Characters and Anatomy

Skull of Kryptobaatar visualised by CT scan. Notice the big lower incisors and the large blade-like premolars. Source: DigiMorph
Click on image to enlarge.

Skull

The skull of multituberculates is wide and massive, compressed from front-to-back rather than from side-to side as in placental mammals. The snout is also wide and bluntly pointed, and bends downwards.

The zygomatic arches, the bones supporting the muscles of the face and extending to the side of the skull - equivalent to our cheekbones, are stout and have prominent ridges where the muscles for chewing would have attached. The solidity of this area of the skull suggests that multituberculates had strong chewing muscles.

The orbits of the eyes are very large, and they have a roof but no floor, unlike in placentals where the orbits have a floor but are not roofed. The eyes were very big and situated quite far back to either side of the head.

Schematic drawing of the skull of Kryptobaatar dashzevegi (Suborder Cimolodonta), seen from above. (from Wible and Rougier, 2000).

Ear

The bones of the inner ear - the stapes, the malleus and the incus - are arranged as in primitive living mammals like marsupials. The inner ear was reconstructed digitally by CT scans of the skulls, and this showed that the cochlea (the auditory branch of the inner ear), which has a different curvature in the different groups of mammals, was quite straight, as in the monotremes.
There has been debate over whether multituberculates could hear high-frequency sounds. Some argue that, because the conduction of sound is done mostly through bone in multituberculates rather than by a vibrating membrane, they would have more difficulty perceiving high-frequency sounds (Meng and Wyss, 1995). Others suggest that the ear is compatible with high-frequency hearing but is less sensitive to low-frequency sounds (Hurum, 1998). Because the scientists arrived to these conclusions on different species, it is possible that all species did not have the same hearing capacities.

Mandible and chewing mechanisms

The mandible is the lower jaw. In multituberculates, it generally looks like what you would find in a rodent.
The way in which multituberculates chewed can be reconstructed from the striation patterns on their teeth. Ptilodus, for example, had two patterns of chewing, which used different movements and different teeth (Krause, 1982). The first pattern, the slicing-crushing cycle, involves the fourth premolar; the second pattern is the grinding cycle, where the molars come into play. The wear on the teeth show that the multituberculates had a backward power stroke, i.e. the jaw moved front-to-back when chewing. This is characteristic of the Multituberculata; in most mammals, the jaw moves from side-to-side.

Dentition

The molar teeth have several longitudinal rows of cusps, which give the order its name (cusps are also called tubercles). When the jaw is closed, the molars come together and the outer row of cusps on the lower molars falls into the valley between the cusps of the upper molars.

The incisors changed during the evolutionary history of Multituberculata, because they were used differently. In Ptilodontidea, the incisors were used to grasp, hold and pierce. In Taeniolabididae, the lower incisors were more robust and used for gnawing.
In advanced multituberculates, the canine is absent. Canines are present in most Jurassic forms, where they resemble the premolars.

The front part of the dentition, the incisors and canines, are separated from the premolars and molars by a gap, the diastema, also seen in rodents. The number of molars and premolars is different for each group; the particularity of multituberculates is that, when the number of teeth is reduced in a lineage, the tooth that disappears tends to be in the middle of the tooth series. Most other mammals lose teeth from the beginning or the end of the tooth series.
The lower premolars distinguish multituberculates because they are blade-like.

Ornamentation of the enamel, such as grooves, pits and ridges, varies within the multituberculates. The Allodontidae, for example, have smooth enamel, whilst Plagiaulacidae teeth can be strongly ornamented.

Lower jaw of Kryptobaatar, showing incisors, premolars and diastema

Lower jaw of Kryptobaatar dashzevegi (suborder Cimolodonta), seen from the side and showing large incisors and the blade-like premolars. The canines are absent and the diastema is large.
(from Wible and Rougier, 2000).
Click on image to enlarge

Author: Aude Caromel
Last updated: 20/11/06
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