The Evolution of Conifers

Conifers such as this pine, are easily recognized. Familiar conifers have needle-like leaves and woody cones. These cones, which are visible here, are the ovulate cones, which produce the ovules/seeds. Smaller, ephemeral cones that bear pollen are produced in the spring.
 

Other conifers, such as the Araucaria, are less familiar. These plants have a southern hemisphere distribution and the only ones you are likely to see are the Norfolk Island Pine, which is widely grown as an indoor pot plant, and the Monkey Puzzle Tree, which is grown in southern California. These plants, with their small, spirally-arranged needles, resemble (superficially) some of the earliest true conifers of the Upper Paleozoic. To find the evolutionary source of conifers, we must examine some common Carboniferous plants which don't seem to have any of the common conifer characteristics.

A very common kind of fossil in Carboniferous deposits is a long, strap-like leaf with parallel veins. These leaves, known as Cordaites, must have been tough and leathery in life, for they are usually very well-preserved as fossils. Shown here is a reconstruction of one species of Cordaites (Cridland, 1964). Plants of this type ranged from shrubs to small trees and, as this reconstruction indicates, many had stems with pronounced adventitious roots. Sedimentological studies suggest that many may have been like the modern mangrove, growing on brackish-water mudflats. The thick leaves, often with sunken stomata, appear to be adapted to dryness (so-called xeric adaptations) which might seem strange for a plant growing with its roots in the water. In fact, marine and brackish water represents extreme physiological stress for most vascular plants, and the xeric features of later conifers can probably be traced back to these mangrove-like Cordaites.

Here is a closer view of a cluster of spirally-arranged Cordaites leaves (Stewart, 1983), which certainly don't seem to be conifer-like. It is the small, cone-like fertile structures, seen on short axes among the leaves, that led a noted botanist, Rudolf Florin, to propose that cordaites were indeed the ancestors of conifers.
 

Here is a reconstruction of a pair of these cone-like structures (Delavoryas, 1953), each born at the junction of a bract (b) and the primary axis (pa). Most of the "scales" (s) of these cone-like structures (modified leaves on a very short axis) are sterile, but the terminal ones in this case bear elongate, finger-like sacs containing Florinites-type pollen. These pollen-bearing "cones" are known as Cordianthus.

The female cone-like structures are even more rudimentary and their terminal, fertile scales bear distinctive, heart-shaped ovules. These female ovules/seeds are widely distributed in Carboniferous sediments and are know as Cardiocarpon or Samaropsis (Stewart, 1983).

The woody cones scales of conifers are thought to have evolved by the shortening and fusion of the entire cone-like branch system shown above, with the cone representing an analog of the entire fertile axis.

Sediments of very late Carboniferous and early Permian age contain the fossils if the first true conifers. In the reconstructions shown above (Florin, 1951), (A) shows the end of a growing branch of a Lebachia with several smaller branches attached. A reconstruction of the ultimate branches, with their dense spiral of small needles is shown in (B). These plants had a strong superficial resemblance to modern foliage of Araucaria (see above).

Although these early conifers looked modern in terms of their foliage, their cones are quite primitive and show a definite link with the cordaites. In the Lebachia cone shown at (B), a single cone scale in the complex bears a single ovule. Still more cordaitean is aspect is the ovuliferous scale at (C), an example of Ernestiodendron. Note the multiple ovules on short stalks. In the Walchiostrobus shoot at (D) the ovules are fully emergent from the sterile cone complex, borne on stalks much like the cordaitean ovules illustrated earlier.

Conifers were one of the major lineages to benefit from the Permo-Triassic mass extinction and we will return to them several more times prior to the end of the course.