LECTURE NOTES IN ZOOLOGY

Module 3. ARTHROPODS, TARDIGRADES, PENTASTOMIDS, ONYCHOPHORANS

Content Module 3.1. Arthropods

Topic 3.1.3. Subphylum Trilobitomorpha

General characteristics of the subphylum. Origin, evolution, paleontological Background, modern diversity, geographic distribution, and their role in nature and human life. Overview of major taxonomic groups essential to human life and the functioning of natural ecosystems. Class Trilobita (Trilobites). These were marine animals that flourished in the early Paleozoic era (Cambrian and Ordovician periods) and went completely extinct by its end. Their sizes were generally small (ranging from a few millimeters to 10 cm), though some species reached 75 cm. Over 10,000 fossil species have been described. They exhibited a distinct combination of highly specialized traits (significant cephalization, uniramous appendages) and primitive features (unspecialized mouthparts, homonomous segmentation of the trunk, anamorphosis, etc.). The body was divided into a HEAD and a trunk. The head bore a pair of antennae and four pairs of mouthparts that did not differ from the trunk appendages. The trunk consisted of a variable number of segments (from 4 to 44), each with a pair of appendages. Frequently, several posterior segments fused with the pygidium to form a tail region. Trilobites serve as index fossils used to determine the age of sedimentary rocks and their belonging to periods of the Paleozoic era. Four classes are known; one of them is Trilobita (comprising over 90% of all described trilobitomorph species).

Class Trilobita (Trilobites).

The DORSAL SIDE OF the body was covered by a hard exoskeleton—a dorsal shield impregnated with calcium carbonate and sometimes calcium phosphate. The surface of the shield often featured various sculptures. The dorsal shield was elongated-oval in shape and divided by two longitudinal furrows into three parts, or lobes, with the middle lobe being more convex. Transversely, the shield was divided into cephalic, thoracic, and pygidial sections. On the sides of the head shield, most trilobites possessed differently developed eyes of two types: facet (compound) and aggregated. Facet eyes consisted of A large number of relatively large hexagonal or quadrangular lenses (facets, or ommatidia) covered by a common cuticle, with the number of facets ranging from a few dozen to 15,000. Aggregated eyes consisted of rounded lenses separated from each other by intervals of a reticulated cuticular Structure. The number of lenses ranged from one or two to several hundred. Many species possessed paired simple ocelli in the middle of the head shield (the "median eye"); another pair of similar eyes was located on the anterior margin of the head shield. Entirely blind forms are also known.