So God created great sea creatures and every living thing that moves, with which the waters abounded, according to their kind, and every winged bird according to its kind. And God saw that it was good. And God blessed them, saying, “Be fruitful and multiply, and fill the waters in the seas, and let birds multiply on the earth.”(Genesis 1:21-22) NKJV
If a sedimentary rock is full of the remains of Graptiloids does it means that the Graptiloids were in their maximum abundance? No, it means that the place where the Graptiloids lived got buried in sediment.
From the evolution perspective: if our premise is true, that life on Earth is the result of biological evolution, then on the chain of simple-to-complex deep sea marine life is pretty simple. We can place it near the oldest part of our timescale of how life developed.
The description of a period of time is an inductive rationalization of circumstantial evidence. Here’s some more description of the life which is attributed to the Ordovician period:
The Ordovician is best known for its diverse marine invertebrates, including graptolites, trilobites, brachiopods, and the conodonts (early vertebrates). A typical marine community consisted of these animals, plus red and green algae, primitive fish, cephalopods, corals, crinoids, and gastropods.From the University of California Museum of Paleontology
Has the penny dropped? This is a description of a marine habitat, not a period of time.