Microbrachius

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

Although two of the three described species are known from Late Emsian-aged strata of Early Devonian China, the type and youngest species, M. dicki, is known from upper Givetian freshwater strata of Scotland. The various species have short thoracic armor, and large heads. There are patterns of small, but noticeable tubercles on the armor, with the arrangement varying from species to species.

Species

M. 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.^[1][2] 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. ^[3]

M. sinensis

This species is from Emsian-aged freshwater strata of the Late Eifelian Qujing Formation in Middle Devonian Qujing, China, and predates M. dicki. M. sinensis differs from M. dicki in that the former has some of its tubercles arranged and fused together to form distinctive lines on its thoracic armor, and that the former is larger, with an estimated total armor length of about 3 to 4 centimeters.^[4]

M. chuandongensis

This species is endemic to the Late Emsian-aged Chuandong Formation in Qujing, China. It differs from M. sinensis primarily by having the tubercles arranged randomly.^[5]

References