Diversification and Relationships of Extant Homosporous Lycopods

16 Pages·2001·8.3 MB·English

by  Niklas Wikström

American Fern Journal 91(3):150-165 (2001)
and
Diversification Relationships of Extant
Homosporous
Lycopods
WlKSTROM
NiKLAS
Department The Museum,
of Palaeontology, Natural History Cromwell Road,
London Kingdom
SVV7 5BD, United
—
A
Abstract. series of phylogenetic analyses using nucleotide sequence data have resolved many
aspects of the relationships in a group of land plants that until recently had received comparatively
homosporous
little attention, the lycopsids (Lycopodiaceae). The family includes no more than
400-500
living species but the group has evolved as an isolated lineage since the Early Devonian
(390 Mya). Despite this ancient history, patterns emerging through the phylogenetic analyses imply
most
that diversification in this group is comparatively recent. The Lower Jurassic stem section
minimum
Lycoxylon indicuin indicates a age for the split between Lycopodium and Lycopodiella
at 208 Mya, and reticulate fossil spores from the Early Jurassic indicate that early cladogenesis in
Lycopodium
is of equivalent age. In the diverse predominantly epiphytic Huperzia group, biogeo-
%
graphic data indicates that 85-90 of all living species result from cladogenic events postdating
the final rifting of S. America and Africa in Mid to Late Cretaceous. The timing of these events
coincides with the radiation of Angiosperms, and the diversification of epiphytic Huperzia was
mediated by development
likely the of broad leaved Angiosperm
rain forests. Results indicate a
single origin of epiphytism in Huperzia, hut there have been two
at least reversals to the terrestrial
The
habit in the neotropics.
diversification of a large secondarily terrestrial clade, including about
montane
80 high altitude species, was likely triggered by the Andean orogeny in the Mid Miocene,
no more than 15 Mya.
an
are
herbaceous among
species most
are the easily recognizable and conspicuous
elements
group was soon
recognized as the product of an early cladogenic event land
in
plant evolution (Banks, 1968, 1975; Bower, Recent
1908). phylogenetic
anal-
yses support view and
this indicate that lycopsids are remaining
sister to all
ants (Doyle, 1998; Duff and Nickrent
Kranz
provides strong evidence
that the three extant
lineages (Lycopodiaceae,
Sela-
and
ginellaceae
Isoetaceae) also originated in the Palaeozoic. The occurrence
of Protolepidodendrales Devonian
in the Early (Kenrick and
Crane, 1997} sug-
minimum
Myr
gests a age of 390 for the split between Lycopodiaceae and
the
ligulate clade including and
Selaginellaceae
Isoetaceae. Furthermore,
the oc-
currence
of Isoetales in the Late Devonian (Kenrick and
Crane, 1997] suggests
Myr
known
these events,
little is
crown
diversification of the groups. Although
the fossil record includes taxa
^
V A K ^ &
imilarities
1992), Selaginellaceae (Thomas, 1992, 1997) and
Isoetaceae
(Pigg, 1992; Re-
tallack, 1997), their specific relationships
to living species remain
unclear.
One
of the major problems has been
the lack of well developed
phylogenetic
WIKSTROM: EXTANT HOMOSPOROUS
LYCOPODS
151
hypotheses. Without such hypotheses, no way whether
there is of telling the
crown
fossils are nested within the groups, or are part of the stem lineages.
Isoetes bestonii Retallack example has been back
for identified as far as the
shown
Triassic [Retallack, 1997). If to be part of the crown group of living
would
Isoetes, this certainly contribute to our understanding of the patterns
of diversification of living species.
Separated from the ligulate lycopsids since the Early Devonian, the homo-
sporous lycopods (Lycopodiaceae) have evolved an independent
as lineage for
show
at least 390 Myr. Morphologically, living species striking similarities to
some
of the oldest vascular plants (Heuber, 1992; Kenrick and Crane,
1997],
but from a palaeobotanical perspective Lycopodiaceae have been
particularly
problematic (Skog and Hill, 1992; Thomas, 1992]. The macrofossil record
is
meager and work hampered by
relatively palaeobotanical recognition prob-
is
lems. Absence of clear morphological synapomorphies for the family makes
assignments of fossil taxa difficult, and this is often based on absence of the
more groups such Another
characteristics of distinctive as Selaginellaceae.
problem involves the superficial similarities between Lycopodiaceae and organ
systems of other plant groups. These problems notwithstanding, Skog and Hill
hypothesized major groups within Lycopodiaceae were
(1992] that the living
established during the Late Jurassic-Cretaceous. In other words, despite
its
modern
apparent species diversity in Lycopodiaceae evolved
relict status,
more
and with
comparatively recently in parallel that in diverse, ecologically
prominent groups such as flowering plants.
my
Over the couple of years, and coauthors have been developing a
last I
Lycopodiaceae (Wikstrom and
phylogenetic hypothesis living species of
for
Wikstrom Using molecular sequence
Kenrick, 1997, 2000a, 2000b; et ah, 1999].
and
data from the plastid rbcL gene, and from the trnl^-truF intron spacer
we number
regions, have conducted a of phylogenetic analyses at different
These analyses provide empirical support
levels of the hierarchy in the family.
for the idea that most cladogenesis in this ancient family is comparatively
made
recent. Here review some of the progress that has been in our under-
I
standing Lycopodiaceae relationships and the implications for interpreting
of
the evolution of the family.
Relationships
monophyly and PnyLLOGLOSSL/M.—Recent analyses
Lycopodiaceae cladistic
weakly supported by
have shown that monophyly of Lycopodiaceae is at best
Kenrick and Crane, Other stud-
morphological characters [Crane, 1990; 1997).
synapomorphies (Wagner and
have unequivocal family
ies failed to identify
may
Lycopodiaceae be paraphyletic to a hetero-
Beitel, 1992), or indicate that
and
sporous lycopsid clade comprising Selaginellaceae, Isoetaceae extinct rel-
Molecular characters, on the other hand, furnish very
(Bateman,
atives 1992).
(Wikstrom and Kenrick, 1997,
monophyly family
strong support of the
for
mono-
Lycopodiaceae
unequivocally support
2000a). Their rbcL sequence data
Lycopodiaceae
phvlv. Furthermore, molecular data resolve a basal split in sep-
AMERICAN FERN JOURNAL: VOLUME NUMBER
152 91 3 (2001)
Marchantia polymorphs
Andraea
rupestris
Sphagnum
palustre
nudum
Psilotum
51
Equisetum arvense
57
Angiopteris evecta
Ephedra tweediana
Zamia
64 inermis
13
99
Pin us edulis
20
100
Psedotsuga menzi
Isoetaceae
melanopoda
Isoetes
63
11
84 apoda
Selaginella
50
Selaginellaceae
100
Sefaginella selaginoides
Phyllogfossum drummondii
94
Huperzia sefago
22
Huperzia wilsonii (Epiphyte)
100
Huperzia hippuridea
53
100
Huperzia campiana (Epiphyte)
Lycopodiaceae
Huperzia cumin
64
gii
Huperzia HnifoHa (Epiphyte)
18
100
Lycopodiella inundata
41
100
Lycopodiella alopecuroides
"Strobilate clade"
88 Lycopodium clavatum
1
Lycopodium obscurum
2
70
Lycopodium
digitatum
A
Fig. 1. single most parsimonious tree indicating relationships of Lycopodiaceae. The
tree
is
reproduced from the analyses of Wikstrom & Kenrick (1997) of rbcL sequence The
data. tree
is
printed as a phylogram with branch lengths proportional to the amount of change along each
branch and branch support is indicated by decay indices (above) and bootstrap values (below)
for
each branch.
arating a "strobilate clade", including Lycopodium and
L. Lycopodiella Holub,
Hup
Phylloglossum Kunze The
clade. single most
parsi-
Wikstrom
shows
branch support measured by decay
as indices (Bremer, 1988; Donoghue
and
et al, 1992) bootstrap values (Felsenstein, 1985). The support mono-
for
phyly Lycopodiaceae
of not entirely unexpected. Perhaps more
is
surprising
Huperzi
m
WIKSTROM: EXTANT HOMOSPOROUS LYGOPODS
153
and morphology made
highly divergent has
its relationships to other spe-
its
cies in Lycopodiaceae understand. Previous taxonomic treatments
difficult to
have with
either implicitly or explicitly classified incertae sedis respect
to
it
other groups in the family (Holub, 1985; 011gaard, 1987; Wagner and
Beitel,
A
between Phylloglossum and Huperzia however
1992). close relationship
is
number
consistent with a of morphological features such as spore morphology
(Breckon and Falk, 1974; Tryon and Lugardon, 1991), sporangial epidermis
morphology phytochemistry (Markham and
(011gaard, 1975), et ah, 1983) chro-
mosome
number
[Blackwood, Other and
1953). characters, in particular the
morphology of the perenniating tuber in Phylloglossum, led workers such as
Bower (1935), Bruce (1976a) and Hackney (1950) to consider a close relation-
ship between Phylloglossum and Lycopodiella. The rbcL sequence how-
data,
ever, lend unequivocal support for the Huperzia-Phylloglossum relationship.
The Phylloglossum from Huperzia Lycopodium-
evolution of either a or a
Lycopodiella like ancestor implies a remarkable architectural transformation.
new
This include the development of a organ (perenniating tuber), a substan-
stem and development
reduction of the microphyll-bearing system, the of
tial
a unique stem anatomy. noteworthy though that the aspects of morphology
is
It
likely to be the least affected by this overall architectural change, such as spore
and sporangial epidermis morphology and phytochemistry, are also the most
consistent with the rbcL data in supporting a Huperzia relationship.
—
The "strobilate clade". The "strobilate clade" includes the two genera Ly-
copodium and Lycopodiella (sensu 011gaard, 1987), and although comprising
than
My
One
and subgeneric names follows that of 011gaard (1987). factor contributing
subgeneric groups the segregation of several divergent
to the inflation of is
m
mor
imilar
Works morpho-
between such groups. looking at various
to find similarities
such stem anatomy (Bierhorst, 1971; Jones, 1905; Ogura,
logical features as
morphology (Tryon and Lugardon, 1991; Wilce, 1972), distribu-
spore
1972),
and
trophophylls sporophylls (Bruce, 1976b),
tion of mucilage canals in
(W,
•phology
and
011gaard
ogy 1976c) have tended to reinforce this pattern, (1990)
(Bruce,
groups represent ancient evolutionary line-
hypothesized the subgeneric
that
ages.
work
taxonomic recognizing subgeneric
in
Despite the success of recent
morphological
knowledge about the variation in
groups and our increased
have been few explicit attempts to investigate the phylo-
these there
plants,
Wagner and
Lycopodium and Lycopodiella. Beitel
of
genetic relationships
American and Wikstrom and
of N. species
analyzed the relationships
(1992)
representatives of both genera in their analyses. Both
Kenrick included
(1997)
AMERICAN FERN VOLUME NUMBER
JOURNAL:
154 91 3 (2001)
selago
H.
1
52
Phyllogfossum
hippuridea
H.
21
100
campiana
H.
L
magellanicum
13. Section Magellanica
100 L
^
fastigiatum
96
L
Pseudodiphasium
volubile Section
L
alpinum A
15 L
wightianum Section Complanata
1 100
L
digitatum
^^
1
L
annotinum
Section Annotina
L
clavatum
82
IS
Lycopodium
Section
1
100 L
vestitum
69
L
Obscura
obscurum Section
L
79 jussiaei
17 Diphasium
8 Section
100 L
93
scariosum
Pseudolycopodium
i Section
L
casuarinoides
Section Lycopodiastrum
58
100 La. alopecuroides
32
Section Lycopodiella
100
La. inundata
cernua
La.
41 42
glaucescens
100 100 La. Section Campyiostacinys
5
99
penduHna
La.
77
4
La. lateralis Section Lateristachys
64
La. caroliniana
Section Caroliniana
Fig. Strict consensus of two most parsimonious
2. trees indicating relationships within the
"stro-
The
bilate clade". tree is reproduced from the analyses of Wikstrom and Kenrick (2000b) of com-
bined rbcL gene and
trnL intron sequence data. Branch support indicated by decay indices
is
and
(above) bootstrap values (below) for each branch. Subgeneric groups
(sections sensti 011gaard,
shown
1987] are the
to right.
Studies however, only included number
a limited and
of species the majority
of the subgeneric groups recognized were not included. To address
the
rela-
tionships of Lycopodium and and
Lycopodiella,
specifically the relationships
among we
subgeneric groups, recently undertook an analysis using combined
taxa
(Wikstrom and The
Kenrick, 2000b]. consensus
from
strict tree this analy.
reproduced in figure 2 showing branch support (decay and
indices
bootstrap
and
values), subgeneric groups sensu
(sections 011gaard, 1987) to the As
right.
seen monophyly
in figure the analyses support
2, of both Lycopodium and
m
low number
relatively a groups and some
for of
of the questions concerning
WIKSTROM: EXTANT HOMOSPOROUS LYCOPODS
155
among
relationships subgeneric groups remain unresolved. There are however
some
interesting patterns that not only provide an opportunity calibrate the
to
tree against the fossil record (see belov^] but that are supported by morpho-
logical data.
Lycopodium casuarinofdes Spring, [section Lycopodiastrum) grouped as
is
sister to all remaining Lycopodium. This basal split in Lycopodium sup-
is
ported by at least two morphological features: the lack of spore muri (Tryon
and Lugardon, 1991; Wilce, 1972] and the presence of thick, lignified and
sinuate sporangium cell walls in L. casuarinoides (011gaard, 1975). Both fea-
tures are reminiscent of Huperzia and Pbylloglossum and are best explained
as retained plesiomorphic features. Another well supported grouping the
is
—
Pseudodiphasium [Lycopodium Magellanica Even
volubile G. Forster) clade.
though there are few similarities in habit between L. volubile and section Ma-
gellanica, features of spore morphology (Wilce, 1972] and the absence of basal
mucilage canals in their sporophylls (Bruce, 1976b; 011gaard, 1987) support
this grouping. In Lycopodium, two more subgeneric group relationships are
supported. Section Obscura groups with section Diphasium and section An-
notina groups with section Lycopodium.
more The
Relationships within Lycopodiella are difficult to resolve. analyses
of Wikstrom and Kenrick (2000b) included datasets from both the plastid rbcL
gene and the plastid trnL intron regions. Results from the separate analyses
and
however with respect subgeneric group relationships the sup-
differed to
combined was moderate. The
port obtained in the analyses results are also
somewhat morphological Section Caroliniana
considering features.
surprising,
example has including isovalvate sporangia (011gaard,
several features,
for
1987) and absence of leaf veinal mucilage canals (Bruce, 1976b], that are best
would
explained retained plesiomorphic conditions. This suggest a basal
as
from remaining
Lycopodiella separating section Caroliniana species,
split in
supported by molecular data (Wikstrom and
but such relationship not
a
is
Kenrick, 2000b].
clade.—The Huperzia-Phylloglossum clade by
Phylloglossum far
Huperzia- is
and Huperzia alone includes
more species diverse than the "strobilate clade",
The occur epiphytes
an 300-400 majority of these either as
estimated species.
montane South America, Africa and
low mid rain forests of
in to altitude
open upper montane
ground dwellers in habitats of the
South-East Asia, or as
Andean
and high altitude alpine vegetation of the
neotropical rain forests
mountains.
and
had been no phylogenetic treatments of the group,
Until recently, there
morpho-
document identifying discrete
most taxonomic treatments difficulties
011gaard
relationships. (1987) for ex-
infrageneric
indicating
logical features
be premature and considered
ample considered formal taxonomic decisions to
K
and Schrank group to be the only reasonably
Martins
his selogo C.
(L.)
He however recognize 22 informal species
did
distinct infrageneric entity.
are
vicanance
AMERICAN FERN VOLUME
156 lOURNAL:
possibly linked to the rifting of Pangea, or more recent transoceanic dispersal.
He
recognized and
also several different epiphytic groups, but has
terrestrial
it
how
been
unclear these are related each other and whether epiphytism
to
evolved once
or
iteratively.
we
To
address these questions, recently developed a phylogenetic hypoth-
esis oi Huperzia based on plastid trnL-trnF intron and spacer sequences (Wiks-
trom The
et al, 1999]. analyses included 46 Huperzia species, representing 18
of 011gaard's 22 informal species groups and most of the geographical regions
where
Huperzia The
species are found. resulting consensus from
strict tree
these analyses reproduced showing branch
is in figure 3 support {decay in-
and
dices bootstrap values), species groups (sensu 011gaard, growth hab-
1987),
it (terrestrial/epiphytic) as well as the occurrence of neotropical and paleo-
The many
tropical clades. results indicate that of the species groups recog-
H.
H. and
taxifolia (Sw.) Trevisan H.
H
^fl
the existence of other Epiphytism
interesting patterns. appears have
to origi-
nated once within Huperzia with two independent
at least reversals
to a
ter-
restrial habit [H. liippuridea (Christ) Holub and the clade including H. sau-
rurus, //. brevifolia (Grev. & Hook.) Holub and H. reflexa groups), both in the
With two
)pics. exceptions [H, ophiodossoides (Lam.) Rothm. and H.
funifi
Within
and
brevifolia H.
refl
groups
constitute a large, secondarily These
terrestrial clade. patterns are
all
well supported by and
the trnL-trnF data
the existence of neotropical and
an
(Wikstrom
Kenrick,
2000a).
Comparing
these with
results respect morphological
to data
difficuh.
is
m
and
groups, characteristics that have been used endpoints
are morphological
in
continua,
resulting in arbitrary decisions on
character
state delimitations. Not-
withstanding some
these problems,
of the patterns obtained
in the molecular
analyses are unexpected. For example, and
neotropical members
paleotropical
K
of the H. phlegmaria and of the groups show
verticillata conspicuous mor-
phological similarities but, based on the molecular
analyses, morpho-
these
logical similarities are the result of convergent evolution. Furthermore,
as ho-
mosporous
plants they have usually been considered
to disperse but
easily,
biogeog
Huperzia
ametophyt
ametophytes
are
(Wikstrom
knowledge
on
aspects of the establishment and
dvnamics
'
WIKSTROM: EXTANT HOMOSPOROUS LYCOPODS
157
H. proVitera H. hippuris group
,
H. fordii H. hamiltonii group
I
B8 H. sieboJdii H. cancellata group
1
78
H. tauri H. phlegmaria group
99 H. hippuris H. hippuris group
H
carinafa group
H. fockyen
phlegmaria group
H. bafansae H.
H
squarrosa group
_4 H. squarrosa
I
97
H. nummulariifolia
Paieotropical
H. phlegmarfoides
dade
67
1
phlegmaria group
62 H. cf phlegmaria H.
75
8
H. horizontalis
99
4 H. phlegmaria
H. funiformls group
H. taxifolia
1
H. vertfciltata H. group
verticillata
1
H. dacrydioides
62
H. gnidioides group
hofsW
96 H.
H. billardieri H. myrtffoHa group
1
mandiocana group
H. wilsonii H.
I
hippmdea group
94 H. H. brongniartii
I H
95
H. dichotoma dichotoma group
54 I
H. tenuis H. vertidllata group
I
H. linifolia H. linifotia group
\
99 H. sarmentosa H. verb'dHata group
I
97 H. capeffae
H. cowpacta
H. crasaa
1 H. hystiix saurunjs and
H.
64
1 H
99 H. polydactyfa
groups
64 brevifolia
H. rufescens
76
Neotropical
H. cwrJingU
clade
H. attenuata
96 H. tetmgona
87
_ H. eversa
62 _ H. reflexa
group
H. reflexa
64
98
_ H. unguiculata
t
99
76
H. reflexa var, reflexa
la H. rosenstockiana H. taxifolia group
100
campiana phlegmaria group
H. H.
group
H. lindenii H. taxifolia
n
H. ericifolia
100
H. heteroclita
H. phlegmaria group
100
94 H. subulata
7
H. ophiogfossoides
L
H. tacidula
selago group
H.
i
77 H. sefago
_ Phyflogfossum
L
magetlanicum
L
fasb'giatum
L
vofvbSe
L
deiAerodensum
_
obscufum
L.
L
afpintm
L
wightianum
_
L
annotifKtm
L
_ vestitum
L
casuaiinddes
La. akjpecuroides
inundata
La.
giaucescens
La.
p&KhiHna
La.
cemua
La.
La. fateraiis
species
Terrestrial
parsimonious indicating relationships within the
most trees
consensus of 16,924
Fig. Strict
3.
Wikstrom
The reproduced from the analyses of et al. (1999J
tree
Hupe'rziaPhylloglossnw clade. is
Branch support indicated by decay indices (above)
and spacer sequence data. is
of trnh-truF intron
each branch. Informal species groups (sensu 011gaard. 1987) are
and values [below) for
bootstrap
and
epiphytic Huperzio into a neotropical a paieotropical
The of
partition
indicated to the right.
and H. ophioglossoides (marked with *) are exceptions to
H. funiformis
clade also indicated.
is
and remaining
Taxa within grayed boxes are terrestrial species
biogeographic pattern.
this clear
are epiphytic.
AMERICAN FERN VOLUME NUMBER
158 JOURNAL:
91 3 (2001)
of lycopod populations however limited, and before such knowledge
ac-
is is
quired, one can only
speculate.
Patterns of
Diversification
Calibration of the phylogenetic tree critical to reconstructing the historical
is
patterns and two approaches have been adopted. Where possible, information
from the fossil record has been used. This includes the use of fragmentary
plant remains, isolated organs and spores. The calibrations discussed here
are
however and based on As
tentative primarily a literature survey. seen in figure
some
further
quired. Ultimately, the fossils should of course not only be used
for calibra-
tions, but should also be included in the analyses. Attempts
to correlate bio-
geographic data on living species with other geological data such major
as
tectonic events have also been done.
The
macrofossil record of Lycopodium and Lycopodiella meager (Skog and
is
The middle
Hill, 1992). Jurassic (Bajocian-Bathonian; 174 Mya) Lycopodites
falcatus Lindley & Hutton is one of the better known species and mor-
its
phology was documented by He
in detail Harris suggested an
(1961), affinity
with section Complanata, based on flattened branch system with
its large
lat-
eral leaves and smaller dorsal and would
ventral leaves. correct, this suggest
If
minimum
Mya
a age of 174 the between Complanata and
for split section the
clade including sections Lycopodium and Annotina Although
the
(Fig.
4). ar-
affinity
stem
position along the stem lineage of section Complanata indicated
as in figure
highly uncertain.
4, is
The mature stem anatomy Lycopodium
of highly with
distinctive the xy-
is
lem
arranged bands
in parallel This
(plectostele). feature characteristic of
is
all living species. Srivastava (1946) documented what he considered be
to a
lycopodiaceous
plectostele [Lycoxylon indicum
from
Srivastava) the
Jurassic
(Lias) of India Although completely
(Fig. decorticated, the arrangement
5). of
xylem
the strands is highly reminiscent of living Lycopodium. This
ev-
fossil
minimum
idence
suesest a ase for the snlit betwRRn J\^rn-nndhim ^nrl
l^rrr^.
Mya
same
Lycopodites
falcatus
two names
if the represent different organ names the same would
for species
certainly contribute to our understanding of lycopod
evolution.
Using evidence
fossil for calibrating the tree associated with number
is a of
problems and
potential sources of error (Doyle and Donoghue,
1993). First of
synapomorphies shared by must
all, living taxa be observable and
recognizable
in fossil taxa. Secondly, fossils only provide estimates minimum
of
<
maximum
meager
WIKSTROM: EXTANT HOMOSPOROUS LYCOPODS
159
H. selago
Phyltoglossum
H. hippuridea
campiana
H.
L
magellanicum
Section Magellanica
L
fastigiatum
L
Pseudodiphasium
volubile Section
L
alpinum
L
Complanata
wightianum Section
I
L
digitatum
t
My
Lycopodites falcatus t (174 a)
L
annotinum
Section Annolina
L
clavatum
Reticulate spores
Lycopodium
Section
L
rl
vestitum
L Obscura
Section
obscurum
L
jussiaei
Section Diphasium
L
scahosum
- Mya)
Reticulatispohtes t (208
L
deuterodensum 1 Pseudolycopodium
Section
Plectosteie
L
t
casuarinoides Section Lycopodiastrum
Mya)
Lycoxylon indicum t (208
alopecuroides
La.
Section Lycopodiella
La. inundata
cemua
La.
Campylostachys
glaucescens Section
La.
pendulina
La.
Section Lateristachys
La. lateralis
La. caroliniana Section Caroliniana
from Wikstrom and Kenrick (2000b)
two most parsimonious trees
consensus of
Fig. 4. Strict
and
The were not part of the analyses
used calibrating the tree. fossils
including the fossils for
basalmost position implied by their characteristics. Ultimately
have simply been inserted the
at
knowledge
current
makes such an inclusion unfeasible at this stage.
underestimate the true age of origin.
can greatly its
observation of feature
this
minimum based on information there-
age estimate fossil
Our
confidence in a
of finding the feature in earlier
depends on the probability that failure
fore
meaning had
absence" not yet origi-
"true
represents a it
records
geological
information calibrating the ages of
most useful for
the
nated. In this respect,
comes the spore record.
clade fi^om fossil
groups within the strobilate
rni J „r r „-i ^T^^T^oc TAHt>i rjntntivR T.vcoDodium affinities is auite ex-