An Eocene leaf flora from the northern Peruvian Andes

Abstract

The fossil forest known as El Bosque Petrificado Piedra Chamana lies in close proximity to the village of Sexi in the Department of Cajamarca in northern Peru. The forest was deposited in either a lahar or pyroclastic flow, and ashfall deposits, as mineralized wood and leaf impressions 39 million years ago (late Middle Eocene). More than 30 types of non-monocot angiosperms have been described from the fossil wood, which is densely embedded in rock outcrops, scattered across the ground, and occasionally found in situ. Anatomical characteristics and identifications of the woods suggest a seasonally flooded lowland tropical forest with a dry period. This site also preserves a small leaf flora which has not received a full treatment in earlier works and is the focus of the present study. The leaves are described in detail using standard leaf architecture terminology. The flora has 31 non-monocot leaf morphotypes. The presence of more than 30 morphotypes suggests high diversity based on having only ∼210 non-monocot leaf specimens. The leaf collection also includes ∼10 monocot specimens. One of the leaf morphotypes shows similarities to the extant mangrove genus Avicennia, possibly confirming one of the fossil wood identifications. Most of the leaves are incomplete, but when the margin is preserved, it is untoothed. The lack of teeth indicates a warm, tropical climate (estimated mean annual temperature of >25 °C based on leaf margin analysis). Leaves in the collection are small with 28 of 31 morphotypes inferred as microphyll or notophyll in laminar size, providing support for the seasonal dryness indicated by the fossil woods. Despite the small size of the collection, this leaf flora is important to document as there are few Eocene plant macrofossil localities in Peru; it provides some insight into the Cenozoic forests in this understudied region.