Archaeomylodon is so far known only from a skull of an adult individual. The skull has a length of 59.7 cm, the width of the skull is 23.8 cm and the height is 21 cm. Thus, it belongs to the largest known mylodontid. Archaeomylodon might have reached around the same dimensions of the giant Lestodon, which weighs up to 4 tons. The skull showed an elongated tube-like shape typical for large ground sloths and was built almost rectangular in top view. Only in the area of the eyes was there a small constriction as well as a small widening in the nasal bone-upper jaw section. The anterior section was narrower than in Lestodon, but broader than in Mylodon. In lateral view, the frontal line was flat, lacking a dome-like bulge such as occurred in Glossotherium or Thinobadistes. The nasal bone was oriented slightly upwards. In anterior view, this resulted in a high nasal opening, reminiscent of Mylodon and differing from the distinctly flat one in Glossotherium. The anterior zygomatic arch originated from the maxilla above the third molar-like tooth. The parasagittal ridges on the parietal bone were widely spaced, comparable to Mylodon but unlike the close position to each other in Lestodon. The occipital bone in posterior view had a high and more rounded shape corresponding to Mylodon. In Lestodon and Glossotherium this was rather flattened and therefore broader. On the underside of the skull, the margins of the palate were largely parallel to each other and not divergently oriented anteriorly as in Glossotherium and Lestodon.
