Extant baleen whales (Mysticeti) have a deciduous foetal dentition, but are edentulous at birth. Fossils reveal that the earliest mysticetes possessed an adult dentition. Aetiocetids, a diverse clade of Oligocene toothed mysticetes, have a series of small palatal foramina and associated sulci medial to the postcanine dentition. The openings have been homologized with lateral palatal foramina that transmit neurovascular structures to baleen in extant mysticetes, thereby implying a co-occurrence of teeth and baleen in aetiocetids. However, homology of the foramina and sulci have been questioned. Using CT-imaging, we report that the lateral palatal foramina of Aetiocetus weltoni are connected internally to the superior alveolar canal, which transmits neurovascular structures to baleen in extant mysticetes and to teeth in extant odontocetes. Furthermore, the lateral palatal foramina of Aetiocetus are separate from the more medially positioned canals for the greater palatine arterial system. These results provide critical evidence to support the hypothesis that the superior alveolar neurovasculature was co-opted in aetiocetids and later diverging mysticetes to serve a new function associated with baleen. We evaluate competing hypotheses for the transition from teeth to baleen, and explore the transition from raptorial feeding in early mysticetes to filter-feeding in extant species.

The skull and partial right dentary (attached to rostrum) of Aetiocetus weltoni (UCMP 122900) were CT scanned by YXLON International in San Jose, CA using a Y.CT Modular cone-beam system. A total of 3866 slices were acquired with a slice thickness of 0.27 mm and pixel dimensions of 0.14 mm by 0.14 mm. 

The data are a merging of two scans, one of which contains most of the cranium, and the other a small portion of the left zygomatic arch. Unfortunately, the section with the zygomatic arch was not matched well with the other dataset. However, this does not affect the internal structure of the cranium and rostrum.