UPDATES FOR CHAPTER 15.
  1. PAGE 222. February 2005. Two separate lines of mammals each evolved the mammalian middle ear
  2. PAGE 222. January 2005. Repenomamus giganticus

The middle ears of monotremes and other mammals evolved independently. The paper was in Science, and is not yet on free access: Rich, T. H., et al. 2005. Independent origins of middle ear bones in monotremes and therians. Science 307: 910-914, and comment, 861-862.

The evolution of the middle ear had looked like a fairly complex sequence of changes from cynodonts to mammals: yet here it seems to have happened twice in very similar (though not identical) ways. I don't yet have to alter the cladogram, Figure 15.8, though it's now possible that the monotreme clade came off the rest of the mammals before Hadrocodium. The new paper implies that the separation of monotremes from other mammals happened early (perhaps early Jurassic). Now that we have these early monotremes that still have adult teeth (unlike their living descendants), we may be able some day to identify the deep ancestors of monotremes.

Having said all that, there is considerable scepticism. Here's a comment from the folks at U. Texas, from Science, September 2005. And here's another comment in the same issue. The original authors maintain their position.The specimens on which the claim is based are few, and rather damaged. Critics essentially are saying that extraordinary claims need extraordinarily good evidence. We really will have to wait and see: probably until more Cretaceous monotremes turn up.

Repenomamus giganticus
This is the largest Mesozoic mammal yet, a triconodont from the Lower Cretaceous of China. It was the size of a raccoon, gigantic for a Mesozoic mammal (but of course nowhere near the size of later mammals). The paper is Hu, Y., et al. 2005. Nature 433, 149-152, and comment, pp. 116-117. In the same paper, a smaller species of Repenomamus is reported to have large chunks of a baby Psittacosaurus skeleton in its body cavity. Presumably it died with its last meal still being digested. This is an exciting and interesting paper. Some comments
1. It's overkill to suggest, as Anne Weil does in her commentary, that birds "got off the ground" to "avoid rapacious mammals". The local coyotes round my place don't seem to make much impact on the (ground-nesting) quail and wild turkey populations. And even if a mammal can eat a baby dinosaur, there are snakes that eat mammals the size of cattle, lizards that eat pigs and people, and flies that eat toads (that's not a misprint).
2. This report doesn't alter our global picture that Mesozoic mammals were small. It's one species, and it's not an accident that it was named giganticus.
3. It's naive of Hooker (in the Nature news piece) to suggest that Repenomamus didn't eat plants (on the grounds that it didn't have grinding molars). Foxes, coyotes, and raccoons don't have grinding molars, and they are officially "Carnivora", but they eat fruit, berries, and seeds as well as the occasional prey. (Check out coyote scat if you don't believe me!)
4. This was not a "prehistoric badger" and it wasn't discovered by "archaeologists" (Nature news service).

NOTES AND LINKS FOR CHAPTER 15.

Cynodonts

Teeth

Mammalian Reproduction

Suckling

The First Mammals

Morganucodonts

Hadrocodium, an Early Jurassic mammal described in 2001.

  • Three things stand out: this animal was TINY, less than 2 grams in weight (15 would add up to an ounce); it had a big brain for its size; and it had a well-developed middle ear. All ot these are astonishing for such an early mammal. It means that mammals evolved basal mammalian characters quickly, long before they split into the tree divisions we have surviving today. The paper was published in Science.
  • Press release
  • BBC News OnLine

    Triconodonts
    In March 1999, the discovery of the complete triconodont Jeholodens from the Chinese locality that also contains feathered theropods (Chapter 12) and the first angiosperm (Chapter 14) posed more questions than it solved. Here's the paper, from the NatureAsia site. Here are some comments:

    Then, in January 2005, came the announcement of the largest Mesozoic mammal yet, the triconodont Repenomamus giganticus, as big as a raccoon. This is gigantic for a Mesozoic mammal (but of course nowhere near the size of later mammals). The paper is Hu, Y., et al. 2005. Nature 433, 149-152, and comment, pp. 116-117. In the same paper, a smaller species of Repenomamus is reported to have large chunks of a baby Psittacosaurus skeleton in its body cavity. Presumably it died with its last meal still being digested. This is an exciting and interesting paper. Some comments
    1. It's overkill to suggest, as Anne Weil does in her commentary, birds "got off the ground" to "avoid rapacious mammals". The local coyotes round my place don't seem to make much impact on the quail and wild turkey populations. And even if a mammal can eat a baby dinosaur, there are snakes that eat mammals the size of cattle, lizards that eat pigs and people, and flies that eat toads (that's not a misprint).
    2. This doesn't alter our global picture that Mesozoic mammals were small. It's one species, and it's not an accident that it was named giganticus.
    3. It's naive of Hooker (in the Nature news piece) to suggest that Repenomamus didn't eat plants (on the grounds that it didn't have grinding molars). Foxes, coyotes, and raccoons don't have grinding molars, and they are officially "Carnivora", but they eat fruit, berries, and seeds as well as the occasional prey. (Check out coyote scat if you don't believe me!)
    4. This was not a "prehistoric badger" and it wasn't discovered by "archaeologists" (Nature news service).

    Fruitafossor

    April 1, 2005. Chipmunk-sized mammal from the Jurassic may have eaten termites. The paper is in Science: Luo, Z.-X., and J. R. Wible. 2005. A Late Jurassic digging mammal and early mammalian diversification. Science 308, 103-107. It will be on the Web in a few months. My only worry about the interpretation is the very small size: the living analogs are MUCH bigger, therefore MUCH more powerful. Voles dig, but they don't dig for termites. I wonder how big a baby anteater (etc.) is when it begins to dig well enough to feed itself...

    Monotremes

    Multituberculates

    The Tribosphenic Molar

    Zhangheotherium
    A mammal from the Early Cretaceous of China, described in 1998, Zhangheotherium is likely to be very important in revising our ideas about the evolutionary radiation of early mammals:

    Tribosphenic Teeth Evolved Twice
    A new analysis of mammal teeth announced early in 2001 showed that the tribosphenic molar evolved separately in two separate groups of mammals. First, it evolved in a group of Southern Hemisphere mammals which have survived as the living monotremes. Their fossil representatives include Steropodon (p. 252), but they also include a new fossil from Australia, Ausktribosphenos. Later, tribosphenic molars evolved again, in Northern Hemisphere mammals whose descendants are modern Theria.

    Marsupials and their ancestors (metatherians)