Given the question: Information: - The Ordovician is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.2 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. The Ordovician, named after the Celtic tribe of the Ordovices, was defined by Charles Lapworth in 1879 to resolve a dispute between followers of Adam Sedgwick and Roderick Murchison, who were placing the same rock beds in northern Wales into the Cambrian and Silurian periods, respectively. Lapworth, recognized that the fossil fauna in the disputed strata were different from those of either the Cambrian or the Silurian periods, and placed them in a period of their own. It received international sanction in 1960, when it was adopted as an official period of the Paleozoic Era by the International Geological Congress. - The Devonian is a geologic period and system of the Paleozoic, spanning 60 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, where rocks from this period were first studied. The first significant adaptive radiation of life on dry land occurred during the Devonian. Free-sporing vascular plants began to spread across dry land, forming extensive forests which covered the continents. By the middle of the Devonian, several groups of plants had evolved leaves and true roots, and by the end of the period the first seed-bearing plants appeared. Various terrestrial arthropods also became well-established. Fish reached substantial diversity during this time, leading the Devonian to often be dubbed the "Age of Fish". The first ray-finned and lobe-finned bony fish appeared, while the placodermi began dominating almost every known aquatic environment. - Ostracoderms ("shell-skinned") are the armored jawless fishes of the Paleozoic. The term does not often appear in classifications today because it is paraphyletic or polyphyletic, and has no phylogenetic meaning. However, the term is still used as an informal way of loosely grouping together the armored jawless fishes. - The class Osteostraci ("Bony Shields") is an extinct taxon of bony-armored jawless fish, termed "ostracoderms", that lived in what is now North America, Europe and Russia from the Middle Silurian to Late Devonian. - The Silurian is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago (Mya), to the beginning of the Devonian Period, Mya. As with other geologic periods, the rock beds that define the period's start and end are well identified, but the exact dates are uncertain by several million years. The base of the Silurian is set at a major Ordovician-Silurian extinction event when 60% of marine species were wiped out. - Witaaspis was an extinct genus of osteostraci fish that lived in the Wenlock and Ludlow stages of the Silurian period . After reading the paragraphs above, we are interested in knowing the entity with which 'witaaspis' exhibits the relationship of 'instance of'. Find the answer from the choices below. Choices: - base - bearing - class - congress - england - environment - era - extinct - extinction event - fish - official - period - rock - seed - six - taxon - term - time
The answer is:
taxon