by Brongniart. The flowers, or some of them, were originally
described by Nathorst as Williamsonia angustifolia. This form of
stem, of a habit entirely different from that of recent Cycads and
extinct Bennettites, points to the existence in the Mesozoic era of
another type of Gymnosperm allied to the Bennettitales
of the Jurassic and Cretaceous periods by its flowers,
but possessing a distinctive character in its vegetative
organs. There is no doubt that the Cycadophyta, using
the term suggested by Nathorst in 1902, was represented
in the Mesozoic period by several distinct families
or classes which played a dominant part in the floras
of the world before the advent of the Angiosperms. In
addition to the bisporangiate reproductive shoots of
Bennettites, distinguished by many important features
from the flowers of recent Cycads, a few specimens of
flowers have been discovered exhibiting a much closer
resemblance to those of existing Cycads, e.g. Androstrobus
Balduini from Bathonian rocks of France; Zamites
familiaris, described many years ago by Corda, from
Lower Cretaceous rocks of Bohemia, and Androstrobus
Nathorsti, from Wealden beds in Sussex. The majority
of the species were, however, characterized by flowers of
a different type known as Bennettites and Williamsonia.
Fig. 17.—Fronds of Williamsonia pecten.
The living Maidenhair-tree (Ginkgo biloba) (see Gymnosperms) remains, like Matonia and Dipteris, among the ferns, as an isolated relic in the midst of recent vegetation. In Rhaetic, Jurassic and Wealden floras, the Ginkgoales were exceedingly abundant (Map B, G1–G17); in addition to leaves agreeing almost exactly with those of the recent Ginkgoales. species (fig. 18), there are others separated as a distinct genus, Baiera (fig. 18, G), characterized by the greater number and narrower form of the segments, which may be best compared with such leaves as those of the recent fern Actiniopteris and of certain species of Schizaea. Male flowers, like those of Ginkgo biloba, but usually characterized by a rather larger number of oval pollen-sacs on the stamens, have been found in England, Germany, Siberia and elsewhere in association with Ginkgo and Baiera foliage. The occasional occurrence of three or even four pollen-sacs on the stamens of the recent species affords a still closer agreement between the extinct and living types. Seeds like those of Ginkgo biloba have also been recorded as fossils in Jurassic rocks, and it is possible that the type of flower known as Beania, from the Inferior Oolite rocks of Yorkshire, may have been borne by Ginkgo or Baiera. The regions from which satisfactory examples of Ginkgoales (Baiera or Ginkgo) have been recorded are shown in Map B (G1–G17). Both Tertiary and Mesozoic localities are indicated in the map.
|
| Fig. 18.—Leaves of Ginkgoales. |
| A, Ginkgodium, Japan (Jurassic). |
| B, C, D, E, F, H, Ginkgo leaves.—B, from Franz Josef Land (Jurassic); C, Greenland (Lower Cretaceous); D, Siberia (Jurassic); E, Germany (Wealden); F, England (Jurassic); H, China (Rhaetic). |
| G, Baiera leaf, Inferior Oolite, England. |
| (A, after Yokoyama; B, after Nathorst; C, D, after Heer; E. after Schenk; H, after Krasser. All the figures 12 nat. size.) |
An adequate account of fossil Mesozoic Conifers is impossible within the limits of this article. Coniferous twigs are very common in Mesozoic strata, but in most cases we are compelled to refer them to provisional genera, as the evidence of vegetative shoots alone is not sufficient to enable us to determine their position within the Coniferae. There are, however, Coniferales. several forms which it is reasonable to include in the Araucarieae; that this family was to the fore in the vegetation of the Jurassic period is unquestionable. We have not merely the striking resemblance of vegetative shoots to those of recent species of Araucaria and Agathis, e.g. species of Nageiopsis, abundantly represented in the Upper Jurassic beds of the Potomac area in North America, species of Pagiophyllum and other genera of Jurassic and Wealden age, but an abundance of fossil wood (Araucarioxylon) from Jurassic and Cretaceous strata in Europe, North America, Madagascar and elsewhere agreeing with that of recent Araucarieae, in addition to several well-preserved female flowers. C. A. Hollick and E. C. Jeffrey have recently shown that some Lower Cretaceous specimens of the well-known genus Brachyphyllum obtained from Staten Island, N.Y., possess wood of the Araucarian type. This genus has long been known as a common and widely spread Jurassic and Cretaceous conifer, but owing to the absence of petrified specimens and of well-preserved cones, it has been impossible to refer it to a definite position in the Coniferales. It is now clear that some at least of the species of Brachyphyllum must be referred to the Araucarieae. In a recently published paper Seward and Ford have given a general account of the Araucarieae, recent and extinct, to which reference may be made for further details as to the geological history of this ancient section of the Coniferales. Some of the fossils referred to the genus Kaidocarpon, and originally described as monocotyledonous inflorescences, are undoubted Araucarian cones; other cones of the same type have been placed in the genus Cycadeostrobus and referred to Cycads. Araucarites Hudlestoni, described by Mr Carruthers from the Coralline Oolite rocks of Malton in Yorkshire; Araucarites sphaerocarpa from the Inferior Oolite of Somerset; also another cone found in the Northampton Sands, which is probably specifically identical with A. Hudlestoni, and named by Carruthers Kaidocarpon ooliticum, afford good illustrations of British Araucarian flowers. A flower of a rather different type, Pseudaraucaria major, exhibiting in the occurrence of two seeds in each scale an approach to the cones of Abietineae, has been described by Professor Fliche from Lower Cretaceous rocks of Argonne. The well-known Whitby jet of Upper Liassic age appears to have been formed to a large extent from Araucarian wood. Among the more abundant Conifers of Jurassic age may be mentioned such genera as Thuytes and Cupressites, which agree in their vegetative characters with members of the Cupressineae, but our knowledge of the cones is far from satisfactory. Many of the small female flowers borne on shoots with foliage of the Cupressus type consist of spirally disposed and not verticillate scales, e.g. Thuytes expansus, a common Jurassic species.
Fossil wood, described under the name Cupressinoxylon, has been recorded from several Mesozoic horizons in Europe and elsewhere, but this term has been employed in a wide sense as a designation for a type of structure met with not only in the Cupressineae, but in members of other families of Coniferae. The Abietineae do not appear to have played a prominent part before the Wealden period; various older species, e.g. Rhaetic specimens from Scania, are recorded, but it is not until we come to the Upper Jurassic and Wealden periods that this modern family was abundantly represented. Fossil wood of the Pinites type (Pityoxylon) has been described from England, France, Germany, Sweden, Spitsbergen, North America and elsewhere; some of the best British examples have been obtained from the so-called Pine-raft, the remains of water-logged and petrified wood of Lower Greensand age, seen at low water near Brook Point in the Isle of Wight. Well-preserved Abietineous female flowers have been obtained from the Wealden rocks of England and Belgium, e.g. Pinites Dunkeri, P. Solmsi, &c.; specimens of seeds and vegetative shoots are recorded also from Spitsbergen and other regions. Hollick and Jeffrey have recently added to our knowledge of the anatomy of Cretaceous
