Placodermi ecology

From Palaeos

Gnathostomata
`--o PLACODERMI
   | ?--Stensioella
   | ?--Pseudopetalichthyida
   `==Acanthothoraci (paraphyletic)
      |--Rhenanida
      |--Antiarchi
      `--+--Ptyctodontida
         |--Petalichthyida
         `--Arthrodira
            `--Phyllolepida

In a lake in Devonian Gondwana, an Austrophyllolepis ritchiei settles in while a trio of Bothriolepis askinae forage, and a school of Culmacanthus antarctica swim by. © Stanton Fink

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 paired fins and a heterocercal tail indicates they could swim quite efficiently when they wanted to, but the heavy armour would've weighed them down, and it is likely many of them spent much of their time sitting on the bottom.

Although the head and thorax were heavily armored with dermal (skin) bone, the rest of their bodies were quite vulnerable, covered with small bony scales or lacking even that. It is not clear what purpose their 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 molluscs) a support for the internal organs (in view of the very weak cartilaginous skeleton this may not be unlikely)

Many placoderms, particularly the Rhenanida, Petalichthyida, Phyllolepida, and Antiarchi, were bottom-dwellers. As such, Placodermi has been popularly misinterpreted as being a tribe of bottom-feeding snails and garbage trucks, nevermind that the placoderms were the dominant vertebrate group during the Devonian. One must remember that the vast majority of placoderms were predators, many of which lived at or near the bottom. Many, primarily the Arthrodira were mid- to upperwater dwellers, and were active predators. The largest known arthrodire, Dunkleosteus telleri, was a 6 to 8 meter long predator and was presumed to have a nearly worldwide distribution, as its remains have been found in Europe, North America and Morocco. Other, smaller arthrodires, such as Fallacosteus and Rolfosteus of Gogo, had streamlined, bullet-shaped head armor, strongly crediting the idea that many, if not most, arthrodires were active swimmers, rather than passive ambush-hunters whose armor practically anchored them to the seafloor.

It was originally thought that the placoderms went extinct due to competition from the first bony fish, as well as the early sharks, due to a combination of the supposed inherent superiority of the bony fish and sharks, as well as the presumed sluggishness of the placoderms themselves. Since then, though, as more accurate summaries of prehistoric organisms have been developed, it is now presumed that the last placoderms died out one by one as each of their ecological communities suffered due to the environmental catastrophes during the Devonian/Carboniferous extinction event.

Palaeos com - Placodermi

[edit] Credits

Stanton 060924, MAK060926