How did sauropod dinosaurs grow so big before the modern grass foods, paleontologists have wondered. It turns out that very ancient spore-bearing horsetail grass (equisetum) is quite nourishing:
From the paywalled article, based on research presented by University of Bonn researchers at the recent annual meeting of the Society of Vertebrate Paleontology:
Horsetails appeared to be poor fodder in previous tests, which simply burned the plants to measure carbon content, Gee says. Instead, her team adapted the Hohenheim gas test, a method for assessing the quality of fodder for farm animals. They fermented modern horsetails for 3 days to simulate the journey through a sauropod’s gut and measured the volume of gas produced—an indicator of energy content. The researchers were astounded to find that horsetails released more energy than any other plant group, including 16 modern grasses. Equisetum is rich in protein, they say, and far more nutritious than the ferns, cycads, and conifers common in the dinosaur era. Gee argues that horsetails by rivers and lakes would have offered sauropods, especially young ones, “a plentiful, accessible, and extremely nutritious food.”John Pickrell, “Sauropods get a new diet and a new look” at Science
Equisetum, considered a “living fossil” is the only surviving member of a large family of spore-bearing vascular plants found as early as 150 mya. It’s still here. The giant sauropods not so much.