Friday, 6 October 2017

Prehistoric plants

Five years ago (almost to the day) I posted an ancient Greek coin that had the extinct silphium plant as its reverse design. I have a group of other extinct plants that I have been meaning to post for a while and have finally got around to photographing the early plant fossils. They are all horsetails or club mosses (or relatives of them) and date to the Carboniferous period, approximately 360-300 mybp.

Annularia is a plant fossil belonging to the order Equisetales or Horsetail. Whorls of small leaflets are arranged concentrically around a thin stem. These were indicative of humid to wet habitats such as along rivers and lake shores.

Sigillaria (relative of club moss)
Sigillaria is a genus of extinct, spore-bearing, arborescent (tree-like) plants. It was a lycopodiophyte, and is related to the lycopsids, or club-mosses.

Reaching a height up to 30 metres, with a tall, single or occasionally forked trunk that lacked wood. Support came from a layer of closely packed leaf bases just below the surface of the trunk, while the centre was filled with pith. The long, thin grass like leaves were attached directly to the stem and grew in a spiral along the trunk. The old leaf bases expanded as the trunk grew in width.

The trunk was topped with a plume of long, grass-like, microphyllous leaves, so that the plant looked somewhat like a tall, forked bottle brush. The plant bore its spores (not seeds) in cone-like structures.

 Lepidodendron cone/strobilus
Lepidodendron (related to club moss)
Lepidodendron had tall, thick trunks that rarely branched and were topped with a crown of bifurcating branches bearing clusters of leaves. These leaves were long and narrow, similar to large blades of grass, and were spirally-arranged.

Lepidodendron has been likened to a giant herb. The trunks produced little wood, being mostly soft tissues. Most structural support came from a thick, bark-like region. This region remained around the trunk as a rigid layer that grew thicker, but did not flake off like that of most modern trees. As the tree grew, the leaf cushions expanded to accommodate the increasing width of the trunk.

Lepidodendron likely lived in the wettest parts of the coal swamps that existed during the Carboniferous period. They grew in dense stands, likely having as many as 1000 to 2000 plants per hectare. This would have been possible because they did not branch until fully grown, and would have spent much of their lives as unbranched poles.

They sometimes reached heights of over 30 metres, and the trunks were often over 1 metre in diameter.

Calamites (horsetail)Calamites is a genus of extinct arborescent (tree-like) horsetails to which the modern horsetails (genus Equisetum) are closely related. Unlike their herbaceous modern cousins, these plants were medium-sized trees, growing to heights of more than 30 metres (100 feet).

Club moss
Club mossThe climate supported lush, swampy forests. Club moss (Lycopods) made up the largest component of these forests and achieved gigantic size, growing to heights of more than 40 metres with supporting trunks measuring up to 2 metres or more in diameter.