Fossil Plants 499 



clearly that the origin of most of the groups is unknown. It also 

 shows that most of the great groups have had a very long his- 

 tory on the earth. Furthermore, there is evidence of progressive 

 changes in structure as we follow the plants in any one group from 

 the earliest records to the present time. One of the large plant 

 groups of the Carboniferous, the cordaites, became extinct. 

 This also has been the fate of many smaller groups not shown on 

 the diagram. These groups disappeared because their structures 

 were not suited to the changed environments of later times. 



The dominant animal groups of the several periods are in- 

 dicated and also the time roughly estimated to have been neces- 

 sary for the deposition and consolidation of the rocks belonging 

 to each period. 



Later Paleozoic forests. During the latter half of the Paleo- 

 zoic there were five great groups of plants that dominated the 

 vegetation. The lycopods were represented by large tree forms 

 with stems that showed secondary thickening, with scale, or 

 lance-shaped leaves, and spores produced on large sporophylls 

 arranged in cones. The ancestral forms of the equisetums are 

 the calamites (Calamariales), with tall, straight, hollow-jointed 

 stems with whorls of branches bearing slender simple or forked 

 leaves, and cones. While abundant in the coal measures, they 

 contributed little material to the coal itself. The calamites seem" 

 to have attained in some instances a height of 90 feet, but most 

 of the forms were smaller and with the ferns formed a conspic- 

 uous part of the vegetation of open places in the lycopod forests. 



The seed-ferns {Pteridos pernio phyta or Cycadofilicales) include 

 several families of plants with fern-like leaves and stems but 

 which produced a simple type of seed. Many of the leaf im- 

 prints of the coal measures that were formerly classed as ferns 

 belong to this group. From the early Paleozoic to Permian 

 there is a gradual increase in the complexity of the stem structure, 

 in the direction of the cycads. This group became extinct about 

 the close of the Paleozoic. 



