Aetiocetus: it’s supposed to be a baleen whale ancestor

This is going to be another paleo story about taxon exclusion.
Earlier we looked at paleoworkers trying to nest turtles among diapsids and pterosaurs among archosaurs… but this only happens when more closely related taxa are overlooked or omitted. In the case of the toothed archaic whale, Aetiocetus (Fig. 1), it sure looks like a good candidate for baleen ancestry. The palate was wide, triangular and flat. Unfortunately there are better candidates that were overlooked. Details below.

Figure 1. Three species attributed to Aetiocetus. Other workers considere these basal to baleen whales, but they don't include tenrecs and desmostylians in their analyses.

Figure 1. Three species attributed to Aetiocetus. Other workers considere these basal to baleen whales, but they don’t include tenrecs and desmostylians in their analyses. The teeth of A. polydentatus are simple cones, as in modern toothed whales.

Wikipedia reports:
Aetiocetus is a genus of extinct basal mysticete (baleen whale (late Oligocene, 30 mya). The large reptile tree (LRT, 1161 taxa) nests it with other toothed whales, far from mysticetes. Some traits that distinguish Aetiocetus from other toothed whales include:

  1. the nostrils of the whale had migrated further back on the skull than seen in archaeocetes, but not so far as in modern whales.
  2. no more than three small denticles on the anterior and posterior margins of the posterior upper teeth.
  3. postcanine teeth heterodont.
  4. 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.
  5. notch by the internal nostrils formed of the palatine, pterygoid, and vomer bones
  6. coronoid process of the dentary well developed
  7. zygomatic arch is expanded anteriorly and posteriorly but is narrow at the middle.
  8. mandibular symphysis not fused.
  9. descending process of the maxilla becomes a toothless plate below the orbit.
  10. wide rostrum

According to Wikipedia,
all these features are functionally related to filter-feeding with baleen and are hallmarks of the Mysticeti. The presence of teeth seems ‘paradoxical’ to these workers. Unfortunately, this hypothetical relationship is recovered only in the absence of desmostylians. 

According to the National Library of Medicine
“All bones possess larger or smaller foramina (openings) for the entrance of the nourishing blood-vessels.”

According to Marx et al. 2016:
“Aetiocetids have previously been proposed as the most basal mysticetes to possess baleen, the key adaptation of modern whales. More specifically, the widespread occurrence of palatal nutrient foramina (in Aetiocetus, Fucaia and Morawanocetus), which in extant mysticetes supply the baleen rack, has been used to infer the existence of an incipient baleen structure between or just lingual to the teeth (Deméré and Berta, 2008; Deméré et al., 2008). While such an interpretation is possible, it also remains untested: just as the origin of feathers in non-avian dinosaurs does not mark the beginnings of flight, so the appearance of palatal foramina in mysticetes need not indicate the presence of baleen. Instead, the foramina of aetiocetids could, for example, have supplied its immediate predecessor – namely, well-developed gums, the presence of which is indicated both by the strongly emergent teeth of early mysticetes (Deméré and Berta, 2008; Fitzgerald, 2010) and, possibly, the largely unworn incisor of NMV P252567.”

Baleen was not found in any specimen of Aetiocetus. 
The presence of baleen is inferred from the presence of nutrient foramina. If so, then baleen had two convergent origins and Aetiocetus represents a dead end, leaving no modern descendants. If nutrient foramina only fed the skin and other tissues that lined the jaws, then no baleen was present.

Baleen WAS found
in the basal mysticete, Miocaperea (Bisconti 2012, skull 1 meter long; Miocene). Traditionally whale workers consider taxa like Caperea and Miocaperea highly derived because desmostylians are not included in their analyses. By contrast, the LRT finds these to be basal mysticetes, Caperea for right whales and Miocaperea for all other mysticetes. Most studies nest Caperea with right whales (clade: Balaenoidea; Demere et al. 2005, Churchill et al. 2012, Ekdale et al. 2011. Marx 2010, did not. All wrongly assume Odontoceti as the outgroup.

Figure 4. The caption for this photo is: "Brian Beatty measuring the jaws of one of our toothed mysticetes. Photo by R. Boessenecker."

Figure 2. The caption for this photo is: “Brian Beatty measuring the jaws of one of our toothed mysticetes. Photo by R. Boessenecker.” This was the traditional view of mysticete origins prior to the addition of desmostylians to the LRT.

The image above
is from  the CCNHM blogsite and the caption for the photo is: “Brian Beatty measuring the jaws of one of our toothed mysticetes. Photo by R. Boessenecker.” Perhaps unknown to Professor Beatty, this is true only in the absence of desmostylians from phylogenetic analysis, an easy mistake to make prior to the entry of desmostylians to the LRT. Various authors have been bending over backwards trying to discover the origin of mysticetes among toothed whales. But they cannot be discovered there. Mysticetes arise from desmostylians, taxa studied by Professor Beatty, but the mysticete connection was not realized.

And that, again demonstrates the value of the LRT,
which permits taxa that have never been tested together to nest together. Taxon inclusion solves every problem in paleontology.

Chronology
This is always a problem due to the rarity of fossils, typically (but not always) recovered as late survivors long after initial radiations. These are taxa recovered by the LRT in the lineage of mysticetes.

  1. Mesonyx: Paleocene or earlier
  2. Ocepeia: Paleocene
  3. Cambaytherium: Eocene
  4. Anthracobune: Eocene
  5. Desmostylus: Oligocene
  6. Cetotherium: Oligocene
  7. Miocaperea: Late Miocene
  8. Caperea and others: Recent

Characters that separate aetiocetids from cetotheres:

  1. Skull shorter than half the presacral length
  2. Snout not constricted
  3. Premaxilla/maxilla notch less than 25º
  4. Naris angle 30-90º but dorsal
  5. Nasals subequal to frontals
  6. Orbit shape in lateral view at least half longer than tall
  7. Frontals without posterior processes
  8. Postparietals angled from the dorsal plane
  9. Tabulars absent
  10. Squamosal descending process at right angle
  11. Frontals and parietals both fused
  12. Occiput far posterior to quadrate articulation
  13. Opisthotics descend
  14. Vomernasal (or other anterior palatal opening) separate from choanae (internal narial opening) by maxilla
  15. Premaxilla teeth present and robust
  16. Last maxillary tooth present and anterior to orbit
  17. Some cervical neural spines taller than centra
  18. 10th caudal length not > chevron depth
  19. Interclavicle/sternal elements present
  20. Scapula not robust
  21. Manual digit 1 present

Interesting tidbits
The coronoid process becomes larger in two tested cetotheres, but not the basal Cetotherium. You wouldn’t think baleen whales would need a tall coronoid process (used for biting), and most don’t have one, but Tokharia and Yamatocetus do.

It only takes the deletion of a few desmostylians to nest baleen whales with toothed whales, due to the large number of similar traits (which is why we call them all ‘whales’. Conversely it only takes the deletion of a few archaic whales to nest odontocetes with baleen whales, as I discovered through testing these options.

For more heretical desmostylian information,
click here.

References
Bisconti M 2012. Comparative osteology and phylogenetic relationships of Miocaperea pulchra, the first fossil pygmy right whale genus and species (Cetacea, Mysticeti, Neobalaenidae). Zoological Journal of the Linnean Society 166: 876-911.
Buchholtz E 2010. Vertebral and rib anatomy in Caperea marginata: Implications for evolutionary patterning of the mammalian vertebral column. Marine Mammal Science 27: 382-397.
Churchill M, Berta A, Deméré TA 2012. The systematics of right whales (Mysticeti: Balaenidae). Marine Mammal Science 28: 497-521.
Deméré TA, Berta A and McGowen MR 2005. The taxonomic and evolutionary history of modern balaenopteroid mysticetes. Journal of Mammalian Evolution 12: 99-143.
Ekdale E G, Berta A and Deméré TA 2011. The comparative osteology of the petrotympanic complex (ear region) of extant baleen whales (Cetacea: Mysticeti). PLoS ONE 6:1-42.
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.
Fitzgerald EMG 2012. Possible neobalaenid from the Miocene of Australia implies a long evolutionary history for the pygmy right whale Caperea marginata (Cetacea, Mysticeti). Journal of Vertebrate Paleontology 32:976-980.
Marx FG 2011. The more the merrier? A large cladistic analysis of mysticetes, and comments on the transition from teeth to baleen. Journal of Mammalian Evolution 18:77-100.
Marx et al. 2016. Suction feeding preceded filtering in baleen whale evolution. Memoirs of Museum Victoria 75: 71–82.
Van Valen L 1968. Monophyly or diphyly in the origin of whales. Evolution. 22 (1):37–41.

wiki/Aetiocetus

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