The Piedra Chamana fossil woods and leaves reveal a tropical forest with some water stress

The Piedra Chamana fossil forest in northern Peru is an assemblage of angiosperm woods and leaves preserved in volcaniclastic rocks dated at 39 Ma (late Middle Eocene). Deborah Woodcock and Herbert Meyer analyzed anatomical and morphological features of the fossils to reconstruct the paleoenvironment during this time of global warmth, taking advantage of the co-occurrence of woods and leaves to compare different proxies and analytical approaches.

The fossil woods show many similarities with modern tropical-forest woods and tropical fossil-wood assemblages; closest correspondence within the Neotropics is to semi-deciduous lowland tropical forest with moderate precipitation (~1000–1200 mm). Features unusual for the modern South American tropics are mainly vessel-related characters (semi-ring-porosity, grouped vessels, helical vessel thickenings, short vessel elements) linked to water stress or seasonal water availability. Leaf analysis indicates mean annual temperature of 31°C (n = 19, 100% entire-margined) and mean annual precipitation of 1290 mm (n = 22, predominantly microphylls and notophylls).

“[A]nalysis of wood-anatomical features of a diverse fossil wood assemblage like the Piedra Chamana fossils allows for 1) placement to vegetation type(s) with known climatic parameters based on characteristics of the anatomy, in this way yielding quantitative estimates of paleoclimate variables, and 2) identification of features that are anomalous with respect to modern associations, which can in turn provide additional climate and environmental information and serve as a source of hypotheses about wood:climate relationships,” write the authors.

Woodcock and Meyer say that the paleovegetation was clearly lowland tropical forest with a dry aspect, but anomalous aspects of the wood anatomy are consistent with the high temperatures indicated by the leaves and most likely explained by differences in seasonality and water stress compared to the present-day Neotropics. A close modern analog may be in very seasonal regions of Asia. Pronounced monsoonal (summer-rain) conditions may relate to a location (paleolatitude of 13°S) outside the near-equatorial tropics.