Mesonychids are an extinct order of carnivorous hoofed mammals. The hooves of Mesonychids are on each toe, unlike most hoofed mammals of today, such as horses or deer. There is a theory that cetaceans (whales, dolphins, and porpoises) are derived from Mesonychids.
Mesonychids lived from the late Paleocene to the Eocene periods (roughly 60-37 million years ago). Species have been found in the American and Canadian west and in northern China and Mongolia. Mesonychids are though to be distantly related to modern cetaceans and hoofed animals, but are an extinct order with no direct descendants.
| Looking at the reconstructions on this page and elsewhere, Mesonychids will appear very similar to canids at first glance, but are actually quite different. In fact, the are more closely related to modern hoofed mammals like deer and pigs than to modern canines like wolves and foxes. For the following comparisons of skulls, the diagram at the left will help in identifying parts of mammalian skull anatomy. The skull depicted is that of a modern red fox. |
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When the skull of a Mesonychid, here that of Sinonyx, is compared with that of a canid, in this case the extinct dire wolf, several differences are apparent. First to be compared is overall skull structure.
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More differences can be seen in the teeth and jaws.
 The most noticeable difference here is the longer mandible, but look at the teeth. The carnassials of a Mesonychid have three cusps, or points. A tall central cusp and two equal sized cusps on either side. There is also much less of a difference in size. The canines are much thinner and slightly longer than in the dire wolf, and the two premolars right behind the canines are small and sharp, and positioned in a way which reduces the size of the lower diastema. The upper diastema is seen in the full skull image, and is much larger than in the dire wolf. |
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The reason for these differences reflect the diet of the two animals. Wolves, including the extinct dire wolf, are pack hunters which bring down large prey. Their thick canines are designed to withstand the pressure of holding struggling prey without breaking, and the large carnassials are designed for cutting meat like shears. It is speculated that some Mesonychids were scavengers, like modern hyenas, and others were possibly fish eaters, like modern seals and toothed whales. Similar three pointed carnassials are present in extinct whales and modern seals and sea lions, and are designed for grasping slippery marine prey, and some Mesonychids, like Andrewsarchus, have bone crushing teeth similar to that of a hyena. The sagital crest on both of these animals indicate strong jaw muscles. The sagital crest is where a mammals lower jaw muscles are attached, and the greater the surface area on the sagital crest, the larger the muscle attached. The strong jaws of the dire wolf are designed for holding on to large prey such as paleo bison. The Mesonychid jaws may have been used for a stronger grip on water bound animals in some species, and in others for crushing bone.
Next to be compared are the feet.
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Mesonychids are an extinct order of hoofed predators, adapted for scavenging and feeding off shorelines. although at first they appear similar to canids, they share dental and facial traits with living whales and dolphins, and the hooves of living hoofed herbivores like horses and goats. This combination of traits makes Mesonychids unlike any other animal alive today.
