The superfood for Mesozoic herbivores is likely some form of plant that was widely available and provided a high level of nutrition. During the Mesozoic era (252.17 to 66 million years ago), the planet was dominated by different vegetation, and the dominant herbivores were dinosaurs, such as stegosaurs, ankylosaurs, sauropods, and ornithischian dinosaurs.

Here are some possible candidates for superfoods for Mesozoic herbivores:

1. Conifers: Conifers, including various types of pines, firs, spruces, and cedars, were abundant during the Mesozoic era. The needles of these conifers would have been a nutritious source of food for herbivores, providing essential nutrients like vitamins, minerals, and fiber.

2. Cycads: Cycads are a group of ancient seed plants that were diverse and widespread during the Mesozoic era. They had large, frond-like leaves and produced nutritious seeds. The leaves and seeds of cycads would have been excellent food sources for herbivorous dinosaurs.

3. Ginkgoes: Ginkgoes are another type of ancient gymnosperm that existed during the Mesozoic era. They had distinctive fan-shaped leaves and produced nutritious seeds. The leaves and seeds of ginkgoes could have been a valuable part of the diet for herbivorous dinosaurs.

4. Ferns: Ferns were also abundant and diverse during the Mesozoic era. They had various types of fronds and could have provided herbivores with essential nutrients like vitamins and minerals.

5. Equisetopsida (Horsetails): Equisetopsida is a group of mostly herbaceous plants that includes the modern-day horsetails. These plants were common during the Mesozoic era, and their silica-rich stems could have been a valuable source of minerals for herbivorous dinosaurs.

It's important to note that our understanding of the specific diet of Mesozoic herbivores is based on fossil evidence and research, but there may have been other plant species that were also significant food sources for these ancient animals.