The carpoid, which lived 360 million years ago, is an ancestor of today's sea urchins and starfish. It closes a 70-million-year gap in the evolutionary record of a group of creatures known as the echinoderms which live on the sea bottom and have protective spikes.
Some evolutionary theorists have suggested that vertebrate creatures including mankind are cousins of the echinoderms.
Until its discovery in Wuppertal, Germany, no carpoid had been known between 390 million and 320 million years ago, geologist Hans Martin Weber of the Rhineland local authority said in Cologne on Friday.
The amateur palaeontologist found several fragments up to 1.2 centimetres in size of the creature's shell and fore end in some rock exposed after a kindergarten had been ripped down. He took them to Weber to identify.
Geologist Weber said the rock had once been silt at the bottom of a tropical sea.
Other fossils found in the rock were the first of their types in Germany. Weber said the site had been well known to fossil hunters for 100 years and it was amazing no one had seen the echinoderm before.