material that comprises claws, hooves, nails, and hair) that grows throughout the whale's life. Development of mysticetes indicate that they had a toothed ancestor, as the fetal baleen whale forms tooth buds which are later reabsorbed and do not develop any further. However, Aetiocetus presents the evolutionary biologist with evidence for this transition in the fossil record. While baleen, as a soft tissue, does not preserve in the fossil record, whale paleontologists are able to identify evidence for baleen attachment in the palates of mysticetes. These are evident in what are known as nutrient foramina. These nutrient foramina, present on the maxillae of the whale, are associated with grooves and sulci, or fissures, which in life are occupied by branches of the superior alveolar artery and nerve. This superior alveolar artery supplies nutrients to the epithelial, or surface cells of the body, from which the baleen continuously develops. In all known archaeocetes and odontocetes, nutrient foramina are absent. These nutrient foramina are most apparent in A. weltoni, whose holotype has the best-preserved palate. The development of nutrient foramina and teeth are closely intertwined in mysticetes: first, an alveolar groove on the palate of the developing mysticete. The deciduous teeth form in the groove, and then are reabsorbed, while development of rudimentary baleen plates begin. The alveolar groove fills with bone until the laterial nutrient foramina form. This close association leads Demere and Berta to hypothesize that Aetiocetus displays an ancient ontogeny, or growth sequence. These nutrient foramina are also present on A. cotylalveus and another related aetiocetid, Chonecetus goedertorum. Compared to other edentulous, or toothless, mysticetes, the pattern of nutrient foramina is most similar to extant balaenopterids (blue whales and other rorquals) and fossil cetotheres. Feeding strategy How did Aetiocetus feed, and what did it feed on? Its teeth are reminiscent of the bite-and-swallow strategy of archaeocetes and odontocetes, but they also possessed expanded palates. Modern mysticetes grow their baleen from this expanded palate and use the baleen to trap arthropods and fish in their mouths. This is known as bulk-feeding, in that the whale is not selecting individual prey items and does not use echolocation to find prey, as odontocetes do. Fossil mysticetes with wide, toothless palates are inferred to bulk-feed and the first occurrence of such whales is in the late Oligocene, approximately 4 to 5 million years after the first toothless mysticetes appeared. A crucial question is,