HELP SOS PLEASE (ONLY CORRECT ANSWERS PLEASE)

1- Option A) describes an index fossil. Trilobite existed for a short time, but was abundant and lived in many locations.

2- Option B) Continents were once joined and have since drifted apart.

Explanation:

1) Fossils are animal and vegetable rests found in different strata of sedimentary rocks. Sedimentary layers deposit chronologically, so they are used to reflect history. They keep in each layer some of the forms of life that inhabited that area in the past. These fossils turn to be very useful while dating ages. The term Index fossils refer to those fossils that only existed in a given era or geological period during evolution.  

Index fossils must:    

•Be easily recognizable and distinguishable from all the other fossils

•Have lived in a relatively short geological period

•Present a wide geographic distribution

•Have lived in different sedimentary basins

•Appear in different types of rocks

•Be abundant

A) Trilobite existed for a short time, but was abundant and lived in many locations. The species accomplish the requirements. It is easily recognizable, had a wide geographic distribution, was abundant, and lived for a short time.  

2) The tectonic plates theory states that there is a continual movement of the crust. It explains the movement of the different plates and their directions and interactions. The continental drift theory explains how these movements have been taking place since millions of years ago. When continents were together in a unique continent, many species used to inhabit it. When plates started to separate, they took some of these species that got apart by the ocean. Some species were already dead and fossilized, while some other organisms died during continental drift and got fossilized after the divergence. The existence of the same fossils, placed in the same layers and of the same age, suggests that they used to inhabit the same area and died during the same time, meaning that continents were together when they got fossilized.

the reason the two sea urchins don't compete with each other for food, resources, etc. is because the urchins have adapted to the environment but they have also adapted to each other. this is most likely because the genes of the sea urchins.

the tricuspid valve is present between right atrium and right ventricle