September 26, 2012 2 Comments
I like horsetails. Equisetum stands alone representing an entire class of land plants, and one could argue that it is the most successful genus of vascular plant in the world. It can be found on every continent except Antarctica. [Note: Equisetum is introduced in Australia today, but extinct sphenopsids including Equisetum grew there during the Mesozoic, as is the case for Antarctica.] Studies of well-preserved fossils from the Early Cretaceous and Jurassic have shown that they have been doing basically the same thing for ~150 million years (Stanich et al. 2009; Channing et al. 2011); and Triassic (~225Ma) compression fossils suggest the genus may be older. Fossil evidence demonstrates that Equisetum is probably the oldest living genus of vascular plant.
Living and extinct horsetails are easily recognized by their distinct jointed stems with whorls of leaves and sometimes branches borne at the joints (nodes). The spore-producing structures are aggregated into a strobilus or cone.
Deep/old branches of the tree of life that have few genera or species today, like horsetails, often turn out to have a rich fossil record of extinct diversity that encompasses a much broader range of morphology, life history, and ecology than can be found among modern representatives.
A recent paper in Review of Palaeobotany and Palynology re-emphasized this point for me. Rößler et al. (2012) described a fossil horsetail from the Permian of Germany (~250-300 million years ago) that they called Arthropitys bistriata. This plant was a large tree (15 meters or more) with wood and growth rings, and a branched crown! Hardly the stream-side herb in the photo above.
There have been other descriptions of fossil horsetails showcasing their extinct diversity in the last few years. Neocalamites horridus (Shuquin et al. 2012) from the Triassic of China looked like a giant Equisetum covered in sharp prickles, and Sphenophyllum costae (Bashforth and Zodrow 2007) from the Pennsylvanian of Nova Scotia was an elaborate bramble with distinct orders of branching that produced a range of leaf types, from fan-shaped leaves for capturing light, and hook shaped leaves for climbing and support.
Last fall my wife and I found our own horsetail fossils while collecting plant fossils in the Cloverly Formation in Wyoming. This specimen is broken at the node, so what you are seeing from the bottom up is a stem, with a whorl of flimsy looking branches in the middle of the photo. The branches subtend (are directly below) a whorl of leaves that are mostly fused into a collar with free tips, kinda like Bart Simpson’s hair. The node is under that collar, and that’s where the top of the plant broke off.
Rößler et al. 2012 The largest calamite and its growth architecture – <i>Arthropitys bistriata</i> from the Early Permian Petrified forest of Chemnitz. Review of Palaeobotany and Palynology 185 p.64-78