This tiny antiarch from the Eday Flags was well described by Susan Hemmings in 1978. The tail has never been found. Possibly it had no tail, since specimens are usually well preserved in well varved fish beds where other species have even the most delicate structures preserved. Microbrachius has many small hooks on the inside and at the end of its little arms (pectoral appendages). Maybe they helped the fish to attach itself to water plants when the current was too high. However, Long et al. 2015 (see below) speculate that they could have aided two fish connecting to one another during the mating process. There are good indications that most of the fish in the Eday Subgroup were living in the rivers or tiny lakes.
The antiarchs in general are typical bottom dwellers and with their tiny mouths on the ventral side of the head. They were probably after everything edible. Maybe the pectoral appendages also helped in finding food on the bottom.
Recently it was discovered that there was a difference between the male and female Microbrachius. The male Microbrachius have claspers at the place of the pelvic fins and they are described in a nature paper (Long, J.A. et al. 2015, Copulation in antiarch placoderms and the origin of gnathostome internal fertilization) to function as a penis for internal fertilization. There were two claspers available and probably both could be used this way as can be seen also in some recent sharks today. On the first webpage, the starting page for the fossil fishes from Orkney a male and a female Microbrachius are figured, see also the pictures below.