Placoderms, antiarchs

From Agassiz 1844

Pterichtyodes milleri
Asterolepis orcadensis
Microbrachius dicki

The antiarchi look like the other placoderms but have a tail which is covered with thin, but clearly visible scales. They have very peculiar pectoral appendages (like arms) instead of pectoral fins. These pectoral appendages are also armoured with bony plates and movements is made possible by a very well-developed joint system, similar to the legs of lobsters or crabs.

The appendages also have tiny hooks at the anterior end.

The eyes were situated on top of the head instead of on the sides like most placoderms. Probably they spend most of the time on the lake bottom, feeding with their tiny ventrally placed mouth on small invertebrates or scavenging.

Morphology and histology

The outer layers of the dermal plates of the antiarchs are mostly covered with tiny tubercles, or sometimes with a more sinuous ridge-like ornamentation. In thin section the outer layer has the tubercles, the medial layer has one or more rather porous layers with big open pores, with the basal layer lamellar in structure.

The Morphology of the dermal plates of Microbrachius dicki looks much like Pterychthyodes and Asterolepis.

Asterolepis orcadensis

Asterolepis orcadensis, posterior medium dorsal plate

Photomicrographs of a thin section of dermal plates

Pterychthyodes milleri

Surface of dermal plate and Photomicrographs of a thin section of dermal plates

Photomicrographs of a thin section of dermal plates

References:

See end webpage on placoderms arthrodira.

Placoderms Antiarchi