Question

The ostracoderms were
A. An ancient group of spiny thin echinoderms
B. A group of primitive protochordates
C. Primitive fishes without jaws
D. One of the first terrestrial vertebrates

Answer 1

Hint: Ostracoderms belong to the Palaeozoic era and lasted till the Devonian period. The group includes extinct animals. These organisms are known to have been aquatic in nature. Ostracoderms were found primarily in freshwater.

Complete answer:
Option (a) is incorrect. Echinoderms belong to the phylum Echinodermata. They are marine organisms. Their endoskeleton is spiny made up of calcium carbonate. They have a well developed water vascular system and locomote with the help of tube feet. These organisms lack respiratory and excretory systems. Water flows in through the tube feet and oxygen is exchanged by diffusion. Waste generated in the body is removed with the help of the tube feet as well. Common members of this phylum are starfishes and sea anemones.
Option (b) is incorrect. Protochordates are lower chordates. They are triploblastic, bilaterally symmetrical and coelomate animals. These animals are marine animals and can be divided into three sub phylum – hemichordata, urochordata and cephalochordata. These organisms have some characteristics of the chordates. For example, the notochord is present at some life stage or the other. Branchiostoma is a protochordate.
Option (c) is correct. Ostracoderms are primitive jawless fishes. They are all extinct at present and were freshwater dwellers. It is believed that the Ostracoderms emerged from the Urochordates. Their body was fish like, but paired fins were absent. the head and thorax were covered with bony armours. Gills were present for respiration and the animals were filter feeder. They gradually became extinct in the Devonian period.
Option (d) is incorrect. The first terrestrial vertebrates evolved from fishes. The fishes developed limbs to evolve into aquatic tetrapods. The push for invasion of land stemmed from unfavourable aquatic conditions along with predation and competition. The migration to the terrestrial environment happened during the late Devonian period and were favoured by abundant prey, less competition and favourable climatic conditions.

So, Option (C) is correct.

Note: Ostracoderms were primitive jawless fishes. Their bodies were covered with bony armours. Ostracoderms were freshwater dwellers. They became extinct after the emergence of the jawed fishes during the late Devonian period. The closest living relatives of the Ostracoderms are lampreys and hagfishes.