Biology : The Time of Ancient Life : Part Twelve
Life on Land:
The Lycophytes became even more significant elements of the world's flora during the Carboniferous. These non-seed plants evolved into trees in the fossil genera Lepidodendron and Sigillaria, reaching heights up to 40 meters and 20-30 meters respectively. Lepidodendron trunks can be over 1 meter in diameter.
We know much about the anatomy of these coal-age lycopods because of an odd type of preservation known as a coal ball. Coal balls can be peeled and the plants that are anatomically preserved within them laboriously studied to learn the details of cell structure of these coal age plants. Additionally, we have some exceptional petrifactions and compressions that reveal different layers of the plants' structure. Estimates place the bulk, up to 70%, of coal material as being derived from lycophytes.
Cross section through a branch (approximately an inch in diameter) of a large lepidodendrid tree. In the very center is a pith, surrounded by primary xylem and a small fringe of secondary xylem [wood, MJF]. Then there is black gunk and an open white area. Phloem and innermost cortical tissues are typically not well preserved, and this black gunk and white areas probably represent their positions in the branch. The outermost part of the stem is gone.
Sigillaria was another arborescent lycopod, and is also common in coals-age deposits. In contrast to the spirally borne leaves of Lepidodendron, Sigillaria had leaved arranged in vertical rows along the stem.
Like the lycophytes, the sphenophytes reached their zenith during the Carboniferous and have declined to but a single genus, Equisetum, surviving today. The largest of the sphenophytes were trees placed in the genus Calamites. Numerous leaf fossils, such as the specimen of Annularia shown below, have been associated with these trees, as have reproductive cones. Smaller shrubby plants were in the genus Sphenophyllum.
Sphenophyllum leaves of a late Cambrian sphenopsid from the Coal Deposits of France.
Annularia stellata, a small stem of a Carboniferous calamite from the Coal Deposits of France.
Ferns were well represented in the coal swamps. Numerous fern leaf fossils have been found, often the mode of preservation makes for striking fossils. Ferns ranged from small, shrubby forms similar to those we see in many places today, to tree-ferns, such as the Paleozoic genus Psaronius.
Pecopteris sp. is the foliage of a tree fern. Pecopteris grew on Psaronius, which was one of the most common fossil tree ferns. Psaronius reached height of about ten meters (32 feet) and resembled a modern palm tree. Although common throughout much of the late Paleozoic, this plant was most abundant in the Late Pennsylvanian and Permian. Their increasing abundance reflects changing climates at the close of the Paleozoic and may be related to an increase in drier habitats.
The seed ferns underwent increases in diversity during the Carboniferous. Often the isolated leaf fossils are difficult to distinguish between ferns and seed ferns. Often seed ferns had the leaf fossil known as Neuropteris associated with the seeds.
Other seed fern groups that characterized the Carboniferous include the medullosans and the cordaitaleans. During the late Carboniferous the voltzialean seed ferns appear. This latter group is considered by paleobotanists to be closest to the gymnosperms. Cycads, a now-minor group of gymnosperms but major components of the world's floras during the Mesozoic, appeared during the Carboniferous.
Specimens of Cordaites, the name-bearer for the cordaitaleans, were large trees with strap-shaped, leathery leaves that often had parallel leaf veins. The cordaites had their reproductive structures and seeds arranged in cones. The cordaites had a number of growth forms and several different types of woody tissues have been observed in cordaite wood. The cordaites have been interpreted as growing in an environment like the modern bald cypress swamp.
The major animals on land during the Carboniferous were the amphibians (and their descendants such as the stem reptiles) and insects. Dragonfly fossils have been found with wingspans up to 75 cm.
With climate changes occurring during the late Carboniferous, the plants changed, as did the terrestrial animals. Gondwanan glaciation has been blamed for this change. One response that tetrapods made was to develop the amniotic egg. With the egg and resulting freedom from water for reproduction (to which amphibians to this day must return to lay their eggs) reptiles (sometimes known as stem-reptiles, could exploit drier environments further from water.
Hylonomus was one of the earliest reptiles. This quadrupedal (four-legged) stem reptile ate insects (which were prolific in the coal swamps). From this group of early reptiles evolved the pelycosaurs, the great reptiles of the Carboniferous-Permian times, and possibly the bipedal thecodonts of the Permian-Triassic. Pelycosaurs included both herbivorous and carnivorous species. An elaborate fin or sail on their back characterized the group.
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