Microbrachius is an extinct genus of tiny, advanced antiarch placoderms closely related to the bothriolepids. Specimens range in age from the Lower Devonian Late Emsian Stage (393.3 Ma) to the Middle Devonian Upper Givetian Stage (382.7 Ma). They are characterized by having large heads with short thoracic armor of an average length of 2-4 cm. There are patterns of small, but noticeable tubercles on the armor, with the arrangement varying from species to species. Specimens of Microbrachius have been found in Scotland, Belarus, Estonia, and China. Microbrachius dicki is the earliest placoderm and vertebrate that shows evidence of internal fertilization. Anatomy The bony armor of placoderms surrounds the head and trunk of the animal, and is divided into several interlocking plates that fused together. Microbrachids are recognized for their large armor plates around the head and shorter thoracic plates. Species Microbrachius dicki The type species is from Upper Givetian freshwater strata of Scotland and Estonia, including the John O'Groats sandstone of Caithness, and the Eday beds of the Orkney islands. It is named after its discoverer, Robert Dick. Total armor length is an average of 3 centimeters. It lived about 385 million years ago and is the earliest gnathostome in which internal fertilization has been identified. Newly discovered fossils of M. dicki found in Estonia in 2013 demonstrate clear evidence of unique male and female sexual organs, with males possessing claspers, and females developing fixed plates to lock the claspers in for mating. Separate male and female organs indicate these fish would have been capable of internal fertilization similar to sharks and other cartilaginous fishes, rather than broadcasting gametes externally into the water column like most modern bony fishes. The finding also served to reveal the unusual mating strategy used by this ancient group, which involved a sideways position with their bony arms locked together. Distinguishing Characteristics: • Mixilateral plate overlapping the anterior median dorsal plate • Anterior ventro-lateral plate has a crista postbranchialis undeveloped • Lateral plates quite broad • Denticles on the mesial edge of the distal segment of the pectoral appendage are large strongly recurved spines • Posterior median dorsal plate which is roughly equal to the anterior median dorsal plate • Well developed lateral corners on the anterior median dorsal plate Microbrachius sinensis This species is from Emsian-aged freshwater strata of the Late Eifelian (387.7 Ma) Qujing Formation in Early Middle Devonian Qujing, China, and predates