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Ctenis biloba, a cycadale |
The leaves of Ctenis were also similar to modern cycads. The broad rachis had compound leaflets attached along it. Some fossil leaves measure six feet in length. The parallel venation in Ctenis had occasional cross connections. Ctenis was distributed all across the Jurassic landmass and so far enough fossils have been discovered to be attributed to approximately 50 species. Pseudoctenis also had a worldwide distribution during the Jurassic and differed from Ctenis in one detail . . . the parallel venation of its leaflets had far fewer cross connections. Harris (1964) has observed that if leaves of the more generalized modern genera such as Dioon, Macrozamia, Lepidozamia, and Encephalartos were to be found as fossils, they would be classified within the Pseudoctenis model. Yorkshire, England, is a fertile area for cycad fossils from the Jurassic. In addition to foliage leaves, many fossils of vegetative structures have been found, enough to hint at the route to both the loose megasporophyll design of Cycas and the tightly closed cone structure of the Zamiaceae. The cones of some of these cycads hung down from the leaf apex like pendulous fruit. Another species of that same time, Pseudoctenis lanei, was quite similar in form to the modern Encephalartos, having a massive and compact male cone (named Androstrobus manis) and regularly pinnate leaves with parallel venation in the leaflets. If further discoveries bear out these similarities, the cycad may be classified within the genus Encephalartos. This would provide evidence for at least one of the early home ranges of Encephalartos, and indicate the antiquity of its origins. Another fossil cone, Androstrobus zamioides, from the Middle Jurassic sediments of North Yorkshire, may be a Stangeria relative. Another Stangeria- like fossil from Argentina suggests that this monotypic and isolated African cycad originated in South America and was once global in range. The leaves of the newly discovered genus Chigua from Columbia are strikingly similar to the forked venation, serrated leaf edges, and strong midrib of Stangeria. It would be a find similar to that of locating a living Spenodon (the Tuatara of New Zealand) if this rare and relict genus should turn out to be a surviving relative of Stangeria still present in the New World. |
Closeup of fossilized leaflets of Ctenis biloba. Note the broad connection at the base of the pinnae to the rachis and how the veins divide creating a bipinnate leaflet. |
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As the Jurassic period came to a close, the continents continued shifting. The Tethys seaway opened a corridor from the present Caribbean through the Straits of Gibraltar and continuing south between Arabia and the Near East to the South Polar sea. Additionally, Africa had begun to break away from South America in the west and Antarctica and India in the east. The worldwide distribution of the Cycadales was being sundered. The unique characteristics of the major genera of the modern cycads would now appear as they began to specialize within these much more restricted ranges. |
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Pseudoctenis lanei-type cycadale
reconstruction, |
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