THE MAIN KINDS OF FOSSILS 



31 



not weathered too long. Our Devonian rocks have yielded no ferns as yet and fern collecting 

 in the Mississippian rocks of Ohio is poor. 



Phylum Arthrophyta 



The horsetails or scouring rushes have a hollow stem, separated into segments, and 

 they look rather like bamboo. Note that the bamboos are not Arthrophyta; they are related to 

 the grasses. The horsetails reproduce by spores. In the late Paleozoic the horsetails (fig. 28) 

 were at their peak and some of them reached the size of trees , with 

 trunks up to a foot in diameter. The present-day horsetails are small 

 and inconspicuous relatives of these Paleozoic giants. The stems, cones, 

 and leaves, the latter borne in rings around the "joints" of the stem, are 

 found as fossils. The best collecting ground for fossil horsetails in Ohio 

 is in Pennsylvanian rocks, as for the ferns. 



Phylum Lepidophyta 



The club-mosses of our present-day woods are not well known 

 plants for they are small and inconspicuous. They are trailing, green 

 plants, with here and there a short stalk with a terminal cone 6 or 8 

 inches long; the terminal cone gives the stalks the appearance of a club 

 (hence the name, club -mosses). Like the horsetails, they are the de- 

 scendants of much larger forms of the same group, the scale trees, that 

 were abundant in the late Paleozoic forests. 



Fig. 28 Catamites 



In the scale trees, the leaves were long and strap-like, attached directly to the stem. 

 When the leaves fell off they left a seal-like scar on the trunk. The pattern of leaf -scars helps 

 to identify fossil scale trees; their remains are not uncommon in the Pennsylvanian rocks of 

 Ohio. Fossil Lepidophyta are known from the Upper Devonian to the present. 



Phylum Pteridospermophyta 



These are the seed ferns, distinguished from the true ferns by the fact that the spores 

 are retained and dispersed within special nutritive and protective structures known as seeds, 

 instead of being liberated without this nutritional ad- 

 vantage as in free-sporing plants. The phylum flour- 

 ished from the late Devonian to the late Paleozoic, 

 when they died out, but not before they had given rise 

 to other kinds of seed-bearing plants ancestral to those 

 now living. 



Seed ferns are found in the Pennsylvanian rocks 

 of Ohio; their leaves cannot be readily distinguished 

 from some of the true ferns unless they can be associ- 

 ated with their reproductive organs (fig. 29). This has happened now and then; when it does the 

 plant may be identified as a seed fern. 



Phylum Cycadophyta 



Living examples of the Cycadophyta or cycads are the sago palms which have long feather- 

 like leaves springing from a short, squat trunk. They bear seeds which are carried in many 

 kinds of cones. Extinct cycadophytes are abundant in Triassic and Jurassic rocks and may 

 represent one persistent line of descent from the seed ferns. No fossil cycads are recorded 

 for Ohio rocks. 



