Geoscience Reference
In-Depth Information
Much has been written concerning the large
'modern' Archaeopteris trees of the Late Devonian
and information concerning these organisms has not
progressed much in recent years (Beck & Wight
1988; Trivett 1993; Meyer-Berthaud et al. 1999).
By contrast, there is a large amount of new data
on the Cladoxylopsida, a class of plants that flour-
ished in the Middle Devonian and includes the
earliest trees. In addition to a review of the trees
that evolved in this group, we propose a model of
cladoxylopsid trees addressing some parameters
that may feed the models cited above, for example,
the growth potential of the trunk and root system and
the amount of lignified tissues they produced.
(Fig. 1b). The Equisetopsida also evolved the tree
habit, but in the latest part of the Devonian
(Fig. 1a). The trunk of Pseudobornia was up to
60 cm wide (Fig. 1b), had distinctive nodes and
produced three orders of branches. Nothing is
known about the anatomy of these trees that, until
now, has only been reported from the Famennian
of Bear Island and Alaska (Mamay 1962; Schweit-
zer 1967; Fairon-Demaret 1986). Interestingly,
and despite the intensive research conducted since
then, Chaloner & Sheerin's (1979) conclusions
remain largely accurate in terms of trend and date
for the earliest evidence of large sized plants
(Middle Devonian) (Fig. 1b). One recent improve-
ment, however, was the discovery of an archaeop-
teridalean trunk about 20 cm in diameter in a new
Givetian (late Middle Devonian) locality near Ron-
qui`res in Belgium (Gerrienne & Meyer-Berthaud
2006). More importantly, the affinities of Eosper-
matopteris that have remained uncertain for more
than 100 years have just been resolved. The trees
supported by these stumps were not lepidosigi-
llarian and related to the Lycopsida as suggested
by Algeo et al. (2001) but, again, are pseudospor-
ochnalean (Boyer &Matten 1996; Stein et al. 2007).
The evidence, therefore, is that the earliest trees
belong to the pseudosporochnalean cladoxylopsids
followed by the progymnosperm Archaeopteridales
with a period of overlap in the Givetian (Fig. 1b).
The Cladoxylopsida and the Archaeopteridales are
two major groups included in the euphyllophytes,
a clade characterized by two derived characters
related to architecture: (1) pseudomonopodial
branching and (2) helically arranged branches
(Kenrick & Crane 1997b) (Fig. 1a). The two
branches issued from the pseudomonopodial div-
ision of an axis are not equal as for those resulting
from the ancestral isotomous pattern of division.
They differ in diameter and, generally, also in orien-
tation (Fig. 1a). This pattern allows the differen-
tiation of lateral branching systems that may
become specialized to perform certain functions
more efficiently, such as photosynthesis and repro-
duction. The helical arrangement of branches
permits a wider range of spatial occupation than
the strict arrangement of successive branches at
908 found in the ancestral taxa. The Cladoxylopsida
are basal among a complex of taxa informally called
'ferns s. l.' that also comprises the Equisetopsida
and diverse groups of Filicopsida. The Archaeop-
teridales are basal members of a strongly supported
monophyletic group, the lignophytes, which also
includes the seed plants (Fig. 1a). The derived
character shared by the lignophytes is the possession
of a cambium that produces both secondary xylem
(i.e. wood) and secondary phloem, the latter tissue
increasing the capacity for the long-distance trans-
port of photosynthates.
When and in what groups did
trees evolve?
If entire trunks are rare in the fossil record, wide
pieces of axes are common and they are used by
palaeobotanists as evidence of trees. This is based
on the mechanical relationship that links height to
diameter in self-supporting columns with a known
distribution of tissues/material. According to one
of the most commonly used models (the elastic simi-
larity model), it is predicted that the maximum
height reached by a self-supporting column before
it buckles under its own weight increases at the 2/3
power of its diameter (Mosbrugger 1990). Other
models, and statistical analyses conducted on a
wide range of axes from different taxonomic groups,
confirm this relationship but differ by the scaling
exponent (Niklas 1993). All studies predict that
the wider a fossil stem is, the taller it might have
been from a biomechanical point of view.
In 1979 Chaloner & Sheerin showed that
the maximum diameter of axes followed an increase
that they described as 'logarithmic' through the
Devonian. An arbitrary 10 cm threshold was
reached in the early Middle Devonian (Fig. 1b).
The earliest taxa to show such stems or branches
belonged to Calamophyton and Pseudosporochnus,
two pseudosporochnalean genera included in the
Cladoxylopsida (Fig. 1a, b). Tall trees were present
by the end of the Middle Devonian, represented by
the stump casts of Eospermatopteris found in
several localities near Gilboa (New York). These
measure up to 1 m in diameter.
In the Late Devonian, most wide axes belonged
to two major but unrelated groups: the Lycopsida
(i.e. Lepidosigillaria, Cyclostigma, Leptophloeum)
and the progymnosperm Archaeopteridales
(Fig. 1a, b). The Cladoxylopsida were still present
but their stems did not reach the dimensions of the
Pseudosporochnales. Indeed, those of Pietzschia,
the largest known cladoxylopsid genus of Late
Devonian age, did not exceed 16 cm in diameter
