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

SUBKINGDOM MULTICELLULAR ANIMALS (METAZOA)

SECTION PRIMITIVE MULTICELLULAR ANIMALS (PROMETAZOA)

PHYLUM SPONGES, OR PORIFERA (SPORGIA ABO PORIFERA)

CLASS ARCHAEOCYATHA (ARCHAEOCYATHA)

These are predominantly marine, extinct animals that differ from true Sponges by having a solid Skeleton and lacking spicules.

For a long time, scientists believed that archaeocyathans represented an independent phylum, closely related to modern sponges. Today, many species of modern sponges have been found which, apart from spicules, possess a massive calcareous skeleton or completely lack spicules.

The body of archaeocyathans was shaped like a goblet, the base of which consisted of a massive calcareous skeleton (Fig. 84). In adult specimens, this skeleton comprised an outer wall pierced by numerous pores, and an inner wall. The space between them was called the intervallum. Inside the body, as in all sponges, was the internal atrial, or paragastral, cavity. Located within the intervallum were horizontal skeletal partitions—floors (tabulae)—or perforated longitudinal plates known as septa.

In other groups of archaeocyathans, the interwall space was open or filled with a system of irregularly arranged, complexly curved, and sometimes branching perforated partitions known as taeniae. The skeleton of certain archaeocyathans is formed by vertical tubes called calicles, or chambers.

According to Electron Cell/15.html">Microscopy data, the skeleton of archaeocyathans consisted of randomly arranged lime crystals measuring 3—9 µm in size.

The soft Tissues in archaeocyathans were distributed as follows.

Externally, the skeletal goblet was covered by a thin layer of pinacoderm; the inner wall bounding the paragastral cavity was also covered with pinacoderm. The mesohyl was located in the intervallum.

Archaeocyathans predominantly exhibited a syconoid or leuconoid type of Structure; the flagellated chambers contained afferent and efferent canals, and an osculum was located at the apex of the goblet.

The animal attached to the substrate by a specialized Skeletal structure—a basal attachment cup (or holdfast), which appeared as a massive calcareous monolith or consisted of numerous calcareous tubules.

The sizes of the skeletons of fossil archaeocyathans ranged from a few millimeters to 1.5 m in length and from 3— 4 mm to 70 cm in diameter.

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Fig. 84. Diagram of archaeocyathan structure:

1 — atrial cavity; 2 — outer wall; 3 — basal holdfast;

4 — inner wall; 5 — septum; 6 — chamber; 7 — intervallum

The body shape of archaeocyathans was exceptionally diverse—regular and irregular goblet-like, tubular, mushroom-shaped, disc-shaped, spherical, and so on. It has been proven that this shape Variability was of an intraspecific nature, as in modern sponges, and depended on current velocity, Water Temperature, and other factors, which indicates a low level of their Organization.

A special form of budding is known in archaeocyathans. On the surface of the cup around the osculum, outgrowths called cribra were formed, appearing as rounded bodies with a strong carbonate shell and slit-like pores. They detached from the parent Organism and, as believed, were dispersed by currents, subsequently settling on the substrate. In addition, certain archaeocyathans exhibited a specific mode of reproduction through The formation of longitudinal partitions that divided the walls and the internal cavity, producing several new individuals. Colonies were formed in this manner, as well as by regular budding. Since specimens of various ages have been found, this helped reconstruct The sequence of skeletal structure formation. Initially, the outer wall and a small basal holdfast appeared. Subsequently, the wall became porous, the inner wall formed, followed by The system of interwall partitions.

Until recently, archaeocyathans were known only from Lower Cambrian deposits. Today, they have also been found in Upper Cambrian rocks of Antarctica.

Archaeocyathans were widespread in the seas of the Early Cambrian and serve as index fossils for this period.

Like spiculate sponges, archaeocyathans were filter feeders. Reconstruction of the functional model of the archaeocyathan body (goblet shape, pore and efferent canal diameters, etc.) indicates that their goblet was ideally adapted to passive water filtration. They inhabited the shallow waters of warm seas, forming (along with calcareous filamentous Algae) reefs of considerable size. It is likely that these formations, much like modern coral reefs, served as the foundation for species-rich, specific ecosystems. This is supported by the discovery of remains of crustaceans, trilobites, brachiopods, Mollusks, Echinoderms, etc., within archaeocyathan settlements. The causes of the rapid flourishing and intensive extinction of archaeocyathans remain unknown.

In 1977, French researcher J. Vacelet discovered a small (a few centimeters long) animal in the equatorial reefs of the Indo-Pacific basin, which was initially assigned to the class of calcareous sponges (Calcispongea). This organism (Vaceletia crypta) possesses the same cellular composition as sponges, specifically containing archaeocytes and choanocytes, and is characterized by a leuconoid type of irrigation system. Its larva is a parenchymula, and its ontogeny exhibits features typical of both demosponges and calcareous sponges.

However, the presence of a continuous calcareous skeleton consisting of two walls and horizontal partitions between them indicates that V. crypta does not belong to calcareous sponges. Furthermore, a fossil group of sphinctozoans (Sphinctozoa) with a similar skeleton is known from the Carboniferous to the Cretaceous. Consequently, these animals, including Vaceletia, should be considered true archaeocyaths. During the Paleozoic era, there existed yet another group of organisms close to typical archaeocyaths and sphinctozoans—the stromatoporoids. Thus, the geological history of archaeocyaths is not over; their descendants still live today, although the heyday of this group has long since passed.

Archaeocyaths are true sponges whose skeleton is not composed of separate spicules, but is monolithic instead. All other features of their organization—the general body plan, cellular composition, and ontogeny—are identical to those of all other sponges.