Glossopterids †

Seed ferns with tongue-shaped leaves

The Glossopteridales were woody seed plants, that existed and dominated in the Permian Period. The name "Glossopteris" means tongue-fern, referring to the tongue-shaped leaves they possessed. They were probably trees or large shrubs with robust conifer-like wood, similar to the Araucariaceae. These trees were restricted to the middle- and high-latitude parts of Gondwana, and an important contributor to the Permian coal deposits.

It is assumed that some of these plants were deciduous, and the abundance of leaves in the fossil strata helped Alfred Wegener, a German scientist during the early 1900s, discover the existence of an ancient super-continent known as Pangaea. Since these tropical leaves are found in Antarctica, and similar leaves are found on South America, Africa, and Australia, Wegener proposed that the continents must have had different positions from the modern arrangement. This along with other geophysical evidence formed the basis of his continental drift theory.

Ecology & Form

Tree or shrubs, and assumed to posses a conical, conifer-like form
These plants probably grew in saturated or wetland soils (McLoughlin 1993; McLoughlin & McNamara 2001)

Stem

Branches possessed long and short shoots similar to Ginkgo
The woody trunk possessed pycnoxylic wood with a pattern similar to Araucaria (e.g. form genus Dadoxylon)
- Dadoxylon wood, in connection with Glossopteris foliage, is indistinguishable from that produced by Cordaites and the modern conifer genus Araucaria
- Mesozoic examples of this wood type placed in the genus Araucarioxylon
- Pycnoxylic wood anatomy in Upper Paleozoic and Mesozoic seed plants is very conserved

Leaves

Tongue-shaped ("glosso-") and ranging in length from 2 cm to 30+ cm
Leaves possess a smooth or entire margin
Anatomically, leaves had a well-differentiated palisade and spongy mesophyll layers
The leaves are angiosperm-like with the following features:
- Netted venation (e.g. Glossopteris)
- Distinct mid-rib
- Reticulate venation
Some species were deciduous, others were evergreen
Leaves were born in spirals or whorls on short shoots

Roots

- Wood had wedge-shaped appearance (form genusVertebraria)

Reproductive structures

The reproductive structures of this group are poorly understood
Seeds were borne on one side of variably branched or fused structures (McLoughlin 1990, 1990, 1995, 2012; Adendorff et al. 2002; Prevec et al. 2008)
Microsporangia were borne in clusters at the tips of slender filaments (Lindström et al. 1997)
Some taxa were dioecious, but it is unclear whether they were also monoecious (DeWitt et al. 2018)
Both organs were partially fused (adnate) to the leaves, or possibly positioned in the axils of leaves
The homologies of the flattened seed-bearing structures have remained particularly controversial with some arguing that the fertile organs represent megasporophylls (fertile leaves) whereas others have interpreted the structures as flattened, seed-bearing, axillary axes (cladodes)

Geologic Age

Permian
Abundant and dominant in Gondwana (southern hemisphere)
Glossopteris is sometimes cited as surviving into the Triassic based on the distribution of taeniate, bisaccate pollen (Hochuli et al. 2016); no macrofossils
This type of pollen is known from various seed plants, and Triassic examples probably belonged to voltzialean conifers (Balme 1995)

Classification

└Lignophytes

└Glossopteridales †

Above: Leaves and roots of glossopterids

Above: Reconstruction of a glossopterid

Below: Transverse section of a glossopterid leaf from the upper Permian of Australia, showing palisade and spongy mesophyll layers.

Above: Distribution of modern day Glossopteris fossils (dark green) on the continents in their positions as parts of Pangaea (1=South America; 2=Africa; 3=Madagascar; 4=Indian subcontinent; 5=Antarctica; 6=Australia)

Additional Resources

Natural History Museum: Paul Kenrick talks about Glossopteris discovery during the Terra Nova expedition to the South Pole in the early 1900s

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