Placodermi
From Paleos
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The Class Placodermi is composed of a group of armoured prehistoric fishes known from fossils dating from the late Silurian to the end of the Devonian Period. Their head and thorax were covered by articulated armoured plates and the rest of the body was scaled or naked. Placoderms were one of the first jawed fish, their jaws likely evolving from the first of their gill arches. Starting with the the studies of Dr Erik Stensio, and supported by uncrushed fossils that preserve their 3-dimensional structures from the Gogo Reef formation in Australia, it is presumed that sharks share a common ancestry with placoderms.
The first identifiable placoderms evolved in the late Silurian; they disappeared in the Late Devonian extinctions. The first appearance of late Silurian placoderm fossils, in China, show the fishes already differentiated into Antiarchs and Arthrodires, along with the other, more primitive groups; apparently Placoderm diversity originated long before the Devonian, somewhere in the middle Silurian, though earlier fossils of basal Placodermi, have yet to be discovered in these particular strata.
Many placoderms, particularly the Rhenanida, Petalichthyida, Phyllolepida, and Antiarchi, were bottom-dwellers. As such, to paraphrase from what was said in Paleos, 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 an 8 to 11 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.
The earliest studies of placoderms were published by Louis Agassiz, in his five volumes on fossil fishes, 1833 – 1843. In those days, the placoderms were thought to be shelled jawless fish akin to ostracoderms. Some naturalists even suggested that they were shelled invertebrates, or even turtle-like vertebrates. The work of Dr. Erik Stensio, at the Swedish Museum of Natural History, Stockholm, from the late 1920s established the details of placoderm anatomy, and identified them as true jawed fishes related to sharks. He took fossil specimens with well-preserved skulls, and ground them away, one-tenth of a millimeter at a time. Between each grinding, he made an imprint in wax. Once the specimens had been completely ground away (and ironically, completely destroyed as a result), he made enlarged, three-dimensional models of the skulls in order to examine the anatomical details more thoroughly. Many other placoderm specialists suspected that Stensio was trying to shoehorn placoderms into a relationship with sharks, but with more fossil specimens found, especially the exceptionally well-preserved fossils from the Gogo Reef formation in Australia, Stensio's theory of sharks and placoderms as sister groups is accepted as fact.
