Graptolites were tiny, marine, colonial animals that lived during the Palaeozoic Era (Cambrian to Carboniferous Periods i.e. around 545 - 290 million years ago) before becoming extinct. They are common fossils in Ordovician and Silurian rocks and because they were widely distributed and underwent fairly rapid evolution they are often used to correlate the age of sedimentary rocks.
The graptolite fossils shown below were found in Wales and the shale rock dates from the Ordovician Period (and so is between 495 - 443 million years old).
The graptolites were a few centimetres long and look like miniature saw blades. The "teeth" that you may be able to make out on the "saw blades" were once small cups that contained the individual animals that made up the colony; all interconnected along a hollow tube.
Graptolites were filter feeders and tiny tentacles would have emerged from the cup when the organism was alive.
A few species may have lived on the ocean floor but the majority drifted or swam weakly in the ocean currents. They have been placed in the Phylum Hemichordata which is distantly related to Chordates (the Phylum which is comprised mainly of vertebrates).
More graptolites can be seen in the photo below at the top of the rock.
The name graptolite means "writing on stone" due to the similarity of the fossils to pencil scribblings.
Stromatolites are the most common early Pre-Cambrian fossils. In fact, the oldest fossils found which are visible to the naked eye are 3500 million years old stromatolites.
Stromatolites formed mounds as mats of cyanobacteria trapped sediments and successive layers would build upwards in shallow marine waters eventually reaching a metre in height. Living stromatolites can be found today in Shark Bay, Australia.
The polished fossilised stromatolite or algal mat shown in the photos below measures 10 centimetres by 7 centimetres and is 225 million years old (Upper Triassic) from Cotham, near Bristol, Somerset. These particular fossils are known as "Cotham or Landscape Marble" although the rock is actually a limestone not a true marble.
This is one of my favourite fossils and Cotham Marble is famous for the miniature landscapes you can see within the pattern of the stone. The patterns were formed by the action of worm-like animals and the growth of algae on the Triassic mud flats and if you click on the photo to enlarge it you may be able to visualise ploughed fields, trees and sky.