INVERTEBRATE ZOOLOGY IN THREE BOOKS - BOOK 1 - H.I. Shcherbak - 1995

SUBKINGDOM MULTICELLULAR ANIMALS (METAZOA)

SECTION TRUE MULTICELLULAR ANIMALS (EUMETAZOA)

PHYLUM CNIDARIA (OR COELENTERATA)

FOSSIL CNIDARIANS

The oldest known cnidarians to science are soft-bodied, medusa-like organisms whose remains have been discovered in Late Proterozoic (Vendian) deposits. Paleontologists have assigned them to the Class Cyclozoa (Fig. 120). These organisms reached several centimeters in diameter, led a predominantly sessile lifestyle, and possessed a well-developed layer of mesoglea that performed a supportive function. Cyclomedusae attached to the substrate by their aboral pole; their gastral cavity lacked the division into a Stomach and gastrovascular canals seen in modern jellyfish, remaining undivided, quite large, and extending to the bell margin. Most cyclomedusae exhibited distinct cyclic structures on the aboral side of the bell. In some forms, concentric striations were combined with radial elements (presumably radial canals).

Fig. 120. Reconstruction of Vendian cnidarians:

a, b — Cyclomedusa plana (Cyclozoa); c — Armillifera parva; e — Bonata septata (Inordozoa); d — Albumares bransae (Trilobozoa)

Radially symmetrical forms were also found among Vendian medusae, but their radial Symmetry was of an indefinite order that could change even during ontogeny—as the Organism developed, new branches of radial canals, tentacles, and other structures grew. Paleontologists grouped these organisms into the class Inordozoa.

Medusae with triradial symmetry—encompassing stomach pockets, gastrovascular system radial canals, oral lobes, and Gonads—were less common and were assigned to the class Trilobozoa.

There were also medusae with tetraradial symmetry characteristic of modern scyphozoans; these organisms were provisionally placed in the Class Scyphozoa.

The Vendian fauna also preserves the earliest hydrozoans. These were solitary, soft-bodied organisms similar to modern chondrophores (Class Hydrozoa, order Chondrophora). They were disc-shaped, with a keel on the upper side resembling the sail of modern Velella. Unlike most sessile cyclomedusae, they floated on the ocean surface.

It is believed that the Vendian fauna existed for a relatively short time and nearly all went extinct before the onset of the Cambrian period.

The beginning of the Cambrian was marked by the appearance of Skeleton-forming animals, though none yet existed among cnidarians. The first skeletal corals appeared in the mid-Ordovician. The oldest corals differed from modern ones in that they resembled small cups, on the inner walls of which true septa were absent, replaced instead by small ridges. They did not form large colonies, but rather crept along the substrate. These organisms were assigned to the Class Anthozoa, subclass Tabulata (Fig. 121). Some of them were already reef builders.

Fig. 121. Fossil corals: a — coral colony of Tabulata; b — solitary tetraradial coral Omphiuma; c — Heliolithoidea colony

Later, another group of corals emerged that possessed true septa within the gastral cavity. In these forms, six primary sclerosepta were laid down first, while subsequent septa formed in four sectors rather than six, leading to their Classification as tetraradial corals (subclass Rugosa).

Yet another group of fossil corals, the heliolithoids (subclass Heliolithoidea), formed massive colonies with a well-developed coenosarc; their corallites were characterized by the presence of twelve sclerosepta.

However, none of the aforementioned coral groups have survived to the present day. Throughout geological history, certain groups went through mass extinctions while others emerged. Coral assemblages died out three times: at the end of the Middle Devonian (Paleozoic), at the Permian-Triassic boundary, and at the beginning of the Cretaceous period (the end of the Mesozoic era). The first true octocorals appeared in the Silurian period of the Paleozoic era, and scleractinians in the Middle Triassic (the beginning of the Mesozoic era). Hydrozoans close to modern forms (hydrocorals) are known from the Ordovician, represented by colonies with chitinoid and calcareous skeletons.