Cycads: the salad of the Mesozoic

For most of the Mesozoic, if you were a large herbivore looking for a meal, you were probably looking at a cycad. They dominated the understorey for roughly 150 million years and are still with us, mostly in botanical gardens.

Slow and toxic

Cycads grow at a pace that makes conifers look impatient — some individuals add a few centimetres per decade. Their defence is chemical: cycasin, a compound that damages the liver and nervous system of most modern mammals.

That raises an obvious question. If cycads are poisonous, how did anything eat them?

• gut microbes capable of breaking down the toxin
• extremely large body size, diluting the dose
• feeding in rotation across many plant types
• eating young leaves and cones rather than mature foliage

None of these is proven for a specific dinosaur, but all are documented in living animals that eat toxic plants today.

Not palms, not ferns

Cycads look like a palm crossed with a fern and are related to neither. They are gymnosperms — closer kin to pines than to anything with a flower. The seeds sit exposed on modified leaves rather than inside fruit.

A cycad cone can weigh over 40 kilograms. Whatever dispersed those seeds was not a small animal.

Why they declined

Flowering plants arrived, grew faster, and reproduced more cheaply. Cycads never disappeared, but they were pushed to the margins — today about 300 species survive, and a large share of them are endangered, mostly because collectors pay well for slow-growing plants.

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Cover image: Pseudopanax at English Wikipedia — Public domain (source).