Rafting mammals or drifting islands?: biogeography of the ... · ancestral American biota as a result of plate tectonics in that region. The purpose of this paper is to extend Rosen's

Journal of Biogeography (1 980) 7, 11-22

Rafting mammals or drifting islands?: biogeography of the Greater Antillean insectivores Nesophontes and Solenodon

BRUCE J. MAC FADDEN Florida State Museum, University of Florida, Gainesville, Florida 32611, U.S.A.

ABSTRACT. During the late Mesozoic and early Cenozoic, certain soricomorph insectivores were distributed throughout North America, Nuclear Central America, and the proto-Antilles. A regional tectonic reorganization during this time resulted in the decoupling of Caribbean lithosphere from surrounding plates and movement of the Greater (proto-) Antilles eastward relative to the Americas. This movement resulted in biogeographic subdivision (vicariance sensu Rosen, 1975) of the ancestral soricomorph distribution. As far as we know, the American part of the distribution, represented by apternodontids and possibly geolabidids, became extinct by the late Oligocene. The Greater Antillean part of the distribution, represented by Nesophontes and Solenodon, has persisted through to the Recent. In short, these Caribbean insectivores are island relicts of a once more widespread ancestral distribution of soricomorph insectivores. Similar distributional evidence from numerous other elements of the Caribbean biota support the biogeographic hypothesis proposed for Nesophontes and Solenodon.

'I do not deny that there are many and grave difficulties in understanding how several of the inhabitants of the more remote islands, whether still retaining the same specific form or modified since their arrival, could have reached their present homes.'

Charles Darwin (1859, p. 396)

Introduction

Biogeographers have long recognized the peculiarities of island biotas. Darwin (1859) discussed this problem as seen in the Gala- pagos. It is known that islands tend to exhibit a high degree of endemism. Biotic diversity is lower on islands than on mainland regions with comparable areas. Frequently islands show an apparent ecological imbalance of trophic group structure, for example, with

the absence of 'top' carnivores. Phylogenetic- ally primitive forms, or 'relicts,' tend to be preserved in island biotas. In recent years much work has been done on quantitative descriptions of island biotas (e.g. MacArthur, 1972; Simberloff, 1974). Hypotheses that explain the origin of these biotas from an external source have been assumed to be adequate when dealing with easily dispersed forms such as plants and mobile animals, but fall short when considering immobile and exclusively terrestrial animals.

The Greater Antilles possess a peculiar assemblage of relict insectivores that include the genera Nesophontes and Solenodon. Workers such as Matthew (1939), Simpson (1956) and Patterson (1962) have tradition- ally explained the presence of these insectivores in the Caribbean by sweepstakes, or overwater dispersal from the Americas. Recently, Rosen (1975) hypothesized that

0305-0270/80/0300-0011 $02.00 ? 1980 Blackwell Scientific Publications 11

12 Bruce J. MacFadden

virtually the entire Caribbean biota, including Nesophontes and Solenodon, was biogeo- graphically separated (by vicariance) from an ancestral American biota as a result of plate tectonics in that region. The purpose of this paper is to extend Rosen's study and focus on the biogeographic origin of Nesophontes and Solenodon through examination of relevant phylogenetic and geological information.

Models of vertebrate biogeography

Matthew (1939) and Simpson (1940, 1952, and numerous other references) were instru- mental in the formulation of a widely used biogeographic model for terrestrial mammals. Simpson's model includes the concept of sweepstakes, filter, and corridor dispersal routes. The 'traditional (Matthew-Simpson) dispersal' model for mammals was founded on the assumption that the relative positions of continents have remained stable throughout geological time. Recent discussions of vertebrate biogeography that modify the ideas of Matthew and Simpson in the light of plate tectonics have been presented by workers such as McKenna (1972, 1973), Hallam (1974), Cracraft (1974) and Cox (1974). Many of these studies incorporate the recognition of centres of origin and subsequent direction of dispersal for faunas or particular faunal constituents.

Centres of origin can be recognized for taxa that have undergone dispersal during historical times, e.g. the introduction of starlings (Sturnus vulgaris) into North America (Krebs, 1972), rabbits (Oryetolagus cuniculus), into Australia (Ricklefs, 1974), and the northward spread of the nine- banded armadillo (Dasypus novemcinctus) into southern North America (Humphrey, 1974). In prehistorical times, recognition of centres of origin becomes somewhat more complex. Some biogeographers working with wholly extant taxa state that, within a monophyletic group, the most primitive taxon will be closest to the centre of origin. Others state that the most derived taxon will be closest to the centre of origin. Dispersal hypotheses used by palaeontologists usually assume that the older fossil record of a group in one area indicates a closeness to the centre of origin.

However, when dealing with groups with relatively poor fossil records, one is never sure if the oldest representatives are in fact closest to the centre of origin. Centres of origin and subsequent dispersal events are hypothesized with some confidence in geo- chronologically controlled sequences for certain abundant groups, such as marine planktonic microfossils and late Cenozoic mammals. The problem remains as to when a fossil group is well enough sampled and supported by enough temporal data so that discussions of biogeography are meaningful. The solution to this dilemma is certainly elusive.

Croizat, Nelson & Rosen (1974) summa- rized the 'vicariance' model by which syste- matists can reconstruct the biogeographic history of monophyletic groups by analysing cladistic interrelationships of constituent taxa (also see more recent discussion by Rosen, 1978, and Platnick & Nelson, 1978). Ancestral geographic distributions of taxa may be separated by the formation of barriers (vicariance). The separation of ancestral distributions results in evolutionary diver- gence (e.g. allopatric speciation) of the taxa involved. This process of biogeographic subdivision could have occurred numerous times, resulting in repeated events of speciation. Within the groups being studied, more recent biogeographic subdivisions (and allopatric speciations) would result in more closely related species (or sister) groups. Evidence from other groups of organisms with similar phylogenetic and biogeographic histories could be used to support an hypothesis of the biogeographic subdivision events. Vicariance has been used to explain the biogeography of spiders (Platnick, 1976), corals, seagrasses and mangroves (McCoy & Heck, 1976), and parts of the Caribbean biota, particularly fishes (Rosen, 1975).

The vicariance model is not a panacea for interpreting all biotic distributions; but it does provide an elegant explanation for organisms whose biogeography has been pro- foundly influenced by barriers, such as those produced by plate tectonics. Despite claims to the contrary, vicariance is not generally applicable to many relatively recent biogeographic subdivisions. A complex biota might have been affected by several major biogeo-

Antillean insectivore biogeography 13

graphic events which produced its present- day taxonomic composition. As McDowall (1978) has shown, the biogeographic history of a complex biota could combine both vicariance and traditional dispersal models.

Antillean insectivores

Knowledge of the Antillean biota has greatly increased since the turn of the century as a result of numerous natural history expeditions to this region. Classic papers dealing with the fossil and extant mammals collected during the early expeditions were published by G. M. Allen (e.g. 1911, 1918) and Anthony (e.g. 1918). Particularly central to the present discussion, Matthew (1939), in his Climate and evolutdon, devoted an entire chapter to the biogeography of Antillean mammals. In recent years numerous Antillean studies have been presented on Quaternary cave faunas (e.g. Koopman & Ruibal, 1955; Varona & Garrido, 1970), general survey of mammals (Varona, 1974), bats (e.g. Silva- Taboada & Koopman, 1964), primates (Williams & Koopman, 1952; Hershkovitz, 1970), rodents (e.g. Varona, 1970), edentates (e.g. Matthew & de Paula Couto, 1959) insectivores (e.g. McDowell, 1958; Patterson, 1962) and zoogeography (e.g. Simpson, 1956; Hershkovitz, 1972). It was soon recognized that the Caribbean mammals demonstrate characteristics peculiar to island faunas. Notably, mammalian ordinal diversity is low, represented by primates, bats, rodents, edentates and insectivores.

The insectivores include the genera Neso- phontes and Solenodon. Nesophontes consists of at least six species found predominantly in Quaternary cave deposits on Cuba, Hispaniola, Puerto Rico, and smaller surrounding islands (Allen, 1918; Koopman & Ruibal, 1955; McDowell, 1958).Nesophontes has been reported to be extant until as recently as the 1930s. A possibility exists that this genus still may be extant. During the begin- ning of this century, some naturalists claimed that Solenodon had become extinct on His- paniola. Verrill (1907, p. 55) stated that: 'For many years it has been commonly considered extinct, and when, in December, 1906, I undertook a collecting trip to San Domingo with the avowed intention of obtaining Solenodon, prominent zoologists stated that the quest was hopeless, one of them saying that I would be as likely to secure specimens of ghosts as of Solenodon paradoxus.' Solenodon is represented by at least two extant and several extinct species from Cuba and Hispaniola (McDowell, 1958). Patterson (1962) named a new solenodon, Antillogale, from Quaternary cave deposits in Hispaniola. Van Valen (1967) questioned the generic validity of Patterson's Antillogale; Solendon is used here in a broad sense to include it.

Nesophontes and Solenodon are shrew-like in their morphology. This similarity is evi- denced in their skulls, dentitions, postcranials, and, for Nesophontes, size. Solenodon is certainly larger than most shrews and has body lengths of about 15-16 cm and weights of 40-46g (data for S. paradoxus born in

FIG. 1. Adult female of Solenodon paradoxus Brandt 1833 (taken from AMen, 1910, plate 3). Approxi- mately a quarter natural size. Reproduced with the permission of the Museum of Comparative Zoology, Harvard University.

14 BruceJ. MacFadden

captivity; see Penia Franjul, 1977). Solenodon is nocturnal and lives in caves, burrows and rotted trees. Its elongate rostrum (Fig. 1) is often used for digging to obtain food, which consists of various small animals, fruits and vegetables (Verrill, 1907; Pefia Franjul, 1977). The morphological resemblance of the Antil- lean insectivores to shrews, although of large size in the case of Solenodon, has been used in support of phylogenetic relatedness (e.g. McDowell, 1958) or has been dismissed as convergent evolution (Van Valen, 1967).

Phylogenetic relationships of soricomorph insectivores

The Insectivora represent one of the greatest problems in systematics. Until recently, many workers had used this group as a polyphyletic, or horizontal, 'wastebasket' for primitive, archetypical, eutherians. In a novel and somewhat controversial approach, McKenna (1975) presented a cladistic analysis of the Mammalia, and in the process reorganized the traditional insectivore groups (see discussion below). For the systematics presented in this discussion, McKenna (1975) and numerous other works (cited therein) have been used. Objections have been raised concerning McKenna's

methodology and classification (see e.g. Hecht, 1976); however, many of the ideas incorporated in his work, whether new or resurrected from previous studies, will un- doubtedly stimulate ideas concerning a group for which higher-level taxonomic innovation has been largely wanting.

There has been a long-standing recognition of Haeckel's two major insectivore subdivisions, the Menotyphla and the Lipotyphla. The Menotyphla includes the extant macro- scelidids and tupaiids and several fossil groups (Simpson, 1945; Patterson, 1965; Van Valen, 1967; Butler, 1972). Essentially following Butler (1972), McKenna (1975) chose to exclude menotyphlans from the Insectivora. In this scheme the Insectivora are equated with Haeckel's Lipotyphla, which can be treated as a natural, or monophyletic, group based on the shared-derived characters presented in Table 1. These characters corroborate dichotomy 1 in Fig. 2.

Saban (1954) proposed two major lipotyphlan subdivisions, the Erinaceomorpha and Soricomorpha. These two are represented as most closely related taxa (sister-groups) by points 2 and 3 in Fig. 2. McKenna (1975), incorporating earlier interpretations (e.g. Gregory, 1910), has changed the taxonomic content of Saban's insectivore subdivisions.

TABLE 1. Synopsis of shared-derived characters used to corroborate branching points in Fig. 2 *

Branching Taxon Shared-derived characters References point

1 Insectivora = Loss of caecum of gut, jugal bone reduced, Butler (1972), McKenna Lipotyphia maxillary bone expanded in orbital wall, (1975) and numerous

medial branch of internal carotid artery others cited therein lost, and reduced pubic symphysis.

3 Soricomorpha Jugal bone lost, ectopterygoid laminae reduced, McDowell (1958), entoglenoid process modified into a post- McKenna (1975) glenoid process.

5 'Solenodontoids' Narrow tubular snout, zygomatic arch lost, McDowell (1958), posteriorly shifted optic foramen, large McKenna (1960), and anteriorly shifted lacrimal foramen, Butler (1972), shortened and anteriorly shifted infra- McKenna & Lille- orbital canal. graven (1979),

McKenna (1979), McKenna (pers. comm. 1977)

7 Unnamed Position of origin of levator labii superioris, McDowell (1958) funnel-like lacrimal foramen, buccal styles on molars.

* Points 2, 4 and 6 in Fig. 2 are primitive arbitrary operational taxa (see discussion in text).


6

5 Solenodontolds

3 Soricomorpha

1 Insectivoras Lipotyphia

FIG. 2. Hypothesis of phylogenetic relationships (cladogram) of insectivore taxa relevant to the present discussion. Points 2, 4 and 6 represent arbitrary operational taxa. Shared-derived characters used to corroborate dichotomies 1, 3, 5 and 7 are listed in Table 1.

He considered the Erinaceomorpha to be plesiomorphous lipotyphlan insectivores. It is not central to the present discussion to corroborate whether or not this subdivision is natural; for our purposes we can treat it as an arbitrary 'operational taxon' that is the primitive (plesiomorphic) sister-group of the Soricomorpha.

The Soricomorpha are set apart from the Erinaceomorpha as a natural taxon at dichotomy 3 in Fig. 2 based on the shared-derived characters presented in Table 1. For the present analysis, the 'Other Soricomorpha', represented by point 4, is also treated as an arbitrary operational taxon whose detailed interrelationships are not central to this discussion. This taxon is merely used as a primitive (and potentially unnatural, or polyphyletic) sister-group of those taxa united at dichotomy 5. The Other Soricomorpha at point 4 include several families listed in McKenna (1975; also see Saban, 1954; Van Valen, 1967; Butler, 1972; Schmidt-Kittler, 1973).

Dichotomy 5 in Fig. 2 represents a monophyletic taxon that is called, for this discussion, the 'Solenodontoids' (the informal suffix is used here to avoid proposing a formal taxon). As envisaged here, the Solenodontoids include the Greater Antillean genera Soleno- don and Nesophontes, apternodontids, and possibly geolabidids. The apternodontids have been described by numerous workers (e.g. Matthew, 1910; McDowell, 1958), and a

revision of this group is in progress (McKenna, 1979). Apternodus and other apternodontids are known from middle Eocene to middle Oligocene deposits of North America (McKenna, Robinson & Taylor, 1962; Emry, Bjork & Russell, 1979; McKenna, 1979). Apternodontids seem to have been biogeo- graphically widespread; they are known to have ranged as far north as Montana (Emry et al., 1979; McKenna, 1979) and as far south as Texas (Novacek, 1976a). The geolabidids are known from early Eocene to late Oligo- cene deposits of North America (McKenna & Lillegraven, 1979).

Many workers have recognized a close relationship between apternodontids and Solenodon. This relationship is most often supported by the shared presence of zalamb- dodont dentitions. On the other hand, Neso- phontes has a dilambdodont dentition (McDowell, 1958) and geolabidids have been characterized as having 'incipient zalamb- dodont' dentitions (McKenna, 1960). Despite problems encountered when dealing solely with dentitions, there are several cranial characters listed in Table 1 that corroborate dichotomy 5 in Fig. 2 and therefore unite apternodontids, geolabidids, Nesophontes and Solenodon as a monophyletic group, the Solenodontoids.

Point 6 in Fig. 2, which includes apternodontids and possibly geolabidids, represents the primitive sister group of Nesophontes and Solenodon. McDowell (1958) concluded that, based on cranial characters, apternodontids were not at all closely related to Solenodon and Nesophnontes. Furthermore, apternodontids were not even considered to be soricomorph insectivores. This view is a rather radical depa*ture from other studies that identify a close relationship between, at least, apternodontids and Solenodon (e.g. Matthew, 1910; Van Valen, 1967; Butler, 1972; McKenna, 1975). In all fairness to McDowell's thorough study, his interpretations were based on a smaller sample of relevant Apternodus than is presently available. Butler (1972) and McKenna (1975, see especially p. 38; pers. comm., 1977; 1979) forcefully argue for a close relationship between apternodontids, geolabidids, Nesophontes and Solenodon. In short, the geolabic1ids are here tentatively placed with the apternodontids at point 6 in


Fig. 4 as an arbitrary operational taxon pending any modification warranted by the studies presently in progress (McKenna, 1979; McKenna & Lillegraven, 1979).

Dichotomy 7 in Fig. 3 unitesNesophontes and Solenodon as most closely related sister groups based on the characters listed in Table 1. This hypothesis of close relationship between Nesophontes and Solenodon has been suggested by workers such as Allen (1918), McDowell (1958) and Weingart (1974). McDowell (1958, p. 121) stated that: 'One author, G. M. Allen (1918), has questioned this prevailing view [based on dental morphology] and pointed out that in most features of osteology Solenodon and Neso- phontes are very similar, and that dental differences are not irreconcilable .. . Solenodon is, indeed, very closely related to Neso- phontes. ..'

It is realized that readers familiar with the problems of insectivore systematics may take exception to certain aspects of the cladistic

analysis presented here. The important con- clusion to be derived from this analysis is that the apternodontids and possibly geolabidids, on the one hand, and Nesophontes and Solenodon, on the other, are almost closely related sister-groups forming the monophyletic taxon Solenodontoids within the shrew- like, or soricomorph, insectivores.

Caribbean plate tectonics

Based on a review of primary sources, Rosen (1 975) presented an analysis of the geological events in the Caribbean region relevant to the present biogeographic considerations. The hypothesis of Caribbean geological history presented here is similar to that discussed in Rosen (1975), with the addition of some more recent primary source references. It should be emphasized that this is merely one hypothesis based on existing data and alter- natives could certainly be proposed.

A B C

NORTH AMERICA

NUCLEAR CENTRAL AMERICA

GREATER ANTILLES

PROTO- LOWER ANTILLES CENTRAL

AMERICA

SOUTH AMERICA

LATE MESOZOIC LATER MESOZOIC- LATER CENOZOIC EARLY CENOZOIC

FIG. 3. Schematic representation of Caribbean plate tectonics showing events relevant to present discussion. (A) During the late Mesozoic the proto-Antilles were in between Nuclear Central America and South America. (B) During the later Mesozoic and early Cenozoic a major regional tectonic reorganization resulted in the relative eastward movement of the Greater Antilles (on the Caribbean plate). This relative movement was certainly initiated by Eocene time. (C) During the later Cenozoic the Greater Antilles have continued relative movement to their present-day position. Also see discussion in text (fi'gure adapted from Rosen, 1975).


During the late Mesozoic an island arc system termed the 'proto-Antilles' (Fig. 3) lay to the south of North America and Nuclear Central America and to the north of South America (see Malfait & Dinkelman, 1972; Rosen, 1975). At this time the Carib- bean lithosphere was part of the Pacific and American plates. During the later Mesozoic and early Cenozoic a major tectonic reorganization resulted in the decoupling of Caribbean lithosphere from the surrounding plates. During the process of decoupling in late Cretaceous and early Cenozoic times the tectonic reorganization resulted in relative eastward movement of the Caribbean plate with respect to the American plates (e.g. Ladd, 1976; Jordan, 1975; Bowin, 1976; Christofferson, 1976). Perfit & Heezen (1978), based on studies of sea-floor spreading in the Cayman trench, state that the decoupling and relative eastward movement of the Caribbean plate was initiated no later than Eocene time. This relative plate movement would result in the eastward migration of the proto- and then Greater Antilles away from Nuclear Central America to their present-day position. Based on rates of sea-floor spreading in the Cayman trench, which have been stated to range from 0.5 to 2 cm/yr (references in Perfit & Heezen, 1978), the absolute amount of eastward relative movement since at least the Eocene of the proto- and Greater Antilles with respect to Nuclear Central America has indeed been significant, with possible estimates ranging from a minimum of several tens of kilometres to a maximum of several hundreds of kilometres.

It is important to note here that most of the island of Cuba has not been part of the Caribbean plate. Malfait & Dinkelman (1972) suggested that the northern boundary of the Caribbean plate is below the southeastern portion of Cuba. There are major problems yet to be resolved concerning the position of the remainder of Cuba with respect to the North American and Caribbean plates. For the purposes of this discussion, it will be assumed that most of Cuba has not had the same geological history as the 'passengers' on the Caribbean plate. As used here, the term 'Greater Antilles' excludes Cuba (see implications for insectivore biogeography below).

With regard to the palaeogeography of the

Greater Antilles, it is difficult to demonstrate that these islands were continuously above sea level prior to the Cenozoic. Since the early Cenozoic the sedimentary record indicates shallow water or terrestrial palaeoenvironments (Schuchert, 1935; Malfait & Dinkelman, 1972; Perfit & Heezen, 1978). Khudoley & Meyerhoff (1971) presented palaeogeographic maps for the Cenozoic of the Greater Antilles. These maps clearly demonstrate that there is no time during the Cenozoic when all of these islands were entirely submerged. In fact, relatively large land masses, with areas in excess of thousands of square kilometres, are indicated thrcughout post-Eocene times for at least Hispaniola and Puerto Rico.

In summary, it is possible to hypothesize that since at least the Eocene the Greater Antilles (nee proto-Antilles, and excluding most of Cuba) have moved a significant relative distance eastward with respect to Nuclear Central America. Furthermore, it is also plausible at least some relatively large areas of the Greater Antilles have been above sea level since the late Cretaceous or early Ceno- zoic when eastward relative movement initiated.

Biogeography of Greater Antillean insectivores

The presence of mammals on the Greater Antilles has generally been attributed to dispersal from the Americas. Some workers (e.g. Barbour, 1914) have suggested dry land connections for this dispersal route, but the majority have suggested overwater, or island- hopping, routes (e.g. Darlington, 1938; Matthew, 1939; Simpson, 1956; Hershkovitz, 1972; Patterson & Pascual, 1972). Many workers have stated that the mammalian fauna of the Greater Antilles probably resulted from several dispersal events during the Cenozoic. Principally because of their phylogenetic primitiveness it has been assumed that Nesophontes and Solenodon resulted from one of the earlier dispersal events. Patterson (1962, p. 10) stated that: 'The possibility, I would go so far as to say proba- bility, exists that solenodontids were derived from relatively unspecialized apternodontids that inhabited the Central American peninsula during the earlier Tertiary.' These hypotheses

2


have generally incorporated traditional dispersal models. The major point of this paper is that plate tectonic movements in the Caribbean provide an active biogeographic mechanism resulting in the origin of Greater Antillean insectivores. The following hypothesis is proposed:

During the late Mesozoic and early Ceno- zoic the proto-Antilles were south of, and probably contiguous with, Nuclear Central America (Fig. 4). During at least part of this time there was a biogeographic distribution of soricomorph insectivores throughout the proto-Antilles, Nuclear Central America and North America. This biogeographic distribution included soricomorphs such as apternodontids, and possibly geolabidids. As mentioned above, these insectivores are known to have been widely distributed throughout western North America during Eocene and Oligocene times (e.g. McKenna,

1960; 1979; McKenna et al., 1962; Novacek, 1976a, b). Patterson (1962) and Novacek (1976b) have suggested that soricomorphs also existed in Central America during this time. As far as we know, this biogeographic distribution did not extend into South America (see Patterson & Pascual, 1972), although it would not detract from this hypothesis if soricomorphs were found in South America during this time. Sometime during the late Mesozoic and early Cenozoic a major tectonic reorganization resulted in the decoupling of Caribbean lithosphere from surrounding plates, and the Antilles moved eastward relative to the Americas. The process of eastward relative movement of the Antilles resulted in biogeographic subdivision, i.e. the ancestral soricomorph distribution was separated into two parts; one that included North America and Nuclear Central America and the other that included the Antilles. As

NORTH AMERICA ..... ... K.- ~ ~ ~ ~ ~ ~ ~ ~

.i::> * :::::.i::B: --: : ';....-..;-2.-;-:-g::*-: ~: . - : . :: .....:: :v .....

5

OT MRC .i:.>:- :>::..................... :.:.>:>:: .... >: -::->.>i-.

NUCLEAR :CENTRAL | AMER IC

:.y ::.-.>>::->:. ..... :. ... ...... . GREATER

ANTILLES,

PROTO- LOWER

ANTILLES CEN~TRAL Liii L W ~~~AMERICA

SOUTH AMERICA

LATE MESOZOIC LATER MESOZOIC - LATER CENOZOIC EARLY CENOZOIC

FIG. 4. Biogeographic model of Greater Antillean insectivores. Shaded regions indicate past and present biogeographic distributions of soricomorph insectivores discussed in text, i.e. Nesophontes, Solenodon, apternodontids and possibly geolabidids. (A) During the late Cretaceous through early Cenozoic there was an hypothetical ancestral distribution of soricomorph insectivores throughout North America, Nuclear Central America, and the proto-Antilles. (B) Biogeographic subdivision (vicariance) of this soricomorph distribution occurred sometime during later Mesozoic through early Cenozoic. (C) During the later Cenozoic the American part of the ancestral distribution became extinct. On the Greater Antilles, inter-is!and dispersal occurred, resulting in, for example, the presence of Caribbean insectivores on Cuba. In short, Nesophontes and Solenodon are island relicts of a once more widespread ancestral distribution of soricomorph insectivores.

A ntillean insectivore biogeography 19

far as we know, the American part of the distribution, represented by apternodontids and possibly geolabidids, became extinct by the late Oligocene. On the Greater Antilles various biotic elements underwent allopatric speciation, which resulted in forms such as Nesophontes and Solenodon. Furthermore, there also was subsequent dispersal of Caribbean insectivores to Cuba. In short, the Greater Antillean insectivores Nesophontes and Solenodon are island relicts of a former biogeographic distribution of soricomorph insectivores that was once found throughout North America, Nuclear Central America and the proto-Antilles during part of the lAte Mesozoic and early Cenozoic.

Evidence from other taxa

There are numerous elements of the Carib- bean biota whose similar distributions can be used to support the biogeographic hypothesis proposed here for Nesophontes and Soleno- don. Olson (1 976) described a new genus of tody, Paleotodus emryi (Aves: Coraciiformes), from middle Oligocene sediments of Wyoming. This represents the only known pre-Quaternary record of todies. The Quaternary todies, represented by the genus Todus, are endemic to the Greater Antilles. Taylor (in McKenna et al., 1962) described the gastropod Planor- bina pseudoammonius from Bridgerian (approximately middle Eocene) sediments of Wyoming. He stated that this gastropod may be conspecific with the West Indian (and South American) species, P. glabrata.

As we know, salt water is an effective biogeographic barrier for many groups of exclusively or predominantly freshwater organisms. Richards (1937) was particularly impressed with the similarities between freshwater and terrestrial molluscs of the Yucatan and the Caribbean. He concluded that the best explanation for this striking biogeographic similarity was the presence of an early Tertiary 'land bridge' connecting these two regions. In his discussion of the freshwater triclad planarian Dugesia, Ball (1971) stated that the presence of this genus in the Americas and the Caribbean probably resulted from (p. 21) '...fragmentation of a previously widespread parent population.' Flint (1 976) attri-

buted the origin of Greater Antillean caddisflies (genus Polycentropus) to biogeographic subdivision of the proto-Antilles from the Americas as a result of plate tectonics. Rosen (1975) stated that the presence of freshwater poecillid fishes in both the Greater Antilles and Central America support the hypothesis of biogeographic subdivision of the once-continuous ancestral distribution. Support for this hypothesis is also suggested by distributional evidence from other groups discussed in detail elsewhere (Rosen, 1975), such as crayfishes, amphibians and reptiles.

In summary, the coincident phylogenetic and distributional evidence from numerous elements of the Caribbean biota, including Nesophontes and Solenodon, would tend to support the hypothesis of an early Cenozoic generalized (i.e. many taxa involved) ancestral distribution that was subsequently biogeo- graphically subdivided. It is true that some of these taxa can be 'more easily' explained by the model presented here, whereas other taxa of the Caribbean biota with different phylogenetic histories can be 'more easily' explained by traditional biogeographic models that incorporate active dispersal mechanisms such as rafting (see below).

Concluding comments

As Darwin (1859) noted, there are difficulties in explaining the origin of island faunas. For example, with regard to Caribbean mammals, it is perplexing that certain groups are not represented in the biota, e.g. medium- to large-sized ungulates. It is particularly curious that there are no carnivorous mammals known from the Greater Antilles. Perhaps the predatory birds of the Caribbean, including owls, eagles and vultures (Arredondo, 1976) have filled this carnivorous adaptive zone on the Greater Antilles. These birds could have functioned as 'ecological carnivores' throughout much of the Cenozoic. Alternatively, carnivorous mammals may have existed in the pre-Quaternary Caribbean biota, and they could have become extinct before the time when mammals are represented by fossils. This hypothesis is not unreasonable as some sixty-five mammals are known to have become extinct from the West Indies (Rosen, 1975).


Rosen (1975) should be applauded for his monumental biogeographic synthesis of the Caribbean biota. However, one criticism of Rosen's strictly interpreted vicariance model is that the origin of virtually the entire Caribbean biota is explained by the early Cenozoic biogeographic subdivision event that resulted from the eastward relative movement of the Greater Antilles. As McDowall (1 978) has discussed, this wholesale use of the vicariance model is as inductive as the wholesale use of the traditional dispersal model. It is clear that the Caribbean biota has had a complex history that resulted from several biogeographic events. These events could have included: (1) an early Cenozoic biogeographic subdivision of the ancestral biota resulting from plate tectonic mechanisms, i.e. the eastern relative movement of the Greater Antilles; (2) a series of dispersal events (between the Americas and the Caribbean and within the Caribbean) in later Cenozoic times resulting from mechanisms such as island- hopping, i.e. rafting; and (3) modifications of this island biota by extinction.

Acknowledgments

The motivation for the present discussion was in a large part due to, and an extension of, ideas contained in Rosen's (1 975) study. This manuscript has been substantially improved through discussions with the following col- leagues: Jon Baskin, Robert J. Emry, George F. Engelmann, John J. Flynn, Richard Franz, John A. W. Kirsch, Malcolm C. McKenna, Larry G. Marshall, Michael J. Novacek, Michael R. Perfit and S. David Webb. I would particularly like to thank McKenna for access to presently unpublished manuscripts on geolabidids and apternodontids. Jon Baskin, Malcolm C. McKenna, Michael J. Novacek and S. David Webb critically reviewed various versions of the manuscript. The synthesis presented here is not necessarily endorsed by all of the persons acknowledged.

Nancy Halliday and Eugene Hanfling pre- pared Fig. 4. I thank Silvie M. Sidaway, Rhoda J. Rybak and Priscilla A. Williams for their editorial and typing skills.

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Documents

Rafting mammals or drifting islands?: biogeography of the ... · ancestral American biota as a result of plate tectonics in that region. The purpose of this paper is to extend Rosen's