The
type species for the genus is
Aetiocetus cotylalveus. It is defined as being the monophyletic group encompassing the closest common ancestor of
A. cotylalveus and
A. polydentatus and all its descendants: the textbook definition for a
monophyletic taxon.
Skull In
Aetiocetus weltoni, the lateral palatal foramina have an internal connection to the superior
alveolar canal, the latter structure being home to
blood vessels and
nerves used to supply baleen with
nutrients and innervate it in present-day mysticetes.
Aetiocetus is a small, toothed whale with no more than three small
denticles on the
anterior and posterior margins of the posterior upper teeth. Their postcanine teeth are somewhat
heterodont. The base of the
rostrum, or snout, of the whale, is greater than 170% of the width of the
occipital condyles where the skull meets the neck. These features are
synapomorphies, or shared derived traits, of
Aetiocetus. There is a distinct notch by the internal nostrils formed of the
palatine,
pterygoid, and
vomer bones; this is a synapomorphy of
Aetiocetus +
Chonecetus. Synapomorphies of the aetiocetids present in
Aetiocetus are: the
coronoid process of the
dentary, or lower jaw, is well developed; the
zygomatic arch is expanded anteriorly and posteriorly but is narrow at the middle.
Aetiocetus also shares several traits with all mysticetes. The
mandibular symphysis, or the connection between both lower jaw bones, is not fused. The descending process of the
maxilla becomes a toothless plate below the orbit. They possess a wide 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. 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 Deméré 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 The teeth of
Aetiocetus resemble those of archaeocetes and odontocetes, which employ a bite-and-swallow feeding strategy, 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. 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. Deméré 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.
Geography and endemism At first glance, the fact that species are known from only one locality, and that may suggest that
Aetiocetus was highly endemic. Deméré and Berta consider
Aetiocetus to be a lineage endemic to the north Pacific Ocean basin. High endemism would be highly atypical of mysticetes. However, a more likely explanation is that the fossil record for
Aetiocetus is poor, or that a sampling bias is present and not enough work has been done in late Oligocene deposits in the south Pacific Ocean. Perhaps there are more specimens of
Aetiocetus that will be discovered as paleontologists continue searching. The other genera in the family
Aetiocetidae are
Ashorocetus,
Chonecetus,
Morawanocetus, and
Willungacetus. All aetiocetids are known from the North Pacific except the Australian
Willungacetus and its taxonomy is disputed. ==See also==