Final answer:
The first vertebrates appear in the fossil record during the (d) Cambrian period, predating the Ordovician period when the first fish and land plants emerged.
Step-by-step explanation:
The first vertebrates are seen in the fossil record of the Cambrian period, which is the first period in the Paleozoic era, occurring between the Precambrian eon and the Ordovician period, approximately 541 to 485 million years ago (Ma). During the Cambrian period, a variety of animals began to appear in the seas, including the first vertebrates, which resembled hagfish. These early vertebrates had notable traits such as a vertebral column, which would become more complete in future fish that evolved later on.
During the subsequent Ordovician Period (505-440 Ma), the oceans were populated with invertebrates of many types, and the first fish continued to evolve. It was only after the Cambrian period that animal life began to diversify further and plants started to colonize the land, laying the foundation for the vertebrate transition from water to land later in the Paleozoic era.