October 29. Cycads and Gingkoes

We’ve touched on the prolific plant life of the Jurassic several times. It resulted at least to some extent from the greenhouse conditions that prevailed over much of the globe for much of the period. And I mentioned yesterday that there were pretty much no flowering plants. The Jurassic forests were still dominated by conifers, ferns, rushes, and cycads. Not too different from the Carboniferous.

Cycads have cones with exposed seeds, and they first appear in the fossil record during the Permian, with some possible Carboniferous examples. They flourished during the Jurassic, so that they were characteristic of many Jurassic forests, to the point that the Jurassic is sometimes called the Age of Cycads. They are the primary constituents of many Jurassic coal beds. 

Jurassic gingkoes from Oregon.
USGS Monograph XLVIII, by Lester Ward, 1905.

Cycads are gymnosperms, with cones containing exposed seeds. They range from tiny plants only a few centimeters tall to large trees, and while many of them looked like palms, they are not closely related. Cycads today live mostly in tropical environments, but some varieties are found in desert conditions. Modern cycads have been called “living fossils” because they seem to be pretty much unchanged from their Jurassic ancestors, but a recent study has suggested that the diversity of modern cycads only dates to the past 10 million years or so – a second wave of cycad diversification. According to that research based on DNA analysis, cycads declined at and after the end of the Cretaceous but then radiated again during the Miocene, about 10 million years ago. (Note: text above modified thanks to the comment, below)

The giant herbivorous dinosaurs, Sauropods and others, undoubtedly munched on cycads as part of their diet. 

Gingkoes were also common in Jurassic forests. They are fairly closely related to cycads and conifers, although their precise relationship is not certain. The earliest fossils of the Gingko genus come from the early Jurassic, and while their abundance and diversity peaked during the Jurassic, gingkoes declined during the Cretaceous and later, so that today, there is only one living species, Gingko biloba, native to China.

Jurassic gingkoes lived across the northern continents, what are now North America, Europe, Siberia, and China. Today’s gingko trees can be well over 100 feet tall, and Jurassic varieties were probably of similar size. Gingko leaves dominate some fossil assemblages.

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It’s appropriate that today’s birthday is Othniel Charles Marsh, one of the most prominent students of Jurassic vertebrate fossils in the United States. He was born October 29, 1831, near Lockport, New York, and he spent most of his career with Yale University and the U.S. Geological Survey. O.C. Marsh’s competition with Edward Cope in the rush to find and identify dinosaur fossils is known as the “bone wars,” but we’d probably say Marsh won the wars – he named at least 43 orders, families, and genera of dinosaur, and described 80 new species to Cope’s 56. The legacy of both Marsh and Cope is dinosaur fossils in museums around the United States.

—Richard I. Gibson

LINK
Modern cycads not so old?  
Cycads