Archaeopteris. Archaeopteris (not Archaeopteryx!) is abundant from the Late Devonian through the Mississippian. The name Archaeopteris ("ancient fern") was applied to frond-like two-dimensional branch systems with terminal branches webbed in with photosynthetic tissue (Figure 7.6) (VG 1:4). In fertile examples, a branch bearing stalked pairs of elliptical sporangia replace webbed leaves. In some species, fertile branches occur on the same axis with photosynthetic leaflets; in others, whole branch systems are dedicated to reproduction.


Figure 7.6: Archaeopteris foliage showing webbed leaflets and fertile branches intermixed on the same axis. Woody stem is Callixylon.


Webbed leaves and two-dimensional branching differentiate Archaeopteris and its relatives from related plants such as Aneurophyton and Tetraxylopteris. Heterospory is also derived in Archaeopteris. Trilete microspores are 33-70 mm in diameter; megaspores range from 110-500 mm were dispersed into the environment, suggesting a free-sporing life history.

The wood or Archaeopteris -- Callixylon(VG 1:5)(VG 1:6) -- was described in 1911 as Late Devonian conifer from Russia. Logs well over a meter in diameter and 10 m long are not uncommon, leading to the interpretation that they represented some of the first true trees (Figure 7.7) (VG 1:7). The interpretive error is understandable because Callixylon wood differs from that conifers by only a few details of the anatomy of its pits. However, it remains controversial whether wood is independently derived in progymnosperm and seed plant lineages.

The relationship between Callixylon wood and Archaeopteris foliage was not recognized until C.B. Beck happened upon a museum specimen of Archaeopteris that was attached to a small portion of the woody stem. By luck, the wood was preserved anatomically (permineralized). Upon sectioning the specimen, Beck recognized the wood as Callixylon. In 1960, Beck reported the organic connection between Archaeopteris and Callixylon and established a new and extinct group of plants -- the progymnosperms -- to contain woody plants with fern-like foliage and




Figure 7.7: Reconstruction of tree-sized Archaeopteris. Note that the foliage name, Archaeopteris, applies to the whole plant because this name was given first.


The evolution of heterospory among the progymnosperms placed them at center stage in the debate over the origin of the seed habit. Many early workers argued that a heterosporous plant like Archaeopteris was the most likely ancestor to the seed plants because the gradualist paradigm of evolution required a series of morphological intermediates. According to this argument, heterospory was the intermediate between homosporous free-sporing reproduction and the retained endosporic gametophyte of the seed habit.

W.A. DiMichele and colleagues (1989) proposed that the seed habit could evolve directly from a homosporous ancestor like Aneurophyton by speeding up sexual maturation of the megagametophyte such that archegonia and eggs were produced before spores were dispersed. This hypothesis bypassed the free-sporing heterosporous ancestor. They also argued that the ecology of free-sporing heterospory -- typically plants growing in standing-water swamps -- was unlikely to have given rise to early seed plants, which were thought to have pioneered in well-drained upland habitats.

At the moment, both hypotheses remain widely discussed. Future fossil discoveries and more detailed phylogenetic analyses of the plants critical to seed plant evolution may help clarify the picture. However, recent discovered in plant developmental genetics have shown that dramatic changes in morphology, like those needed to evolve seeds from a homosporous aneurophyte, are possible. The linking of the apparently unrelated fields of paleobotany and developmental genetics may hold many important clues to this major evolutionary event -- the origin of the seed habit.