Mean atmospheric O2
Geologic Time Scale
Close-up of a three-foot-long tetrapod of the genus Ichthyostega 365 million years ago in what is today the Canadian Arctic. Flanking the Ichthyostega are Rhacophyton ceratangium, a species of unclassified ancient shrub that are thought to be one of the earliest ferns. The reddish fruit-like nodules attached to the fronds on the right are sporangia, enclosures in which spores are formed. The large tree-like trunk on the far left is the base of a young Archaeopteris.
In the foreground are prehistoric arthropods--a millipede on the left and on the right "roachoids" on trunk of a decaying Lycopsid. Arthropods had been walking the Earth for 40 million years before vertebrates like Ichthyostega began venturing ashore.
Ichthyostega was one of the earliest tetrapods, a descendent of lobe-finned fishes and ancestor of amphibians. Ichthyostega had lungs and seven-toed limbs that allowed it to move about the shallow waters and shores of swamps and floodplains. It was among the first terrestrial vertebrates.
Copyright © Walter B. Myers. All rights reserved.
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