[Illustration: Fig. 12. Two types of bryozoans or “moss animals.” (a)
Section of the lacy type bryozoan. (b) The spiral axis of _Archimedes_
(Mississippian).]
Bryozoans are colonial animals that are often referred to as “sea mats.”
They have been called this because they are commonly found matted on
shells, rocks, fossils, and other objects. The living animal is quite
small, has a tentacle-bearing ridge surrounding the mouth, and secretes
a tiny cup-like exoskeleton composed of calcareous or chitinous
material. These little chambers, known as _zooecia_ (or _autopores_),
are seen as small pits on the surface of the bryozoan colony
(_Rhombopora_, Pl. 17). The zooecia grow together to form the bryozoan
colony, and some fossil colonies grow to be as much as 2 feet across.
Such colonies may be spiral (fig. 12b), branching, or lace-like (fig.
12a), and the latter two types are very common in many of the
fossiliferous strata of Texas. Undoubted bryozoan fossils have been
recorded in rocks of Lower Ordovician age, but questionable Cambrian
forms have also been reported. Bryozoans are abundant in the seas of
today, but only a few forms inhabit fresh waters.
[Illustration: Plate 14]
[Illustration: SPONGE SPICULES
(GREATLY ENLARGED)]
[Illustration: PALEOZOIC SPONGES]
MEANDROSTIA × 1
HELIOSPONGIA × 1
ASTRAEOSPONGIUM × ½
ASTYLOSPONGIA × ½
RECEPTACULITES × ½
GIRTYOCOELIA × 2
[Illustration: Plate 15
PENNSYLVANIAN CORALS]
CLADOCHONUS × 1
STRIATOPORA × 1
LOPHOPHYLLIDIUM PROLIFERUM × 1
MICHELINIA × 1
CANINIA × 1
LOPHOPHYLLIDIUM RADICOSUM × 1
[Illustration: Plate 16]
[Illustration: CRETACEOUS CORALS]
CLADOPHYLLIA × 1
PARASMILIA × 1
PLEUROCORA × 1
[Illustration: TERTIARY CORALS]
ENDOPACHYS × 1
ASTRHELIA × 1
FLABELLUM × 1
MICRABACIA × 2
TROCHOSMILIA × 1
In Texas one may expect to find bryozoan remains in the Pennsylvanian
rocks of north-central and Trans-Pecos Texas where they are abundant in
certain of the marine shales and limestones. Bryozoans may also be
collected from some Cretaceous and Tertiary beds, but their remains are
small and fragmental and they are easily overlooked. Bryozoans have also
been found matted on the shells of fossil mollusks and other
invertebrates.