INVERTEBRATE ZOOLOGY IN THREE VOLUMES - VOLUME 3 - H.I. Shcherbak - 1997

PHYLUM MOLLUSCA (MOLLUSCS)

FOSSIL MOLLUSCS

All the Major Classes of Molluscs (Polyplacophora, Bivalvia, Monoplacophora, Gastropoda, Cephalopoda) emerged during the Cambrian period, with Scaphopods appearing in the Ordovician. Consequently, from their very inception, molluscs underwent a wide adaptive radiation, successfully adapting to various environmental conditions. Representatives of nearly all major taxa within these classes have survived to the present day.

Throughout its geological history, the Class Cephalopoda has exhibited the greatest diversity of forms. Although only two of the seven subclasses have survived to the present day, this does not mean that cephalopods are a dying group. Across changing eras and periods, some cephalopod groups were replaced by others, yet overall, these molluscs have consistently flourished and, then as now, remained in a state of biological progress.

Only Representatives of the subclass Nautiloidea are known from the Cambrian period, with a few species surviving to this day. Their shells were spiral, straight, horn-shaped, etc. (Fig. 121, a). The subclass comprises several orders, of which only Nautilida, which emerged in the Devonian, has survived to the present day. Nautiloids never achieved significant species diversity, and it was only in the Devonian and Mesozoic eras that their relative Abundance among other cephalopods increased.

The subclass Orthoceratoidea includes a small number of fossil species (Ordovician–Triassic) that were quite widespread in Silurian and Devonian deposits. Their shells are straight (Fig. 121, b), ranging from several centimeters to 1 m in length, with lens-shaped curved septa and a centrally located siphuncle.

Fig. 121. Fossil molluscs:

a–e — class Cephalopoda, subclasses: a — Nautiloidea; b — Orthoceratoidea; c — Endoceratoidea; d — Ammonoidea; e — Coleoidea (reconstruction of a belemnite); f — class Xenoconchia; g — class Tentaculita; h — class Hyolitha; i — class Stenothecoida

The subclass Endoceratoidea includes species that lived exclusively in the Ordovician. Their straight shells (up to 4 m long) were flattened in the dorsoventral direction (Fig. 121, c). The septa are concave, and the suture line is straight. The siphuncle was located along the lateral margin of the shell and was exceptionally wide (up to 0.3 of the shell diameter). Additionally, the center of the siphuncle contained calcareous cones (endocones) with openings at their apex, forming a canal known as the эндосифон (endosiphuncle). It is believed that these endocones served to counterbalance the posterior part of the body relative to the anterior, allowing the animal to move horizontally.

The subclass Actinoceratoidea (Ordovician–Carboniferous) comprises a minor number of species with straight shells. These molluscs possessed a central siphuncle; the septal necks and septa were interconnected by a complex system of canaliculi branching off from the endosiphuncle.

The subclass Bactritoidea (Devonian–Permian) included molluscs with straight or curved shells, with the siphuncle positioned on the ventral side. The initial chamber housing the animal's body was hemispherical in shape.

The subclass Ammonoids (Ammonoidea) is the most numerous group of fossil cephalopods. The shell typically consisted of several whorls, which in most species lay in a single plane, though in some they were heavily modified during ontogeny and varied widely in shape. The body resided in the body chamber, which, unlike that of nautiloids, was more or less elongated, giving the animal a worm-like appearance. The siphuncle was located on the ventral, and occasionally on the dorsal, side of the body. The aperture of the shell was closed by a calcareous operculum. The shell diameter ranged from 1–2 cm to 2–3 m. The subclass includes over 10 orders and several thousand species, with many new ammonoid taxa being described annually.

Ammonoids are known starting from the Devonian period, and their evolutionary history is clearly divided into four stages. At each stage, specific orders dominated before dying out and being replaced by others. Such faunal turnovers occurred at the BOUNDARIES OF THE Devonian and Carboniferous, the Permian and Triassic, and the Triassic and Jurassic. Ammonoids went completely extinct at the end of the Cretaceous.

The most widespread representatives in modern seas belong to the subclass Coleoidea. They are known from the Devonian onwards, when the order Belemnitida emerged, persisting until the early Paleogene. Thanks to exceptional fossil impressions, the soft-body anatomy of these animals is generally well known: they possessed 10 arms equipped with hooks instead of suckers, large eyes, a caudal fin, and an ink sac, bearing a strong external resemblance to modern squids (Fig. 121, b). They possessed a large, cone-shaped shell that preserves well in the fossil record. Remnants of belemnite shells, commonly known as "devil's toenails" or "thunderstones," are frequently found along sandy riverbanks and in clay bluffs in Ukraine.

Shells discovered in deposits of various geological epochs are believed by many scientists to belong to distinct groups of molluscs that did not survive to the present day. Let us examine some of them.

The class Xenoconchia brings together species distributed in Carboniferous seas, which became extinct at the beginning of the Permian period. They possessed a conical shell with an aperture at the widened end, which was not divided into chambers. Near the apex on the inner surface of the shell ran a ridge, to which Muscles were likely attached. The shell length did not exceed 10 cm.

The class Tentaculita unites marine organisms that existed from the Silurian to the Devonian. They left behind narrow, conical shells ranging from 2–3 to 30 mm in length. The outer surface of the shell often featured complex sculpture in the form of transverse annular Ribs and depressions. In some species, the shell was not divided into chambers, suggesting they were benthic forms. In most, however, the distal portion of the shell contained a series of consecutive chambers separated by septa; there are no connecting pores or siphuncles between the chambers. It is believed that these chambers were filled with gas, enabling these species to lead a planktonic lifestyle.

Hyoliths (Hyolitha) are an enigmatic group of marine animals that appeared in the Cambrian, flourished during the Ordovician and Silurian periods, and went extinct at the end of the Paleozoic era. They left behind bilaterally symmetrical conical shells up to 15 cm in length. The ventral side of the shell is flat, with a semicircular anterior margin that projects forward. The dorsal side is convex and often bears longitudinal wrinkles; in some forms, this side of the shell features a double wall enclosing transverse ribs. The aperture was closed by a calcareous operculum, on the inner side of which Muscle attachment scars are visible. They are thought to have been benthic organisms. The systematic position of hyoliths remains unclear; the majority of scientists consider them a distinct taxon (class) of molluscs, while others classify them as a separate phylum or ally them with annelid worms.

Stenothecoids (Stenothecoida) were quite widespread in the first half of the Cambrian and went extinct by the end of the period. Most scientists consider them a separate class of molluscs that resemble bivalves due to the presence of a bivalved shell. Each valve exhibited bilateral Symmetry, similar to Brachiopods (see below), though this symmetry was frequently disrupted (e.g., the longitudinal sulcus could be shifted off-center, etc.). One of the Valves was always smaller than the other. A ridge, known as the "keel," ran medially along each valve, often flanked by other sculptural elements (grooves, ridges, etc.). It remains unknown whether the valves covered the body laterally as in Bivalvia, or if one functioned as a dorsal valve and the other as a ventral one (as in Brachiopoda). These were benthic organisms that presumably fed by filtration.