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Got That? The Complex Story of African Mammal Evolution 12/03/2003
The article by Jean-Jacques Jaeger in the Dec. 4 issue of Nature¹ is pretty upbeat about the evolutionary history of African mammals, but takes a bit of untangling to follow.
He begins confidently, “For some 40 million years [sic], the Afro-Arabian landmass existed in splendid isolation. A newly described fossil fauna from the end of that time provides a window [sic] on the evolution [sic] of the continent’s large mammals.” (He refers to a fossil group named the Chilga biota, found in the Ethiopian highlands by Kappelman et al., described in the same issue.²) Let’s take a look out said window and see how evolution has unfolded:

During most of the Cenozoic era, from the Cretaceous–Tertiary boundary 65 million years ago [sic] until roughly 24 million years ago [sic], Afro-Arabia was [sic] an island continent drifting steadily [sic] northwards towards Eurasia. Fossil mammals documenting this period are scarce and belong almost exclusively to endemic forms restricted to Afro-Arabia, such as proboscideans, hyraxes and elephant-shrews. But by around 24 million years ago [sic], a permanent land bridge had formed between the two landmasses. A burst of faunal interchange followed: many Eurasian mammals, such as rhinos and ruminants, dispersed into Africa, and some Afro-Arabian mammals, such as elephants, migrated in the opposite direction. (Emphasis added in all quotations.)

That forms the plot line, but there are problems. The Chilga specimens he describes seem to fit the story, but there are puzzles among the bones:

Among the proboscideans recorded are primitive [sic] forms such as Palaeomastodon and Phiomia (also known from older deposits in Egypt). But there are also representatives of modern families, for example taxa such as Gomphotherium, the earliest proboscidean on the branch leading to extant elephants. Another surprise is the oldest occurrence of deinotheres, peculiar proboscideans with downward-curved lower tusks, which were previously recorded only from rocks younger than 24 million years old [sic]. The new species of deinothere displays molars that are more ‘bunodont’ in form (that is, made of several distinct cusplets arranged in transverse crests) than its descendant, whose molars display plain transverse crests. This discovery seems to rule out the possibility that deinotheres are derived from an ancestor bearing plain, transverse-crested molars, as was formerly supposed, and provides new evidence about proboscidean evolution [sic].

Jaeger bemoans the scarcity of the fossil record for this period, but claims, “Nonetheless, considerable information has been inferred from the evidence we do have.” He talks about how systematists have grouped the African fauna into a superorder Afrotheria based on fossil and molecular evidence. Though “African mammalian faunas are dominated by these endemic forms,” a few other groups did get over to the big island somehow, including our alleged remote ancestors, the catarrhine primates, fathers of hominoids. These “newcomers” went through “rapid evolution” on the landmass, he claims.
Even though the Chilga fossils are supposed to pre-date the land bridge, Jaeger says, “The Chilga mammals also yield insights into the dynamics of the faunal interchange between Afro-Arabia and Eurasia.” How is that possible? By seeing what pre-existed before the interchange, he feels it is possible to document that “the ensuing ecological competition ended with winners and losers.” I.e., some animals were destined to fall in numbers, others to multiply and diversify.
The Chilga fossils do leave a few research items for paleontologists:

Finally, the discoveries of Kappelman et al. highlight two other palaeobiological issues. First, on northern continents glaciation caused a significant cooling around 33 million years ago [sic], which resulted in numerous extinctions [sic] among mammalian communities. From these new data, however, it seems that large Afro-Arabian herbivores were not affected, either at that time or later, implying that the climatic changes were less severe on southern continents. Second, the fossil record of the Afro-Arabian continent is not only scanty but also largely concentrated on the northern edge. This has led to the proposal that other groups of mammals existed in Afro-Arabia during its period of isolation, but that they were restricted to more southern latitudes. However, the Chilga mammal community is rather like that found at Fayum in Egypt, which is some five million years older [sic], providing hints that there was little provinciality among Afro-Arabian mammals at that time. As yet, though, we have unveiled [sic] only a few of the secrets of mammal evolution on the Afro-Arabian continent. Many more surprising discoveries are to be expected.

Got that?

¹Jean-Jacques Jaeger, “Mammalian evolution: Isolationist tendencies,” Nature 426, 509 - 511 (04 December 2003); doi:10.1038/426509a.
²Kappelman et al., “Oligocene mammals from Ethiopia and faunal exchange between Afro-Arabia and Eurasia,” Nature 426, 549 - 552 (04 December 2003); doi:10.1038/nature02102.

We’re going to give our readers an opportunity to go baloney detecting on this story before we have at it. Hint: separate data from storytelling, and look for the damaging admissions.

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