The Missing Explosion.

In the March 29th issue of Nature, molecular biologists look at mammalian diversity through time using modern species lineages and molecular clocks, with abnormal results. Paleontologists have observed that modern orders of mammals do not appear in the fossil record until the beginning of the Cenozoic, when they seem to explode in diversity. The ten million year Paleocene Epoch captures this increase in diversity, but many of these fossils belong to "archaic" mammalian orders that no longer live today. It is not until the following Eocene Epoch, that we observe members of living mammalian orders in the fossil record. Molecular biologists have argued for years now that the great explosion of diversity of mammals occurred much earlier in the Cretaceous.

Why? Molecular biologists recognize mammalian orders in their data very differently then paleontologists. Here is a good example: The order Artiodactyla (pigs, deer, camels, sheep) are recognized by having a unique morphological feature in the ankle that allows them to run fast. So the oldest fossil having this feature is the first artiodactyl, dated at 55 million years ago.

Molecular biologists due something strange and take all the living artiodactyl DNA and compare it with all the other living mammals. The first artiodactyl is identified as when the group split away from another living group. However, they don't have a date when this split occurred and must reconcile it with the fossil record using calibration points. In this particular study cited below, 30 fossils were selected to pin the branching points to dates. Now if we assume that DNA changes at a regular rate (highly unlikely), then a divergence time can be estimated. But this date is not when artiodactyls become artiodactyls, but when they split from another group that just happened to make it to the modern day, such as the Perissodactyls. Furthermore, the more ancestral arctocyonids may have given rise to only artiodactyls. Lacking that unique character of the ankle paleontologists would not recognoize arctocyonids as artiodactyls, but molecular biologists would since they gave rise to only artiodactyls. This pushes the date back for molecular biologists. A "hotspot" is mentioned in the article when 100 million years ago, mammals really split into surviving lineages.

The authors of this study further conclude that in addition to this "hotspot", diversity of surviving mammalian lineages did not increase until later during the middle Eocene. Does this completely controdict the fossil record?

Remember that molecular biologist are limited to only living species and do not see the explosion of mammals during the Paleocene Epoch. Instead they are limited to just the survivors, and extinction has greatly pruned their tree.

Bininda-Emonds et al. 2007. The delayed rise of present-day mammals. Nature 446, 507-512 (29 March 2007) | doi:10.1038/nature05634.