Dolphins, porpoises and whales together comprise the order Cetacea. This order can be further split into two suborders; the odontocetes (“toothed whales” such as orca), and the mysticetes (“baleen whales” such as the blue whale). Baleen whales do not possess teeth, instead they have plates of baleen, a tough bristly substance which hangs from the upper jaw and is used in filter feeding. Unlike teeth, baleen is not made from dentine and enamel but from keratin, the same substance as your fingernails.
During feeding baleen whales open their mouths to take in massive quantities of water containing zooplankton such as small fish and crustaceans, they then close their mouths and, using their tongue, expel the water through the baleen plates trapping their prey in the process. Using this feeding technique the blue whale (Balaenoptera musculus) may take in and expel more than 70 tons of water in one go!
Despite their planktonivorous diet baleen whales can grow to be extremely large. At 180 tons the blue whale is the largest animal ever to have lived.
Baleen does not fossilise well, however, the ancestry of mysticete whales can be deduced from other skeletal features and from DNA. What we now know is that although all modern day mysticetes possess baleen, ancestral species, such as Janjucetus, did not, they possessed teeth as modern day odontocetes do. This means that at some point in the evolution of mysticete whales teeth were replaced by baleen. How this transition occurred is the subject of a 2008 paper by Thomas Deméré et al.
The paper provides two lines of evidence to show that baleen whales did indeed evolve from toothed ancestors. The first is palaeontological evidence based on a fossil whale, Aetiocetus weltoni, that has been dated to be 24-28 million years old. The second line of evidence is molecular, based on DNA sequences.
But first the palaeontological evidence. Deméré and his colleagues examined the fossilised skull of the mysticete whale Aetiocetus weltoni, which, unlike modern day species, possessed teeth. This was a species that existed before teeth had been replaced by baleen. However, upon closer examination of the jaw, small grooves known as “nutrient foramina” were found. These grooves are also found in modern baleen whales and conduct the passage of nerves and arteries to the epithelium of the palate to provide nourishment for the baleen which grows continuously. These slits are not found in any odontocete whales.
Nutrient foramina were also found in two other fossil mysticete whales of a similar age, Aetiocetus cotylalveus and Chonecetus goedertorum. What this suggests is that these species had both teeth and baleen. They are transitional forms.
The second line of evidence is molecular and based on DNA sequences. Given the hypothesis that toothless mysticetes evolved from toothed ancestors, it was predicted that the genes for making teeth (the secretory calcium binding phosphoprotein (SCPP) gene family) would persist in the genomes of modern day baleen whales but in a non-functional state. In other words, once released from the selective constraints of natural selection (due to disuse) random mutations acquired over time should have rendered these genes inactive but still be clearly recognisable to biologists as SCPP genes.
This is exactly what was found. DNA sequences taken from 12 species of modern day baleen whales showed the presence of enamel specific SCPP genes (AMBN, ENAM and DMP1). In mysticetes two of these genes had become completely non-functional due to frame-shift mutations, DNA insertions and deletions and premature stop codons which cause the synthesis of proteins to terminate too early. As a result of mutations the authors suggest that the genes AMBN and ENAM are now decaying pseudogenes.
One final piece of compelling evidence for the toothed ancestry of mysticete whales comes from development. Below is an image of a fin whale foetus with a section of the jaw dissected away. Fin whales do not have teeth, however, what this image shows is that in the embryo baleen whales do develop tooth buds. These develop from cells called odontoblasts which secrete dentin, but, unlike in odontocetes, this is never covered with enamel to form complete teeth. In mysticetes the teeth buds do not break the gumline but are reabsorbed before birth. Tooth buds no longer serve any function to mysticete whales but do provide strong evidence as to their ancestry.
So what does this all mean? We now know that around 30 million years ago the transition from teeth to baleen was underway in the mysticete lineage. At this time species such as Aetiocetus weltoni would have had both teeth and baleen. By acting as a sieve baleen would have enabled these species to catch smaller prey and thereby have access to a greater range of food sources. They may have been better fed enabling them to produce more offspring and so increase the proportion of animals carrying genes for baleen in the next generation. As filter feeding proved to be a highly successful strategy it gradually came to replace teeth entirely.
The fantastic image below is by Carl Buell and shows what Aetiocetus weltoni may have looked like with both teeth and baleen.
Demere T.A., McGowen M.R., Berta A. & Gatesy J. (2008). Morphological and Molecular Evidence for a Stepwise Evolutionary Transition from Teeth to Baleen in Mysticete Whales, Systematic Biology, 57 (1) 15-37. DOI: 10.1080/10635150701884632