The Pinaceae family representing the most common conifers like firs, spruces, hemlocks, and pines, are principally a north temperate family. Today's largest conifer family Pinaceae has 10 genera and over 200 species. Judging from the diversity of the seed cones present during the Cretaceous, the family Pinaceae evolved in the northern hemisphere during the early Cretaceous or Jurassic Period of the Mesozoic Era, 130 to 200 million years ago.
Compsostrobus, a cone from the late Triassic of North Carolina, may be the oldest fossil of the Pinaceae family. The genus Abiocaulis exhibits anatomic features similar to those of the modern Abies. Without stem-attached leaves, Abiocaulis verticillatus has long shoots with helically arranged, decurrent leaf bases. Certain features of the wood of Abiocaulis verticillatus, Keteleeria and Abies, like checking of the tracheid walls and the pattern of rays, suggest a wood of the Cedroxylon type. The presence of bud scales on the fossil specimens also shows similarities within the Pinaceae.
Several excellent studies of pinaceous cones give important information about the diversity of the family. Today a large number of preserved cones are said to be members of the Pinaceae. They are separated into two broad groups based on whether they can be assigned to modern genera or outside the extant taxa. P. belgica, from the Lower Cretaceous of Belgium is the oldest cone of Pinus. The cone scale with abundant sclerenchyma and resin canals, is similar to the modern cone of P. sylvestric. Vasculature of Pinus cliffwoodensis from the Late Cretaceous in the scale occurs as a series of traces, each with a resin canal on the abaxial surface. Also, the needlelike leaf bases and the dwarf shoots of the genus Prepinus represent a transitional stage between the Paleozoic cordaites and modern members of the Pinaceae. However, these leaves are distinct from those of Pinus and thus cannot be considered an early stage in the evolution of a Pinus leaf. Based on the fossil leaf characters, some subsections of Pinus did not exsit during the Late Cretaceous. Foliage characters suggest that the subgenus Pinus may be the oldest within the genus.
Studies and the fossil record of cones tell that the genus Pinus was a distinct Pinaceae member during the Cretaceous. Also, the reproductive systems are located in similar sites, enabling us to see clear understanding of some conifer genera. It is hard to see the origins of the Pinaceae clearly, but what has been known about the family suggests that there was considerable diversity during the Cretaceous.
There is a nice exhibit on Plants of Mazon Creek at the Illinois State Museum. These were fossilized during the Pennsylvanian, and have been a rich source of information. You may also try the Plant Fossils of West Virginia.
Source:
Taylor, T.N. and Taylor, E.L. The Biology and Evolution of Fossil Plants. New Jersey: Prentice Hall, 1993.
