Placodermi

The Placodermi were a class of armored prehistoric fish, known from fossils, which lived 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, depending on the species. Placoderms were among the first jawed fishes; their jaws likely evolved from the first of their gill arches. A 380 million year old fossil of one species represents the oldest-known example of live birth.

The first identifiable Placoderms evolved in the late Silurian; they began a dramatic decline during the Late Devonian extinctions, and the class was entirely extinct by the end of the Devonian.

Fossil record
The earliest identifiable placoderm fossils are from China and date to the Mid to Late Silurian. They are already differentiated into Antiarchs and Arthrodires, along with the other, more primitive groups. Apparently Placoderms diversified long before the Devonian, somewhere in early or mid Silurian, though earlier fossils of basal Placodermi have not been discovered in these particular strata.

The Silurian fossil record of the placoderms is both literally and figuratively fragmented. All known Silurian placoderms exist today only as fragments, either scraps of armor, or isolated scales, of which some have been tentatively identified as either antiarch or arthrodire due to histological similarities. Although they have been identified, many of the Silurian arthrodire and antiarch species have not yet been formally described, or even named. Paradoxically, the best known, or rather, most commonly cited example of a Silurian placoderm, Wangolepis of Silurian China, is known only from a few fragments that currently defy attempts to place them in any of the recognized placoderm orders.

Paleontologists and placoderm specialists suspect that the scarcity of the Silurian fossil record of placoderms is due to placoderms living in environments unconducive of fossil preservation, rather than a genuine scarcity. This hypothesis helps to explain the placoderms' seemingly miraculous appearance and diversity at the very beginning of the Devonian.

During the Devonian, the placoderms went on to inhabit and dominate almost all known aquatic ecosystems, both freshwater and saltwater, in stark contrast to the Silurian. But this diversity ultimately suffered many casualties during the extinction event at the Frasnian–Famennian boundary, the Late Devonian extinctions. The remaining species then died out during the Devonian/Carboniferous extinction event; not a single species survived into the Carboniferous.

Ecology and lifestyles
Many placoderms, particularly the Rhenanida, Petalichthyida, Phyllolepida, and Antiarchi, were bottom-dwellers. As such (paraphrasing Palaeos), the entire class has been popularly misunderstood as being merely a tribe of heavily armored bottom-feeders, even though they were actually the dominant vertebrate group during the Devonian. The vast majority of placoderms were predators, many of which lived at or near the bottom. Many, primarily the Arthrodira, were mid- to upper-water dwellers, and were active predators. The largest known arthrodire, Dunkleosteus telleri, was 8 to 11 meters long, and is presumed to have had a nearly worldwide distribution, as its remains have been found in Europe, North America and Morocco. In fact, it is regarded as the world's first vertebrate "super-predator". Other, smaller arthrodires, such as Fallacosteus and Rolfosteus of Gogo, had streamlined, bullet-shaped head armor, strongly supporting the idea that many, if not most, arthrodires were active swimmers, rather than passive ambush-hunters whose armor practically anchored them to the sea floor.

Extraordinary evidence of internal fertilisation in a placoderm was afforded by the discovery in the Gogo Formation, near Fitzroy Crossing Kimberley, Western Australia, of a small female placoderm, about 25 cm in length, which died in the process of giving birth to a  6 cm live young one and was fossilised with the umbilical cord intact. The fossil, named Materpiscis attenboroughi (after scientist David Attenborough), had eggs which were fertilised internally, the mother providing nourishment to the embryo and giving birth to live young. With this discovery, the placoderm became the oldest vertebrate known to have given birth to live young ("viviparous"), pushing the date of first viviparity back some 200 million years earlier than had been previously known.

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

History of study
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 Stensiö, 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 so completely destroyed), he made enlarged, three dimensional models of the skulls in order to examine the anatomical details more thoroughly. Many other placoderm specialists suspected that Stensiö was trying to shoehorn placoderms into a relationship with sharks, but with more fossil specimens found, the theory of placoderms being the sister-group of chondrichthyians became accepted as fact. However, with the discovery and examination of the exquisitely preserved Gogo reef placoderm fossils, it became apparent that the placoderms shared anatomical features not only with chondrichthyians, but with other gnathostome groups, as well. For example, Gogo placoderms show separate bone for the nasal capsules which are incorporated into the braincase of both sharks and bony fish. Because of these new insights provided by the Gogo Reef specimens, coupled with the fact that placoderms also share anatomical features only with the jawless Osteostracans, the theory that placoderms are the sister group of chondrichthyians has been replaced in favor of the theory that placoderms are a group of stem gnathostomes, in other words, they are the sister group of all other known gnathostomes.