Placodermi

Placodermi (from the Greek πλάξ = plate and δέρμα = skin, literally "plate-skinned") is an extinct class of armoured 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 fish; 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.

Taxonomy and phylogeny
Currently, Placodermi is divided into 9 or 10 recognized orders.

One order is the monotypic Stensioellida, comprising of the enigmatic Stensioella. Another order is the equally enigmatic Pseudopetalichthyida, which comprises three, poorly preserved and poorly studied genera. Both Stensioellida and Pseudopetalichthyida are considered to be basal or primitive groups within Placodermi, though, their precise placement within the class remains unsure. Fossils of both are currently only known from the Hunsruck lagerstatten.

The Petalichthyida are a group of flattened placoderms, known primarily from skulls dating from the Lower to Middle Devonian. It is assumed that they were all benthic predators, though, no mouth parts have ever been found or observed. They have a superficial resemblance to the Phyllolepida.

Brindabellaspida is typified by Brindabellaspis, though, various Early to Middle Devonian placoderm incertae sedis have been inserted in the order, too. Brindabellaspida is considered to be either a stem group, or closest to the stem placoderm, as the cranial anatomy of Brindabellaspis displays many similarities with the cranial anatomies of osteostracan and galaeaspid agnathans. The gross anatomy of Brindabellaspis resembles that of the Weejasperaspid Acanthothoracids.

The Acanthothoraci are a group of generalized placoderms with a stout spine emanating from the median dorsal plate, and (with most), an elongated head. One family, Weejasperaspidae, is considered monophyletic, being the sister-group of Brindabellaspida. Another family, Palaeacanthaspidae, however, is considered paraphyletic, as various genera have been demonstrated to have several anatomical similarities with Rhenanida, Arthrodira, and Antiarchi, thus implying that these genera are the sister-groups of these orders. Brindabellaspid fossils are found mostly in Early and some Middle Devonian strata. The fossils of Acanthothoraci are found in Early and Middle Devonian strata, though, if the fossil Grazosteus is really an acanthothoracid, and not an arthrodire, their range is extended into the Late Devonian.

The Rhenanida are a group of distinctive, superficially ray-like fishes with large, upturned mouths. Their fossils range from near the beginning of the Devonian until the Late Devonian. As with Stensioellida, the armor of Rhenanids are not solid plates, but a mosaic of scales and small plates arranged in patterns corresponding to the various plates typical to placoderms. Upon death, these mosaics come apart, and it is suggested that the rhenanids' rarity in the fossil record reflects postmortem disassociation, and not an actual rarity of species.

The Arthrodira are the most successful of the placoderm groups, their fossils being found from the Late Silurian until the very end of the Devonian (though, no Silurian arthrodires have been officially described or named). The Arthrodires inhabited both fresh- and saltwater environments, and had a wide array of body-shapes, generally ranging from flattened to bullet-shaped. Many arthrodires were benthic-dwelling, though many were also actively swimming nekton, as well. The Arthrodires include many giant members, including the earliest known "giant vertebrate," the 2.5 meter long Tityosteus of Early Devonian Germany, and the first vertebrate superpredators, including Dunkleosteus. According to fossils of Incisoscutum containing unborn fetuses, it is strongly implied that arthrodires gave birth to live young.

The Phyllolepida are a group of freshwater bottom-dwellers that superficially resemble the Petalichthyids. The anatomy of the Phyllolepids, however, suggest that they are actually either a sister-group of the Arthrodires, or are a sub-group within Arthrodira, closely related to the Wuttagoonaspids.

The Ptyctodontida are a group of lightly armored placoderms that bear a strong, albeit superficial likeness to chimaeras. Because only their heads were armored, most ptyctodontids are known from their beak-like toothplates, though, many genera are known from whole-body fossils. These whole-body fossils also show that ptyctodontids had sexual dimorphism, with the males having pelvic claspers. The articulation of headplates in Campbellodus and Ctenurella also suggest that the males had head-claspers, as well.

After Arthrodira, the second-most successful placoderm group are the Antiarchi, a group of bizarre, box-like marine and freshwater benthic creatures whose pectoral fins were modified into caliper-like appendages. The earliest known antiarch is Silurolepis, from the Ludlow epoch of Yunnan. Antiarchs would persist in the fossil record for the entirety of the Devonian.

Below is a cladogram showing the interrelationships of placoderms from Carr et al. (2009):

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 already diversified into their current groups before the start of the Devonian, somewhere during the 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. With one exception, 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. So far, the only officially described Silurian placoderm, and the only Silurian placoderm known from more than scraps is the basal antiarch, Silurolepis, from the Ludlow epoch of Yunnan, China.

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

During the Devonian, in stark contrast to the Silurian, the placoderms went on to inhabit and dominate almost all known aquatic ecosystems, both freshwater and saltwater. 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. In particular, the antiarchs, with their highly modified, jointed bony pectoral fins, were highly successful inhabitants of Middle-Late Devonian freshwater and shallow marine habitats, with the Late Devonian genus, Bothriolepis, known from over 100 valid species. The vast majority of placoderms were predators, many of which lived at or near the substrate. Many, primarily the Arthrodira, were active, pelagic predators that dwelled in the middle to upper portions of the water column. The largest known arthrodire, Dunkleosteus telleri, was 8 to- long, and is presumed to have had a nearly worldwide distribution, as its remains have been found in Europe, North America and Morocco. Some paleontologists regard it 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. Some placoderms were herbivorous, such as the Middle to Late Devonian arthrodire Holonema, and some were planktivores, such as the gigantic, 7 to- long arthrodire, Titanichthys.

Extraordinary evidence of internal fertilization 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 ($2 1/2$ in) live young one and was fossilized 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. The arthrodire Incisoscutum ritchei, also from the Gogo Formation, have been found with embryos inside them indicating this group also had live bearing ability. The males reproduced by inserting a long clasper that was fused to part of the pelvic girdle, the basipterygium. Long basipterygia are also found on the phyllolepid placoderms, such as Austrophyllolepis and Cowralepis, both from the Middle Devonian of Australia, suggesting that the basiptergia were used in copulation.

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.