Aetiocetus, Desmostylus and the origin of baleen

Aetiocetus
(Emlong 1966; late Oligocene, 30 mya; Fig. 1) has been the poster-child for basal baleen whales for several decades, embraced by all whale workers. That needs to change.

When Aetiocetus was first published,
Emlong wrote:“If it were not for the presence of functional teeth on this mature specimen, this cetacean could easily be placed in the order Mysticeti.” (= baleen whales). Nevertheless, Emlong placed his discovery within the Archaeoceti, based on its primitive dentition. He also noted the nares had migrated further back on the skull than in typical archaeocetes, though still anterior to the orbits.

In the pre-cladistic era
Van Valen 1968 placed Aetiocetus as a basal mysticete despite the presence of teeth.

Of course, this assumes
that baleen whales were derived from toothed whales, which they are not, according to a wider gamut study, the large reptile tree (LRT, 1011 taxa).

Figure 1. Palates of two baleen whales, one toothed whale and Neoparadoxia, a desmostylian. See text for details.

Figure 1. Palates of two baleen whales, one toothed whale (Aetiocetus) and Neoparadoxia, a desmostylian. See text for details, but note the vomer splitting the maxilla in three related taxa and the pointed premaxilla in only one.

Aetiocetus is remarkable
for having both a full set of teeth and nutrient foramina. According to cetacean workers,t he presence of nutrient foramina (tiny holes and grooves in the palatal portion of the maxilla) indicates the presence of baleen in Aetiocetus. That’s because nutrient foramina are otherwise absent in archaeocetes and odontocetes (so far). In baleen whales some of the lateral foramina are located in the alveolar grooves, where the tooth roots used to be and where the teeth are in Aetiocetus. Other nutrient foramina radiate along the roof of the mouth (palatal portion of the maxilla). What Aetiocetus was growing there, we may never know. Based on the LRT, that soft palatal tissue in Aetiocetus was not homologous with baleen, though it would have been convergent.

According to Wikipedia
Aetiocetus shares several traits with all mysticetes. The mandibular symphysis is not fused. The descending process of the maxilla becomes a toothless plate below the orbit. A wide rostrum is present.” There is no doubt that among toothed whales, aetiocetes share more traits with baleen whales than any other toothed whales. However, distinct from prior studies, the LRT permits baleen whales to nest wherever they want to. And they don’t want to nest with toothed whales.

Re: Aetiocetus traits
It should be noted that even though the mandibular symphysis is not fused, it is narrow with parallel tips, unlike the anteriorly wide jaws of mysticetes and desmostylians. As in Aetiocetus, sperm whales likewise have a wide palate, at least posteriorly,  Aetiocetes were contemporaries of basal baleen whales like Cetotherium. In the LRT cetotheres are not basal mysticetes, but gray whales are.

We looked at other mysticete traits
and the desmostylian/mysticete connection earlier here, here and here.

The LRT nests Aetiocetus
with NMV P252567 (Marx et al. 2016) and these two nest between the archaeocete Zygorhiza and the extant odontocetes, Orcinus and Physeter. Archaeocetes have teeth of several shapes. Extant odontocetes have simple cones only. The genus Aetiocetus could be phylogenetically transitional in that the species A. weltoni has different tooth types, but A. polydentatus has simple cones only. I say ‘could be’ because those species have not yet been tested in the LRT.

Perhaps overlooked by whale workers 
the desmostylian Neoparadoxia (Fig. 1) also has nutrient foramina in a toothless groove between its anteriorly directed tiny canine and four posterior teeth (premolar #3 and three molars). I am hard pressed to see foramina elsewhere in the palate from available photos. Barnes 2013 wrote: “There are two small centrally placed nutrient foramina in each palatine bone. One is near the maxillary–palatine suture, and the other is located more medial to that one.” Note, Barnes did not report foramina in the maxilla.

Like baleen whales,
the palate is wide and the premaxilla is transversely oriented in Neoparadoxia. The palate already includes a long toothless groove. We don’t know what was growing in that groove. In the heretical LRT Neoparadoxia and other desmostylians are baleen whale outgroups, so that could be where a baleen precursor was growing. The rostrum is downturned slightly. The nares are telescoped to the back creating a blowhole.

Marx et al. nested NMV P252567
within the Aetiocetidae and thought they were shedding new light on the origin of baleen. From their abstract: “Baleen is thought to have appeared in archaic tooth-bearing mysticetes during a transitional phase that combined raptorial feeding with incipient bulk filtering. Here we show that tooth wear in a new Late Oligocene mysticete belonging to the putatively transitional family Aetiocetidae is inconsistent with the presence of baleen, and instead indicative of suction feeding. Our findings suggest that baleen arose much closer to the origin of toothless mysticete whales than previously thought. In addition, they suggest an entirely new evolutionary scenario in which the transition from raptorial to baleen-assisted filter feeding was mediated by suction, thereby avoiding the problem of functional interference between teeth and the baleen rack.” Desmostylians also suck, apparently (see below). And then you don’t have to explain away tooth wear!

Geisler and Sanders 2003
did not consider anthracobunids, desmostylians and tenrecs in their phylogenetic analysis of whales, but instead relied on Sus, the pig, as an outgroup. A larger gamut analysis finds Sus is not related to whales of any sort. When you don’t have the correct outgroups, you can’t place confidence in the order of appearance of derived traits and the order of derived taxa.

Figure 3. Old, toothless Desmostylus mandible with single downturned canine compared to the empty alveolus and mandibles of the gray whale (Eschrichtius).

Figure 2. Old, toothless Desmostylus mandible with single downturned canine compared to the empty alveolus and mandibles of the gray whale (Eschrichtius).

Santos, Parham and Beatty 2016
described an old toothless (save for one canine) Desmostylus mandible (Fig. 2). It bears comparison to the mandible of Eschrichtius, the gray whale, which has an anterior alveolus for an absent canine. Feeding strategies were probably similar. Wikipedia reports, “The (gray) whale feeds mainly on benthic (= sea floor) crustaceans, which it eats by turning on its side …and scooping up sediments from the sea floor.”  Desmostylians were also sea floor feeders, but with their shovel-like jaws, did not have to turn on their sides. Chiba et al 2015, suggests desmostylians used suction to pull in their food, and did not chew it.

Figure 4. Comparison of several desmostylian mandibles with that of Eschrichtius, the gray whale. As this lineage of desmostylians get larger, they more closely match the mandible of the gray whale.

Figure 3. Comparison of several desmostylian mandibles with that of Eschrichtius, the gray whale. As this lineage of desmostylians get larger, they more closely match the mandible of the gray whale. Notice the increasing extent of toothlessness and the gradual lengthening of the mandible. Drawings from Chiba et al. 2015.

Chiba et al. 2015
were kind enough to include a set of desmostylian mandibles to scale (Fig. 3) which, as they grew phyllogenetically larger, more closely approximated the much larger mandible of the gray whale, Eschrichtius, an extant basal baleen whale. That Sanjussen specimen, in particular, comes as a confirming relegation! (Unfortunately this was completely lost on Chiba et al.)

Hypothetical evolution of a feeding strategy
If desmostylians were not vegetarians, but fed by digging their anterior tusks into crustacean-laden sediments, then forcing out the excess water before swallowing, they were essentially doing the same sort of benthic feeding as the gray whale.

But what about those odd-looking desmostylian teeth that disappear in old age?
When individual desmostylians were younger and smaller, their benthic prey would have been relatively larger and teeth may have been necessary for crushing the hard shells before swallowing them. On the other hand, older and larger specimens did not need molars because they could swallow their tiny prey without chewing. In desmostylians even a little bit of baleen would have improved the filtering ability. That humble genesis would ultimately evolve to become the giant strips of baleen found in giant mysticetes as the teeth disappeared phylogenetlcally (Fig. 3), not side-by-side with sharp teeth, as imagined in Aetiocetus.

Figure 4. Baleen highlighted in this gray whale skull. Brighter green is below the gum line.

Figure 4. Baleen highlighted in this gray whale skull. Brighter green is below the gum line.

Next steps in this feeding strategy hypothesis:
Baleen whales, other than the gray whale, have left their benthic feeding grounds for the open seas where some fill their expandable throats with sea water and sieve for prey as that volume is forced out past their lips. Others. like the right whale, use their tongue to force out the sea water through deeper baleen filters. Still others probably perform a combination of the two, tongue and throat.

Time to clear out a few Wikipedia misconceptions

  1. Wikipedia reports, “Desmostylians are the only known extinct order of marine mammals.” — not supported by the LRT. Mysticeti are a clade within Desmostylia.
  2. Wikipedia reports, “The Desmostylia, together with Sirenia and Proboscidea (and possibly Embrithopoda), have traditionally been assigned to the afrotherian clade Tethytheria” — not supported by the LRT. Desmostylia are related to anthracobuinds, Hippopotamus and mesonychids in order of increasing distance. Not sure why this isn’t obvious.
  3. Wikipedia reports, “Aetiocetus is a genus of extinct basal mysticete, or baleen whale.” — not supported by the LRT. Aetiocetus nests between archaic toothed whales and extant toothed whales in the LRT.

Next time someone runs a cladistic analysis on whales,
please use the cetacean taxa and outgroups recovered by the LRT to check their validity. If they are again excluded from future studies the present LRT hypothesis of relationships will never have a chance to be verified or become the consensus. Let’s not let pigs stay the ancestors of whales when we already know better!

References
Barnes LG 2013. A new genus and species of Late Miocene Paleoparadoxiid (Mammalia, Desmostylia) from California. Contributions in Science 521:51-114.
Chiba K et al. 2015. A new desmostylian mammal from Unalaska (USA) and the robust Sanjussen jaw from Hokkaido (Japan), with comments on feeding in derived desmostylids. Historical Biology 28(1-2): 289 DOI: 10.1080/08912963.2015.1046718
Cope ED 1872. Descriptions of some new Vertebrata from the Bridger Group of the Eocene. Proceedings of the American Philosophical Society 12:460-465
Ekdale EG and Berta A 2015. Vascularization of the gray whale palate (Cetacea, Mysticeti, Eschrichtius robustus): Soft tssue evidence for an alveolar source of blood to baleen. The Anatomical Record Advances in Integrative Anatomy and Evolutionary Biology. February 2015; DOI: 10.1002/ar.23119
Emlong D 1966. A new archaic cetacean from the Oligocene of Northwest Oregon. Bulletin of the Museum of Natural History, University of Oregon. 3: 1–51.
Geisler JH and Sanders AE 2003. Morphological evidence for the phylogeny of Cetacea. Journal of Mammalian Evolution. 10: 23–129. doi:10.1023/A:1025552007291
Gray JE 1864. “Eschrichtius“. Annals of the Magaztine Natural History. 3 (14): 350.
Kimura T and Ozawa T 2002. A new cetothere (Cetacea: Mysticeti) from the early Miocene of Japan. Journal of Vertebrate Paleontology 22(3):684-702
Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.
Reinhart RH 1959. A review of the Sirenia and Desmostylia. University of California Publications in Geological Sciences 36(1):1–146.
Santos G, Parham J and Beatty B 2016. New data on the ontogeny and senescence of Desmostylus (Desmostylia, Mammalia). Journal of Vertebrate Paleontology. doi: 10.1080/02724634.2016.1078344
Van Valen L 1968. Monophyly or diphyly in the origin of whales. Evolution. 22 (1):37–41.

wiki/Balaenoptera
fossilworks/Isanacetus
wiki/Eschrichtius
wiki/Paleoparadoxia
wiki/Neoparadoxia

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