Biology : The Time of Ancient Life : Part Ten
Once-dominant elements in the Paleozoic forests, sphenophytes (also known as the equisetophytes) are today relegated to minor roles as streamside herbaceous plants. The group is defined by their jointed stems, with many extremely small leaves occurring at a node, production of spores in cones borne at the tips of stems, and spores bearing elaters (devices to aid in spore dispersal). The fossil members of this group are often encountered in coal deposits of Carboniferous age in North America and Europe. The first sphenophytes show up during the late Devonian, most likely evolving from some group of the trimerophytes.
Ferns reproduce by spores from which the free-living bisexual gametophyte generation develops. There are 12,000 species of ferns today, although the fossil history of ferns shows them to have been a dominant plant group during the Paleozoic Era. The first ferns also appear by the end of the Devonian. Some anatomical similarities suggest that ferns and sphenophytes may have shared a common ancestor within the trimerophytes.
The progymnosperms are an extinct group of nonseed plants that have fern-like leaves and reproductive structures attached to gymnosperm-like stems with large amounts of wood.
The most famous of the progymnosperms, Archaeopteris, was first assigned to the ferns. During the 1960s paleobotanist Charles B. Beck convincingly demonstrated that this common late Devonian leaf fossil was actually produced by the same plant that also produced a common Devonian petrified wood known as Callixylon.
The Seed Ferns
The seed ferns, fancy name pteridosperms (literally fern-seed), appeared during the late Devonian. One of the more important groups of seed ferns was the lyginopterids, small vine-like plants that probably grew on other plants. This group produced seeds in a cupule-like structure.
Perhaps the earliest animals ashore were the invertebrates, most likely groups of arthropods. The oldest known terrestrial arthropod is a trigonotarbid from the Silurian. Trigonotarbids superficially looked like spiders. The major groups of terrestrial arthropods were present during the Devonian: chelicerates including mites, spiders, and arachnids; and made the transition to land life, and uniramians including flightless insects, centipedes and millipedes.
The earliest terrestrial vertebrate fossils are from the late Devonian. These "stem tetrapods" as they are known, were little more than fish out of water. The late Devonian lungfish had two groups, the Dipnoi (which includes the living African, Australian, and South American lungfishes) and the Crossopterygii, whose sole living member is the coelacanth Latimeria. Comparison of the skull, forelimb, and tooth structure of Devonian crossopterygians such as Eusthenopteron with similar areas in a more specialized tetrapod such as Ichthyostega supports descent of amphibians from Devonian crossopterygians. However, some molecular studies point to descent of the amphibians from the lungfish, the Dipnoi.
The Devonian Extinction
Near the end of the Devonian another mass extinction occurred. This one was more severe on marine creatures than on the newly established terrestrial forms. The corals were quite seriously decimated, and extensive reef building did not return until the Triassic with the evolution of a new group of reef-building corals, the scleractinians. Brachiopods, trilobites and primitive fish groups either were diminished or completely snuffed out by this extinction event.
Global cooling tied to glaciation in the southern supercontinent of Gondwana has been proposed as the cause of the Devonian extinction, as it was also suspected of causing the terminal Ordovician extinction. Rocks in parts of Gondwana suggest a glacial event. The forms of marine life most affected by the extinction were the warm water to tropical ones. Another hypothesis for this extinction is that an asteroid impact caused a global cooling. There are several impact sites known to be of the right potential age to have been involved in this extinction. Neither hypothesis, glaciation or impact, is unequivocally supported by the available data.
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