Microbrachius is an extinct genus of tiny, advanced antiarch placoderms closely related to the bothriolepids.

[1] The bony armor of placoderms surrounds the head and trunk of the animal, and is divided into several interlocking plates that fused together.

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.

[2][1] 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.

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.

[5] Distinguishing Characteristics:[4] • Poorly developed lateral corners on the anterior median dorsal plate • Ornamentation on thoracic consists of tubercles fused into distinct lines • Breadth/Length ratio of 100 of anterior median dorsal plates This species is endemic to the Late Emsian-aged (393.3 Ma) Late Early Devonian Chuandong Formation in Qujing, China.

M. kedoae is currently recognized as the youngest member of the genus along with M. dicki that occurs in laterally correlative beds.

[7] The clasper of M. dicki is a deeply grooved dermal bone that curves laterally at almost a 90 degree angle to the side of the animal.

For example, the clasper of Austroptyctodus gardineri is a dermal bone that curves laterally to the side of the animal and resembles a hook.

Female specimens of M. dicki show paired blade-like structures in the same region of the animal that the claspers are in the males.

These blades are ornamented with curving ridges and marginal tubercles on their dorsal (internal) surfaces facing the cloacal chamber and are fused to each other and posterior ventral lateral plate as well.

display semicircular plates located in the same anatomical area suggesting that these specimens are females and structures used for mating.

[7] New specimens of M. dicki have started the conversation of which is the primitive state for reproduction in vertebrates, internal or external fertilization?