From the evolutionary point of view, which one among the following is the most primitive animal?

From an evolutionary perspective, the turtle is the most primitive animal among the options. Turtles belong to the class Reptilia, which evolved during the Paleozoic and Mesozoic eras [2]. Fossil records indicate that turtles appeared as a distinct lineage with their characteristic shell structure long before the rise of modern mammals. In contrast, dolphins (cetaceans), otters (mustelids), and walruses (pinnipeds) are all mammals that evolved much later during the Cenozoic era [2]. These three groups represent secondary adaptations where terrestrial mammals returned to aquatic environments [4]. While cetaceans like dolphins diverged early in the Cenozoic [3], and pinnipeds like walruses followed, they are significantly more derived and complex in their evolutionary history compared to the ancient reptilian lineage of turtles [4]. Therefore, turtles represent a more basal or 'primitive' vertebrate group in the context of evolutionary chronology.

Sources

1. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 3: Geological Time Scale The Evolution of The Earths Surface > 3.4. Phanerozoic Eon (550 mya to present) > p. 44
2. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 3: Geological Time Scale The Evolution of The Earths Surface > Cenozoic Era (65 mya to present) > p. 48
3. [4] https://www.britannica.com/animal/turtle-reptile/Origin-and-evolution
4. [3] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 12: Animal Diversity of India > Do ycu know? > p. 188