In 1909 a vacationing paleontologist named Charles Wolcott happened upon some very interesting looking plant and animal fossils in the Canadian Rockies, near the border of Alberta and British Columbia. The fossils were exciting (to a paleontologist) in that they preserved soft features extremely well, which is a rarity – usually features like bones and hard shells are distinct in a fossil but not much else. Over the next 30 years Wolcott returned often to locate, study, and categorize the species he found. Eventually he determined (correctly) that they dated from the Middle Cambrian. Thus they excited a handful of paleontologists with a niche interest in that time period, and the fossils from the site, which became known as the Burgess Shale, held the oh-so-thrilling distinction of being the oldest soft tissue fossils ever discovered.
Wolcott and his fossils remained in that strange limbo of academic fame-obscurity, his work well known to a small handful of people but otherwise wholly ignored. Then in the 1960s some enterprising graduate students re-examined the thousands and thousands of Burgess Shale fossils and found something curious. In keeping with scientific orthodoxy of his era, Wolcott had carefully, meticulously – obsessively, even – categorized every one of his finds into an existing family of plants or animals. The students noted that much of what he found was so utterly bizarre and unlike anything alive today that Wolcott had actually found a lot of species the likes of which have never been seen before or since.
What is particularly interesting, presuming one understands evolution, is how wacky some of the body plans nature "experimented" with during that era appear today. The Smithsonian keeps a nice gallery of what some of the Burgess Shale creatures looked like; suffice it to say that the whole four legs, two eyes, one mouth thing hadn't quite caught on yet. One – an arthropod called Opibinia – had five eyes on stalks and a long elephant-like trunk with a clawed mouth on the tip.
There is an active debate among people who understand such things better than I do just how unique the specimens of the Burgess Shale truly are. Some scientists believe that they are far less bizarre than some early interpretations have suggested, while others maintain that it indeed represents a snapshot of the "Cambrian explosion" which saw the phenomenally rapid expansion of forms of complex life on Earth. I think the understanding of what happened 600 million years ago will always involve a healthy amount of conjecture, but it stands to reason that any sudden proliferation of kinds of life forms would involve a great deal of trial and error. Especially at such an early stage in the evolution of life, there had to have been a lot of false starts before a general blueprint of animal life – adapted to breathing oxygen, resistant to the deleterious effects of solar radiation, and with the basic senses necessary to sustain life long enough for reproduction – was settled upon.