page_title: Aetiocetus
section_title: Feeding strategy
hierarchical_section_title: Aetiocetus / Description / Feeding strategy
caption_attribution_description: English: whale filter
context_page_description: Aetiocetus is a genus of extinct basal mysticete, or baleen whale that lived 33.9 to 23.03 million years ago, in the late Oligocene in the North Pacific ocean, around Japan, Mexico, and Oregon, U.S. It was first described by Douglas Emlong in 1966 and currently contains known four species, A. cotylalveus, A. polydentatus, A. tomitai, and A. weltoni. These whales are remarkable for their retention of teeth and presence of nutrient foramina, indicating that they possessed baleen. Thus, Aetiocetus represents the transition from teeth to baleen in Oligocene mysticetes. Baleen is a highly derived character, or synapomorphy, of mysticetes, and is a keratinous structure that grows from the palate, or roof of the mouth, of the whale. The presence of baleen is inferred from the fossil record in the skull of Aetiocetus.
  Aetiocetus is known from both sides of the Pacific Ocean: it was first documented in Oregon, United States, but it is also known from Japan and Mexico. The genus is currently constrained to the Northern hemisphere and has little value in biostratigraphic studies of the Oligocene due to its limited occurrences across the Pacific.
context_section_description: 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, then, did Aetiocetus feed by bulk-feeding or by bite-and-swallow? If they fed only by selective prey capture, were they simply pre-adapted for the specialized feeding behavior of extant mysticetes? Structurally, Aetiocetus possesses teeth that are quite similar to primitive odontocetes, such as Squalodon. These odontocetes have an inferred bite and tear style of eating with limited mastication. Both primitive and extant odontocetes find their prey through the use of echolocation; however, mysticetes have no evidence in their fossil record of ever evolving or initially possessing the ability to echolocate. Piscivory, or a diet based solely on fish, is likely the primitive condition for Cetacea, and it seems most parsimonious that Aetiocetus fed like an archaeocete, locating fish without the use of echolocation.
  However, an argument exists that Aetiocetus was in fact a bulk feeder, who fed by gulping and straining prey from the water through their interlocking cusped cheek teeth. This is supported by the presence of a lack of mandibular symphysis, meaning the jaw was loosely articulated, and by the presence of the wide palate. This feeding method has an analog in crab-eater seals. This hypothesis combines the idea of bulk feeding and retention of the dentition. Aetiocetus might have been a functional mysticete. Lending credence to this interpretation is the presence of mandibular kinesis in Aetiocetus, though they lack the rostral kinesis seen in more derived mysticetes. This cranial kinesis, or ability of the skull bones to move relative to one another, permit the mysticete skull to decrease the strain exerted on the skull during bulk feeding.
  Fitzgerald argued against the model of tooth-aided filter feeding, based on the lack of closely pressed teeth and the presence of simple postcanine crowns.  Deméré argues that this assumes a very small prey size (i.e., krill). The distinction here is that Aetiocetus was a bulk feeder, and prey size does not enter into this definition of feeding strategy. There is no reason to assume a priori that all bulk filter-feeders eat small prey, given the large diversity of food items consumed by modern mysticetes. Demere hypothesizes that Aetiocetus’ bulk feeding behavior could have targeted large prey, such as schooling fish or squid. With prey items of this size, Aetiocetus’ teeth would still have served well as a coarse sieve.
caption: Schematic of the bulk feeding method employed by modern mysticetes. It is plausible that Aetiocetus used a variation of this method using a combination of baleen and teeth.
exif: {}