rostrum. All these features are functionally related to filter-feeding with baleen and is a hallmark of the Mysticeti. The presence of teeth, as Barnes et al. remark, seems paradoxical. Lastly, Aetiocetus shows some symplesiomorphic traits with more archaic whales. The do not experience the same degree of telescoping as modern whales, so their nares, or nostrils, are still relatively anterior. Contrary to the image of the modern baleen whales, Aetiocetus still possessed developed, enamelized adult teeth . This indicates that loss of functionality in relevant enamel genes, such as ameloblastin (AMBN), enamelin (ENAM), and amelogenin (AMEL), had not yet taken place in Aetiocetus. Dentition For the most part, Aetiocetus retains a primitive tooth count of 11 upper teeth and 11 lower teeth, abbreviated 11/11. This is interpreted to be the basic mammalian dental formula with 3 incisors, 1 canine, 4 premolars, and 3 molars on both upper and lower jaws. However, A. weltoni and A. polydentatus show variation from the plesiomorphic mammalian dental formula. A. weltoni possesses an 11/12 dentition. True to its name of “many toothed”, A. polydentatus possesses more teeth than any other aetiocetid, and is remarkable in that the number of teeth are asymmetrical. On the right side of the upper jaw, A. polydentatus has 13 teeth, and on the left it has 14 teeth. The bottom jaw is also asymmetric, with 14 teeth in the right dentary and 15 in the left. This is the first polydont aetiocetid, meaning that it had more teeth than the standard mammalian formula. Embryonic baleen whales have polydont teeth before birth. A. polydentatus indicates that this condition was also present in tooth-bearing mysticetes, as expected from developmental data. In addition to its polydont dentition, A. polydentatus is unique in that these teeth are not differentiated into different teeth types as seen in other members of Aetiocetus. Paleontologists refer to this condition as being homodont, or “same tooth”. Presence of baleen Aetiocetus is unique in its representation of transition from toothed archaeocete to toothless mysticete. However, Aetiocetus is not a transitional form in the strictest sense, that is, it cannot be an ancestor to extant Mysticeti. More derived forms, such the Cetotheriidae, a family of toothless baleen whales, are contemporaneous with Aetiocetus. Hence, whales whose feeding relied entirely on baleen made their stratigraphic appearance before Aetiocetus, meaning that “true” baleen whales existed before Aetiocetus. Baleen is made of keratin (the same