accommodation in WroclawThe class Monoplacophora has recently been divided into two classes and this name has been abandoned. “Monoplacophorans” range from the Lower Cambrian to the Holocene (Recent), and are considered the most primitive group of mollusks. The shell in both helcionelloids and tergomyans are oval-shaped with a subcentral apex making it look cap- or spoon-shaped. Only two genera of “monoplacophorans” (Vema and Neopilina) and eight species are known to live today. The majority are extinct, although the genus Neopilina is recorded back to the Middle Devonian Period. Helcionelloida are the most primitive of the two classes, represented at the Burgess Shale by the most common fossil found there, Scenella amii (Matthew, 1902).

The class Coniconchia (Cambrian to Permian) are small conoid single-valved organisms, open at one end with or without a cap or opeculum. The hyolithids and tentaculitids have been included in this class, but they may belong in their own classes, or even their own phyla. Hyolitha Henningsmoen (1952) are tapering, calcareous shells coming to a closed point, have bilateral symmetry, and are trigonal in cross section. They have an operculum (also called a cap) covering the aperture. From the aperture when preserved, are a pair of curved calcareous “appendages” called “helens” or “wings”. These animals are assumed to be benthic (living on the ocean bottom) because of their thick shell, and are often found in the gut of the Middle Cambrian priapulid worm, Ottoia prolifica, at the Burgess Shale. Tentaculita Boucek (1964) are elongate, straight, tapering tubes that are calcareous. More than 150 species have been described. They may be represented by some Middle Cambrian fossils, and are very abundant in the Silurian and Devonian Periods. Some were benthic while others were pelagic.

The Polyplacophora (chitons) range from the Cambrian to Recent. They have an ovoid-shaped shell subdivided into eight separate plates, and are highly adapted for attaching themselves to rocks and other shells. The Aplacophora (solengasters) are worm-like animals with no fossil record. They have calcareous spicules imbedded in their epidermis. Scaphopoda (tusk or tooth shells) are burrowing marine mollusks that are elongate and cylindrical tubes often shaped like an elephant tusk. Unlike Coniconchia, both ends of the tube are open. The scaphopods have a fossil record from Devonian to Recent.

The class Bivalvia (also known as Pelecypoda and Lamellibranchia) first appeared in the Ordovician Period. Common names for some bivalves include clams, mussels, scallops, and oysters. As the name bivalve implies, they have two valves (shells) attached by a hinge and hinge ligaments. The valves are opened by adductor muscles relaxing and the natural elasticity of the hinge ligaments cause the shell to open. The adductors contain smooth and striated muscle fibers to all for prolonged and rapid closing of the valves. Bivalves may be infaunal (soft-bottom dwellers), epifaunal (attached surface dwellers), boring species, or unattached surface dwellers. Some species of scallops and file shells have the ability to “swim” in order to escape predators.

The Cephalopoda (squid, octopus, cuttlefish, Nautilus, ammonites) are an extremely diverse group of predaceous marine mollusks that have adapted many unique structural complexities and metabolic efficiencies. The have a distinct head, bilateral symmetry, an undivided mantle, a u-shaped digestive tract, and a molluscan foot that has evolved into a series of tentacles surrounding the mouth and a muscular funnel (hyponome) beneath it. The funnel is used to propel the animal backward through the water by expelling a powerful jet of water from it. Some species also eject ink from this funnel to aid in escape from predators. There are two general body plans with in this class: (1) the ectocochliate; and (2) the endocochliate cephalopods. The ectocochliates have an external chamber shell, while the endocochliates have a much simpler looking internal shell. The cephalopods date back to the Upper Cambrian Period.

The Gastropoda (slugs and snails) first appeared in the Lower Cambrian. They are the second highest class of invertebrates in terms of diversity behind the insects. Gastropods make up approximately 80% of the species known within the phylum Mollusca. They live in terrestrial, freshwater, and marine environments. General characteristics are an elongate, flattened, broad foot surface; recognizable head with eyes and tentacles; mouth with radula (rasping apparatus). Most species have a coiled shell, symmetrical cap, or no shell at all. Gastropods are divided into three subclasses: (1) Prosobranchia; (2) Opisthobranchia; and (3) Pulmonata. The Prosobranchia are divided into three orders: (1) Archaeogastropoda; (2) Mesogastropoda; and (3) Neogastropoda.

Spirifer sp.

Guangi Province, China
Ordovician, about 440 million years old.
About 4 mm (1.5 inches) across.