in the representatives of the Mylodontidae and Megalonychidae. The teeth had the structure typical of the Megatheriidae. They were consistently molar-like in shape (molariform) and thus homodont, unlike most other sloths. The occlusal surfaces showed two sharp, transverse ridges separated by a V-shaped indentation. A special feature of Megathericulus is the extreme front and rear narrowing of the teeth, so that they appear very rectangular in cross section. This made the last upper tooth 0.7 cm long and 1.6 cm wide. Similarities to Anisodontherium can be seen in this feature, while the phylogenetically younger forms in particular had more square to trapezoidal tooth cross-sections. In accordance with most megatheres, the teeth were very high-crowned (hypsodont), which also explains the special shape of the lower jaw with the bulge of the lower edge. The hypsodonty index (ratio of the height of the horizontal bony body of the mandible to the length of the row of teeth) for Megathericulus was 0.92 and thus indicates an average value for Megatheria (Megatheriops and Pyramiodontherium below 0.9, Megatherium and Anisodontherium above 1.0). The length of the upper row of teeth was about 8 cm. Parts of the front and hind legs are documented from the body skeleton. The humerus was 36 to 38 cm long and generally slim. The large and small bony prominences on the head (tuberculum majus and tuberculum minus) reached large dimensions. A strong and extensive deltopectoral ridge ran along the shaft, which no longer appears clearly in later megatherias. The humerus widened considerably downwards and measured between 13.6 and 16.2 cm at the end of the joint (elbow joint). The ratio of the lower width to the total length of the bone thus corresponded approximately to that of Megatherium and was significantly larger than in Pyramiodontherium and Megatheriops. Most of the other long bones are, with exceptions, only fragmented. The ulna was characterized by a rather short and narrow upper articular process (olecranon). In the first aspect, this agrees with other megatheriines, the latter is different from the mostly wide olecranon of the other members of the family. As in almost all megatheriids, but unlike other sloths, the lower end of the tibia and fibula were fused together. The shin itself measured about 28 cm in length and was also slender. It broadly corresponded to that of Pyramiodontherium. Based on the joint facets on the second metacarpal that has been handed downward it can be concluded that a metacarpal-carpal complex (MCC) was also formed in Megathericulus. This consisted of the fused first metacarpal and the large polygonal bone and is a typical feature of megatheriids and some other large ground sloths.
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