Whales are beautifully ridiculous. They are majestic divers, in some cases plunging nearly two miles underwater. And yet sooner or later they must rise back to the surface to breathe air. They breathe through a rather ridiculous-looking hole on top of their head. Unlike fish, which often reproduce by spraying millions of eggs and swimming away, whales give birth to one calf at a time, which they proceed to nurse for months. Some whales are like underwater bats, shrieking through their blowholes and listening to the echoes. And perhaps most ridiculous of all are whales that turn themselves into giant filters, thanks to a ridiculous tissue called baleen.
Baleen is a giant frond-like growth that sprouts from the jaws of 11 species of whales. Baleen whales open up their toothless mouths, sucking clouds of krill and other animals. They then ram the water out with their massive tongues, trapping food in their overlapping plates of baleen. Licking off the food, they open their mouths for another gulp.
Whales are ridiculous thanks to their history.They evolved from mammals on land. Their swimming, reproduction, breathing, and other adaptations to life in water are all the result of tinkering with a terrestrial animal’s body. Fossil discoveries have documented how coyote-like mammals moved into the water about 45 million years ago and became more and more adapted to the marine life. The evolution of whales was not a single leap, however, but a long series of transitions. Even after whales had abandoned life on land, they were still not yet like whales today. None of them, for example, had baleen.
Among living whales, baleen is an all-or-nothing affair. If you’re a whale you either have baleen or you have none. All other whales are profoundly different, with teeth instead of baleen. And while toothed whales can all echolocate, baleen whales cannot. Studies on whale DNA only reinforce the sharp divide between baleen whales and other whales. All baleen whales share genetic markers not found in toothed whales. In other words, the evolutionary tree of living whales is split into two branches. Paleontologists have found many extinct members of those two branches from the past 30 million years, bearing the hallmarks of either baleen whales or toothed whales.
In a sense, then, the origin of baleen whales is as remarkable as the origin of all whales. Yet that fact does not represent a real challenge to evolution. After all, there was a time when scientists had not yet found walking whales, and now they’ve found plenty. Other scientists have meanwhile been searching for the fossils of the earliest baleen whales. And, as I’ll describe below the fold, they’ve now found a particularly interesting one: a baleen whale without the baleen.
The whale in question is called Janjucetus hunderi, named after the Australian town of Jan Juc where it was found, and a Mr. S. Hunder who found its fossils. Among its 25-million year old remains are a nearly complete skull, some vertebrae, ribs, and a bone from its flipper. As you can see from the skull, which I’ve reproduced here, this was an animal with big eyes and plenty of sharp teeth. To understand where it fits in the history of life, Erich Fitzgerald, a graduate student at Monash University in Australia made a careful study of its bones. He then compared over 200 fine anatomical details in Janjucetus to 23 other whale species. Some of these whales are living, and some are extinct, including a few that have yet to be fully described by scientists. Fitzgerald also compared these whales to pigs and hippos, which are among the closest terrestrial relatives to whales. The results of his study appear in the Proceedings of the Royal Society of London. Here I’ve reproduced the evolutionary tree that sums up his work.
The new results support previous studies, showing that living whales only represent the crown of branches on a very deep tree. The common ancestor of toothed and baleen whales lived about 35 million years ago, about 10 million years after early whales began moving into the water. As I decribed in my book, At the Water’s Edge, whales lost their hind legs almost completely during those ten million years, except for a few vestigial bones in their body wall. Their nostrils moved partway up their snout. Their ears adapted for hearing underwater. The lineage that gave rise to the toothed and baleen whales is the only one that survives today, while all the other whales became extinct.
Janjucetus is a whale with teeth. And yet Fitzgerald found that they belong to the baleen lineage, not the lineage of living toothed whales. (Nor do they belong to an earlier branch of the whale tree.) Its teeth may bear no resemblance at all to the frond-filled mouths of living baleen whales, but Janjucetus shares with them some key traits not found in other whales, such as very wide lower jaw bones that were joined at the front not by a bony chin but by a mesh of cartilage. (See update here.)
Fitzgerald points out that Janjucetus’s sharp teeth, powerful biting muscles, and big eye sockets make it resemble a leopard seal. He argues that it got food in the same way, hunting after individual fish and tearing their bodies apart. On its hunts, Janjucetus may have also relied on a sharp sense of hearing. It had a large hollow space in its lower jaws, which may have been stored with fat that could have conducted sound to its ears. But it shows no trace of the equipment toothed whales use for echolocation.
In other words, baleen whales evolved baleen long after splitting off from other whales. Their baleen-free ancestors apparently thrived as leopard-seal-like hunters for millions of years. Over time, their descendants evolved some of the traits that are found in all baleen whales today. Their jaws grew flatter and pointier. They still had teeth, which they may have been able use to filter food. Their teeth had changed shape, so that they were no longer good for shearing. Instead, they locked together. Crab-eating seals have similar teeth, which they use for filtering prey. As these whales shifted away from leopard-seal lifestyle, their eyes got smaller as well.
Some of these transitional whale fossils not only have teeth but also have marks suggesting they also held baleen. (Baleen plates are not giant teeth. They are made of keratin, the stuff in our hair and fingernails, rather than enamel.) As Fitzgerald’s tree shows, the mixed-mouth whales gave rise to new species that kept the baleen and lost the teeth. They had become fully adapted to a new style of filter-feeding, and the results were dramatic: baleen whales proceeded to evolve to much bigger sizes. With the emergence of the blue whale, they became the biggest animals to ever exist on Earth.
Yet even the first “true” baleen whales were not like today’s baleen whales. Some fine-tuning still remained, such as pushing the blow hole all the way to the top of the head, for example. But as Darwin himself noted, today’s true baleen whales still preserve signs of their distant toothy past. Their embryos develop tooth buds, which are absorbed into the jaw as plates of baleen grow over them.
This study on Janjucetus is hardly the last word on baleen whale evolution. Paleontologists have found a number of other early baleen whale fossils that have yet to be carefully studied–a process that can take years. The branch marked ChMTM represents some whale fossils at the Charleston Museum of Natural History that have yet to be named, for example. Fitzgerald’s analysis suggests that these fossils are even more primitive than Janjucetus. You can expect more work on baleen whale DNA and even on the evolution of their development–which embryonic signals changed to produce baleen and to kill off teeth? And paleoecologists will be offering insights into the changing environment in which baleen whales emerged–a cooling ocean in which krill and other plankton began to produce in bigger concentrations. Whale evolution is a very big picture, and one that’s still coming into focus.
But Janjucetus already points to some important rules about major evolutionary transformations. As species adapt to new ecological niches, they become mosaics of primitive and advanced traits. It’s much the same story for baleen whales as for land vertebrates, as demonstrated by the fish with legs, Tiktaalik, that made news earlier this year. Lurking in the Earth are strange beasts that straddle the divides of life as we know it today.
Source: Erich M. G. Fitzgerald, A bizarre new toothed mysticete (Cetacea) from Australia and the early evolution of baleen whales. Proc. R. Soc. B doi:10.1098/rspb.2006.3664 1
Links to this Post
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