This post has been a long time in coming but hopefully I’ll be able to start posting regularly after this and get a steady stream of updates going. Here, finally, is a primer on placoderms since I’m likely to be posting about them more.
Placoderms are an entirely extinct group of armoured fishes that were most common in the seas of the Devonian Period (419-459 mya). They were an impressively diverse group of organism containing both ray-like bottom feeders (Rhenanids) and giant predators (Dunkelosteus). Placoderms also include some of the smallest known vertebrates (Minicrania lirouyii )  with a head and thoracic shield of only 20mm) and the largest vertebrates to have evolved up to the Devonian (Titanichthys; probably maxed out at over 5 meters long). Despite their diversity and dominance for over 40 million years no placoderms survived past the end of the Devonian for reasons unknown. So this is a whole branch of life that originated, diversified, and went extinct just as vertebrates were starting to move onto land!
There are nine orders of placoderms currently recognized  Stensioellida, Pseudopetalichthyida, Petalichthyida, Ptyctodontida, Acanthothoraci, Rhenanida, Antiarchi, Phyllolepida, and Arthrodira with the first two being poorly known. They were long thought to be a monophyletic group but recent discoveries have turned the old relationships on its head, especially the discovery of a mid-Silurian vertebrate, Entelognathus primordialis, that has a combination of characters seen in placoderms and bony fishes (osteichthyes) . I might do a separate post just about that strange beast sometime. Anyway, the different orders of placoderms are now thought to be paraphyletic with Antiarchs as the most basal, arthrodires as intermediate, and ptyctodontids as the sister-group to the remaining jawed vertebrates , figure 1 although this is still in flux.
In terms of morphology placoderms as a whole have an ossified dermal skeleton, that is the bones form from the dermal layer near the surface of the skin rather than internally like most of the bones in our own bodies (Interestingly most of our skull bones are dermal in origin). This external skeleton is commonly referred to as armor and it protects the soft internal body of the fishes including the internal skeleton which is rarely preserved and appears to be largely composed of cartilage. The armor of the placoderm is typically broken down into four groups of armor plates that are tightly connected to each other: head, cheek, thoracic, and ventral armors (figure 2). In antiarchs the pectoral fins are enclosed by jointed appendages reminiscent of arthropods.
The head shield in most arthrodires consists of seven paired plates (postnasal, central, preorbital, postorbital, paranuchal, marginal, and postmarginal) and three median plates (rostral, pineal, and nuchal). In some primitive arthrodires the rostral and pineal plates are fused (Buchanosteidae) to form a rostropineal and in other forms the postnasals appear to have been lost. The head shield is connected to the thoracic shield by a hinged contact between the paranuchal and the anterior dorsal lateral. The thoracic shield is composed of five to seven plates in arthrodires (median dorsal, anterior dorsal lateral, anterior lateral, posterior dorsal lateral, posterior lateral, interolateral, and spinal). The interolateral and spinal plates are lost in independent lineages, coccosteids and aspinothoarcids (surprise surprise) respectively. Depending on the species the thoracic armor is articulated to varying degrees with the ventral armor which would have protected the soft underbelly. This part of the armor consists of four plates (anterior median, posterior median, anterior ventral lateral, and posterior ventral lateral) which are typically found in isolation or fragmented. Finally, the cheek shield is connected to the head shield either loosely with the preorbital or intimately with both the preorbital and lateral portion of the head shield. The cheek shield consists of three plates (suborbital, postsuborbital, and submarginal).
I’ll stop there for now. I’ve updated the placoderm occurrence database so I might do something about placoderm diversity next. I also have a paper on Titanichthys under review so maybe more on that soon too!
1. Zhu, M. & P. Janvier. 1996. A small antiarch, Minicrania lirouyi Gen. et sp. nov., from the Early Devonian of Qujing, Yunnan (China), with remarks on antiarch phylogeny. Journal of Vertebrate Paleontology 16:1-15.
2. Denison, R.H. 1978. Handbook of Paleoichthyology: Placodermi. Gustav Fischer Verlag, Stuttgart, New York, 128 pp.
3. Zhu, M., X. Yu, P. E. Ahlberg, B. Choo, J. Lu, T. Qiao, Q. Qu, W. Zhao, L. Jia, H. Blom, and Y. Zhu. 2013. A Silurian placoderms with osteichthyan-like marginal jaw bones. Nature 502:188-193.
4. Davis, S.P., J.A. Finarelli, and M.I. Coates. 2012. Acanthodes and shark-like conditions in the last common ancestor of modern gnathostomes. Nature 486:247-250.
5. Dunkle, D.H. and P.A. Bungart. 1947. A new genus and species of arthrodiran fish from the Upper Devonian Cleveland Shale. Scientific Publication of the Cleveland Museum of Natural History 8:103-117.