Fenestrate bryozoan in limestone
Bryozoans are colonies of tiny individuals, approximately a millimeter or less, living from tide-level to abyssal depths in today’s oceans. They have been abundant and diverse throughout geologic time since the Ordovician Period. Bryozoan colonies vary in shape and can be stick-like, massive, or lacy and net-like. Lacy bryozoans are also called fenestrate bryozoans. The lacy bryozoan in the picture is approximately 2 inches (5 cm) long and is present in Pennsylvanian age rocks in northwest Arkansas.