Mammalian Reproduction


The First Mammals


Hadrocodium, an Early Jurassic mammal

  • 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

    Triconodonta, from Palaeos

    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. Carnegie Museum site. You will also find very nice illustrations on this site.

    The came Repenomamus giganticus, 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!)


    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. 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...



    The Tribosphenic Molar

    A mammal from the Early Cretaceous of China, described in 1998, Zhangheotherium is important in framing our ideas about the evolutionary radiation of early mammals:

    Tribosphenic-type teeth may have evolved twice
    Molar teeth that look "tribophenic" 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, but they also include a new fossil from Australia, Ausktribosphenos. Later, similar molars evolved separately in Northern Hemisphere mammals whose descendants are modern Theria.

    Marsupials and their ancestors (metatherians)