Chapter 17: IMAGES FOR LECTURES
The great groups of placental mammals
Confirming four great clades of placental mammals. Briefly, the four great groups are
Afrotheria, a group including elephants, hyraxes, manatees, aardvarks, and others, which must have evolved in Africa, or perhaps in a larger section of Gondwanaland that included Africa.
Xenarthra, a group including sloths, anteaters, and armadillos, which must have evolved in South America, or perhaps in a larger section of Gondwanaland that included South America.
A group including primates and rodents, which probably evolved in Asia
All the other placental mammals, including Carnivora and the ungulate herbivores and whales, bats, and hedgehogs. These are probably Asian as well.
Of course, the challenge is to fit fossil placental mammals (for which we have no DNA) into the scheme. A major implication of the classification is that mammal evolution was strongly affected by geography. This is not a surprise, but the extent of the dependence is surprising (to me).
By 2004, people were saying that if you forget the idea of a strictly accurate molecular clock, and massage enough data, and rely on fossils for calibrating geological time, you can end up with something you believe. Molecular and fossil data can be "partially reconciliated", in the immortal phrase of the authors.
Douzery, E. J. P., et al. 2004. The timing of eukaryotic evolution: Does a relaxed molecular clock reconcile proteins and fossils? PNAS 101: 15386-15391.
The Paleocene/Eocene boundary
The arrival of modern mammals in North America at 55 Ma. Research program of Christopher Beard.
The Fossils of Messel
Ecological Replacement: The Guild Concept
Cenozoic Mammals in Dinosaur Guilds
Evolution by Improvement
Walking Whales, September 2001. In no particular order:
National Geographic News site
University of Michigan press release with a totally inaccurate heading.
OK, what do these two new papers say, and what do they mean? The papers document the earliest whales (Cetacea), which were terrestrial mammals walking on land. They have ankles that are matched only among living artiodactyls (Artiodactyla: sheep, cattle, deer, hippos, pigs). This nails down a close evolutionary linkage between the first artiodactyls and these first whales: they were sister groups forming a clade that people call Cetartiodactyla. The next most closely related group (by skeletal structure) is an extinct group of early mammals called mesonychians, which had been thought to be the nearest to whales until we found these new specimens. But mesonychians did not have an artiodactyl ankle, so that hypothesis has to be abandoned in the light of the new evidence.
Molecular evidence says that among living artiodactyls, hippos are the nearest to living whales. The new evidence says nothing new about that hypothesis, despite the headline in the press release cited above. It would take a dramatic re-appraisal of skeletal evidence to get hippos to be primitive artiodactyls (which is what the molecular evidence implies), and I cannot see that happening unless some very strange new fossils are found. So again, molecular evidence from living mammals, 50 million years after the evolutionary event that separated the clades in question, has to give way to real fossil evidence from the time it happened.
May 10, 2002. The evolution of whale ears. Whales, it turns out, have extraordinarily narrow semicircular canals in their ears. This is basically to stop them getting dizzy as they do their extraordinary acrobatic swimming in water. No land mammals have ever evolved this feature. So paleontologists checked the ears of very early whales, including those that were still capable of walking on land, in the Eocene. And ALREADY they had aquatically adapted ears, long before the rest of the body fully adapted to marine life. The paper was in Nature 417: 163-166.
August 12, 2004. Clearer picture of the evolution of whale ears. This is a new paper by a team headed by Hans Thewissen. It's in Nature, so won't be generally available on the Web. It's a really fine study.
While we're talking about origins of marine mammals, how about the discovery of a fossil sea cow with legs? National Geographic News, October 11, 2001.
While we're talking about origins of marine mammals, how about the discovery of sea sloths. Blog by Carl Zimmer, June 2004.
Early elephants were nowhere near the size (and probably did not look or behave like) living elephants. However, we probably are nowhere near their first ancestors, because arguments from the embryology of living elephants suggest an aquatic stage in their ancestry.
The ecology of the shovel-tusked mastodon Platybelodon, a reconstruction written nearly 70 years ago by Roy Chapman Andrews. From Natural History.
P 253. Carnivorous deer. For the deer that eat baby birds, see this Web page
References for Chapter 17
This page last updated April 8, 2013
Links last checked October 5, 2005.
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