Furcacauda is a genus of thelodont from the Silurian to Devonian Periods. It had a large caudal fin which made it look very odd. Furcacauda was closely related to the Silurian thelodont Lanarkia.
Since the largest thelodont, Thelodus, was 30 cm, Furcacauda was smaller than that. Furcacauda and other thelodonts were jawless. Scientists have found a fossilized part of Furcacauda that looks like a stomach. So Furcacauda could have had the first stomach of any vertebrate.
This image shows a fossil of Furcacauda on the bottom, and the top fossil is Lanarkia. Although Furcacauda didn't look anything like Lanarkia, they were still both thelodonts. Thelodonts were covered in tooth-like scales rather than having armor on their head like most jawless fish did.