[2][3][4] Mammalodon fossils have been found in Australia and New Zealand Mammalodon was, at first, considered to be a member of Archaeoceti, an ancient group of whales, which was evidenced by its apparent ancient features, such as the variety of differently shaped teeth in its jaw (heterodonty) that modern whales lack.

The left maxilla—upper jaw—of specimen NMV P199986 preserved four premolars and three molars, and the space between the teeth (diastema) increased towards back into the mouth.

The top-most breastbone, the manubrium, is T-shaped and wider than is long like archaeocetes, but plate-like and compressed like modern baleen whales.

Unlike in modern whales though similar to archaeocetes, the thyrohyoid bone of the hyoid apparatus used to hold up the tongue is large and tubular as opposed to plate-like.

It probably had a fused mandibular symphysis linking the two halves of the jaw together, unlike in later and modern baleen whales.

As such, it was not able to filter-feed in the same manner as extant baleen whales, making its diet and ecological niche a mystery.