Palaeobotany

Wielandiella angustifolia & Williamsoniella coronata

During much of the Mesozoic Era, gymnosperms were a major component of many Mesozoic floras, with the Cycadopsida (true cycads) and the Cycadeoideopsida the Bennettitales, being a conspicuous and significant component of low level and understory vegetation.

Once globally diverse, only the true cycads are extant today, their current distribution confined to a limited band of tropical and subtropical climates.

Yet some cycad families, the Stangeriacae and Zamiaceae have a good fossil record recognised from early in the Mesozoic era, with several extant genera, Dioon, Encephalartos and Stangeria, closely resembling their prehistoric counterparts.

The Bennettitales, however, are now completely extinct, and unfortunately, due to the often poor and fragmentary preservation of their remains, have presented a problem to confidently reconstruct. It is generally agreed, however, that they should be divided into three families :- the Cycadeoidaceae with stout trunks and Cycadolepsis or Zamites type foliage; The Williamsoniaceae with Ptilophyllum or Otozamites type foliage; and the Wielandiellacea, with leaves of an Anomozamites or Nilssonipoteris type.

Current research indicates that Williamsonians were generally small to medium, dense, busy plants exhibiting dichasial, multi-branching growth morphology with some species possessing leaves in the forks of branches and others limited to their terminal apex.

Wielandiella angustifolia presented here is a Williamsonian bennettitale with dichasial branching morphology, possessing a small cone at the branching fork and a spiral of leaves at the terminus of each branch. The leaves of Wielandiella are assigned to the Anomozamites leaf type. Examples of Wielandiella have been found in Russia, Turkmenistan, Ukraine, Mongolia and Sweden.

Williamsoniella coronata is another Bennettitale with dichasial branching. The leaves of Williamsoniella, are of the simple Nilssoniopteris type and sprout from the trunk and terminal ends of branches, with small, spherical cone-like flowers emerging from below each leaf axil.

Ink and graphite drawings

For further reading please see:-

Divaricate growth habit in Williamsoniaceae (Bennettitales): Unravelling the ecology of a key mesozoic plant group. Palaeobiodiversity and palaeoenvironments. June 2014 Christian Pott & Stephen McLoughlin.

A revision of Wielandiella angustifolia, a shrub-sized bennettitite from the Rhaetian-Hettangian of Scania, Sweden, and Jameson Land, Greenland. Int. J. Plant Sci. 2014.

Eight Common Conifer Foliage Morphologies

Carboniferous – Recent 300-0 Ma.

Here I present a series of illustrations based on information presented by Stewart & Rothwell in their 1993 publication, Palaeobotany and the Evolution of Plants, contrasting some common Mesozoic conifer leaf types alongside their extant morphologically similar counterparts. The fossil species are the lighter of the paired illustrations.

The genetic affinities between living and extinct conifers are hard to understand, with many extant species bearing superficial similarities to extinct genera. Recent taxonomic studies have reclassified the position of the conifer families, recognising the Taxaceae as a distinct monophyletic group within the Pinales, rather than a separate order.

For the purpose of illustrating prehistoric plants however, where we are concerned with appearances on the macroscopic rather than the microscopic level, it is useful to be able to observe and compare similarities between extinct and extant foliage morphotypes, with many prehistoric genera possessing fronds and leaves closely resembling living plants today.

Both Brachyphyllum and Pagiophyllum are recognised as belonging within the Araucariaceae. Brachyphyllum has small helically arranged pyramidal leaves with a broad basal cushion, resembling closely the shoots of Athrotaxis cupressoides.

Pagiophyllum leaves are wider and longer than the width of their basal cushion. Leaves of this kind are observed in Araucaria bidwillii.

Both genera form an overlapping boundary with the morphological characteristics of the fossil taxon Cyparissidium, the latter having narrow spirally arranged leaves held adjacent to the leaf axis. Glyptostrobus pensilis, the Chinese swamp cypress is a living example of a conifer with leaves of this kind, as are Taxodium ascendens and the podocarp Dacrydium cupressinum.

Another notable foliage morphotype recognised from the Permian period onwards, is Elatocladus. Elatocladus leaves can be arranged helically or opposite on the leaf stem with elongated dorsally flattened leaves narrowing where they adjoin the basal cushion. Examples of extant Elatocladus type foliage are Cephalotaxus, Amentotaxus, Taxodium distichum and Metasequoia.

Unlike the other conifer families, Pityocladus and its living morphological representatives the Pinaceae are better represented in the Cenozoic than the Mesozoic, having evolved from their ancestors in the Jurassic to become the largest family of extant conifers. Pityocladus leaves belong in an order distinct from the other conifer families and bear foliage with two or more long acicular leaves radiating from a fascicle sheath. Podozamites has a leaf morphology superficially resembling that of a cycad; its large multi-veined leaves are lanceolate in shape and radiate alternately or helically, tapering where they meet the stem. Agathis is a good representative of the Podozamites morphotype. The fossil taxon Geinitzia has leaves morphologically similar to those of Pagiophyllum, being helically arranged along the leaf axis. However, Gentizia leaves are thinner needle-like structures that do not taper where they join the stem. Geinitzia leaf types are represented in the Taxodiodeae by Cryptomeria and in the Araucariaceae by Araucaria heterophylla, both species having shoots of this type.

Cupressinocladus is a non-committal term assigned to fossil conifer foliage with shoots similar in their morphological characteristics to some members of the extant Cupressaceae, possessing small scale-like or dorsally flattened leaves arranged in whorls or decussate pairs appressed or spreading from the leaf stem. Thuja and Thujopsis are examples of living conifers with shoots that agree with the Cupressinocladus morphotype.

Although we are discussing here the morphological similarities between extinct fossil foliage and living plants, it must be noted that these similarities may be purely superficial, and future fossil discoveries and microscopic research may disprove anything but a distant association, but for now being able to make these comparisons offers the artist the opportunity to reconstruct the external appearance of these dominant Mesozoic gymnosperms with a reasonable degree of certainty.

For further reading refer to

Stewart W.N & Rothwell G.W 1993 Paleobotany and the Evolution of Plants 2nd Edition.

Stippled ink drawings

Conspicuous Jurassic Filicales

Late Triassic/Early Jurassic Worldwide

During the Late Triassic and Early Jurassic periods, when the great landmass of Pangaea had only begun to fragment, the world’s flora was globally similar, and a wonderful representation of the diverse range of ferns from these periods can be found worldwide. In this illustration I have depicted three notable Late Triassic/ Early Jurassic fern species.

Cladophlebis, foreground and lower right, was a member of the Osmundeace and may well have exhibited arboreal characteristics.

Coniopteris, background, certainly grew to considerable heights, and is recognised as a member of the Dicksoniaceae, a family of notable extant Treeferns.

Marattia, middle ground, are today large tropical ferns represented by a variety of species, some, like Angiopteris growing to considerable size.

Ink & graphite drawing

Mesozoic Cycads & Cycadeoids

Early Jurassic - Late Cretaceous

Understanding the relationships between the component parts of prehistoric plants can be difficult at best. Here I have prepared three illustrations of Mesozoic cycads and cycadeoids, the bennettitales. Cycadophytes are gymnosperms (naked-seed plants) and comprised a significant part of many Mesozoic floras.

Pseudoctenis is a true cycad and was common during the Jurassic period, its leaves being found worldwide. Like many living cycads, it exhibited arboreal characteristics, its trunk growing to several metres in height and leaves growing to two metres in length. Indeed, its foliage is so similar to the extant genus Encephalartos, that it may prove to be of the same family. The cone (strobilus) of Pseudoctenis is called Androstrobus.

Superficially resembling cycads in appearance, the bennettitales were a sister taxon that flourished from the Early Jurassic to the Late Cretaceous. They can be divided into two groups, the Cycadeoidaceae with stout barrel shaped stems and the Williamsoniacea with taller trunks. The name Williamsonia refers to the seed-bearing cone of the plant, and like other bennettitales, Williamsonians differ from true cycads in several ways, most notably microscopic breathing pores (stomata) on the underside of their leaves and the presence of pollen-bearing flowers called Weltrichia.

Confusion has arisen among researchers, however, as to which leaves belong to which trunk and further problems arise from historic misidentification. Indeed, the hypothetical reconstruction of Williamsonia sewardina I present here, based on the now famous reconstruction by Sahni 1932, may prove to be inaccurate. Current research indicating that Williamsonians were generally small to medium, dense, busy plants exhibiting a dichasial, multi-branching morphology with some species possessing leaves in the forks of branches and others limited to their apex.

Unlike true cyads, bennettitalian leaves detached from the trunk at their base, resulting in a column of compressed leaf scars. Williamsonia leaves are of the pinnate Pitilophyllum type while Cycadeoidea leaves are of the Zamites foliage type. The flowering structures of the bennettitales and how they attached to the main body of the plant are still poorly understood.

Stippled ink drawings