Thallophytes are simple plants without roots, stems, or leaves. They
include the fungi, algae, and diatoms (Pl. 12). Diatoms are microscopic
fossils that are found in many of the rocks of Texas, and they are quite
abundant in Recent sediments as well. Certain of the Paleozoic
limestones of central Texas contain banded spherical masses of algae
called “algal biscuits.” Although not particularly useful fossils,
thallophytes have a long geologic history and are known in rocks ranging
from Precambrian to Recent in age.