Bennettitales

Bennettitales were an extinct order of seed plants that once dominated much of the Earth.

These plants first appeared during the Permian period. They became very common during the Mesozoic era. Most of them eventually went extinct toward the end of the Cretaceous period. Scientists classify them as gymnosperms, which are plants that produce seeds in cones. However, their exact place in the tree of life is still a subject of intense debate. They are famous for looking very similar to modern cycads, but they possess unique features that set them apart.

These plants are divided into two main families with very different growth habits.

The first family is Cycadeoidaceae. These plants had stout, thick trunks that looked much like modern cycads. They produced bisporangiate cones, meaning a single cone contained both pollen and ovules. The second family is Williamsoniaceae. These plants had slender, woody trunks that branched out in a divaricate habit. This means their branches spread out at wide angles, similar to the modern Banksia plant. Some scientists believe Williamsoniaceae might be a paraphyletic group. This means it may include all Bennettitales that do not belong to the Cycadeoidaceae family.

Understanding how Bennettitales functioned requires looking closely at their reproductive structures.

Their reproductive organs are often called cones, though some researchers refer to them as flowers. In many species, the pollen is held in paired sacs called synangia. These sacs are located on the adaxial, or inner, edge of leaf-like structures called microsporophylls. This is a major difference from cycads, which bear pollen on the outer surface. In female cones, the ovules sit at the tips of sporophylls. Once fertilization occurs, the microsporophylls wither away. The ovules then transform into seeds that contain two embryonic leaves, known as dicotyledonous seeds.

Even the leaves of Bennettitales provide clues about their identity.

Many species had pinnate leaves, which look like feathers, with small segments attached to a central shaft. Other forms, like Nilssoniopteris, had narrow, smooth blades. Some even had sawtooth-shaped edges. While the leaves look like cycad leaves, scientists use the cuticle to tell them apart. They look at the stomata, which are tiny pores used for breathing. Bennettitales have syndetocheilic stomata. This means the guard cells and the surrounding subsidiary cells develop from the same mother cells. This is different from the haplocheilic stomata found in cycads and conifers.

The history of discovering these plants spans many years. In 1828, the English geologist William Buckland named the family Cycadeoidea after finding fossil trunks in England. Later, in 1870, William Carruthers and William Crawford Williamson described the first reproductive organs. Carruthers was the first to realize these were not cycads. He named a tribe Bennettiteae to honor the botanist John Joseph Bennett. Finally, in 1892, the German botanist Adolf Engler established the official order name, Bennettitales. These researchers helped move the study of these plants from simple observation to complex classification.

The relationship between Bennettitales and other plants has changed as technology improved. In 1907, researchers proposed the Anthophyte hypothesis. This suggested that flowering plants, or angiosperms, evolved from Bennettitales. However, modern molecular data has contradicted this idea. Current evidence suggests that angiosperms are actually the sister group to all living gymnosperms. Some scientists once grouped Bennettitales with gnetophytes in a group called Anthophyta. Now, molecular evidence shows gnetophytes are more closely related to conifers. A 2017 study suggested Bennettitales might be more closely related to the Ginkgo and cycad clade.

Finally, Bennettitales played a vital role in ancient ecosystems through interactions with animals. Some species may have been pollinated by insects. For example, pollen from the Early Cretaceous has been found on the proboscis of an extinct fly. This suggests that flies from the family Zhangsolvidae helped move pollen. Some reproductive structures, like those in Williamsoniella, had long central receptacles. These might have produced nectar for insects with long mouthparts. It is even possible that the fleshy scales on some cones attracted animals to help spread their seeds. This shows how complex these ancient plant-animal relationships truly were.