Norman Barker is an assistant professor of pathology and art as applied to medicine at The Johns Hopkins University School of Medicine. Ancient Microworlds Promicoceras is an ammonite that lived 200 million years ago. This fossil was found in England. Here the heart of a trunk lies in the center of the picture. Archaeocidaris is a sea urchin that lived 300 million years ago in a shallow sea covering what is now Texas.
Some sea urchins crawl using their spines, which they can manipulate like stilts.
Under the right conditions, these mats trap sand and sediment, building up in layers to form mounds called stromatolites. First appearing in rocks three and a half billion years old, stromatolites are evidence of the earliest known communities of life. Until 500 million years ago, stromatolites were the most prominent sign of life on earth. The stromatolites in these photographs are all 2.5 billion years old, and were found in Minnesota. These sand granules are made of the minerals quartz and hematite.