Janjucetus Hunderi Explained
A reconstruction of Janjucetus hunderi by Carl Buell. Unlike its modern baleen-bearing relatives, Janjucetus likely lacked baleen and instead had large eyes, probably used for locating prey.
A small, new genus and species of toothed whale was recently named, Janjucetus hunderi, from the Late Oligocene (ca. 24-27 Ma) of Australia (Fitzgerald, 2006). Detailed analyses by Fitzgerald indicate that Janjucetus represents one early representative of Mysticeti. Mysticeti includes the modern whales that lack teeth but use baleen to filter out food from large quantities of seawater and/or bottom sediments. Baleen is composed of long plates that are suspended from the upper jaw and made of keratin (the protein that is abundant in hair and fingernails). Janjucetus is one of a small, but growing, group of early mysticetes that retained teeth. While the skulls of toothed mysticetes are quite different from their toothless living relatives, the skull of Janjucetus is even further removed from its toothy contemporaries. For example, unlike most other toothed mysticetes (such as aetiocetids), Janjucetus differs by having a much shorter, broader snout, a “rosette” of teeth at the front of the upper jaw (a bundle of outwardly pointing, long sharp teeth), and extremely large eyes.
Fitzgerald (2006) logically points to these features as evidence against bulk feeding in Janjucetus. Bulk feeding is seen in living baleen-bearing mysticetes, and it has been proposed for some toothed mysticetes such as aetiocetids and Llanocetus from Antarctica. Instead, Fitzgerald (2006) argues that the shorter, broader snout with outwardly pointing teeth at the business end is most similar to other aquatic animals that eat fish or other small vertebrates.
A hypothesis that Fitzgerald (2006) tests here is whether or not the melon is unique to the Odontoceti (extant toothed cetaceans) or whether early mysticetes also had a melon, and subsequently lost the organ. The melon is a large fatty sac on the face of modern odontocetes that is thought to play a role in echolocation. In echolocation, odontocetes interpret echoes of their vocalizations to produce an audio “picture” of their environment. The melon is thought to focus and direct the sound waves of odontocete vocalizations. Dissections of modern mysticetes have discovered a patch of fatty tissue that has been interpreted as a rudimentary melon. The huge orbit for the eye in Janjucetus does not support this, according to Fitzgerald (2006). Instead, the large orbits indicate that Janjucetus was probably not capable of echolocation; modern echolocating odontocetes have comparatively smaller eyes because echolocation replaces the eyes as the primary organ system for finding prey. Thus, Fitzgerald (2006) concludes that the melon only evolved once, within the Odontoceti.
Janjucetus indicates the existence of far greater diversity among early mysticetes, and Oligocene Cetacea in general, than has been generally recognized. According to the study, the discovery of Janjucetus suggests that early mysticetes radiated into numerous niches with distinct feeding ecologies before some ‘settled’ into filter feeding. Given this early experimentation in diet and feeding, the evolution of filter feeding was probably a gradual process.
Summary written by Robert Boessenecker.