Important Fossil Genera : a short description

• Study of Paleobotany gained momentum with the discovery of certain fossils that were well  preserved and had resemblance to living forms and it was easy to describe and place  them in a correct systematic position.  
• Such fossils were also helpful in explaining a few missing links in tracing the phylogeny of plant groups
• Understanding  some such fossils will help us appreciate the importance of Paleobotany and its role in study of  plant evolution .

LEPIDODENDRON

Systematic Position

Sub-Division: Lycopsida

Order           : Lepidodendrales

Family         : Lepidodendraceae

• This is a fossil appearing similar to Class Lycopsida of Pteridophytes which was a heterosporous form
• It   belongs to an age that ranged from Upper Devonian to Upper Carboniferous period of the Paleozoic era
• This genus was particularly dominant in the swamp forests of the Carboniferous period
• It comes under the Order Lepidodendrales which includes a number of  other Form genera
• This order is important because the fossils coming under this group are very well preserved 
• Most of the forms are preserved as impressions and casts
• The first fossil found under Lepidodendrales was the stem portion named as form genus Lepidodendron
• Later on all the other parts  such as :
• Leaves called Form genus Lepidophyllum,
• Underground axes called Form genus Stigmaria,
• Strobili under the name form genus Lepidostrobus and
• Megasporangium with seed named as form genus Lepidocarpon were found
• Thus the whole plant was reconstructed as the natural genus Lepidodendron
• As per the rules of nomenclature in Palaeobotany, the reconstructed plant was  called  Lepidodendron  as it had to be named after the first found fossil form
• The reconstructed plant natural genus Lepidodendron was a large tree more than 30 m in height with an unbranched main trunk and a crown of leaves at the top that showed repeated dichotomous branching
• The tips of leaves bore the strobili called form genus Lepidostrobus and leaves were called form genus Lepidophyllum
• The leaves were arranged spirally and appeared linear or acicular, simple and showed the presence of ligules

Form-genus  Lepidodendron

• The main trunk  was the stem which was the earliest recorded fossil in this group
• The surface of the trunk showed the presence of leaf scars  with leaf cushions that followed a set pattern which was helpful in identifying the species
• The middle of the leaf cushion had  the leaf scar and the region of the bundle or the position of the stalk was also visible
• The bundle scar was  flanked by the parichnos strands which formed part of the parichnos  system
• Parichnos system is a special system of a network of parenchyma tissue with air spaces which is a special feature  of Arborescent  (Tree-forms) Lycopods
• Sometimes two additional scars have been observed which  are formed due to  shrunkem parenchyma tissue
• The  underground support system for this group of Lepidodendrids was the Stigmarian system  which was seen as a bifurcation of the trunk once and another time to form four branches called the Stigmarian bases
• These were further branched and these appendages were called Stigmarian rootlets
• A transverse section of the fossilized  stem revealed several layers of tissue
• The outermost layer of the stem showed a well developed Periderm that was essentially produced from the underlying layer of Phellogen
• Phellogen had formed the Cortex on the inner side that had three regions
• Outer – middle and inner cortex
• The leaf traces pass through the middle cortex and on entering the leaf base split into two parichnos strands
• Following the inner cortex a layer of phloem was seen and inner to this a  region of secondary Xylem was  seen which showed poor development in the upper part of the  trunk
• Secondary Xylem is composed of tracheids with scalariform thickenings
• The central part of the stem shows a prominent pith and medullary rays
• The stele is protostelic  and pith was absent in the smaller branches of the stem

Form –  genus Lepidocarpon

These are the seeds belonging to the natural genus Lepidodendron and have been collected in the form of compressions.

The megasporangium shows a covering of the sporophyll on both sides  leaving a small opening  in the centre between the halves of the integument.

This opening could be considered as equivalent to a crude micropylar opening.

The prominent feature of Lepidocarpon  is a functional megaspore that developed and matured into gametophyte even before being shed from the strobilus.

It showed the three aborted spores out of the four in the spore tetrad, megaspore and megagametophyte which is similar to that of a Gymnospermous ovule.

The megasporophyll covering the megasporangium and the sporangium inside it can  be considered homologous to  integument and megasporangium respectively. t

The megaspore wall showed sporopolleninin in the form of strands

But Embryos were not observed in Lepidocarpon seeds

Lepidocarpon is called as a seed by many Paleobotanists while some feel it is a seed-like structure as it does not possess a true integument

It is an example for  Convergent evolution as it could be fore-runner of the modern –day ovules  possessing a single megaspore that is functional

Role in Phylogeny of Pteridophytes and Gymnosperms

It is a giant Lycopod or arborescent form that is the ancestor of the extant Lycopsids and stands a  testimony for a high level evolution occurring  due to sudden mutations on a large scale

Order Lepidodendrales with its seed form genus Lepidocarpon is a pioneer for heterospory and seed habit that arose at a later stage in the evolution of plants