Home page >>> History of Insects
by K.Yu. Eskov
Geographical distribution of any taxon in any period of its history has formed by the two main factors: by the actual climatic gradients (equatorial-temperate, humid-arid etc), and by the previous dispersal routes (sea bariers for the terrestrial groups, continents for the marine ones). The significance of every component in this couple of factors seems to be quite different for various groups. For instance, terrestrial vertebrates (tetrapods) seem relatively autonomous from environment, and their distribution pattern would reflect mainly the direct isolation in past. In contrast, the vascular plants are able to cross the marine barriers as seeds or spores, and their distribution pattern at every period would formed mainly by the actual climatic boundaries. In this respect the winged insects seem to be much closer to the plants than to the vertebrates. Due to this reason the geographical history of insects is anticipated to be quite similar to one of the vascular plants, and we should base our reconstructions mainly on the palaeofloristic regions of the World.
Darlington (1957) have supposed that formation of the new taxa is restricted mainly to the tropical equatorial zone, and proliferated descendants oust the ancestors to the extratropical regions; he called this model the "tropical pump". Meyen (1987b) presented the similar model, the "phytospreading", and supported it by huge palaeobotanical data. He demonstrated that during the Late Palaeozoic (i.e. the time of establishment of the main evolutionary lines of vascular plants) almost all new taxa (of familial or higher rank) appeared in the low latitudes at the more deep stratigraphic levels than at high latitudes. Hence, new taxa, as a rule, really originate in the equatorial zone and later "spread" to the extratropical latitudes. The opposite situation, i.e. the origin of a taxon in boreal zone and its subsequent invasion to the tropics, never have been recorded (Meyen 1987b).
Later, Rasnitsyn (1989b) and Eskov (1994, 1996) have modified Meyen's model. They supposed that the phenomenon of "phytospreading" is rather ecological than geographical in nature. It means the localisation of macroevolutionary processes in regions of the world where the climatic conditions are most favourable (that is, the abiotic component of natural selection is weakest), followed by anisotropic penetration ("spreading") of the newly formed taxa into regions with harsher abiotic conditions. So, in the epochs of the sharp climatic zonation, such as the recent time or the Late Palaeozoic studied by Meyen, the main area of macroevolution is really localised in the wet and hot equatorial zone. But at the epochs of effaced climatic zonation, such as the Mesozoic, the restriction of the centres of faunogenesis to the equatorial zone would be less expressed. A "falsifiable prediction" has been made (Eskov 1994): the number of exceptions to Meyen's scheme would have increased with the weakening gradient in global climatic zonation; this supposition was tested on the distribution patterns of various terrestrial arthropod groups (Eskov 1996) and seems to be confirmed.
Very few insect remains are known from Devonian strata (Fig. 496). All these remains are supposed to belong to Archaeognatha or other wingless forms. No strict geographical conclusions may be based on this scanty material; however, it should be mentioned that all the three Devonian insect localities lies within 10° from the Devonian equator (Labandeira, Beall & Hueber 1988).
This time is known as the period of sharp climatic zonation ('cryo-era'), similar to the modern one. The main palaeofloristic regions were the tropical Euramerican and Cathaysian Realms, northextratropical Angaran Realm and south-extratropical Gondwanan Realm (Fig. 497). The climate of both Euramerica and Cathaysia was wet and hot, the climate of Angaraland and Gondwanaland was varied from cool-temperate to glacial; at the Late Carboniferous several huge continental glaciations developed in Gondwanaland and, according to the modern data, in Angaraland as well (Chumakov 1994). Periglacial plains of Gondwanaland were covered by so-called 'Bothrichiopsis tundra'; landscape of Angaraland, by analogy, usually called the 'Cordaites taiga' (Meyen 1987b).
All 15 insect orders known from the Carboniferous originated in the tropical zone of the Euramerican region. Two of them (Triplosbida and Syntonopterida) never occurred outside of it, and the remainder appeared in the higher latitudes only after some interval of geological time. Nine (Paoliida, Eoblattida, Blattida, Caloneurida, Grylloblattida, Hypoperlida, Dictyoneurida, Diaphanopterida and Mischopterida) appeared in Angaran and/or Gondwanan realms in the Late Carboniferous, while five (Blattinopseida, Palaeomanteida, Ephemerida, Odonata and Orthoptera) 'spread' from tropics only in the Permian. For example, the insect order Hypoperlida, which arose in the tropics, penetrated to the extratropical regions of both Gondwanaland and Angara, and later (in the Early Permian) flourished in the very northern extratropical latitudes — a situation like that noted by Meyen (1987a) in the history of cordaites and glossopterids. Of the insect families found in the Angaran and Gondwanan faunas, only four (the Psychroptilidae, Aykhalidae, and Tschirkovaeidae of the Dictyoneurida and the Hypoperlidae of the Hypoperlida) have no confirmed tropical origins.
The similar distribution pattern is demonstrated by other Palaeozoic arthropods. The arachnids were extremely abundant and diverse during the Late Devonian and the Early and Middle Carboniferous in the Euramerican region; all 14 of their orders known in the Palaeozoic originated in this region (Scorpionida, Thelyphonida, Phrynida, Araneida, Kustarachnida, Chernetida, Ricinuleida, Solpugida, Acarida, Haptopodida, Architarbida, Opilionida, Trigonotarbida and Anthracomartida). Of these, only the Scorpionida and Araneida subsequently appeared in the Late Carboniferous of the Angara region, the Trigonotarbida in the Late Carboniferous of Angaraland and Gondwana, and the Opilionida in the Permian of the Subangara region. The Acarida and Chernetida did not appear in extratropical regions until the Jurassic and Cretaceous, respectively, but this lag may be due merely to taphonomic causes. The remaining 8 orders have never been found outside the tropical zone.
The diplopods are represented in the Early and Middle Carboniferous by three orders, which originated in Euramerica: the Amynilyspedida, Spirostreptida, and Spirobolida. The first two of these never occurs outside the tropics, and the last appears only in the Late Permian of Subangara and the Early Triassic of Siberia. Thus, both these classes of arthropods fully correspond to Meyen's model for the Late Palaeozoic. At the beginning of the Permian, however, they practically disappeared from the fossil record, and their Mesozoic-Cainozoic history is known so fragmentarily that no generalisations are possible.
The structure of insect population in the tropical and extratropical regions in Carboniferous is extremely different. Extratropical faunas consists of few species represented by relatively huge series of specimens. For instance, in Chunya locality (Angaran Realm) the great majority (87%) of specimens belongs to: the single species of dictyoneurids (more than 100 imprints), the single species of eoblattidans (more than 60 imprints), and a series of highly uniform blattid fragments (more than 30 imprints). The situation in tropical Euroamerican Realm is opposite. For instance, in Commentry (France) 13 species of dictyoneuridans are found, and only two of them are present by two imprints; 35 insect remains from the Middle Carboniferous of the Saar Basin presents 31 species. So, the species diversity of insects in tropic belt in the Carboniferous was much higher than in the temperate one; in extratropical regions a strong dominance of a few species is observed, known as a character of recent extremal habitats, such as tundra etc. (Zherikhin 1980a).
Like the Late Carboniferous, the Early Permian was a cryo-era: it was a period of sharp climatic zonation accompanied with the continental glaciations, at least in Gondwanaland. At the beginning of the Early Permian the arrangement of the main phytogeographic regions have still almost as in the Late Carboniferous (Meyen 1987a). The type of faunogenesis did not changed, and the "equatorial pump" continued to operate steadily without interruption.
All the many higher taxa that appeared in the Early Permian seem to have originated in the tropical equatorial zone of Euramerica; these were the Jurinida, Neuroptera, Coleoptera, Trichoptera, Panorpida, Psocida, Thripida and Hemiptera, as well as the suborders Ephemerina in the mayflies, Grylloblattina in grylloblattidans, and Protelytrina in earwigs. The majority of the above orders and suborders have spread to the extratropical regions only in the latest Early Permian and often flourished then and in the Late Permian in Angara and possibly in Gondwanaland (the existing records for Australia and South America are not enough for a sure conclusion). These very taxa have formed the basis of Late Palaeozoic – Early Mesozoic global fauna of the insects (though the taxa of the Carboniferous origin, i. e. Odonata, Palaeomanteida, Blattida, Grylloblattida (other than Grylloblattina), Orthoptera also contributed appreciably to that fauna). Of them, only ditaxineurine and kennediine dragonflies and protelytrine earwigs have not survived the Permian-Triassic boundary. Thus, the entire Carboniferous and Early Permian history of the higher taxa of insects shows not a single (!) deviation from Meyen's "phytospreading" scheme.
The global climatic pattern was somewhat changed towards the end of the Early Permian (beginning from the Kungurian). The temperature contrasts were obliterated: there was a frostless climate in both Gondwana- and Angaraland. The southern part of the latter (the Urals, Kazakhstan, Dzhungaria, Mongolia, Manchuria) formed a new palaeofloristic region – the Subangaran one (Fig. 498). Subtropical (or warm-temperate) SubAngaraland is regarded by palaeobotanists as the very place of origin for the majority of taxa, that composed later the bulk of the Mesozoic flora of the Earth (Meyen 1987a). So the role of "generating forge" had changed hands: from tropics to warm-temperate zone. Probably it was because the abiotic conditions in equatorial zone became quite hard due to global aridisation.
The biogeographic pattern observed in insects seems to be very similar to the plant one. The orders Corydalida, Perlida, and Phasmatida, as well as the earwigs (Forficulina), the hemipteran suborders Cicadina and Psyllina, the grasshoppers (superfamily Tettigonioidea) first appeared in Subangaran region. Stoneflies, in particular, were presented here by both of its main infraorders, the Perlomorpha and the Nemouromorpha. The third stone-fly infraorder, the Gripopterygomorpha, appeared during the Late Permian in south-extratropical Gondwanaland (the recent gripopterygomorph stoneflies are endemics of the "Gondwanan" continents, but in the Mesozoic the infraorder was widely spread in Eurasia. So, the main sources of new high-rank insect taxa were extratropical (warm-temperate) zones, both northern and southern; for instance, Panorpida flourished in both these zones. On the other hand, in the Late Permian in both Subangaran and Gondwanan regions the cockroaches, the dominant carboniferous group, have almost disappeared.
In the Triassic all the continents reunited to form the super-continent, Pangea. The climate displayed intermediate contrasts (with a huge arid belt in the equatorial zone), and the super-continent was peculiar by its homogeneous biota. Palaeobotanists emphasise the loss of all obvious floristic boundaries (Meyen 1987a); the palaeofloristic pattern sketched out by Dobruskina (1982), may be tentatively referred to as a system of diffuse florogenetic centres. Relatively small climatic gradients together with spatial unity of the Triassic continents account for the very quick dispersion of both insect and plant taxa across the Pangaea; new taxa appearing almost simultaneously over all the present-day continents. Hence, it becomes practically impossible to identify the place of origin for many taxa, and this is also true for all high-level Triassic taxa. The dispersion pattern of Vertebrates is governed by the same regularities.
The Triassic is known as one of the key stages of the insect evolution. Numerous taxa arose at that time, and almost all of them have appeared synchronously in geographically distant and/or climatically different localities: Diptera in the France (Vosges), North America, the Balearic Islands, and Central Asia, Hymenoptera in Central Asia and Australia, Mesotitanida in Central Asia, Australia, and Ukraine, the advanced dragonflies (suborder Libellulina) and the crickets of the superfamily Grylloidea (Central Asia, Australia, Argentina, South Africa and, only dragonflies, in Japan), the annulipalpian caddis-flies (England, Central Asia and Argentina), the true bugs (Hemiptera: Cimicina) (Vosges, North America, Central Asia and Australia), the aphids (Hemiptera: Aphidomorpha) (Vosges, Central Asia and Australia); the mesoblattid cockroaches quickly spread all over the world, so that in the poor and highly uniform East Asian faunas they became a main, or sometimes even the only component. It should be noted, that the age of Vosges is now appreciated to be more ancient than the other above localities; however, the spread of such taxa as dipterans, bugs, etc., from the equatorial zone may be stated only with reservations, due to absence of contemporary extratropical faunas.
The most intensive faunogenetic processes, however, may be supposed for the palaeofloristic Central Asian Area, which quite strictly corresponds to the Late Permian Sub-Angaran Area (cf. Figs. 498 and 499) and, probably, has inherited its role of "generating forge" from the latter. Subtropical Central Asia seems to be an area of appearance of the above Triassic high-rank taxa; several insect taxa were in the Late Triassic the endemics of this region and spread out sufficiently later: the infraorder Coccomorpha (Hemiptera), the true thrips (suborder Thripina), the curculionoid beetles (the most advanced and proliferated group of coleopterans). On the other hand, there is the first precedent of the appearance of high-rank taxon in high latitudes and its subsequent spread to the low latitudes, the locusts (suborder Acridina) that first appeared in the Late Triassic of Siberia.
The Jurassic was a time of drastic tectonic differentiation of the land masses. The Tethys Ocean separated Laurasia (more or less corresponding modern Eurasia) from the block of southern continents, i.e. Gondwana. The newly formed Indian and Atlantic Oceans had divided the Southern continental block into the Western Gondwanaland (South America, Antarctica, Australia, New Zealand) and Eastern Gondwanaland, which began dividing further into Africa, Hindustan and Madagascar. On the other hand, just from the Late Triassic the phytogeographic differentiation of the world vegetation became almost the same as in the Jurassic (Meyen 1987a): the main areas were nothal, equatorial and Siberian (Fig. 500).
The majority of high ranking taxa originated during the Jurassic in the equatorial belt, and subsequently appeared at higher latitudes (in Siberia and Gondwana). There were: moths (England and Kazakhstan), snake-flies (Rhaphidiida) (England and Central Asia), advanced dragonflies (higher Libellulina) (Germany, Kazakhstan), the bark-lice of the suborder Psocina and hemipterans of the infraorder Aleyrodomorpha (Kazakhstan), and the culicomorph dipterans (Central Asia). The parasitic hymenopterans ("Parasitica") present a contrasting history in that they appeared almost simultaneously (though they are very rarely found) in both the equatorial zone (Germany and Central Asia) and in Siberia; later, in the Late Jurassic, they flourished in Kazakhstan.
On the other hand, it was the time when apparent movement occurred in the opposite direction, from high latitudes to low ones. There was only one such instance in the Triassic – the Acridina (see above). During the Early and Middle Jurassic, however, there are a whole series of taxa which had originated in the temperate Siberian region (including Mongolia and northern China) and some time later penetrated into the Equatorial region. These include caddis-flies of the most advanced suborder Integripalpia (Phryganeina), both the leading superfamilies of mayflies (the Ephemeroidea and Siphlonuroidea) and several important hymenopteran superfamilies (Proctotrupoidea, Evaniomorpha, Tenthredinoidea). Due to this reason Zherikhin (1980a) notes that the Siberian entomofaunas of Early Jurassic are very advanced and seem to be more similar with the Late Jurassic ones than to the equatorial (Indo-European) faunas of the Early Jurassic.
Both the above trends can be regarded as departures from the operating regime of the "equatorial pump"; they can be directly attributed to the establishment of a less contrasting world climate in the geologic time under consideration.
The Cretaceous is known both as the time when continental masses were most fragmented, and when climatic gradients were most obliterated: everywhere on the Earth the climate was similar to that of the present-day subtropics to warm-temperate regions ("thermal-era"). The arrangement of the main palaeofloristic regions in the Cretaceous generally correspond to those found in the Jurassic, named Siberio-Canadian, Euro-Sinian, Equatorial and Nothal (Fig 501).
No results of the operation of the “tropical pump” have been found in the Cretaceous with its almost uniform climate and very weak latitudinal zonation. The only case of the origination of a taxon in relatively low latitudes and its farther propagation into the higher latitudes is represented by the order Termitida. The first termites appeared in the Neocomian of Spain and the Weald of England, but it appears in extratropical territories of eastern Asia (the Neocomian of Transbaikalia and Aptian of Mongolia) so rapidly, that we cannot establish the region of termite origin with a sureness; probably, it is the same case as in the Triassic taxa (see above).
All the other higher-rank taxa originated in the extratropical regions of Gondwanaland or Siberio-Canadian Province. Mantida (as well as "pre-fleas" Saurophthirus – see below) appeared during the Neocomian in Transbaikalia (Baissa locality), precisely in the layers corresponding to cold climatic episodes. It should be noted that these same layers also contain the first advanced cockroaches (Blattellidae), whereas the "warm" layers are dominated, as before, by archaic groups. The most advanced and flourishing group of the higher muscomorph flies – the muscoid group ("Calyptrata") – appeared in the Campanian and the Maastrichtian of Canada. All the infraorders of higher butterflies (Eriocraniina, Incurvariina, Nepticulina, Papilionina) originated in various points of Siberio-Canadian Province. Near the Jurassic- Cretaceous boundary in Mongolia appeared the first stinging hymenopterans (the "Aculeata") and the most advanced and flourishing groups of parasitic hymenopterans — the superfamilies Platygasteroidea, Chalcidoidea and Ichneumonoidea. The first ants appear in the Albian of the Okhotsk region (NE Russia), while the true, high social Formicidae are found in the Turonian of New Jersey (USA). The true weevils (Curculionidae—the largest family of the animal kingdom, with about 70,000 Recent species) originated in the Aptian of Mongolia. Siberio-Canadian Province was also the region of proliferation of several key post-Jurassic groups, such as aphids. A remarkable situation is observed in Pulicida: the first representatives of so-called "pre-fleas" (Strashila and Saurophthirus) were found in the Late Jurassic and the Neocomian of Siberia, respectively, and, some later, in the Aptian of the high-latitude part of Gondwanaland, in Koonwarra (Tarwinia), whereas in the equatorial zone this order have not recorded up to appearance of the true fleas (Baltic and Dominican ambers).
The trichopteran fossil cases demonstrate the same pattern as the imago. The first cases appeared in the Mid- or Late Jurassic in Laurasia, both extratropical (Transbaikalia and eastern Mongolia) and tropical (western Mongolia), and remained there until the end of the Jurassic. It is worth mentioning that the construction of cases has been started by Annulipalpia and Integripalpia practically simultaneously. Reliable data on the case construction activities in the Cretaceous are unknown for Annulipalpia, whereas the Integripalpia cases became much more diverse and numerous in the north-eastern Asia. Later in the Early Cretaceous they appeared for the first time in Gondwanaland (Brazil and Australia), although they remain infrequent there. The Early Cretaceous cases were not discovered in the low latitude Laurasia until they have been encountered quite recently in the Weald deposits in England. Extremely rare are the Low Cretaceous cases in Manchuria, and they are completely absent in the more southern China regions despite their considerable proximity to the Siberian records. In any case, a real extension of the cases construction area doubtless took place only in the Late Cretaceous. Since that time the cases have been reported from a number of sites in Central Asia, Europe, eastern North America.
In the Cainozoic the arrangement of the continents was similar to the present, except for Australia (which separated from Antarctica in the Eocene), Hindustan (connected with Eurasia in the Eocene) and South America (isolated from North America up to the Pliocene) (Fig. 502, 503) The Palaeogene was still the “thermo-era”, and beginning from the Neogene the climatic regime of “cryo-era” (with sharp thermal gradient and polar glaciations) begin established. So, it may be expected that the main faunogenetic pattern in the Palaeogene was quite similar to that of the Cretaceous.
Zherikhin (1978, 1980a) have supposed that during the Cretaceous and the Palaeogene the global climate was extremely obliterated, and the analogues of both modern boreal and equatorial climates were absent. If it is true, the tropical ecosystems of the modern type (e.g. hylean rain forests and savannahs) should appear very recently, simultaneously with the boreal ones (e.g. tundra and taiga). This hypothesis seems to be supported by the composition of a single huge entomofauna from the equatorial zone of Palaeogene, i.e. Burmese amber (unless its age is the Late Cretaceous). Rasnitsyn (1996) concluded that "past Burmese forest that has yielded the fossiliferous amber differed fundamentally from the tropical rain forest as we know it".
In fact, a number of groups which played an important role in the functioning of the modern tropical ecosystems originated in the Palaeogene outside the tropical zone. Among these, for example, were the termites of the family Termitidae (Oligocene of Europe), the stingless meliponine bees (Eocene of Europe), and the tsetse flies Glossinidae (Oligocene of North America).
No taxa of ordinal, subordinal or even superfamilial rank have originated during the Cainozoic (even in the Palaeogene), and it is impossible to test the hypothesis of the “tropical pump” on the modern insect faunas. The situation in the Cainozoic is complicated by the insufficient availability of fossil material from the tropics. It should be noted, however, that the fauna of the Miocene Dominican amber displays several features typical of the modern tropical faunas: it includes some families that are now virtually limited to the tropics, (e.g. brentid beetles), or which are now key elements of the present tropical ecosystems (Termitidae), as well as the almost complete absence of aphids, known as the one of the most important element of the post-Jurassic temperate entomofaunas.
Zherikhin (1978, 1980a) have emphasised that numerous "far-southern" taxa, now restricted to the Southern Hemisphere,were, however, present in the Cretaceous and Palaeogene entomofaunas of Eurasia and/or North America. Such is, for instance, chironomid midge subfamily Aphroteniinae, found as fossils in the Late Cretaceous amber of Yantardakh in North Siberia, while its recent members are restricted to South Africa, Australia and South America. These findings put a new perspective on the problem of the so-called "Gondwanan ranges".
Modern biogeographers prefere to explain the origin of such distribution patterns (as in aphroteniines), in relation to the existence in the Mesozoic of the protocontinent Gondwanaland and to its subsequent fragmentation due to the continental drift (cf. Figs. 496-503). The fragments are thereby often believed to have retained until our days representatives of the initial Gondwanan biota. This approach, recently called "mobilistic biogeography", attempts to fit the cladogram of a studied taxon to the sequence drawn by geophysics for dispersal of the fragments of Pangea and Gondwanaland. This viewpoint, proposed by J. Hooker, now seems to be a basic paradigm of the historical biogeography.
Data on the fossil insects, however, rather testify to an alternative hypothesis proposed by A. Walles. His "ousted relics" concept identifies the Southern Hemisphere disjunct ranges as the results of extinction of "intermediate links" at northern continents. Numerous "Gondwanan" insect taxa have been encountered as fossils in Eurasia and North America, and their number is still increasing (see the check-lists in Eskov 1987, 1992). There are, for instance, such classic objects of the "mobilistic biogeographers", as aphroteniine midges, gripopterygomorph stoneflies, coleorrhynchan bugs, nannochoristid scorpion-flies.
So, it may be supposed that most far-southern, so-called "Gondwanan" groups are only relics of a broader, probably, global distribution. The present-day Southern Hemisphere fauna of many insect groups inherited several elements of the Mesozoic and Palaeogene faunas. Attempts to consider their places of origin as being the same as their present-day distribution seem to be inadequate (Eskov 1984b, 1987; Rasnitsyn 1996).
It should be emphasised that the conservation of Mesozoic and Palaeogene relicts is not a monopoly of the Southern continents. Several "Laurasian" taxa, restricted to Eurasia and/or North America, were found as fossils at southern, Gondwanan, continents (e.g. the snake-flies, the most archaic hymenopterans, Xyelidae). These cases seem to be much less common than the "Gondwanan ranges" (such as Aphrotheninae), but it is probably an artifact due to a relative deficiency of palaeoentomological material from the Southern Continents. In fact, study of new, rich Gondwanan localities (e.g. Santana and Koonwarra) over the past decade have enlarged the check-list of the taxa that are now extinct in Gondwanaland by more than a factor of two (Eskov 1992), and the number of such patterns is increasing (e.g. the currently Mediterranean Scoliidae-Proscoliinae found as fossils in Santana: Rasnitsyn & Martínez-Delclòs 1999).
The above conclusions don't refute the existence of real Gondwanan groups of insects, which were connected with the Southern Continents during all their history, such as penguins among the vertebrates and Nothofagus among the plants. For instance, the modern ranges of the hymenopteran families Pergidae and Monomachidae, as well as of the basal Xiphydriidae (subfamily Derecyrtinae), are restricted to the Southern Continents, and their past ranges are either southern as well (in case of Monomachidae) or totally unknown [184.108.40.206.5]. But the conclusion about the Gondwanan origin of a taxon should be based on an adequate number of fossils; the value of the modern range seeming to be of very limited significance.
Thus, the "falsifiable prediction" made earlier seems to have been confirmed. The number of exceptions to Meyen's scheme actually increased as the global gradient of climatic zonation weakened, and in the maximally azonal interval - the Cretaceous period - the model ceased to work entirely. This supports the hypothesis that the phenomenon of "phytospreading" is not so much geographic as ecological in nature. In the present context, phytospreading is the localisation of macroevolutionary processes in regions of the world where the climatic conditions are most favourable (that is, the abiotic component of natural selection is the weakest), followed by anisotropic propagation ("spreading") of the newly formed taxa into regions with harsher abiotic conditions. This mechanism can function in different regimes dictated by external conditions, and its operation in the equatorial pump regime is no more than a widespread but partial case. There are three such regimes (determined by the global climatic gradient), and they form a closed three-element cycle.
1. The Late Palaeozoic was characterised by a sharply manifested climatic zonation that included the glaciations of both Gondwanaland and Angaraland. The very powerful equatorial-polar temperature gradient reduced all the other climatic gradients to the status of fluctuations. Phytospreading operated in the equatorial pump regime: all the new taxa of high rank originated in the tropics and then migrated into the higher latitudes, where they sometimes survived even after their disappearance from the regions of their origin. The faunal changes that took place in this interval (including such a radical one as the Carboniferous-Permian) had no effect on the type of faunogenesis.
2. In the Mesozoic and Early Palaeogene, the climatic zonation became increasingly weaker. The lessening and disappearance of the single equatorial-polar temperature super-gradient led to the appearance (and increasing manifestation) of second-order gradients - humid-arid, lowland-highland, etc. As a result, the single tropical-polar phylogenetic gradient also gradually broke down; the equatorial pump began to operate intermittently and then ceased to function altogether. Phytospreading then began to operate in the regime of Dobruskina's diffuse centres of speciation.
3. The Late Palaeogene and Early Neogene was characterised by a transition from an azonal to a zonal epoch. Establishment of the new climatic regime was due to redistribution of the thermal balance resulting from tectonic movements (the separation of Antarctica from South America, leading to the inception of the Antarctic circulation, etc.). The biotic reorganisations (mass extinctions in the circumequatorial and circumpolar regions, concentration of the Mesozoic-Palaeogene relicts in the subtropical and warm-temperate zones) can be characterised by Zherikhin's zonal stratification mechanism.
4. During the interval from the Neogene to the present, the three-stage cycle was completed, and we are now returning to the initial phase of a zonal epoch; phytospreading again operates in the regime of the equatorial pump. Darlington, in fact, proposed this term for the current period of geologic history. The principal macroevolutionary events are thus taking place in the tropics once again. One might even attempt to guess which of the "monsters" that abound in the present tropical faunas will become the initial forms of new orders and suborders to propagate over the earth after the next 10-15 Myr. Unfortunately, of course, it is now obviously impossible to make falsifiable predictions.
On the other hand, the relict survivors from preceding epochs turn out to be localised chiefly in the extratropical regions; their future fate will be governed by its own quite rigorous laws. The successive stages in the reduction of the initial amphitropical (bipolar) geographic ranges of such relicts (for example, the "circum-Pacific domino scheme"; Eskov & Golovach 1986) have been considered on the examples of both terrestrial (Eskov 1987, 1992) and marine (Newman 1986) arthropods.
|Main||History of Insects||Personnel||Field work||Collections||Publications||Library||Events||In memoriam|