Relationship With Whales
Mesonychids possess unusual triangular molar teeth that are similar to those of Cetacea (whales and dolphins), especially those of the archaeocetids, as well as having similar skull anatomies and other morphologic traits. For this reason, scientists had long believed that mesonychids were the direct ancestor of Cetacea, but the discovery of well preserved hind limbs of archaic cetaceans, as well as more recent phylogenetic analyses now indicates that cetaceans are more closely related to hippopotamids and other artiodactyls than they are to mesonychids, and this result is consistent with many molecular studies.
Most paleontologists now doubt that whales are descended from mesonychids, and instead suggest that whales are either descended from, or share a common ancestor with, the anthracotheres, the semi-aquatic ancestors of hippos. However, the close grouping of whales with hippopotami in cladistic analyses only surfaces on deletion of Andrewsarchus, which has often been included within the mesonychids. One possible conclusion is that Andrewsarchus is not a mesonychid, but rather closely allied with hippopotamids. The current uncertainty may, in part, reflect the fragmentary nature of the remains of some crucial fossil taxa, such as Andrewsarchus.
Read more about this topic: Mesonychids, Phylogeny and Evolutionary Relationships
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