Botanical Journal of the Linnean Socie[y (1988). 98: 21 1-224. With 8 figures
Bryophyte disjunctions in the Northern
Hemisphere: Europe and North America*
W. B. SCHOFIELD
Department of Botany, Universig of British Columbia, Universip Boulevard, Vancouver,
B.C., V 6 7 2 B 1 , Canada
Keceiired June 1987
SCHOFIELD, W. B., 1988. Bryophyte disjunctions in the Northern Hemisphere: Europe
and North America. Two major disjunctions are recognized between the bryofloras of Europe and
North America: Amphi-Atlantic and Western Europe-Western North America. Each of these has
specifically segregated floras related to climatic regimes and correlated with different historical
development. An interpretation of the disjunctions is based on reconstruction of past continental
interconnections coincident with reconstructed climates and an understanding of the biology of the
bryophytes.
ADDITIONAL KEY WORDS:-Biogeography.
CONTENTS
Introduction . . . . . . . . . .
Amphi-Atlantir disjunction.
. . . . . .
Predominantly northern or arctic . . . .
Prrdominantly oceanic
. . . . . .
Prcdominanrly southern (not necessarily ocranir)
Widrsprrad . . . . . . . . .
Wrstcrn Europe Wrstrrn North America disjunrtion
Arctic or suhalpinr rlimatrs . . . . .
Ocranir climatcs . . . . . . . .
Mediterranean rlimates . . . . . .
Stcppc climates . . . . . . . .
Discussion . . . . . . . . . . .
Acknowlrdgcmrnts
. . . . . . . .
Rcfrrenccs.
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INTRODUCTION
A major disjunctive pattern in plant distribution was noted in the thesis of a
student of Carl Linnaeus, Jonas P. Helenius (Graham, 1972). I n 1750 this thesis
noted species of flowering plants found in eastern North America and
disjunctively in eastern Asia. Since the Professor (i.e. Linnaeus) was responsible at
that time for the content, and probably the writing, of the thesis, the idea of
disjunctions therefore originated with Linnaeus. Certainly Linnaeus would have
+This paper is dedicated to Dr William C. Steere in celebration of his 80th birthday.
0024-4074/88/11021 I + 14 $03.00/0
211
101988 Thc Linncan Society of London
212
W. B. SCHOFIELD
been unique at that time in the possession of detailed background concerning
distribution of plant species.
A high proportion of the holarctic bryoflora is very widely distributed in the
northern part of the Northern Hemisphere. Indeed, in Arctic regions the majority
of the species are circumarctic. The European moss flora, as listed in the most
recent check-list (Corley et al., 1981), has at least 70% of its species known also in
the North American flora. At least 64% of the hepatics of Europe as noted in the
check-list of Grolle 1983a) are present in North America.
Among the species that these two major geographic areas share, however, a
number show a decidedly disjunctive distribution of considerable magnitude.
Two major disjunctions are considered herein:
1. Amphi-Atlantic defined by the taxa that are found in Europe and sometimes
eastward into Asia, and in North America are confined mainly to areas east of the
hundredth meridian.
2. Western Europe-Western North America defined by taxa that are found in
Europe predominantly in the western areas nearest the Atlantic, and in North
America are confined mainly to areas west of the hundredth meridian, and are
predominantly near the Pacific coast. These bryophyte disjunctions have been
discussed by Schofield (1969, 1980, 1984, 1988) and Schofield & Crum (1972).
Evans (1914) first brought attention to hepatics that showed a disjunction
between Western Europe and the Pacific coast of North America, and Harvill
(1950) discussed it briefly for the mosses.
At the present time these two major disjunctive elements are prevented from
merging by both topographic and climatic barriers. Those confined to the Pacific
coast are restricted by the hyperoceanic climates in part, and inhibited from
extending eastward by mountain chains (specifically the Coastal and Cascade
Mountains) and by the semi-arid climates that characterize the region between
these mountain chains and the Rocky Mountains. Bryophytes of Mediterranean
climates are similarly confined in western North America where this climatic
region occupies a limited belt from southwestern British Columbia southward to
southern California.
From eastern North America the taxa are prevented from extending westward
by the climates of the Great Plains. Although the boreal climates provide a
relatively continuous corridor, the disjunctive bryophytes are not found in this
region. It is probable that both climate and the absence of appropriate habitats
combine to exclude the species from this potential corridor of range expansion.
Undoubtedly other factors, including reproductive biology and historical events,
have also been important in shaping the ranges of these disjunctive species.
It will become apparent immediately that this paper has a strong North
American bias, based on the author’s greater experience in that area and lesser in
Europe. Nomenclature, except in very few cases, is based on the check-lists of
Corley et al. (1981) and Grolle (1983a). The exceptions are species unknown from
Europe or unrecognized by the compilers of the European check-lists.
AMPHI-ATLANTIC DISJUNCTION
The species that exhibit this basic pattern show several segregated units within
North America. T o a certain degree, these segregates are shown by the same
BRYOPHYTE DISJUNCTIONS IN THE NORTHERN HEMISPHERE
213
Figure I . Distribution of the hepatic Scapania kaun'nii Ryan in North America and Europe
(information from Schuster, 1974; Arnell, 1956).
species in Europe. Approximately 8% of the European and 7% of the North
American moss flora and 15% of the European and 12% of the North American
hepatic flora show this pattern.
Predominantly northern or arctic
These species are best represented in Greenland; others have a wider
distribution in North America. Many are uncommon in North America and are
restricted to few localities. Others are uncommon in both Europe and North
America. Many of the hepatics are probably under-recorded; their seeming rarity
reflects this.
The following species are representative of those that show this pattern:
HEPATICAE: Barbilophoria atlantica (Kaal.) Schiffn., Cephalozia loitlesbergii Schiffn.,
Cephaloriella stellulifera (Tayl. ex Spruce) Schiffn., C. grimsulana Uack ex Gott. &
Rabenh.) K. Muell., Cladopodiella francisci (Hook.) Joerg., Jamesoniella undulifolia
(Nees) K. Muell., Jungermannia gracillima Sm., 3. jenseniana Grolle, Lophoeia
ascendens (Warnst.) Schust., Prasanthus suecicus (Gott.) Lindb., Scapania calcicola (H.
Arn. & J. Perss.) Ingham, S. carinthiaca Jack ex Lindb., S. kaurinii Ryan (Fig. l ) , S.
lingulata Buch., S. massalongi (K. Muell.) K. Muell., S. obcorduta (Berggr.) S. Arn.,
S. parviflia Warnst., S. tundrae (H. Am.) Buch.; MUSCI: Homalothecium lutescens
(Hedw.) Robins., H. sericeum (Hedw.) B. & S., Hypnumjutlandicum Holmen &
Warnke, Philonotis calcarea (B.S.G.) Schimp., P. seriata Mitt., Seligeria diversifolia
Lindb., Sphagnum angermanicum Melin, Thuidium tamariscinum (Hedw.) B.S.G. and
a g o d o n conoideus (Dicks.) Hook. & Tayl.
Predominantly oceanic
These species are found either near the Atlantic or are confined mainly to
regions of high precipitation in eastern North America. They are predominantly
of more southerly distribution and reach greatest diversity and abundance in the
Southern Appalachian Mountains of North Carolina, Tennessee and Georgia (see
Sharp, 1941). Belland (1987) has provided an assessment of this disjunction in the
moss flora of the Gulf of St Lawrence area of Canada.
214
W. B. SCHOFIELD
Figure 2. Distribution of the moss Sphagnum pjhisii Brid. in North America and Europe.
The following are representative species: HEPATICAE: Cephaloziella massalongi
(Spruce) K. Muell., Cololejeunea minutissima (Sm.) Schiffn., Harpalejeunea ovata
(Hook.) Schiffn., Lejeunea ulicina (Tayl.) Gott. et al., Leptoscyphus cuneifolius (Hook.)
Mitt., Plagiochila exigua (Tayl.) Tayl., Radula voluta Tayl. ex Gott. el al.; MUSCI:
hacamptodon splachnoides (Brid.) Brid., Andreaea crassinervia Bruch., Atrichum
crispum Uames) Sull. & Lesq., Bartramidula cernua (Wils.) Lindb., Bryhnia novaeangliae (Sull. & Lesq.) Grout, Hygrohypnum eugyrium (Schimp.), Broth., H.
monlanum (Lindb.) Broth., Rhabdoweisia crenulata (Mitt.) Jamieson, Sematophyllum
demissum (Wils.) Mitt., Sphagnum pylaisii Brid. (Fig. 2 ) and S. strictum Sull.
A number of these show very restricted ranges in North America (e.g.
Plagiochila exigua, Radula volula, Bartramidula cernua and Rhabdoweisia crenulata).
These are confined to one or very few localities in the Southern Appalachian
Mountains. Others are more widely distributed, including Atrichum crispum,
Bryhnia novae-angliae and Sphagnum pylaisii.
Predominantly southern (not necessarily oceanic)
These species appear to have been derived mainly from a subtropical or
tropical flora. Some have a very wide range throughout the tropics and subtropics
(e.g. Dumortiera hirsuta (Sw.) Nees., Haplohymenium triste (Ces. ex De Not.) Kindb.,
Trematodon longicollis Michx.) . Some of these species barely extend into temperate
latitudes. Nearly all of these species have their North American range entirely in
unglaciated terrain while several extend somewhat further northward in Europe.
The following species are representative: HEPATICAE: Dumorliera hirsuta, Lejeunea
Java (Sw.) Nees, Marchantia paleacea Bertol., Telaranea nemalodes (Gott. ex Aust.)
Howe (Fig. 3 ) ; MUSCI: Acaulon triquelrum (Spruce) C . Muell., Fabronia ciliaris (Brid.)
Brid., Haplohymenium trisle (Ces.) Kindb., Pleurochaete squarrosa (Brid.) Lindb. and
Trematodon longicollis.
Widespread
A number of species range widely in eastern North America, and are not
confined to either northern or southern localities. These tend to have broad
BRYOPHYTE DISJUNCTIONS IN THE NORTHERN HEMISPHERE
215
Figure 3. Distribution of the hepatic Telaranea nematodes (Gott. ex. Aust.) Howe in North America and
Europe (information from Schuster, 1969; Mueller, 1951-1958).
habitat diversity and are predominantly species within forests. Representative
examples are: HEPATICAE: Metzgeria furcata (L.) Dum. (Fig. 4), Nowellia curvlfolia
(Dicks.) Mitt., Pallavicinia lyellii (Hook.) Carruth., Porella pinnata L., P .
pla~phylloidea(Schwein.) Lindb., Scapania nemorea (L.) Grolle, Trichocolea tomentella
(Ehrh.) Dum.; MUSCI: Anomodon rostratus (Hedw.) Schimp., Dicranurn fulvum Hook.,
Ditrichum lineare (Sw.) Lindb., Fissidens cristatus Wils. ex Mitt., Leucobryum glaucum
(Hedw.) Aongstr., Mnium hornurn Hedw., Polytrichum pallidisetum Funck, Thuidium
erecturn Duby, Ulota crispa (Hedw.) Brid., U. hutchinsiae (Sm.) Hammar.
WESTERN EUROPE-WESTERN
NORTH AMERICA DISJUNCTION
These species also show several segregated patterns within North America that
are reflected also in their European distributions. Approximately 7 yo of the
European and 6% of the North American moss flora and approximately 504, of
the European and 4% of the North American hepatic flora show this pattern.
<
>
V.
Figure 4. Distribution of the hepatic Metrgeria furcata (L.) Dum. in North America and Europe.
216
W. B. SCHOFIELD
Figure 5. Distribution of the moss Pohlia erecfa Lindb., in North America and Europe (after Shaw,
1982; Nyholm, 1954-1969).
Arctic or subalpine climates
In North America these species are confined mainly to areas above tree line,
either altitudinally or latitudinally, and are predominantly in the northern
Cordilleras. In Europe the species tend to be more widespread in distribution,
although a number are rare (e.g. Pohlia erecta Lindb., Bucegia romanica Radian).
The following are representative: HEPATICAE: Asterella lindenbergiana (Corda ex
Nees) Lindb., Bucegia romanica, Frullania jackii Gott.; MUSCI: Mnium spinosum (Voit.
ex Sturm) Schwaegr., Myrinia pulvinata (Wahlenb.) Schimp., Pohlia erecta (Fig. 5 )
and Schistidium pulvinatum (Hedw.) Brid.
Oceanic climates
In Europe the greatest concentration of species showing this pattern is in
western Great Britain and the Norwegian coast. A very large portion of this
disjunction is in oceanic portions of western North America. A number of the
species are widespread and abundant along the Pacific coast (e.g. Hepaticae:
Bazzania pearsonii Steph., Douinia ovata (Dicks.) Buch, Porella cordaeana (Hueb.)
Moore, Musci: Dicranoweisia cirrata (Hedw.) Lindb. ex Milde, Hookeria h e n s
(Hedw.) Sm.). Others are locally abundant (e.g. Hepaticae: Marsupella alpina
(Mass. ex Limpr.) H. Bern., Pleurozia purpurea Lindb., Scapania ornithopodiodes
(With.) Waddel., Musci: Andreaea megistospora B. Murray, Dicranodontium
subporodictyon Broth., Hymenostylium insigne (Dicks.) Podp.) . A number are
extremely rare (e.g. Hepaticae: Cephaloziella phyllacantha (Mass. & Carest.) K .
Muell., Herbertus sendtneri (Nees.) Lindb., Musci: Daltonia splachnoides (Sm.) Hook.
& Tayl., a g o d o n gracilis Wils. ex Berck.).
Representative species include: HEPATICAE: Anastrepta orcadensis (Hook.)
Schiffn., Anastrophyllum donnianum (Hook.) Steph., Bazzania pearsonii Steph.,
Cephaloziella phyllacantha (Fig. 6 ) , Douinia ovata, Herbertus sendtneri, Mastigophora
woodsii (Hook.) Nees., Porella cordaeana, Scapania ornithopodioides; MUSCI: Andreaea
sinuosa B. Murray, A. megistospora, Campylopus schwarzii Schimp., Daltonia
BRYOPHYTE DISJUNCTIONS IN THE NORTHERN HEMISPHERE
217
Figure 6. Distribution of the hepatic Cephulozicllu plyllucantha (Mass. & Carest.) K. Muell., in North
America and Europe.
splachnoides, Dichodontiumjavescens (With.) Lindb., Dicranodontium subporodictyon, D .
uncinatum (Harv.) Jaeg., Dicranoweisia cirrata, Ditrichum zonatum var. scabrifolium
Dix., Hymenostylium insigne, Hookeria lucens, Leptodontium recuruifolium (Tayl.) Lindb.,
Plagiothecium undulatum (Hedw.) B.S.G. and Qgodon gracilis.
These oceanic species show, in North America, a decidedly northerly (but not
Arctic) distribution. This is true also for Europe. Coastal mountains are a
conspicuous influence in generating the very moist climates that provide the
suitable habitats. The greatest diversity of these species is in north coastal British
Columbia, especially the Queen Charlotte Island archipelago (Schofield, 1988).
The biology and distribution of many of these species are discussed for Europe by
Stcarmer ( 1969) and Ratcliffe (1968).
Mediterranean climatej
These species are also mainly near-coastal in western North America and are
more southerly in distribution. They are most abundant in California, especially
in the San Francisco Bay region, but extend northward to southwestern British
Columbia and occasionally further north to southwestern Alaska. Most are found
on outcrop knobs with widely spaced trees, especially Quercus. In Europe they are
confined mainly to near-coastal areas of the Mediterranean.
Representative species include; HEPATICAE: Cephaloeiella turneri (Hook.) K.
Muell., Lunularia cruciata (L.) Lindb., Targionia hypophylla L.; MUSCI: Antitrichia
califrnica Sull. (Fig. 7 ) , Bartramia stricta Brid., Claopodium whippleanum (Sull.) Ren.
& Card., Dicranella howei Ren. & Card., Entosthodon fascicularis (Hedw.) C . Muell.,
Epipterygium tozeri (Grev.) Lindb., Funaria muhlenbergii Turn., Metaneckera menziesii
(Hook.) Steere, Orthotrichum lyellii Hook. & Tayl., Pterogonium gracile (Hedw.) Sm.,
Scleropodium touretii (Brid.) L. Koch, S. cespitans (C. Muell.) L. Koch, Tortula
amplexa (Lesq. & James) Steere, T. bolanderi (Lesq. & James), Howe and T.
subulata Hedw.
Another species, Crumia latifolia (Kind. ex Macoun) Schof., can be mentioned
in this context, although not strictly European in its distribution outside North
218
W. B. SCHOFIELD
-..
Figure 7. Distribution of the moss Antitrichia californica Sull., in North Amercia and Europe.
America. It shows a wide distribution within Mediterranean climates within
North America, but extends wider into moist habitats of both drier and wetter
climates. Its presence in the extant flora of the Caucasus is of extreme interest,
especially in conjunction with the presence there, in the fossil record, of the
Mediterranean climate species, Claopodium whippleanum (see Abramova &
Abramov, 1958).
Steppe climates
These climates are semi-arid and characterize local continental areas. In
Europe, steppe vegetation occupies a large portion of Hungary and the southern
part of the U.S.S.R. adjacent to the northern shores of the Black Sea. The steppe
species, in North America, appear to reach their greatest diversity in southwestern British Columbia. This may reflect the more comprehensive
understanding of the bryoftora of this area than of similar steppe areas in the
United States. This information has been gathered by Mcintosh (1986), whose
detailed results will be published in the near future. However, it is possible that
the species, if they were once present in the United States, have been extinguished
through destruction of the habitat. Certainly land use has drastically reduced
their potential breadth of distribution in Canada. No disjunctive hepatics have
been noted; among the mosses, the following are representative: Aloina bifrons
(DeNot.) Delgadillo, Desmatodon guepinii B. & S., Phascum vlassovii Lazar. (Fig. 8).
Pseudocrossidium revolutum (Brid.) Zander, Pterygoneurum koz/ovii Lazar. (Fig. 8) and
Tortula intermedia (Brid.) DeNot.
Most of these species occupy very circumscribed sites within the steppe and are
elusive to discovery. Many are visible only during a very brief period during the
year. All are terrestrial and most grow in microsites where moisture persists longer
than in the general terrain.
DISCUSSION
While the non-disjunctive circumpolar species of arctic and boreal regions show
wide ranges and have had a relatively continuous corridor for effective dispersal
BRYOPHYTE DISJUNCTIONS IN THE NORTHERN HEMISPHERE
219
Figure 8. Distribution of the mosses Phascum vlassovii Lazar., and Pterygonrurum kozlovii Lazar., in North
America and Europe (courtesy T. T. McIntosh); both species show essentially the same distribution.
for a long period of time, undoubtedly both preceding and following the most
recent glaciations, those that are disjunctively distributed are restricted mainly to
very specific climatic regions that have probably been interconnected in the very
distant past and are markedly disjunctive a t present. Furthermore, the disjunctive
species are found mainly a t or near regions that escaped the harshest extremes of
the last glaciations. The non-disjunctive species of northern regions are mainly in
terrain that was fully glaciated during the last glaciations.
When the ecology of the disjuncts is examined, a number of features become
apparent. I n the Amphi-Atlantic species, those that grow in relatively sheltered
sites (i.e. within forests or in narrow canyons) tend to be almost equal in number
to those that grow mainly in exposed sites (sunny outcrops or non-forested areas).
However, for the Western North American-Western European disjunction there
is a clear predominance of species restricted mainly to open sites.
An examination of the entire disjunctive flora reveals the number of species
with easily dispersed diaspores to be almost equal to those that produce no easily
dispersible diaspores. However, if one explores the diaspore production in species
of more northern distribution in the Amphi-Atlantic disjunction, compared with
those of more southerly distribution, it becomes apparent that southern ranging
species have a strong tendency to more frequent sporophyte production, while the
northerly ones are inconsistent in sporophyte production. This reflects, in part,
sexuality, with dioecious species predominating among those of more northerly
distribution. In the Western North America-Western Europe disjunction the
species of oceanic restriction show very reduced production of readily dispersible
diaspores. This contrasts markedly with species of mediterranean and steppe
climates, where sporophyte production is frequent.
I t is apparent that the Amphi-Atlantic disjuncts in North America have not
been subjected to the same historical constraints as the Western North AmericaWestern Europe disjuncts. Based on distributional pattern within eastern North
America and the reproductive biology and ecology of the species, those of more
southerly range appear to be relictual from time well preceding the Pliocene.
Those of more northerly distribution, using the same criteria, appear to be more
recent immigrants, some possibly postglacial; a few have been introduced by man.
220
W. B. SCHOFIELD
An analysis of the Western North America-Western Europe disjuncts reveals
several distinctive historical schemes that possibly generated portions of the
disjunctive distribution. The entire disjunction appears to be relictual. Relictual is
defined as including species found in or near where they have been present
continuously for many millenia. Species confined to oceanic environments appear
to be extremely ancient. Their world patterns of distribution, their frequent lack
of any readily dispersible diaspores, their affinities or identity with taxa confined
to areas that have had a long history undisturbed by drastic environmental
changes, and the diversity of habitats that they occupy, attest to this.
Alien bryophytes introduced to Europe from western North America are
essentially those of mediterranean climates in western North America. Nursery
stock of horticultural plants from California may have been the main source of
these species. All are terrestrial species of Tortula that occupy disturbed sites in
their indigenous, as well as their adventive ranges (Whitehouse & Paton, 1963;
Paton, 1966; Side & Whitehouse, 1974; Crundwell & Whitehouse, 1976). Alien
bryophytes introduced from Europe to North America include the woodland
species Thuidium tamariscinum in Newfoundland (Crum & Anderson, 1981 ) and
Pseudoscleropodium purum in various localities in eastern and western North America
(Brassard, 1983). Both species are mainly in urban sites in North America.
A consideration of the entire bryflora of North America indicates that the flora
east of the Appalachian Mountains and the flora west of the Rocky Mountains,
have been isolated from each other for a very extended period. In the eastern
North American bryoflora there are numerous species of neotropical affinity (e.g.
Brachilejeunea bahamensis Evans, Chonocolea doellingeri (Nees) Grolle, Fossombronia
brasiliensis Steph., Plagiochila dubia Lindenb. & Gott., Radula mollis Lindenb. &
Gott., Porella swartziana (Web.) Trev., Lopholejeunea muelleriana (Gott.) Schiffn.,
Brachymenium syslylium (C. Muell.) Jaeg. & Sauerb., Cyclodictyon varians (Sull.)
Kuntze, Campylopus arctocarpus (Hornsch.) Mitt., Erpodium domingense (Spreng.)
Brid. ex C. Muell., Fissidens kegelianus C. Muell., Groutiella tumidula (Mitt.) Vitt,
Hypoplerygium tamariscinum (Hedw.) Brid., Schizomitrium pallidum (Hornsch.) Crum
& Anders.). The Southern Appalachian flora has been noted frequently for its
large number of neotropical relicts (see Anderson & Zander, 1973, for a recent
review). The state of Florida, in particular, has many neotropical species (Crum
& Anderson, 1981).
In western North America, on the other hand, the affinity of the bryoflora is
with the palaeotropics (e.g. Apotreubia nana (Hatt. & Inoue) Hatt. & Mizut.,
Chandonanthus hirtellus (Web.) Mitt., Dendrobazzania grzffithiana (Steph.) Schust. &
Schof., Herbertus sakuraii Steph., Lepidozia Jilamentosa (Lehm. & Lindenb.)
Lindenb., Plagiochila semidecurrens Lehm. & Lindenb., Takakia lepidorioides Hatt. &
Inoue, Dicranodontium subporodictyon Broth., Gollania turgens (C. Muell.) Ando,
Hypopterygiumfauriei Besch., Didymodon nigrescens (Mitt.) Saito). The palaeotropical
species in western North America are mainly northern and oceanic in
distribution, with the largest concentration known from the Queen Charlotte
Island archipelago and adjacent islands along the Pacific coast. This area has a
documented history as a biotic refugium, with areas that escaped the Pleistocene
glaciations and that have been available continuously for plant habitation for
many millenia.
It seems probable that long-distance dispersal is unlikely to have had a major
impact upon the origin of the disjunctions discussed here. A high proportion of the
BRYOPHYTE DISJUNCTIONS IN THE NORTHERN HEMISPHERE
22 1
species are found in particular niches in well-developed vegetation; others, of the
open sites, tend to lack readily dispersible diaspores; the coincidence of so many
species that show essentially the same disjunction or other species that show
affinities with taxa of the disjunct area, would argue against long distance chance
dispersal. For such dispersal, bryophytes that occupy open sites and that produce
abundant exposed sporophytes would be the more reasonable candidates. Such
species are generally not represented in these disjunctions.
Unfortunately the fossil record for bryophytes is too incomplete to be helpful for
the time periods for which such a record could be extremely useful, i.e. the
Mesozoic and through the Cenozoic. Even the available information concerning
all bryophyte fossils is regrettably inadequate (see Krassilov & Schuster, 1984;
Miller, 1984; for recent summaries), and identifications of many of these are
dependent on an assumption that material is of the right size and form to be
considered to be a gametophore of a bryophyte. Rarely are gametangia or
attached sporophytes available. I n consequence, it is necessary to rely on
information from other fossil organisms, especially vascular plants, and upon the
reconstruction of past vegetations, climates, continental positions and land
interconnections.
There is convincing evidence that genera of the order Jungermanniales were
present in theTertiary (Grolle, 1980, 1981a, 1981b, 1982, 1983b, 1984), and these
are identical to modern genera. The same appears to be true for the mosses,
although preservation of the material leaves considerable room for uncertainty.
If the preceding assumptions are accepted, it is possible to speculate that these
‘modern’ genera of bryophytes were associated with the vascular plant vegetation
on the Laurasian continental mass preceding the break-up of that continent and
the formation of the Atlantic that separated the continents of the Eastern and
Western Hemispheres. Reconstructions of climate for the late Mesozoic and early
Cenozoic indicate that the northern portion of this Laurasian continent was
wetter and milder than that of the present continents derived from it (Parrish,
1987; Parrish & Curtis, 1982; Parrish, Ziegler & Scotese, 1982; Spicer & Parrish,
1986). The influence of the inland sea of North America during the Cretaceous
would have had considerable impact on the climate and biota of the land masses
east and west of it, and would have isolated these land masses from ready
exchange of biota (see Parrish, Gaynor & Swift, 1984, for comments on this
seaway). After the seaway withdrew to form the interior of this large continent,
this area would have possessed a drier climate than the coastal areas. T h e
appearance of the Cordilleras in western North America, with their interception
of wet air-masses, would have exaggerated this drier climate even more.
It is possible to speculate, therefore, that the oceanic bryofloras now present in
western North America could have once extended across the northern portion of
the palaeocontinent of Laurasia, connecting northern European and North
American ranges. As climate cooled in the northern areas and as the Atlantic
opened up isolating the continents, the bryophytes could have extended their
ranges southward to the suitable climates both in Pacific North America and in
Atlantic Europe. Some species were able to extend their ranges even further into
mountainous Asia (e.g. the Himalayas, high mountains of Japan).
The distributions of the steppe species are especially intriguing. These are
always continental in distribution and appear to be restricted to steppe vegetation
or to semi-arid climates. Undoubtedly their distribution is inadequately
222
W. B. SCHOFIELD
documented. It is reasonable to assume that steppe climates have existed
throughout geologic time in continental areas, but it is impossible to locate a
corridor for dispersal of the species between the steppe areas of Eurasia and North
Amercia. A possibility can be suggested: the steppe bryoflora may have been
present in the supercontinent preceding break up. Upon separation of Eurasia
from North Amercia, these steppe species could have entered steppe areas as they
appeared on the new continental masses. This scenario would assume little or no
evolutionary change in the separated populations.
The enigma of Mediterranean climate disjuncts is equally difficult to resolve. If
there were once a connection around the early Atlantic coasts to the Tethyan area
and to both of the northern continental masses of the Eastern and Western
Hemispheres preceding the connection of North and South America, it is
reasonable to suggest that the coastal climates bounding this narrow seaway
might have been suitable for the expansion of ‘Mediterranean’ bryophytes along
this corridor and thence to the west coast of North America. It is equally possible
that the transfer of the floras could have been from western North America to the
Mediterranean area via the same route. The elimination of the species from the
eastern North American portion of the corridor could be attributed to climatic
change in that area and its persistence in the areas where the species are now
found. One major flaw can be noted in such a scenario: many of the
‘Mediterranean’ species show relatively wide ecological tolerances, especially in
western North America. It would be expected, therefore, that at least some of
these species would still persist as relicts in south-eastern United States or in the
Gulf of Mexico coast of Mexico where suitable habitats exist.
Finally, the northern and arctic Amphi-Atlantic disjuncts may be relatively
recent immigrants to one or the other of the continents, possibly expanding their
ranges step-wise across the northern Atlantic. Their limited distribution in one or
the other continent could be utilized as supportive evidence.
It is apparent that the understanding of the evolution of these disjunctive
patterns remains in a very uncertain state. A satisfactory understanding of their
origin must rely on more comprehensive information concerning past history for
which fossil material would be invaluable, but will not solve the problems. The
solution, if it is ever revealed, will rest upon information from palaeobotany, ideas
concerning continental positions through time, topography and climate through
time, in the biology of the bryophyte species, and on the very careful synthesis of
this information.
It is still necessary to apply Linnaean procedures of acquiring and studying
collections of specimens, followed by interpretations of the interrelationships of
the taxa discovc red. As modern technology assists researchers in reaching
inaccessible areas, new techniques are also made available for tabulating and
correlating the data assembled. The use of computers cannot be over-emphasized.
International cooperation is vital and the intellectual challenge is both exciting
and rewarding.
ACKNOWLEDGEMENTS
The research upon which this paper is based has been supported by the Natural
Sciences and Engineering Research Council of Canada. I acknowledge my debt
to Dr RenC Belland, who gave useful comments and advice on the manuscript. I
BRYOPHYTE DISJUNCTIONS IN T H E NORTHERN HEMISPHERE
223
acknowledge with gratitude the kindness of the Linnean Society and the British
Bryological Society.
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