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.
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.
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 anterior edge of the ascending branch inserted at an obtuse angle to the alveolar plane of the horizontal body of bone.
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.
A special feature of Megathericulus is the extreme front and rear narrowing of the teeth, so that they appear very rectangular in cross section.
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 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.
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.
Other finds were documented in western Chubut, including a left mandibular branch and also a right ankle bone from the Collón Curá Formation.
[4] In the Río Mayo Formation, numerous fragmented remains were recovered, including parts of the lower jaw and various elements of the musculoskeletal system.
The fossils came from two different find sites and each represented a single individual per locality, which could be deduced from the matching sizes and preservation states.
A partial skeleton of Megathericulus consisting of a complete skull without the lower jaw and remains of the forelimb was found in an outcrop on the north bank of the Río Rosario.
The deposits in which the fossils were embedded date back to what was once a large wetland of swamps, lakes and rivers that drained north into the Caribbean Sea.
In a classical system based on skeletal anatomical features, the Megatheriidae, together with the Megalonychidae and the Nothrotheriidae, form a more closely related group, the superfamily of the Megatherioidea.
Megathericulus is considered a basal form of the phylogenetically more modern branch of the Megatheriidae, which are summarized in the subfamily of the Megatheriinae and also includes the aforementioned Eremotherium and Megatherium.
[10] In 1939, Ángel Cabrera named the species M. primaevus using individual postcranial skeletal parts (ribs, ulna , tibia , talus, and heel bone) from the Cañadon Ftamichi near Paso Flores in the Argentine province of Río Negro.
[4] Another species had already been established in 1930 by Lucas Kraglievich with M. friasensis, citing a partial skull from the Río Frias in Chile, but according to most authors it is a representative of the Scelidotheriidae, which in turn belong to the Mylodontidae family.
In 2013, a research team led by François Pujos referred M. cabrerai and M. andinum to Megathericulus, two species originally in the genus Eomegatherium stood.