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Characters and Anatomy


Skull     Dentition     Skeleton     Posture

Skeleton

Vertebral Column

The neck is quite short relative to the fairly large head. The skull is somewhat longer in proportion to the body length than in modern rodents and marsupials of a similar size.
The vertebral count is as follows: 7 vertebrae in the neck, 13 in the upper back, 7(?) in the lower back, and many in the tail. The tails of some specimens are not preserved so it is difficult to estimate the number of vertebrae in the tail, but they tended to be long. In Ptilodus (Suborder Cimolodonta), there are about 23 tail vertebrae.
The vertebrae are distinctive in that the various knobs which help articulate and limit mobility in the spine are poorly developed.
The sacrum, the bone linking the vertebral column of the back to the tail, and on which the pelvis articulates with the spine, is quite long.

Hindlimbs

The pelvis possesses extra bones projecting towards the front of the pelvis, into the body cavity; these bones are quite common in Mesozoic mammals, but have been lost in placental mammals. The bones in the pelvis are fused such that the right and left halves of the pelvis could not separate when giving birth.
The feet are very mobile on the ankle.

The ankle of several species possesses two extra bones, which have been compared to the extra bones supporting a spur in monotremes. In the male platypus, these spurs are used to deliver poison. Whilst the spur itself is not preserved in multituberculate fossils, the presence of the two extra bones suggest that it may also have been present. Defence through poison may have been a strategy used by smaller mammals in a land dominated by dinosaurs at the time (Hurum et al., 2006).

Reconstruction of Catopsbaatar, showing ankle spurs and sprawling posture
Reconstruction of Catopsbaatar catopsaloides (suborder Cimolodonta) ready for attack, exhibiting the sprawling posture, the ankle spurs and the long tail.
(from Kielan-Jaworowska and Hurum, 2006, reproduced with authorisation).
Click on image to enlarge

Posture

The posture of multituberculates has been a point of controversy. Two types of postures are seen in terrestrial animals: a sprawling posture, where the limbs stick out on the sides of the body, and an upright posture, where the limbs are tucked under the body. In animals with a sprawling posture, the gait is symmetrical - one of the forelimbs moves at the same time as one of the hindlimbs, after which the next hindlimb and forelimb move. In an asymmetrical gait, the forelimbs move in sequence, then the hindlimbs. This gait is seen in animals with an upright posture. Most mammals have an upright posture, except for the monotremes, which have a sprawling posture, like most reptiles.
The posture of multituberculates has been alternatively reconstructed as upright and sprawling. The sprawling posture hypothesis has received more support in recent years (Kielan-Jaworowska and Hurum, 2006). In this case, the body would have been held closer to the ground than in mammals today.

The strong vertebrae in some species indicate that some may have moved around in jumps, in which case they would have had an asymmetrical gait as well as a sprawling posture. As this is not seen in any modern mammal, it makes comparison and reconstruction difficult.

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