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Mamm. biol. 68 (2003) 1±15 Mammalian Biology ã Urban & Fischer Verlag http://www.urbanfischer.de/journals/mammbiol Zeitschrift fuÈr SaÈ ugetierkunde

Review Major mammalian : a review under consideration of molecular and palaeontological evidence

By K. M. HELGEN South Australian Museum, Adelaide, and Department of Environmental Biology, University of Adelaide, Adelaide, Australia

Receipt of Ms. 30. 10. 2001 Acceptance of Ms. 05. 06. 2002

Abstract

Recent advances in mammalian higher-level molecular have generated support for the recognition of four fundamental superordinal clades of living placental . These super- ordinal hypotheses are used as a framework for exploring patterns and trends that have occurred during mammalian to give rise to the contemporary placental fauna. Potential relationships of recent mammal groups not previously referred to one of these four clades are exam- ined here. Patterns of comparative disparity and diversity, historical and modern , and morphological and ecological convergence between and within placental superorders are discussed.

Key words: Phylogenetics, eutherian, convergence, diversity, disparity

Introduction

ªThe stream of heredity makes phylogeny; exons) representing every eutherian , in a sense, it is phylogeny. Complete genetic have generated well-resolved phylogenies analysis would provide the most priceless that suggest extant placental mammals can data for the mapping of this stream . . .º be partitioned into four superordinal clades. One of these fundamental superordinal Simpson (1945) groups is , a of probable African origin comprising six orders of mostly African and Madagascan mammals: Comprehensive molecular studies of high- Hyracoidea, , , Tubuli- er-level mammalian phylogenetics (Mad- dentata, Macroscelidea, and sen et al. 2001; Scally et al. 2001; Murphy (, otter , and golden moles). et al. 2001 a, b) have recently infused fresh The superorder , in turn, is a tra- perspective into long-debated questions re- ditionally recognized assemblage compris- garding relationships and radiations among ing Folivora (sloths, see Delsuc et al. the living placental mammal lineages. These 2001), Vermilingua (vermilinguas), and Cin- studies, the first to employ very large - gulata () (all of which are recog- cular datasets (dominated by nuclear nized here as full orders) with greatest di-

1616-5047/03/68/01-001 $ 15.00/0. 2 K. M. HELGEN versity and possible origin in South Ameri- Correct higher-level relationships among ca (Delsuc et al. 2001; or possibly Antarcti- placental orders, among orders ca, see Vizcaino et al. 1998). A third clade (Palma and Spotorno 1999; Phillips et was dubbed by Murphy al. 2001; Springer et al. 1998), and be- et al. (2001 b) to recognize that its member- tween placentals, , and mono- ship consists of the apparently monophy- tremes (Janke et al. 1997, 2002; Killian letic clades (Rodentia and La- et al. 2001; Kirsch and Mayer 1998), are gomorpha) and (, currently the subject of very active mole- Scandentia, and Dermoptera ± i. e. Archon- cular and morphological investigation. Re- ta sensu Gregory 1910, minus and ele- searchers employing varied data sets have phant shrews). The last of the four placental arrived at many conflicting hypotheses for superorders was designated placental relationships in particular (see by Madsen et al (2001 a) (to denote the ap- Liu et al. 2001). In this light, the potential parent origin of many of its member groups phylogenetic arrangements delineated by on northern ); it includes the or- Murphy et al. (2001 b) should be consid- ders Lipotyphla (shrews, moles, and hedge- ered working hypotheses that will undergo hogs), Chiroptera, , Pholidota, additional refinement (see Novacek 2001). Perissodactyla, and Artiodactyla (including Notably, superordinal relationships sup- cetaceans; n. b. the name Cetartiodactyla, ported by Murphy et al. (2001 b) (and dis- as currently employed by molecular sys- cussed here) strongly contradict the results tematists, should probably be considered a of many morphology-based phylogenetic term of convenience rather than a valid arrangements (e. g. Shoshani and McKen- superordinal name. Cetartiodactyla could na 1998; Fischer and Tassy 1993; Novacek be a valid name if cetaceans were consid- 1992; Novacek and Wyss 1986). However, ered the sister group of all other artiodac- molecular phylogenetic studies can em- tyls. However, cetaceans are properly ploy vastly larger datasets and potentially nested within the order Artiodactyla as tra- more powerful analytical methods, and ditionally defined, which is otherwise para- are thus of fundamental utility in resolving phyletic ± cf. Arnason et al. 2000; Lum et relationships between taxa when morpho- al. 2000; Shimamura et al. 1997 ± thus, the logical methods offer ambiguous results name Artiodactyla can be retained for this (cf. Murphy et al. 2001 b; Shoshani and assemblage, just as the order Carnivora, McKenna 1998). In addition to clarifying paraphyletic without pinnipeds, does not seemingly intractable phylogenetic pro- change its name upon their inclusion or ex- blems, acts in tan- clusion). dem to enrich macroevolutionary studies This eutherian arrangement complements by pointing out anatomical convergences support previously generated from molecular that would be difficult to conclusively and morphological studies for the recognition identify otherwise (such as those used in of two marsupial superorders (Phillips et al. the past to unite paraphyletic superorders 2001; Szalay 1982, 1994; Temple-Smith 1987): such as Volitantia, Anagalida, Archonta, , including four Australasian Altungulata, and Edentata; cf. Shoshani orders as well as the enigmatic South Ameri- and McKenna 1998). This provides insight can marsupial Dromiciops (in the monotypic into the relationship between ecological order ); and , similarity, homology, and homoplasy comprising the American (order Di- through . delphimorphia) and -mice (order Pauci- Assuming that there are four fundamental tuberculata). Entertaining these superordinal placental clades provides a fresh framework hypotheses, a total of seven fundamental for exploring origins, comparative success, groupings can be recognized among living historical biogeography, and convergent mammals: four placental superorders, two of these major groups, and for marsupial superorders, and . addressing the unresolved relationships of Major mammalian clades 3 several poorly-known placental lineages The recent sundering of insectivore-grade within this framework. Some of these topics mammals to all branches of the placental have been discussed elsewhere (Eizirik et mammal tree, the suggestion that erinaceids al. 2001, Madsen et al. 2001, Murphy et al. closely resemble tribosphenic mammals 2001 a,b; Novacek 2001; Scally et al. 2001) from the Early of Australia but are explored in greater detail here. (Rich et al. 2001 b, 2002), and lingering ar- gument over relationships (Cao et al. 2000; Mouchaty et al. 2000; Nikaido Unresolved clade membership et al. 2001) all emphasize the need to fur- ther clarify the phylogenetic position of Several poorly-known placental groups of both and . uncertain affinity were not included in the One last recent eutherian omitted collective analyses of Murphy et al. from molecular investigations of higher-lev- (2001 b). While these are groups that ap- el placental relationships (due to lack of pear only peripherally in the Holocene, samples) is the endemic Madagascan order their phylogenetic relationships have signif- Bibymalagasia. MacPhee (1994) erected icant bearing on historical biogeography. this order to house Plesiorycteropus,anex- The first of these are the and tinct genus of medium-sized mammals that nesophonts, insectivore-grade mammals en- survived late into the Holocene, probably demic to the Greater Antilles. Nesophonts until about 1 000 ybp. The relationship be- have apparently become extinct in recent tween Bibymalagasia and other placental centuries and are known only by skeletal mammals is uncertain (MacPhee 1994; material, but solenodons survive as highly McKenna and Bell 1997). However, Ple- endangered species in Cuba and Hispanio- siorycteropus was earlier classified as a tu- la. The relationships of solenodons have bulidentate (Patterson 1975; Thewissen never been very clear (McDowell 1958; 1985), and MacPhee (1994) discussed mor- Van Valen 1967; Whidden and Asher phological resemblances between Plesioryc- 2001). They are probably referable to the teropus, , and . In light of laurasiatherian order Lipotyphla (in the re- current knowledge about major placental stricted sense, i. e. in recent groups, it seems highly possible that Plesior- literature) along with shrews, moles, and ycteropus is part of the radiation of hedgehogs (see Stanhope et al. 1998 b), afrotheres, all of which share a traditionally but further study is needed in the context enigmatic phylogenetic position and a dis- of the arrangement of Murphy et al. tribution restricted to Africa and/or Mada- (2001 b). The true affinities of nesophonts gascar. Interestingly, MacPhee (1994) remain almost completely obscure; pointed out astragalar features shared although commonly arranged as sister taxa, (though not exclusively) by Plesioryctero- it remains to be demonstrated that neso- pus, elephants, hyraxes, aardvarks, and ele- phonts and solenodons are more closely re- phant shrews (but not sirenians, which of lated to each other than to any other insec- course lack hindlimbs), calling these ªpossi- tivore-grade mammals (see Whidden and ble derived traits of a superordinal group- Asher 2001). ing that includes Plesiorycteropusº, a point Another little-known group of insectivore- that deserves additional study. Despite grade mammals are the Indomalayan gym- overwhelming molecular evidence for nures and (Galericinae, formerly Afrotherian monophyly (Madsen et al. Hylomyinae). Morphological analyses con- 2001; Murphy et al. 2001 a,b; Springer et clusively place gymnures as the sister group al. 1997, 1999; Stanhope et al. 1998 a,b; to hedgehogs (Erinaceinae) (see Corbet van Dijk et al. 2001), it is a commonly 1988; Frost et al. 1991; Neveu and Gasc raised point that painstaking studies of 2002), but apparently no molecular studies comparative (e. g. Shoshani and have included gymnures (Novacek 2001). McKenna 1998) have uncovered no mor- 4 K. M. HELGEN phological synapomorphies coherently unit- published; Wilson and Reeder 1993). Dis- ing this otherwise seemingly hodgepodge parity, a measure of gross morphological assemblage (Asher 1999; Hedges 2001, No- variation, is generally more difficult to vacek 2001). It is possible that the correct quantify (Neige et al. 2001; Wills et al. relationships of Bibymalagasia will only be 1994). Nevertheless, disparity is probably resolved using methods for analyzing an- more easily measured in living mammals cient DNA (reviewed by Hofreiter et al. than in some other taxonomic groups, as 2001). placental mammal orders (sensu McKenna and Bell 1997; with ªinsectivoranº groups Comparative success of major as arranged by Murphy et al. 2001 b) gener- mammalian clades ally encompass species of similar morpho- type often strikingly divergent in morphol- Extant diversity, disparity, and distribution ogy from representatives of the most are all potentially useful measures of cur- closely-related living order (a major reason rent and historical success in cross-clade why mammalian superordinal relationships comparisons. Diversity is usually simply de- have been so difficult to unravel). Thus, fined as the number of species in a clade the number of orders within a mammalian (although here number of genera is used, superorder is considered here to be a rea- because this measure is less dynamic over sonable proxy measurement of disparity. time, and assessments of species diversity By these measures, laurasiatheres and eu- in mammals have changed considerably archontoglirans have been more successful since the most recent review volume was than xenarthrans and afrotheres, especially

Diversity" Disparity^ Distribution

46% Laurasiatheria 484 7d Worldwide 33%

24% Euarchontoglires 522 Worldwide

e 14% Xenarthra 13 Neotropics

1 29% Afrotheria 31 6 Africa, Madagascar

Fig. 1. Comparative success of major placental mammal clades. Diversity, disparity, and distribution of placental mammal superorders. Percentages of total placental mammal diversity and disparity in each superorder are shown above and below the branch for each clade, respectively. Figures are based on Recent mammals of the world (WIL- SON and REEDER 1993). Placental mammals constitute 93% of extant mammalian genera and 70% of extant mam- malian orders. a. Number of genera (less sensitive to taxonomic change than number of species). b. Number of orders (proxy measurement for number of distinctive morphotypes; see text). c. Orders recognized as follows: in Laurasiatheria: Lipotyphla, Chiroptera, Carnivora, Pholidota, Perissodactyla, Artiodactyla, (); in Euarchontoglires: Rodentia, , Scandentia, Dermoptera, Primates; in Xenar- thra: Folivora, , and ; in Afrotheria: Afrosoricida, Macroscelidea, Tubulidentata, Hyracoidea, Pro- boscidea, Sirenia. d. Artiodactyla (including cetaceans and ªtraditional artiodactylsº) is counted twice for measuring disparity, be- cause of the unusual morphological divergence of these two groups. e. Includes one wide-ranging species in North America, the Dasypus novemcinctus. f. Includes one terrestrial Asian representative, the elephant Elephas maximus. Major mammalian clades 5

Table 1. Modern distribution and generic diversity of major placental mammal clades. Figures are percentages of placental mammal totals. Current range is based on the approximate distributions given in figure 1; ranges of marine mammals are not considered. If A is the sum of the areas of all continents occupied by a clade, then range percentages are calculated as: A/(sum of the values of A for all four clades). Figures for continental size are taken from WORLD ALMANAC (2002).

Superorder Current range Diversity

1 Xenarthra 5.6 1.2 2 Afrotheria 9.5 3.0 3 Euarchontoglires 42.5 49.7 4 Laurasiatheria 42.5 46.1 in terms of diversity and distribution Laurasiatheria (307 genera excluding Chir- (Fig. 1). These two superorders have high optera) greatly exceeds that of Afrotheria (and nearly equivalent) diversity and dis- or Xenarthra (Fig. 1). parity, together accounting for 96% of ex- A classic tenet of ecology is that species tant placental mammal diversity and 51% richness increases with increasing area. A of extant disparity. Living xenarthrans show correlation might then be expected between the least diversity and disparity of the four the size of a higher taxon's modern geo- major placental groups. Extant afrotheres graphic range and its modern diversity. This also exhibit little diversity, but extremely is neatly demonstrated by comparisons be- high disparity (almost one-third of the pla- tween mammalian superorders. Distribu- cental total) nearly equivalent to that of tional indices for each of the four placental Laurasiatheria. Interestingly, among pla- superorders are good general predictors of centals, disparity is rather equally distribu- modern diversity (Tab. 1), especially for Eu- ted between ªGondwananº (Xenarthra and archontoglires and Laurasiatheria. The Afrotheria) and ªLaurasianº superorders smaller modern ranges of xenarthrans and (Laurasiatheria and Euarchontoglires) ± a afrotheres are correlated with considerably striking fact given the extremely unequal lower diversity, although these groups ap- diversity of these two groups (a discrepancy pear to be even more depauperate in reality of two orders of magnitude). This could than expected on the basis of geography. suggest that these two large groupings are Recent may partially explain sister taxa and thus equally old assem- this discrepancy. Compared to other super- blages, despite markedly uneven current orders, xenarthrans and afrotheres suffered success (see Scally et al. 2001; Cao et al. proportionally greater losses of diversity 2000; Madsen et al. 2001; Murphy et al. during late Quaternary extinctions (Ander- 2001 a); however, Murphy et al. (2001 b) son 1984), although these declines are did not support this hypothesis. (Inciden- partly accounted for already because the tally, a superordinal assemblage embracing distributions of many such extinct species afrotheres and xenarthrans [in part] dates (e. g. North American taxa such as ground to Linnaeus [1758] who included elephants, sloths and mastodons) lay outside the mod- sea cows, sloths, vermilinguas [and pango- ern distributions of these clades. lins] in a group he named ªBrutaº). Unba- Given congruence between number of low- lanced diversity among placental superor- er-level taxa (i. e. diversity) and modern dis- ders is due in part to the disproportionate tribution within mammalian superorders, it success of two orders in particular: Chirop- is interesting to compare also number of tera (bats) and Rodentia () (laura- higher taxa (i. e. disparity) and the esti- siatheres and euarchontoglirans, respec- mated ancient distribution of each superor- tively). Nevertheless, even with their der at about the time they began to diversi- exclusion, modern diversity of Euarchonto- fy (i. e. roughly 100 Mya, see Murphy et al. glires (79genera excluding Rodentia) and 2001 b). Currently xenarthrans are posited 6 K. M. HELGEN

Table 2. Ancient distribution and modern disparity of major placental mammal clades. Figures are percentages of placental mammal totals. Distribution indices are calculated as in table 1.

1 Superorder Cretaceous range A Disparity Cretaceous range B

1 Xenarthra 10.8 14.3 15.3 2 Afrotheria 18.3 28.6 29.1 3 Euarchontoglires 28.1 23.8 22.0 4 Laurasiatheria 42.8 33.3 33.6 to have been restricted to , ing a relatively rapid early radiation. afrotheres restricted to Africa, euarchonto- Furthermore, xenarthrans (like afrotheres) glirans restricted to Eurasia, and laura- are also thought to have split from the re- siatheres to have been fully Laurasian in mainder of placentals relatively early, but distribution (inhabiting Eurasia and North as a group (both living and ) are con- America) (Archibald et al. 2001; Delsuc siderably less morphologically variable than et al. 2001; Flannery 2001; Murphy et al. afrotheres. 2001 b; McKenna and Bell 1997), although Given that the early fossil record of evidence for some of these assumptions is afrotheres is essentially unknown, an alter- slim. Cretaceous distribution calculations nate explanation for high afrothere dispar- are approximations based on modern conti- ity could be that the presumed early restric- nental sizes (with the exception of Eurasia, tion of this group to Africa is not real. from which the area of the Indian subconti- Perhaps some afrothere orders that survive nent was subtracted), although continental only in Africa or Madagascar originated in areas have fluctuated since the Middle Cre- other parts of Gondwanaland, dispersed to taceous with tectonic movements and shift- Africa prior to complete Gondwanan frag- ing sea levels and coastlines (see Smith et mentation, and subsequently became ex- al. 1994). tinct on smaller Gondwanan fragments. In- Despite this impreciseness, this set of geo- terestingly, if Cretaceous afrotheres (which graphic assumptions approximately re- occupy a phylogenetic position among covers the modern distribution of disparity living placentals according to Murphy et al. among placentals (Tab. 2, ªCretaceous 2001 b) are assumed to have originally ran- Range Aº). These comparisons also suggest ged into parts of Gondwanaland that are to- that afrotheres (and to a lesser extent, xe- day , Australia, and the Indian narthrans) exhibit disproportionately high subcontinent, and Cretaceous xenarthrans modern disparity (with representatives as are assumed to have extended to Antarcti- different as golden-moles, elephants, and ca, these geographic assumptions recover sea cows) for having been restricted to Afri- the modern distribution of placental dispar- ca in the Middle Cretaceous. This discre- ity much more accurately (Tab. 2, ªCretac- pancy could have more to do with time than eous range Bº). Of course no such tight geography; if afrotheres are the sister group relationship need exist, and without addi- to all living placental mammals, they might tional fossil evidence (and a better under- exhibit disproportionately high disparity standing of the timing of Gondwanan frag- simply because they have been diversifying mentation events), such conjectures about slightly longer than any one of the other ancient distribution are practically baseless. three fundamental placental clades. How- These comparisons simply demonstrate that ever, molecular clock estimates of diver- current ideas about the ancient geographic gence at each successive node early distribution of these groups are broadly on the placental tree (corresponding to the concordant with patterns of disparity seen fundamental superordinal splits) are simi- in placental clades today (with the possible larly ancient (Eizirik et al. 2001), suggest- exception of Afrotheria). Major mammalian clades 7

Gondwanan prodigal sons like a home field advantage, for their Gondwanan persistence? Is this analogous In addition to Afrotheria and Xenarthra, to a pattern commonly seen at much lower Monotremata is a third major mammalian taxonomic levels, where, after initial or per- clade with low extant diversity and a lim- iodic expansion, species contract from ited modern range essentially limited to a more remote, marginal habitats to a central Gondwanan fragment. Interestingly, all or core range? (See Brown 1957.) Of three of these groups are thought to have course this pattern is far from universal originated on the continents to which they among higher taxa, but a potential explana- are now restricted (however, future fossil tion for these particular cases may be that discoveries on Gondwanan landmasses, lineages originating on smaller landmasses especially Antarctica, may overturn these are generally less successful than those views; see Vizcaino et al. 1998; Simpson from larger landmasses in colonizing and 1978). Although none of these clades ever competing in new areas (Matthew 1915; matched the diversity of modern laura- Janis et al. 1998), a phenomenon Flannery siatheres or euarchontoglirans, all have de- (2001: 78) claimed to be ªone of biology's clined in diversity over time and have dis- more iron-clad rules.º Higher taxa that ex- tributions considerably more limited than hibit the pattern described here (i. e. wide at some point in their past. For example, expansion only to contract back to the xenarthrans are known from the point of origin) can be aptly labelled ªpro- of Antarctica (VizcaõÂno and Scillato- digal sonº taxa. Yane 1995; Vizcaino et al. 2001) and achieved high diversity in the North Amer- ican Pleistocene (Anderson 1984; McKen- Convergent adaptations among na and Bell 1997) but today are almost major mammal groups wholly restricted to the Neotropics. Af- rotheres were likewise once more cosmo- Debates involving mammalian phyloge- politan in distribution, with proboscideans netic relationships have frequently focused in particular inhabiting most of the world's on establishing whether certain derived land surface for much of the Cenozoic ecomorphological traits have evolved once (Shoshani and Tassy 1996). Today, aside (representing a unique synapomorphy) or from the aquatic sirenians and one multiple times (representing convergent that extends into the Middle East, only a evolution) during mammalian . For single afrothere, the Asian elephant (Ele- example, in challenging chiropteran mono- phas maximus), survives outside of Africa/ phyly (and, thereby suggesting that pow- Madagasacar; coincidentally, the center of ered flight originated not once but twice this species' historical distribution (the In- among mammals, i. e. separately in mega- dian subcontinent) was also originally a chiropterans and microchiropterans), Pet- Gondwanan fragment. Monotremes, too, tigrew (1986) excited considerable interest once ranged into South America and pre- in relationships. Studies based on myr- sumably Antarctica (Pascual et al. 1992), iad systems, molecular and morphological, but are now found only in Australia and have since upheld chiropteran monophyly New Guinea. Although causal factors of (see Simmons 1994), although ªmicrochir- and range contraction in these opteransº are now thought to be a grade three Gondwanan lineages have no doubt rather than a clade (Hutcheon et al. 1998; been varied and largely separate in space Teeling et al. 2002). Given that gliding and time, coincidence of their modern and has evolved separately at least six times presumed ancient distributions (despite an among mammals (Jackson 2000; but note interval of expansion, then contraction) that these six events occurred only in Eu- raises an interesting question. Is there some archontoglires and Australidelphia, three underlying ecological factor, something times in both lineages), it is rather surpris- 8 K. M. HELGEN ing that powered flight evolved only once (Myrmecobius, Australidelphia), in mammals. The evolution of flight in aardvarks (Orycteropus, Afrotheria), and other mammal lineages may have been pre- aardwolves (Proteles, Laurasiatheria). cluded by complete domination of aerial Marked adaptations for aquatic living have guilds by both (especially in diurnal evolved multiple times among laura- niches) and bats (especially in nocturnal siatheres (whales, pinnipeds), afrotheres niches), whose high geographic mobility (sirenians), and to a remarkable (albeit les- may have rapidly granted them global ser) extent, even in the xenarthran Thalas- monopolies after developing flight (see socnus (a semi-aquatic sloth; see de Mui- Speakman 2001). Bats are highly diverse, zon and McDonald 1995). A notable and it is tempting to call the evolution of for arboreality, the prehensile flight a ªkey innovationº (as done for birds tail, has evolved many times within both and insects; see Chatterjee 1997; Kesel marsupial superorders and within all pla- 2001). However, this might be technically cental superorders except Afrotheria (see incorrect (see Slowinski and Guyer 1993; Emmons and Gentry 1983). In fact, despite de Queiroz 1999), because the sister group otherwise outstanding disparity in morphol- of Chiroptera (thought to comprise the car- ogy and lifestyle (see above), and in con- nivores, , perissodactyls, and ar- trast to other therian superorders, afro- tiodactyls including whales; Murphy et al. theres show almost no adaptations for 2001 b) is also quite diverse and successful arboreality (potentially an insight into com- (with more extant genera but fewer species petitive forces faced during early afrothere than in bats). Therefore, rates of diversifi- history), with only a single (the tree cation may not be biased in favour of bats hyraxes, i. e. the extant genus Dendrohyrax) on account of their ability to fly. Whether living in trees. Finally, the many groups of or not flight is directly responsible for ac- small mammals adapted to a burrowing life- celerated diversification in bats (an intui- style underground, such as golden moles tive proposition nonetheless), it is certainly (Chrysochloridae, Afrotheria), true moles the source of their wide geographic success, (, Laurasiatheria), marsupial moles which has ultimately allowed the superor- (Notoryctes, Australidelphia), and (albeit to der Laurasiatheria to match Euarchonto- a lesser extent) numerous lineages of sub- glires (with the spectacularly diverse order terranean rodents (Euarchontoglires) pro- Rodentia, in which the small body size and vide an especially remarkable example of generalist habits of many of its representa- morphological and ecological convergence. tives promotes rapid dispersal) in nearly Another contemporary debate regarding cosmopolitan distribution. the uniqueness of an important morpholo- Madsen et al. (2001) briefly discussed some gical trait involves tribosphenic molars (i. e. notable convergent adaptations among dif- molars with a talonid basin that occludes ferent placental superorders (reviewed in with a large protocone, a functional com- additional detail by Scally et al. 2001), but plex allowing for grinding and shearing akin these parallel radiations are even more to a mortar-and-pestle; see Bown and spectacular than those authors note. For ex- Kraus 1979). This condition is considered ample, independently-acquired adaptations synapormorphous for extant therian mam- for insect- or worm-eating have resulted in mals and their close fossil relatives (tribo- the complete loss of teeth and extension of sphenidans), and is traditionally held to the tongue in three very distantly-related have first appeared in early therians on groups of mammals: (Tachyglossi- northern continents (Luo et al. 2001). With- dae, Monotremata), pangolins (Manidae, in the last decade, a number of fully tribo- Laurasiatheria), and vermilinguas (Myrme- sphenic fossil mammals have been discov- cophagidae, Xenarthra), as well as the evo- ered on Gondwanan continents (e. g. lution of a reduced or unspecialized denti- Sigogneau-Russell 1991; Flynn et al. tion in numerous other mammals, including 1999; Rich et al. 1997; Rich et al. 2001 a; Major mammalian clades 9

Rauhut et al. 2002) that predate the ap- al. 2001). This makes a claim by Rauhut pearance of Laurasian tribosphenic mam- et al. (2002), who recently described a new mals. Most notably, two tribosphenic taxa tribosphenic mammal from the of from the Cretaceous of Australia, Ausktri- Argentina, more extraordinary than those bosphenos and Bishops (together, the Ausk- authors alluded. Rauhut et al. (2002) allied tribosphenidae), have been identified as their new taxon Asfaltomylos with ªaustra- early placentals (Rich et al. 1997, 1998, losphenidansº (see above), but noted of 2001 a,b, 2002), although this arrangement the holotype that ªthe subdivision of the has received little support from other post-dentary trough indicates the presence authors (see Kielan-Janworowska et al. of distinct post-dentary bonesº. If this in- 1998; Luo et al. 2001, 2002). Luo et al. ference is correct, Asfaltomylos would be (2001) instead argue that ausktribosphenids an example of a rather highly-derived and other Gondwanan tribosphenid mam- mammal without the typical mammalian mals share a more recent ancestor with middle-ear arrangement, suggesting either monotremes (together comprising the clade a dual origin or a singular reversion of this ªAustralosphenidaº) than with extant ther- condition. Given that a number of other ian mammals, suggesting separate origins Mesozoic mammals without post-dentary of tribosphenic molars on northern and bones retain distinct (though not necessa- southern continents during the Mesozoic. rily subdivided) post-dentary grooves (see This hypothesis has since been widely em- Luo et al. 2002; Rich et al. 1998), it should braced (see Rauhut et al. 2002; Sigog- probably be assumed (contra Rauhut et neau-Russell et al. 2001; Luo et al. 2002), al. 2002) that Asfaltomylos possessed the but has been strongly criticized recently derived ear arrangement typical of other (Rich et al. 2002; see also Butler 2001), crown-group mammals, unless or until a and whether the tribosphenic molars of tri- specimen is discovered with post-dentary bosphenidans and ªaustralosphenidansº bones intact within the post-dentary are separately derived should probably be groove. considered unresolved (Rich et al. 2002). Interestingly, current debate over whether placentals originated in (particu- Early placental biogeography larly Far Eastern Eurasia) or - land (particularly Australia) is currently As Murphy et al. (2001 b) noted, their paralleled by similar debates regarding the well-resolved phylogeny of extant placen- origins of modern birds (see Cooper and tal mammals provides an excellent frame- Penny 1997) and angiosperms (see Sun et work against which biogeographic hypoth- al. 2002), two other major groups that may eses about mammalian radiations can be have originated at about the same time. tested. These authors developed an explicit Finally, in the context of distinguishing hypothesis about the sequence of Cretac- homology from homoplasy in mammalian eous biogeographic events to explain the history, a brief note about the unique orga- presence of afrotheres in Africa, xenar- nization of the mammalian middle-ear is thrans in South America, and the remain- in order, in response to a recent comment der of living placentals (designated Bor- by Rauhut et al. (2002). Incorporation of eoeutheria) in present-day North America the ªreptilianº postdentary bones (i. e. the and Eurasia. Given evidence that quadrate and articular) into the middle- afrotheres split off from other placentals ear, such that the middle-ear contains three first, followed by xenarthrans, Murphy et bones and the jaw comprises only the den- al. (2001 b) suggested that the root of the tary, is generally acknowledged to be a crown-group placental tree lay in Gondwa- synapomorphous arrangement uniting naland, that the split between afrotheres crown-group mammals (Shoshani and and all other living placentals corre- McKenna 1998; Luo et al. 2002; Wang et sponded to the vicariance event separating 10 K. M. HELGEN

Africa from South America, and that the dispersal events out of a southern Gond- next major subsequent split (between Xe- wanan fragment that included Antarctica, narthra and ) corresponded Australia, and some contact with South to a Laurasian invasion from South Ameri- America (see Storey 1995): the first to an ca. This scenario offers an intuitive link be- already-isolated Africa at roughly 100 tween the tree topology generated by Mya (Eizirik et al. 2001; also note that Murphy et al. (2001 b) and traditional as- the afrotheres of Madagascar [i. e. tenrecs] sumptions about the geographic origins of arrived there during the Tertiary; see these major groups (see above), and poten- Douady et al. 2002); the second to Asia, tially corroborates both the Garden of perhaps either by transferral via the Indian Eden hypothesis of Foote et al. (1999 a) subcontinent (see Murphy et al. 2001 (but see Foote et al. 1999 b; Archer et al. [supp. info.]; Hay et al. 1999) or by disper- 1999; Rich et al. 2001 b) and the discovery sal from Australia to southeastern Asia of putative Cretaceous placentals from via mechanisms envisioned by Rich et al. Australia (see above). This scenario is also (1998); and finally, that of an already attractive in that the molecular clock date well-differentiated fauna including xenar- generated for the split between afrotheres thrans from Antarctica to South America and other mammals (101±108 Mya) is at the Cretaceous-Tertiary boundary broadly concordant with the date attribu- (Pascual 1998). This model is extremely ted to the separation of Africa and South concordant with the presence of early pla- America (100±120 Mya) (Smith et al. centals in Australia 115±120 Mya (Rich et 1994; Hay et al. 1999). al. 1998, 2002). Whether contemporary or One aspect of this hypothesis ± a trans- slightly more ancient placental mammals hemipheric dispersal event from South from Laurasia (e. g. Cifelli 1999; Averia- America to Laurasia, is problematic given niov and Skutschas 2000; Kielan-Jawo- current fossil evidence. No placental mam- rowska and Dashzeveg 1989; including mals have been recorded from South the 125 Mya Eomaia, Ji et al. 2002) were America prior to the early Tertiary, when close relatives or even direct ancestors of a diverse placental mammal fauna includ- early Gondwanan placentals (whether ing xenarthrans, notoungulates, and a ausktribosphenids are included or disre- number of other enigmatic ungulate-forms garded) remains to be clarified. appeared there (see McKenna and Bell 1997) (given this absence, it is difficult to imagine that South America played such Acknowledgements a vital role in the taxonomic and geo- graphic radiation of Cretaceous placen- I thank T. Flannery for sharing many useful in- tals). The key to understanding South sights and reading several drafts of this manu- America's Tertiary fauna may thus be an script. I also benefited from discussions with (as-yet undocumented) history of taxo- A. Roca, E. Eizirik, and W. Murphy (on molecu- nomic and ecological differentiation that lar phylogenetics, divergence times, early biogeo- graphy, and nomenclature), T. Rich (on Mesozoic occurred in the Antarctic Cretaceous (see mammals), and M. Lee (on disparity). T. Dikow Vizcaino et al. 1998; Pascual 1998). If this assisted with German translation. I am supported is the case, early placental mammal diver- in part by a grant from the Australian-American sification may have proceeded by three Fulbright Commission. Major mammalian clades 11

Zusammenfassung

GroÈûere Taxa der Mammalia: eine ÛberpruÈfung nachErwaÈgung molekularer und palaÈontologischer Befunde In der aktuellen Diskussion der Phylogenie der Mammalia werden vier Taxa als Ûberordnungen der rezenten anerkannt. Anhand dieser Stammbaumhypothesen werden sowohl die Vertei- lung und Trends von Merkmalen innerhalb der Placentalia, als auch der Ursprung der Placentalia un- tersucht. MoÈgliche Beziehungen rezenter SaÈugetiertaxa, welche nicht in dieses System integriert werden konnten, werden diskutiert. Muster morphologischer und oÈkologischer Konvergenzen, hi- storischer und rezenter Biogeographie und vergleichender DisparitaÈt und DiversitaÈt zwischen und unter den vier Taxa werden diskutiert. Die gegenwaÈrtige Verteilung der DisparitaÈt innerhalb der Pla- centalia wird zu einer Analyse der geographischen Verteilung dieser Taxa waÈhrend der Kreide heran- gezogen. Monotremata, Xenarthra und Afrotheria sind wichtige SaÈugetiertaxa, die auf Gondwana- land entstanden sind. Obwohl jedes dieser drei Taxa einst eine viel groÈûere Verbreitung hatte, stimmt ihre rezente, eingeschraÈnkte Verbreitung genau mit den Landmassen uÈberein, auf denen sie entstanden sind.

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