Afrotheria And Breakup Of Gondwana

Molecular evidence has now made it clear that there was also a unique radiation of mammals in Africa (see Box 10.6). Modern mammals as different as elephants, golden moles, tenrecs and aardvarks all appear to share a common ancestry, and indeed this clade, termed the Afrotheria, appears to have been one of the first to diverge from the other placental mammals (Springer et al., 1997,2003; Murata et al., 2003).

10.6.1 Geography and dating of the basal placental divergence

The new molecular understanding of basal placental relationships indicates clearly that biogeography was important, although there are two models. The first, the 'southern origins' model (Murphy et al., 2001), is that eutherians arose in the southern continents (Gondwana) in the Early Cretaceous, and the split of Africa from South America about 120-100Myr ago led to the disjunct distributions. Archibald (2003) notes the absence of Late Cretaceous placentals in Africa and South America, and he prefers a 'northern origins' model that matches the timings of the fossil record. Basal eutherians were present first in Laurasia (Eomaia from 125 Myr ago; see Box 10.4) and Boreoeutheria evolved from them in that area (zalambdalestids and zhelestids in Uzbekistan, 85-90 Myr ago). Afrotheres must have split off somewhat earlier, reaching Africa at least by the end of the Cretaceous. Xenarthrans then split from Boreoeutheria and migrated into South America at least by the Palaeocene.

Fossils and molecules appear to agree reasonably well about the timing of these early splitting events (Archibald, 2003; Benton and Ayala, 2003). Earlier estimates appeared to suggest otherwise, with claims that the placentals began to split 120 Myr ago, although the oldest fossils were basal Tertiary, 65 Myr old. At present, molecular estimates offer a wide range of dates for the split of eutherians from marsupials, 130-185.3 Myr ago. Eomaia, a definite eutherian (see Box 10.3), is 125 Myr old, just slightly younger than the top estimate. Then, modern placentals split about 101-108 Myr ago, according to molecular dates, with the branching of Afrotheria. Xenarthra branched from Boreoeutheria about 88-100 Myr ago, and Laurasiatheria from Euarchontoglires from 88 to 79 Myr ago (see Box 10.6). There are no afrotherians or xenarthrans as old as this, but the zhelestids and zalambdalestids from Uzbekistan indicate the existence of the latter two clades some 85-90 Myr ago.

There is perhaps less agreement about the timing of splitting of the placental orders—molecular dates tend to be well down in the Late Cretaceous, although the fossils resolutely sustain the classic view that mammal orders appeared and radiated only after the KT event (Archibald, 2003). Taking rodents as an example, many molecular dates suggest the order originated from 112 to 100Myr ago. Huchon et al. (2002), however, find a date of 65Myr ago, in accord with the fossils. It is not

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