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The largest living mammal on Earth, the blue whale, survives on only minuscule prey such as plankton and krill by filter feeding using baleen plates. How did this strange ecological relationship come about? The evolution of filter feeding was a result of the combination of selection for large body size, irregular distributions of schooling prey, and an uneven accessibility of prey resources. Understanding the evolution of baleen plates is crucial to understanding modern marine mammal biology because all present-day mysticetes (baleen whales) have baleen plates instead of teeth (Croll et al. 2002). These plates are not homologous to teeth (Deméré et al. 2008), but are made of keratin and project ventrally from the outer edges of the mouth. They grow continuously, but are worn by the tongue, causing hair-like fringes to emerge. These fringes are coarser on the outside and overlap with adjacent plates on the inside. They have evolved to be very efficient in capturing common mysticete prey, which includes plankton, copepods, krill, and pelagic schooling fish (Croll et al. 2002). Mysticetes have evolved to utilize baleen plates in three different ways, causing the coarseness of the fringes, density of the fibers, and number and length of the plates to vary. Gray whales feed on sediment-dwelling benthic amphipods, so they have short and coarse baleen, which gives them the ability to separate amphipods from sediment. Right and bowhead whales consume prey by slowly swimming into populations, so they have developed long, fine-textured baleen that allows them to minimize pressure. Rorquals have short baleen, which along with their streamlined bodies, allows them to swim faster and consume more prey in a shorter amount of time (Croll et al. 2002). The number of plates also varies, ranging from about 320 plates in gray whales to about 720 plates in blue whales, a member of the rorquals. Not surprisingly, right and bowhead whales fall somewhere in the middle (Steele 1970). This is important because it shows the variation in which natural selection has acted on various species, which allows them to utilize different ecological niches. When mammals first returned to the seas, natural selection acted on larger body size because it allowed for more efficient thermoregulation. This change also helped the organisms to sustain longer periods of time between meals. In higher latitudes, schooling prey was much more abundant in the spring and summer. Therefore, mysticetes needed to consume a large amount of prey during this short time it was available to be able to store up enough energy to migrate in the winter. Larger body size allowed them to utilize this ecological niche (Croll et al. 2002). Sexual selection and selection for higher reproductive fitness naturally tend to choose for larger body size (Blanckenhorn 2000), so assuming the organism is able to meet high energy requirements, it is natural for the larger body size to evolve. A combination of increased thermoregulation efficiency and an ability to meet high energy demands via filter feeding allowed this change to take place in mysticetes. It is proposed that filter feeding first evolved during the Ogligocene in the southern hemisphere, following the formation of the Atlantic Circumpolar Current (Croll et al. 2002). This caused the southern oceans to cool, which led to a large scale glaciation event in the previously warm and forested Antarctica. Therefore, previously photosynthesizing plankton died out when the ice spread, leaving only plankton that fed on algae (Marshall 2013). The cooling and subsequent glaciation happened because the Atlantic Circumpolar Current isolated the area that is now Antarctica and kept warm ocean waters away. The area where the warm waters meet the cold creates communities of highly productive zooplankton because of the steep gradients in temperature, salinity, light, and nutrients. The reason behind the increased productivity in the spring and summer is because as the ice melts, it releases accumulated nutrients into the environment. This shift to intense seasonal productivity led to an increase in algal blooms during the spring and summer, which in turn led to higher photoplankton levels (Houben et al. 2013). Because of this shift, a new ecological niche was opened up for cetaceans to adapt. Marine ecosystems are much more variable in nutrients than terrestrial ecosystems, causing patches of high density primary producers in some areas and little to no resources in others. It is proposed that nitrogen is the main nutrient responsible for this variation (Howarth and Marino 2006). This causes the populations of krill and plankton, which feed on these primary producers, to also vary in spread out patches. Therefore, this unique prey distribution prompted the marine mammals to adapt by developing filter feeding mechanisms such as baleen plates so they could exploit a large number of prey in one single attempt (Croll et al. 2002). Now that there is a better understanding of why baleen plates have evolved, researchers are beginning to piece together their stepwise evolutionary history. Erich M.G. Fitzgerald (2006) reported a new Obligocene toothed mysticete found in marine deposits in Austrailia from 23.9-27.0 million years ago that became a breakthrough for scientists studying the evolutionary past. This new species, Janjucetus hunderi, is believed to be older than any other known toothed mysticete, or odontocete. It is small with large eyes and sharp teeth, an indication that it was a specialized macrophagous predator. Therefore, this discovery proves false previous ideas that all early mysticetes were filter feeders and that the initiation and adaptive radiation of mysticetes was related to the evolution of filter feeding. Instead, Fitzgerald proposes that the adaptative radiation of crown group cetaceans was caused by climate change and the increase of productivity following the initiation of the Antarctic Circumpolar Current. He also presents that the extinct baleen whale genus Aetiocetidae is a paraphyletic taxon, unlike a previous study done by Deméré and Berta (Fitzgerald 2006). In this previous study, Thomas A. Deméré and Annalisa Berta (2008) had claimed that Aetiocetids, an ancestor of modern day mysticetes, represents a transitional state between macrophagous predators and filter feeders. This proposal is important because it is the basis for understanding phylogenetic relationships of mysticetes and is critical for generating and analyzing hypotheses explaining the loss of dentition and development of baleen plates. Aetiocetids are toothed mysticetes, seen as a continuation of the ancestral state, and also possessed lateral palatal foramina. These foramina are correlated with providing vascular supply to baleen plates, suggesting that baleen was also present. Deméré and Berta proposed that Aetiocetidae is a monophyletic clade (Deméré and Berta 2008), but Fitzgerald’s discovery of Janjectus hunderi presents otherwise. Fitzgerald argues that Janjucetus hunderi is unlike all other mysticetes, excluding it from Aetiocetidae and making Aetiocetidae a paraphyletic taxon. Although this may seem trivial, it suggests that macrophagy is not primitive for all cetaceans and that the evolution of filter feeding adaptations happened in a stepwise pattern as opposed to in one single explicit adaptation (Fitzgerald 2006). Felix G. Marx (2013) also disagrees with Deméré and Berta’s hypothesis that Aetiocetids were a transitional taxa that possessed both teeth and early proto-baleen. He argues that there is no evidence for the presence of baleen plates found on the bones of Janjucetus and Mammalodon that are found on other Aetiocetids. Assuming that Aetiocetidae is a monophylic clade, like  Deméré and Berta did, there are three possibilities to explain this. The first is a loss of baleen in Janjucetus and Mammaladon, the second is an independent gain of baleen by Aetiocetids, and the third is an assumption that neither mammalontids nor aetiocetids had baleen. Recent studies have doubted that the aetiocetid palatal foramina that Deméré and Berta linked to the presence of baleen weret actually related to baleen possession. If it is proven that they are not related, this evidence would further support the third hypothesis, that Mammalodontids and Aetiocetids did not possess baleen and therefore were not transitional species (Marx 2010). Many studies, such as the previous Marx study, suggest that early odontocetes did not have baleen, but either filtered their prey with specialized teeth or simply engulfed their prey like modern day odontocetes. They propose a direct transition from teeth to baleen (Deméré et al. 2008). However, this transition would be a huge evolutionary leap. Instead, Deméré proposes that the evolutionary change happened stepwise, from teeth to a hybrid to baleen. Deméré further argues his claim that Aetiocetids were a transitional taxa in another study based on molecular evidence as opposed to his previous research on bone structure. It is known that modern mysticetes have teeth initially and then develop baleen plate germs in utero, but lose their dentition and have only baleen during juvenile and adulthood. However, developing mysticetes do not produce enamel because at some point this trait evolved to become a pseuodogene. This is likely to have occurred about 28 million years ago and proves that dentition is an ancestral state of mysticetes. Using parsimony to study this and other ancestral characters suggest that the common ancestor of aetiocetids and edentulous mysticetes evolved lateral nutrient foramina, which are believed to have provided blood vessels and nerves a way to reach developing baleen. Further research suggests that the baleen of Aetiocetus was arranged in bundles between widely spaced teeth. If true, this combination of baleen and dentition would act as a transition state between odontocetes and mysticetes. This intermediate step is further supported by evidence of other changes that occurred with the evolution of baleen that make it possible for the organisms to survive using filter feeding, such as skull structure and throat elasticity. It would be highly unlikely for all of these changes to occur at once (Deméré et al. 2008). Therefore, this study shows that Oligocene aetiocetids possess both ancestral and descendent character states regarding feeding strategies. This makes them a mosaic taxa, showing that either baleen evolved before dentition was lost or that the traits for filter feeding originally evolved for other functions. It also shows that the evolution could have occurred gradually because the ancestral state was originally maintained. Therefore, the mosaic whales could have exploited new resources using filter feeding while not abandoning their previous prey strategies. The result of this stepwise transition is apparent in modern day baleen whales, because of their enamel pseudogenes and their in utero development and reabsorbing of teeth (Deméré et al. 2008). Intense research has been carried out to sort out the evolution and phylogenetic history of mysticetes, but it is clear that there is still much debate surrounding this issue. More work needs to be done to characterize extinct ancestral fossils so that future scientists will be able to piece together a more accurate phylogenetic tree. Determining whether or not a fossil organism possessed baleen can be tricky, though, because baleen rarely fossilizes (Deméré et al. 2008). All hypotheses of baleen presence must be made on the basis of bone dentitions and foramina. However, compiling and analyzing all studies to date seems to show that the evolution of baleen plates happened in a stepwise pattern with intermediate transitional species that possessed both dentition and baleen, such as the aetiocetids proposed by Deméré (Deméré et al. 2008). This adaptation was likely driven by selection for large body size and the intense seasonal supply of krill and plankton following the initiation of the Antarctic Circumpolar Current. Further research will sort out these relationships, but regardless it is a huge feat of evolution to bring about such a unique predator-prey relationship.

FINAL ARTICLE ADDITIONS FOR https://en.wikipedia.org/wiki/Baleen

This occurred because the current kept warm ocean waters away from the area that is now Antarctica, producing steep gradients in temperature, salinity, light, and nutrients where the warm water meets the cold.

The transition from teeth to baleen is proposed to have occurred stepwise, from teeth to a hybrid to baleen. It is known that modern mysticetes have teeth initially and then develop baleen plate germs in utero, but lose their dentition and have only baleen during juvenile and adulthood. However, developing mysticetes do not produce enamel because at some point this trait evolved to become a pseuodogene. This is likely to have occurred about 28 million years ago and proves that dentition is an ancestral state of mysticetes. Using parsimony to study this and other ancestral characters suggest that the common ancestor of aetiocetids and edentulous mysticetes evolved lateral nutrient foramina, which are believed to have provided blood vessels and nerves a way to reach developing baleen. Further research suggests that the baleen of Aetiocetus was arranged in bundles between widely spaced teeth. If true, this combination of baleen and dentition in Aetiocetus would act as a transition state between odontocetes and mysticetes. This intermediate step is further supported by evidence of other changes that occurred with the evolution of baleen that make it possible for the organisms to survive using filter feeding, such as a change in skull structure and throat elasticity. It would be highly unlikely for all of these changes to occur at once. Therefore, it is proposed that Oligocene aetiocetids possess both ancestral and descendent character states regarding feeding strategies. This makes them a mosaic taxa, showing that either baleen evolved before dentition was lost or that the traits for filter feeding originally evolved for other functions. It also shows that the evolution could have occurred gradually because the ancestral state was originally maintained. Therefore, the mosaic whales could have exploited new resources using filter feeding while not abandoning their previous prey strategies. The result of this stepwise transition is apparent in modern day baleen whales, because of their enamel pseudogenes and their in utero development and reabsorbing of teeth.

Intense research has been carried out to sort out the evolution and phylogenetic history of mysticetes, but it is clear that there is still much debate surrounding this issue. More work needs to be done to characterize extinct ancestral fossils so that future scientists will be able to piece together a more accurate phylogenetic tree.

Article Edit for: https://en.wikipedia.org/wiki/Baleen

Sentence and Citation added: The initial evolution and radiation of baleen plates is believed to have occurred during Early Oligocene when Antarctica broke off from Gondwana and the Antarctic Circumpolar Current was formed, increasing productivity of ocean environments.

Suggestions added to talk page: 1. Regarding the fossil species referred to in the Evolution section, this can be expanded to include a specific fossil species, such as Janjucetus hunderi and aetiocetids, which were proposed to be a transitional taxa that had both teeth and an early form of baleen. (Fitzgerald, E.M.G. (2006). A bizarre new toothed mysticete (Cetacea) from Australia and the early evolution of baleen whales. Proceedings of the Royal Society: Biological Sciences: 273, pp 2955-2963.)

2. With that being said, it should also be noted in this section that the phylogeny and evolutionary relationships of mysticetes is still being studied and debated. For example, it has been argued that these species did not actually possess any form of baleen and therefore were not transitional. (Marx, F.G. (2010). The more the merrier? A large cladistic analysis of the mysticetes, and comments on the transition from teeth to baleen. Journal of Mammalian Evolution: 18, pp 77-100.)

3. It can be added that baleen plates likely evolved because of the interaction between a new niche for schooling prey and selection for larger body size that allowed for better thermoregulation when mammals returned to the ocean. Larger body size also helped mammals sustain for a longer period of time before meals of the patchy populations of schooling prey. (Croll, D. A., Newton, K.M., & Tershy, B. R. (2002). Filter feeding. Encyclopedia of marine mammals, pp 428-432.)

Courtney Huck EEOB 3310 Wikipedia Project – Annotated Bibliography TA: David Salazar Valenzuela (Thursday 10:20)

Topic: How did teeth evolve into baleen plates in mysticetes?

References:

Deméré, T.A., McGowen, M.R., Berta, A. & Gates, J. (2008). Morphological and Molecular Evidence for a Stepwise Evolutionary Transition from Teeth to Baleen in Mysticete Whales. Systematic Biology: Vol. 57, pp. 15-37

This study looks at the anatomy of mysticete skull fossils and genotypes to infer a step by step transition from a toothed mysticete to a toothed-baleen mosaic mysticete and finally to the baleen mysticetes that are present today. It also observes vestigial qualities present in modern mysticetes that suggest the genes responsible for enamel production are still present in baleen whales, but have become pseudogenes. The research and conclusions will be used as molecular and fossil evidence for a detailed explanation of how toothed mysticetes transformed into baleen mysticetes.

Demere, T. A. & Berta, A. (2008). Skull anatomy of the Oligocene toothed mysticete Aetioceus weltoni (Mammalia; Cetacea): implications for mysticete evolution and functional anatomy. Zoological Journal of the Linnean Society: 154, pp 308–352.

This study focuses on a specific species of toothed mysticete from the family Aetiocetidae from the North Pacific. This species is important because they are believed to be transitional between ancient toothed and modern toothless whales. Skulls were prepared of these mysticetes, and it was found that many formina and grooves were preserved in this species that were present in the ancient toothed whales. Similar structures are also found in modern baleen whales that are used to allow arteries to reach developing baleen racks. The author states that this suggests baleen was previously present in the studied toothed mysticete species. This research will be used to further investigate how teeth and baleen plates have evolved and interacted with each other over the gradual evolution of mysticetes.

Fitzgerald, E.M.G. (2006). A bizarre new toothed mysticete (Cetacea) from Australia and the early evolution of baleen whales. Proceedings of the Royal Society: Biological Sciences: 273, pp 2955-2963.

The author reports a discovery of a new species of toothed mysticete that dates further back than any other known mysticete. This suggests that toothed whales came before baleen, and that filter feeding via baleen plates was a gradual evolution. Also, the author argues that this ancient species of mysticete was macrophagous. This source will be used to refute the previous idea that the evolution of filter feeding was the first step of the radiation of mysticete species, but rather many feeding strategies were present before baleen plates evolved.

Houben, A.J.P., Bijl, J.P., Bohaty, S.M., Pross, J., Passchier, S., Stickley, C.E., …, Rohl, U. (2013). Reorganization of southern ocean plankton ecosystem on the onset of Antarctic glaciation.

This article discusses a study on the effects of the first Antarctic glaciation on biotic evolution. Specifically, it observes how the glaciation affected plankton populations, causing seasonal influxes in plankton numbers. The authors test this effect using dinoflagellate cyst fossil records. They found that when the glaciation occurred, photosynthesizing plankton died out but photosynthesizing algae was able to survive by thriving during the warm season and surviving the cold season. Therefore, algae-eating plankton were able to increase their populations because they could feed off of the algae year round. The research will be used to demonstrate the rapid development of an ecological niche during the warm season that likely sparked the evolution for baleen plates.

Marx, F.G. (2010). The more the merrier? A large cladistic analysis of the mysticetes, and comments on the transition from teeth to baleen. Journal of Mammalian Evolution: 18, pp 77-100.

This research strives to clarify confusion about the taxonomic placement of extant species of mysticetes. It also challenges the proposal that aetiocetids were a transitional species that possessed both teeth and baleen plates. It suggests that instead, aetiocetids may have evolved on a separate evolutionary path independent of the toothed to baleen transition. This article will be used to counter the evidence showing that teeth evolved directly into baleen plates through mosaic species such as aetiocetids.