SIZE, LAMINA THICKNESS AND VEIN DENSITY OF THE EARLY PENNSYLVANIAN ALETHOPTERIS PINNULE FROM LEWIS CREEK SUGGEST A PRODUCTIVE PLANT WITH SHORT-LIVED LEAVES

Knowing the width and thickness of the photosynthetic lamina and the vein density of pinnules and leaves enables ecologists to predict leaf longevity and productivity for both living and fossil plants. The Lewis Creek Alethopteris, which occurs in coal balls from the Early Pennsylvanian of North America (Hamlin coal zone) is the oldest known permineralized Alethopteris pinnule. However, this alethopterid has defied characterization because it has a thin pinnule lamina (160-180 μm thick) that generally occurs curled around a shriveled midrib, making the original configuration of the pinnule difficult to reconstruct. Curled lamina, shriveled midribs and fractured pinnules suggest that these leaves dried on the plant or the peat surface before permineralization.

We used image analysis of scanned peels to determine the width of the lamina in orthogonal (symmetric) cross-sections and measured the lamina thickness in each leaf with a stereoscopic microscope. Our preliminary data set consists of 46 pinnules. In the two pinnules with relatively well-preserved midribs, the widest part of the midrib coincides with the midrib-lamina junction. Average pinnule width is 4.5 mm but varies from 1.3 – 7.8 mm, which may reflect triangular pinnules (wide at the base, narrow at the tip). The lamina of the Lewis Creek Alethopteris is thin (≈180 μm) for the width of the pinnule. Its lamina thickness/pinnule width ratio is 0.03, suggesting a shade plant with relatively short-lived leaves. Other permineralized alethopterid species have higher lamina thickness/pinnule width ratios (0.12 to 0.5). The vein density averages 8.4 veins per mm, (range 5.4 - 12.5) is high for an alethopterid, and rivals the vein density observed in modern angiosperm leaves, suggesting rapid growth rates.