The Isolation Of Madagascar

Madagascar is separated from the southeastern African coast by the 350- to 750-mile-wide Mozambique Channel, and with a surface area of 230,000 square miles it is the world's largest oceanic island (Greenland, New Guinea, and Borneo are all larger, but are connected to the adjacent mainlands at times of lowered sea level). This isolation has evidently had a strong effect on the composition of Madagascar's fauna which, when compared to those of the continents and even to other very large islands, shows an unusual combination of low diversity at high taxonomic levels with high within-family diversity. Clearly the waters surrounding Madagascar have acted as a powerful faunal filter, albeit a slightly porous one.

The fragment of continental crust we know today as Madagascar once lay deep within the ancient supercontinent of Gondwana, with India to its east. But by the time that Gondwana began actively to fragment in the middle Jurassic, about 160 myr ago, the western edge of the island was already underwater and Madagascar, still attached to Antarctica in the south and to India in the east, began to move south-southeast away from Africa along a slip-strike fault, the modern remnant of which in the Mozambique Channel seafloor is called the Davie Fracture Zone (see review by Wells, 2003). This movement had ceased by the middle Cretaceous, about 125 myr ago, leaving Madagascar in roughly its present position vis-à-vis Africa (Coffin and Rabinowitz, 1988). It is not certain whether at this point Madagascar still retained a land connection to Africa and Antarctica via India (contrast Krause, 2003, with Smith et al., 1984). India parted company with Madagascar in the late Cretaceous, about 88 myr ago (Storey, 1995; Storey et al., 1997), definitively completing the island's isolation well before the beginning of the Age of Mammals at around 65 myr ago.

Although the current record of Cretaceous mammals in Madagascar consists of little more than a small handful of teeth, a remarkably wide range of taxa is represented. Among them are the world's oldest tribosphenic mammal, Ambondro mahabo (Flynn et al., 1999) from the middle Cretaceous, and the earliest marsupial, from the latest Cretaceous (Krause, 2001). The balance of late Cretaceous specimens includes a multituberculate and two gondwanatheres (Krause, 2000, 2003). But as impressive as this variety may be compared to the size of the collection, no modern placental groups are represented; and it is clear that none of Madagascar's modern mammalian groups (or any plausible precursor) is represented among Mesozoic fossils discovered so far, and that none can be shown to represent a Gondwanan remnant. Instead, it appears that all must be descended from ancestral forms that somehow contrived to cross a substantial water barrier (Krause et al., 1997). Terrestrial mammals are notoriously poor overwater dispersers (Lawlor, 1986), and the only even remotely plausible mechanism for getting them to Madagascar is by rafting on tangled mats of vegetation such as those that are swept out to sea by the floodwaters of African rivers.

Today's terrestrial Malagasy mammals belong to four orders: Primates, Lipotyphla (broadly, Insectivora), Carnivora, and Rodentia, all of which also occur on the African and Asian continents. A fifth order, the enigmatic and endemic Bibymalagasia (MacPhee, 1994), was also represented on Madagascar until recently, as was Artiodactyla in the form of pygmy hippopotamuses. However, large-bodied semiaquatic forms like the (probably quite recently arrived) hippopotamuses disperse by different rules from the strictly terrestrial forms, and the same is true for the volant Chiroptera. Among the strictly terrestrial groups, the ancestral primates (see below) and lipotyphlans probably arrived early in the Tertiary period, while the ancestral carnivores and rodents most plausibly reached Madagascar early in the Miocene epoch (see reviews by Tattersall, in press a and b). The general feeling at present is that the other Malagasy mammal groups are most likely monophyletic (see, for example, Goodman et al., 2003; Jansa and Carleton, 2003; Olson and Goodman, 2003; Yoder, 2003; Yoder and Flynn, 2003), and this is probably also true for the primates (e.g., Yoder and Yang, 2004, but see discussion below). If such is the case, then a minimum of five colonization events is still necessary to explain Madagascar's endemic terrestrial mammal diversity. Crossings were possibly concentrated into two periods of time, the early Tertiary and the early Miocene, which makes it necessary to look again at Madagascar's historical biogeography.

To say that Madagascar has been stable in its position relative to Africa since well before the beginning of the Age of Mammals is not to say that the geography of Madagascar and its surrounding crust has necessarily remained static throughout the Tertiary. Despite the fact that today most of the Mozambique Channel is of oceanic depth, it is possible that parts of its seafloor were raised in the past. Thus, McCall (1997) has argued that uplift along the Davie Fracture Zone in the period between about 45 and 26 myr ago resulted in its partial sub-aerial exposure, with subsequent tensional conditions returning the topographic highs. This scenario is based on core samples reported by Leclaire et al. (1989) and Bassias (1992) suggesting that subaerial sediments were deposited along the ancient fault line during late Eocene and Oligocene times. deWit and Masters (2004) have recently raised this possibility once more, and have also suggested a potential late Cretaceous or early Tertiary migration route along the set of fracture zones, known as the Antarctic-Africa Corridor, that lie between Antarctica and Africa/Madagascar. They have also proposed an alternative migration route from India, along the Deccan hotspot corridor to Madagascar's north and east. The latter suggestion evokes the notion of a potential Chagos/Laccadive filter connection between India and Madagascar that was recently mooted by Marivaux et al. (2001), echoing an earlier suggestion by Gingerich (1975).

The fact that Madagascar's modern mammalian fauna is so unlike Africa's (or Asia's) certainly suggests that the water barrier around the island has existed continuously throughout the Tertiary. Indeed, Krause (2003) has rejected the possibility of a landbridge at least partly because of the "extreme dissimilarity" of the African and Malagasy faunas This difference undeniably shows that crossings were rare, and therefore extremely difficult, but it is possible to read this evidence another way. As far as we know, no strictly terrestrial mammal has contrived to cross the water barrier surrounding Madagascar for at least the last 15-20 myr or so. In that case, it seems necessary at least to ask whether, under current geographical conditions, any crossing at all is possible for such inefficient overwater dispersers as placentals of this kind. And should this prove to be the case, the ephemeral existence of island-chain "steppingstones" at points during the Tertiary would clearly have been absolutely essential for the transfer to Madagascar of any terrestrial mammals at all. On the other hand, in the absence of a Tertiary terrestrial fossil record in Madagascar we have no way of knowing how many groups of mammals might have crossed the water barrier during this period without managing to establish themselves permanently on the island. If Madagascar's modern fau-nal composition is biased by selective extinction the implication is, again, that the barrier was more permeable in the past than it appears now, presumably also as a result of ephemeral land connections. It is because of such considerations that future clarification of the mode of primate colonization of Madagascar, and its source, is as likely to come from geological studies of the surrounding seafloor as it is from an enlarging fossil record, or from improved systematic knowledge of the island's endemic mammals and their closest continental relatives.

MADAGASCAR'S PRIMATES

By far the most renowned and diverse group of Madagascar's mammals is its primates, the lemurs. There is general agreement that the lemurs, including the recently extinct "subfossil" forms, should be classified into seven families: Cheirogaleidae, the dwarf lemurs, with five living genera; Lemuridae, the "true" lemurs and their close relatives, with five genera (one extinct); Lepilemuridae, with two genera (one extinct); Indriidae, with three living genera; Archaeolemuridae, with two genera (both extinct); Palaeopropithecidae, with four genera (all extinct); and Daubentoniidae, the aye-ayes, with a single living genus and species. Depending on whose classification one accepts, the living lemurs alone may embrace well over 40 species, and an astonishing total of up to (and possibly exceeding) 72 primate taxa if subspecies are included. Thus, even excluding the recently extinct lemurs from the count, Madagascar ranks third-highest on the list of high-primate-diversity countries worldwide, even though it is less than a tenth the size of the world leader, Brazil (Mittermeier et al., 1994).

This amazing diversity is potentially due to a combination of several factors (see Tattersall, 1982). First, while paling in comparison to the huge area of Brazil, Madagascar is nonetheless extremely large: at 1600 km long, and with a surface area of almost 600,000 km2 it is the world's biggest oceanic island. When first colonized by humans, the island was largely if not entirely forested, providing primate-friendly habitats in virtually all but the most open areas and the most extreme montane environments. Second, due to both its geographical position and its varied topography, Madagascar offers a huge range of forest habitats. Lying almost entirely within the southern tropical zone, Madagascar lies in the path of the easterly trade winds. Its narrow eastern coastal plain is paralleled by a steep and rugged escarpment which captures the moisture borne by those winds, and as a result is naturally clothed by luxuriant rainforest. Madagascar's raised central plateau is deeply dissected, and offers a large range of microenvironments. Toward the west it gradually yields to drier and more seasonal coastal plains where forest cover varies from riverine gallery forests to dry brush and scrub habitats. Madagascar's northern and southern extremities are very dry indeed, the far south supporting the unique "spiny forest" where plant endemism is as high as 98% at the species level. Altogether, this unique island offers primates and other mammals a diversity of ecological settings that is unmatched in any comparable area elsewhere.

All of Madagascar's primate families are completely endemic to the island, and merely on the basis of systematic diversity it is clear that primate evolution there has taken an independent course for a very long time. But in the absence of a Tertiary fossil record, exactly how long is debatable. Because of the overall distinctiveness of the Malagasy primate fauna, it has generally been assumed that the lemurs form a monophyletic group. At the same time, the suborder Strepsirhini to which the Malagasy primates belong is not unique to Madagascar, since there is no question that it also contains the African galagos (Galagidae) and the Afro-Asian pottos and lorisis (Lorisidae). All living strepsirhines share a suite of features that includes the package of characteristics, primitively typical of macrosmatic mammals, that includes retention of a rhinarium and a fully functional vomeronasal organ. Additionally, all strepsirhines lack bony posterior closure of the less than fully frontated orbits, and share possession of an unfused mandibular symphysis, a relatively small brain-to-body size ratio, and extremities bearing divergent first digits. There are flat nails on all digits except the second pedal, which bears a "toilet" or "grooming" claw. The most prominent hard-tissue synapomorphy of the group is the presence of a procumbent toothcomb in the lower jaw. This unusual structure (unique in its morphological details if not in its existence) consists of four teeth in the indriids, archaeolemurids, and palaeopropithecids, and of six teeth in all the other lemurs except for the highly derived Daubentonia, in which it is autapomor-phically replaced by a single pair of constantly growing anterior teeth.

Until recently, there was no ancient fossil record of toothcomb-bearing primates anywhere in the world before the African early Miocene, by which time a substantial fossil record has long shown that both modern Afro-Asian strepsirhine families were already well established (Simpson, 1967). Recently, however, an earlier strepsirhine record has begun to emerge, both in Asia (Marivaux et al., 2001) and in Africa (Seiffert et al., 2003; Martin, 2003).

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