Aetiocetus Weltoni Explained
A reconstruction of Aetiocetus weltoni by Carl Buell. Note the presence of teeth and baleen in this early mysticete whale.
Significant discoveries have been made in the last twenty years regarding the early evolution of whales. However, as research on Eocene whales has marched forward, the history of early Neoceti (a story hiding largely within Oligocene strata) has remained elusive. Spurring focus on this part of whale evolution was the impetus for a special symposium at the 2006 meeting of the Society of Vertebrate Paleontology. A study presented at this symposium was recently published by Deméré et al. (2008), an intriguing analysis that reconstructed the evolution of baleen and teeth in mysticete whales.
Modern baleen whales possess large racks of keratinous baleen that hang from the palate and serve to filter plankton, crustaceans, and even fish out of seawater. A series of arteries on the palate afford blood to supply the gingival epithelia which produce the baleen. These arteries pass onto the palate via holes called foramina and then continue in grooves . As baleen is keratinous, it does not fossilize well, and it is only known to do so in extremely rare circumstances. However, the presence of palatal foramina and grooves allows paleontologists to infer whether or not a fossil whale had baleen.
Unlike modern baleen whales, the most primitive mysticetes still had teeth. While this may seem counterintuitive, teeth are a primitive feature for all mammals, baleen whales included, so the presence of teeth in early mysticetes should not be surprising. The transition from teeth to baleen was a major headache among cetacean researchers, until the skull of Aetiocetus weltoni (from the late Oligocene of Oregon) was further prepared and reinvestigated by Deméré et al. (2008). In addition to teeth, foramina were discovered on the palate of A. weltoni, suggesting the simultaneous presence of baleen and teeth in this animal. In addition to A. weltoni, the authors also observed palatal foramina in A. cotylalveus and Chonecetus goedertorum, additional Oligocene whale fossils from the Pacific Northwest. A second presentation at the same 2006 symposium announced similar foramina in the palate of Morawanocetus, an aetiocetid mysticete from the Oligocene of Japan.
In addition to the fossil data, evidence of a toothed ancestry for Mysticeti resides in both the sequence of ontogeny (the growth history of an organism from fertilized egg to adult) and in the genome of a living baleen whale. Modern mysticete fetuses develop rudimentary teeth in utero that are subsequently resorbed before birth, but not until after baleen appears. Thus, baleen and teeth briefly co-occur in the fetus. This is possibly an example of ontogeny recapitulating phylogeny (the evolutionary history of an organism). In addition to this anatomical evidence for teeth in the early life of baleen whales, Deméré et al. (2008) found molecular evidence for the transition from teeth to baleen. Modern mysticete whales were found to retain inactive dental ‘pseudogenes’ that originally controlled the formation of tooth enamel. As these genes were no longer functional, they were referred to by the authors as ‘molecular fossils’ within the genomes of modern baleen whales. It is difficult, without reference to the theory of evolution, to explain teeth in a mysticete fetus that are never used for eating. It is possibly even more difficult to explain the presence of degraded vestiges of genes whose primary function in modern mammals is the formation of tooth enamel.
The transition from teeth to baleen within Mysticeti appears to be of a stepwise pattern. The transformation of the mouth in these whales was undoubtedly affected by a change in feeding style. The most primitive mysticete whales were likely catching individual fish with their teeth like modern dolphins. In contrast, all modern baleen whales are filter feeders and often gulp swarms or ‘clouds’ of krill and fish. In addition to the inferred presence of baleen, the feeding apparatus of Aetiocetus weltoni includes a lower jaw that lacks a sutured bony connection at the ‘chin,’ and instead bears a groove for a symphyseal ligament like modern mysticetes. This loose joint at the chin allows expansion of the gape in living baleen whales, and permits these mammals to ingest a large volume of water and tiny prey. All this evidence taken together suggests that A. weltoni was a bulk feeder, but the presence of wear facets on the teeth hint that this animal may have retained the primitive ability to snap at larger, individual food items with its teeth. Deméré and coauthors suggest that an ability to both filter feed and to capture individual, larger fish with the dentition may have eased the evolutionary transition from capturing prey items with the teeth to feeding solely with baleen. Thus, they interpret A. weltoni as a mosaic intermediate that bridged primitive and derived modes of feeding in the evolutionary history of mysticete cetaceans.
Summary written by Robert Boessenecker.