Palæos:		Unit 60: Placodermi
The Vertebrates		Overview

Placodermi: Overview

THELODONTI
|--+--CHONDRICHTHYES
|  `--TELEOSTOMI
|
Placodermi
|--Arthrodira
|  |--Actinolepidoidei
|  `--Phlyctaenioidei
`--+--+--Petalichthyida
   |  `--Ptyctodontida
   `--+--Rhenanida
      `--Antiarchi
         |--Yunnanolepidoidei
         `--Euantiarcha

Introduction

The placoderms are among the most ancient of jawed fish, and, along with the Acanthodii, the only class of (gnathostomes) to become completely extinct.  The name "placoderm" is from the Greek and means "tablet + skin", referring to the heavy armoured bony plates that completely covered the head and thorax of these curious prehistoric fish.

Bothriolepis.  Late Devonian - worldwide.  Length about 20 to 30 cm

With their armored head shield and trunk shield composed of overlapping bony plates, the placoderms appear at first glance extremely similar to the ostracoderms (armoured jawless fish).   They are easily distinguished however by their paired fins and presence of jaws, an adaptation that made them quite different from their earlier jawless cousins, for it enabled them to bite solid food rather than simply suck up organic particles from the mud.  Yet still these creatures were equally different from othe jawed fishes.  The jaws were supported in a distinct way, with the palatoquadrate (upper jaw bone) fitting inside the arms of the lower jaw.  This gave the placoderms a distinctively narrow gape.  However, the head and trunk shields of most placoderms were articulated by bony joints, allowing the forward part of the skull to tilt up, increasing the gape and allowing them to take in larger food.  It used to be believed that placoderms lacked teeth and relied on biting or grinding structures formed by dermal bones lining their mouths.  The current view is that these are true teeth, formed by fusion of multiple dental elements as in extant holocephalians.  The backbone consisted of a notochord (an embryonic feature in higher vertebrates) that persisted throughout life, and vertebra consisting only of Y-shaped spines above and below it.  These were often cartilaginous, like the skeleton of  sharks (Chondrichthyes), a group with which the Placoderms have sometimes been considered to share a common ancestry.  

Although the head and thorax were usually armored with dermal bone, the rest of the body was quite vulnerable, covered with small bony scales or lacking even that.  It is not clear what purpose this armour may have served.  It has been suggested that it was a defense against the giant sea-scorpions that inhabited brackish water environments around this time.  Another explanation is that it may have served as an exoskeleton and (as with arthropods and mollusks) a support for the external organs (in view of the very weak cartilaginous skeleton this may not be unlikely)

Placoderms evolved into a variety of ecomorphs (body types), including torpedo-shaped swimmers, flattened bottom-dwellers, and armoured box-like types.  They included both carnivorous and detritivorous types.  The pared fins and a heterocercal tail suggest that they could swim quite efficiently when they wanted to, but the heavy armour would have weighed them down, and it is likely they spent much of their time sitting on the bottom..

Most placoderms were quite small, about 10 or 15 cm in length, but one specialized group of carnivorous types, Dinichthyidae included a few genera reached 4 to 9 meters in length, making them the largest carnivore of the Devonian.  Images of the enormous Dunkleosteus are common on the web.  

Although they first appeared during the early Silurian period, placoderms did not become common or widespread until the Early Devonian, when they underwent an extraordinary evolutionary radiation.  They soon came to dominate most brackish and near-shore ecosystems, and also spread to marine and freshwater environments.  More than 250 genera are known in all, with almost 100 species of one genus, Bothriolepis, making them the most diverse and important of Devonian vertebrates.

The placoderms were seemingly little affected by the extinction at the end of the Frasnian Age, yet the group dwindled during the Famennian and died out at the Devonian/Carboniferous boundary, without leaving any descendents.

Text by MAK.  Last revised ATW050825.

Web Annotations

The UCMP site has coverage equivalent to ours -- better organized but now a bit dated.   Steven Carr's placoderm page has the placoderms as paraphyletic ancestors of the sharks.  We're not sure where this comes from.  The internal arrangement of the various placoderm groups is unclear.  Compare Carr's phylogeny with the two trees in Mikko's Phylogeny.  

Coccosteus site (Placodermi).

checked ATW030919