Just as the animal kingdom lost some remarkable designs during the mass extinction events that punctuated the Palaeozoic and Mesozoic (consider the disappearance of the novel carapaces of trilobites and the aerofoils of pterosaurs), so too the plant kingdom lost some majestic groups that, had they survived until today, would no doubt have been cultivated as centrepieces in many domestic gardens. One such group is the Bennettitales.
The Bennettitales were enigmatic, seed-bearing plants (gymnosperms) characterised by complex reproductive structures, some of which are not yet fully understood. Bennettitales are historically divided into two families, the Cycadeoidaceae (or Bennettitaceae) and the Williamsoniaceae. The two families are distinguished primarily by their growth habit and the arrangement of their reproductive organs. The former have short, stocky trunks somewhat like modern cycads, whereas the latter had slender, profusely branched stems. 1, 2, 3. The former appear to have been restricted to the Jurassic-Cretaceous of western Laurasia, whereas the latter had a global distribution and greater temporal range. They were neither the smallest plants of the Mesozoic nor the largest. They were one of the important, mid-storey elements of the vegetation. If you care to view almost any artist’s reconstruction of a Jurassic landscape you will no doubt see bennettitaleans growing around the feet of (or being eaten by) a large sauropod or ornithischian dinosaur. 4, 5.
Flowers before there were flowers
Apart from their growth habit, the most remarkable character of Bennettitales is their reproductive structures, which can be most simply described as “flowers”. Although these “flowers” differed in the details of their anatomy from the true flowers of angiosperms, they clearly represented an experiment in plant architecture that preceded, but paralleled in architecture, some of the adaptations of modern flowering plants.
The so-called “flowers” of Bennettitales may be either unisexual or bisexual. The most elaborate forms have a central column bearing a tightly packed array of ovules separated by sterile scales. This central complex is surrounded by a ring of male bracts bearing valve-like pollen sacs. These may be surrounded by further ranks of sterile bracts to form a compact, cup-like, showy “flower”. 6, 7