Middle Eocene trees of the Clarno Petrified Forest, John Day Fossil Beds National Monument, Oregon

Abstract

One of the iconic fossils of the John Day Fossil Beds National Monument, Oregon, USA, is the Hancock Tree—a permineralized standing tree stump about 0.5 m in diameter and 2.5 m in height, embedded in a lahar of the Clarno Formation of middle Eocene age. We examined the wood anatomy of this stump, together with other permineralized woods and leaf impressions from the same stratigraphic level, to gain an understanding of the vegetation intercepted by the lahar. Wood of the Hancock Tree is characterized by narrow and numerous vessels, exclusively scalariform perforation plates, exclusively uniseriate rays, and diffuse axial parenchyma. These features and the type of vessel-ray parenchyma indicate affinities with the Hamamelidaceae, with closest similarity to the Exbucklandoideae, which is today native to Southeast and East Asia. The Hancock Tree is but one of at least 48 trees entombed in the same mudflow; 14 others have anatomy similar to the Hancock Tree; 20 have anatomy similar to Platanoxylon haydenii (Platanaceae), two resemble Scottoxylon eocenicum (probably in order Urticales). The latter two wood types occur in the nearby Clarno Nut Beds. Two others are distinct types of dicots, one with features seen in the Juglandaceae, the other of unknown affinities, and the rest are very poorly preserved and of unknown affinity. Leaf impressions in and immediately below the layer containing the trees include the extinct genera Macginitiea and Platimeliphyllum (Platanaceae), and Trochodendroides (Saxifragales).