From Tearing Teeth to Baleen

In 1850, the Honolulu Friend published an unusual editorial from an unexpected source. Whalers who scoured the seas for the giant aquatic mammals knew well that large whales were beginning to become scarce, but this was the first time one of the whales decided to speak up in print about the plight of its kind. A bowhead whale from the Arctic who went under the simple moniker the “POLAR WHALE”,  the concerned cetacean wished to report on a meeting recently held by “the knowing old inhabitants of this sea” in efforts “to avert the doom that seems to await all of the whale Genus throughout the world, including the Sperm, Right, and Polar whales.” The whale continued:

Our enemies have wondered at our mild and inoffensive conduct; we have heard them cry,”there she blows,” and our hearts have quailed as we saw their glittering steel reflecting the sun beams, and realized that in a few moments our life-blood oozing out, would discolor the briny deep in which we have gamboled for scores of years. We have never been trained to contend with a race of warriors, who sail in large three-masted vessels, on the sterns of which we have read “New Bedford,” “Sag Harbor,” and “New London.” … We Polar whales are a quiet inoffensive race, desirous of life and peace, but, alas, we fear our doom is sealed; we have heard the threat that in one season we shall all be “cut up,” and “tried out.” Is there no redress?

The Polar Whale’s Appeal had little effect. Whaling continued apace, but the more effective Save the Whales movement of the 20th century would draw on some of the same sentiments expressed by the hapless Bowhead. Whales were “gentle giants” – intelligent, curious creatures with not a hint of malice in their hearts. In addition to the ecological reasons not to kill whales, slaughtering the enormous marine mammals became a moral taboo. To murder such charismatic creatures turned any whaler into a monster. (Though I hasten to add that some whale-hunting continues today, often under the guise of so-called scientific surveys.)

With the exception of the sperm whale, the majority of large whale species that have been hunted were baleen whales. These are enormous filter feeders that sift out staggering amounts of tiny creatures from the sea by way of hair-like structures hung off of plates in their mouths. They are still hunters, but of a cuter sort (hence our affinity for them). Many millions of years ago, though, baleen whales were quite different. In fact, had some of the very earliest varieties of baleen whale survived to the modern era, I have to wonder if their predaceous habits would have hindered attempts to extend their kind protection from harpoons and gaffs.

Modern whales are divided into two groups – there are the “toothed whales”, or odontocetes, and the “baleen whales”, technically called mysticetes. This organizational split only makes sense because over 53 million years of extinction have wiped out all the archaic varieties of whales that would complicate this distinction. After all, baleen whales evolved from ancestors who had mouths full of piercing, slicing teeth, and the origins of baleen occurred well after the split between the odontocete and mysticete lineages over 34 million years ago. The earliest “baleen whales” had more in common with their sharp-toothed forerunners than the Blue and Grey whales of today, and among the most fearsome was the approximately 25 million year old Janjucetus.

First described in 2006 by paleontologist Erich Fitzgerald from fossils found in Australia, Janjucetus had an impressive set of jaws set with large, differentiated teeth. The smooth, recurved teeth at the front of its mouth snatched prey, which the whale then sheared through with the array of multi-cusped teeth which followed along the tooth row. Despite this formidable dentition, though, we know that Janjucetus undoubtedly belonged to the mysticete lineage because of the organization of its skull bones. One of the upper jaw bones, the maxilla, had a portion in the cheek region that jutted out from the part of the same bone in the snout, and on the underside of the skull, near the back, there were two wide bulbs of bone called basioccipital crests. Together, the anatomical details of these bones help scientists distinguish between fossil mysticetes and odontocetes.

Janjucetus was clearly feeding in a different way than its later, baleen-bearing relatives, and the disparity between the earlier and later mysticetes raises questions about when and how the specialized jaw anatomy of modern species evolved. As Fitzgerald notes in a paper recently-published in Biology Letters, Janjucetus had lower jaws that were fused to each other in the middle at an anatomical point called the mandibular symphysis, but in modern baleen whales the point where the jaws meet is flexible and allows the whales to vastly expand their mouths. To borrow the more technical phrasing Fitzgerald used in his new paper, how did the “dynamic oral cavity expansion” of mysticetes evolve?

Our understanding of early mysticete evolution is still a bit fuzzy. The creatures that exhibit transitional features and document the establishment of what researchers thought were standard baleen whale characteristics are still being sought after. This makes Janjucetus all the more significant because its anatomy indicates that the package of characteristics associated with living mysticetes evolved in a mosaic pattern. Even though Janjucetus had fused lower jaws that were incapable of expanding, this whale had the expanded head shape commonly associated with baleen whales. Janjucetus still had a big, wide mouth, even though it couldn’t gulp quite like its later cousins.

As strange as it may seem, the ability of baleen whales to filter feed may owe something to the hunting habits of forerunners like Janjucetus. Fitzgerald notes that “the wide and blunt head shape” of whales such as Janjucetus “are capable of generating significantly greater negative pressures during suction feeding than those with elongate and narrow jaws.” The wide heads of mysticetes may have initially evolved because such shapes increased the abilities of whales like Janjucetus to slurp up squid and fish. If this is true, then mysticetes may have been an evolutionary modification for feeding on fast-moving prey, but was later co-opted for filter feeding through the evolution of more flexible jaws and baleen. The peaceful Polar Whale and its allies may owe their ability to sieve krill and small fish from the sea to sharp-toothed, suction-feeding ancestors that were likely among the most formidable hunters of the Oligocene seas.

Top Image: The skull of Janjucetus. Modified from Fitzgerald, 2006.

References:

Fitzgerald, E. (2006). A bizarre new toothed mysticete (Cetacea) from Australia and the early evolution of baleen whales Proceedings of the Royal Society B: Biological Sciences, 273 (1604), 2955-2963 DOI: 10.1098/rspb.2006.3664

Fitzgerald, E. (2011). Archaeocete-like jaws in a baleen whale Biology Letters DOI: 10.1098/rsbl.2011.0690

Marx, F. (2010). The More the Merrier? A Large Cladistic Analysis of Mysticetes, and Comments on the Transition from Teeth to Baleen Journal of Mammalian Evolution, 18 (2), 77-100 DOI: 10.1007/s10914-010-9148-4

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