Question

what are stromatolites?
fossils of early reptiles
layered mounds of calcium carbonate deposited by early algae
evidence of asteroid impacts
fossils of early mammals

Explanation:

Stromatolites are formed by the activity of cyanobacteria (a type of early algae). These organisms trap sediment and secrete calcium carbonate, creating layered structures. Early reptile fossils would be of actual reptile remains. Evidence of asteroid impacts would be things like impact craters or specific chemical signatures (e.g., iridium - rich layers). Early mammal fossils would be remains of early - evolved mammals. Stromatolites' key characteristic is the layered calcium carbonate structure from early algae.

Answer:

layered mounds of calcium carbonate deposited by early algae