Temporal range: 430–360Ma Early/Mid Silurian – Late Devonian
|Fossil of Bothriolepis panderi showing its caliper-like pectoral fins|
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.
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. A study of the arthrodire Compagopiscis published in 2012 concluded that placoderms (at least this particular genus) likely possessed true teeth contrary to some early studies. The teeth had well defined pulp cavity and were made of both bone and dentine. However the tooth and jaw development were not as closely integrated as in modern gnathostoma. These teeth were likely homologous to the teeth of other gnathostomes.
One of the largest known arthrodires, Dunkleosteus telleri, was 8–11 m (26–36 ft) long, and is presumed to have had a large distribution, as its remains have been found in Europe, and North America and possibly 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, 7–8 m (23–26 ft) 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 (10 in) 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.
Evolution and extinction 
It is 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.
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.
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.
Taxonomy and phylogeny 
Currently Placodermi are divided into eight recognized orders. There are two further controversial orders. One order is the monotypic Stensioellida, containing the enigmatic Stensioella, and the other order is the equally enigmatic Pseudopetalichthyida. These orders are considered to be basal or primitive groups within Placodermi, though their precise placement within the class remains unsure. Fossils of both are currently known only from the Hunsruck lagerstatten.
Arthrodira (jointed-neck) were the most diverse and numerically successful of the placoderm orders, occupying roles from giant apex predators to detritus-nibbling bottom dwellers. They had a movable joint between armour surrounding the head and body. As the lower jaw moved down the head shield moved, allowing for a larger opening. Like all placoderms they lacked teeth, and used instead the sharpened edges of a bony plate as a biting surface. The eye sockets are protected by a bony ring, a feature shared by birds and some ichthyosaurs. Early arthrodires, such as the genus Arctolepis, were well-armoured fishes with flattened bodies. The largest member of this group, Dunkleosteus, was a true superpredator of the latest Devonian period, reaching 3 to as much as 9 metres in length. In contrast, the long-nosed Rolfosteus measured just 15 cm. Fossils of Incisoscutum have been found containing unborn fetuses, indicating that arthrodires gave birth to live young.
||Antiarchi (opposite-anus) were the second most successful order of placoderms known, after the Arthrodira. The order's name was coined by Edward Drinker Cope, who mistakenly thought the eye-hole was the mouth, and the opening for the anal siphon was on the other side of the body, as opposed to having both oral and anal siphons together at one end. The front portions of their bodies were heavily armoured, to the point of literally resembling a box with eyes, with the sometimes scaled, sometimes naked rear portions often becoming sinuous, particularly with later forms. The pair of pectoral fins were modified into a pair of caliper-like, or arthropod-like limbs. In primitive forms, such as Yunnanolepis, the limbs were thick and short, while in advanced forms, such as Bothriolepis, the limbs were long and had elbow-like joints. The function of the limbs are still not perfectly understood, but, most hypothesize that they helped their owners pull themselves across the substrate, as well as allow their owners to bury themselves into the substrate.|
||Brindabellaspida (Brindabella's-shield) was a long-snouted placoderm from the Early Devonian. When it was first discovered in 1980, it was originally regarded as a Weejasperaspid acanthothoracid due to anatomical similarities with the other species found at the same locality. According to Philippe Janvier, anatomical similarities in the brain of Brindabellaspis stensioi and the brain of a jawless fish suggest it is a basal placoderm closest to the ancestral placoderm. Various Early to Middle Devonian placoderm incertae sedis have also been inserted in the order|
||Phyllolepida (leaf-scales) were flattened placoderms found throughout the world. Like other flattened placoderms they were bottom-dwelling predators that ambushed prey. Unlike other flattened placoderms, they were freshwater fish. Their armour was made of whole plates, rather than the numerous tubercles and scales of Petalichthyida. The eyes were on the sides of the head, unlike visual bottom-dwelling predators, such as stargazers or flatfish which have eyes on the top of their head. The orbits for the eyes were extremely small, suggesting the eyes were vestigial and that the Phyllolepids may have been blind.|
||Ptyctodontida (beak-teeth) were lightly armoured placoderms with big heads, big eyes and long bodies. They has a strong but superficial resemblance to modern day chimaeras. Their armour was reduced to a pattern of small plates around the head and neck. Like the extinct and related acanthothoracids, and the living and unrelated holocephalians, most of the ptyctodontids are thought to have lived near the sea bottom and preyed on shellfish. On account of their lack of armour, some paleontologists have suggested that the Ptyctodontida were not placoderms, but holocephalians or the ancestors of holocephalians. Anatomical examinations of whole fossil specimens have shown that the similarities between these two groups are superficial. The major differences were that holocephalians have shagreen on their skin while ptyctodontids do not, the armoured plates and scales of holocephalians are made of dentine while those of ptyctodontids are made of bone, the craniums of holocephalians are similar to sharks while those of ptyctodontids are similar to those of other placoderms, and most importantly, that holocephalians have true teeth while ptyctodonts have beak-like tooth-plates. Ptyctodontids were sexually dimorphic, with the males having pelvic claspers and possibly head-claspers as well.|
||Rhenanida (Rhine-fish) were flattened, ray-like, bottom-dwelling predators with large, upturned mouths that lived in marine environments. The rhenanids were once presumed to be the most primitive, or at least the closest to the ancestral placoderm, as their armour was made of unfused components, a mosaic of tubercles, as opposed to the solidified plates of "advanced" placoderms, such as antiarchs and arthrodires. However, through comparisons of skull anatomies, rhenanids are now considered to be the sister group of the antiarchs. When rhenanids die, their mosaics come apart, and it has been suggested that the rarity of rhenanids in the fossil record reflects postmortem disassociation, and is not an actual rarity of the species.|
||Acanthothoraci (spine-chests) were a group of chimaera-like placoderms closely related to the rhenanid placoderms. Superficially acanthoracids resembled scaly chimaeras or the relatively heavily armoured ptyctodonts. They were distinguished from chimaeras by a pair of large spines that emanate from their chests, the presence of large scales and plates, tooth-like beak plates, and the typical bone-enhanced placoderm eyeball. They were distinguished from other placoderms due to differences in the anatomy of their skulls, and due to patterns on the skull plates and thoracic plates that are unique to this order. From what can be inferred from the mouthplates of fossil specimens, acanthothoracids were shellfish hunters were ecologically similar to modern-day chimaeras. Competition with their relatives, the ptyctodont placoderms, may have been one of the main reasons for the acanthothoracids' extinction prior to the Mid Devonian extinction event.|
||Petalichthyida (thin plate fish) were small, flattened placoderms, typified by their splayed fins and numerous tubercles that decorated all of the plates and scales of their armour. They reached a peak in diversity during the Early Devonian and were found throughout the world. The petalichthids Lunaspis and Wijdeaspis are among the best known. There was an independent diversification event that occurred in what is now Southern China, producing a handful of unique genera that were once placed in their own order, "Quasipetalichthyida," named after the first discovered species there, Quasipetalichthys haikouensis. Soon after the Petalichthids' diversification, they went into decline. Because they had compressed body forms, it is supposed they were bottom-dwellers that chased after or ambushed smaller fish. Their diet is not clear, as none of the fossil specimens found have preserved mouth parts.|
||Pseudopetalichthyida (false petalichthyids) is group of ray-like fishes comprising three, poorly preserved and poorly studied genera. It is known only from rare fossils in Lower Devonian strata in Hunsrück, Germany. Like Stensioella heintzi, and the Rhenanida, the Pseudopetalichthids had armour made up of a mosaic of tubercles. Like Stensioella heintzi, the Pseudopetalichthids' placement within Placodermi is suspect. The matter is not easy to resolve because there are no complete, undamaged and articulated specimens. The anatomical studies done on the crushed specimens that have been found indicate that if they are placoderms consider then they are a group more advanced than the Ptyctodonts. As such, pro-placoderm experts consider Pseudopetalichthyida to be the sister group of the Arthrodires + Phyllolepida + Antiarchi trichotomy and the Acanthothoraci + Rhenanida dichotomy.|
||Stensioellida (Heintz's-little-Stensio) is another problematic placoderm of uncertain affinity, known only from the Lower Devonian Hunsrück slates of Germany. It was a squat fish that resembled a pug-nosed chimaera-stargazer, or a liberated Gemuendina with broad, wing-like pectoral fins. Like Gemuendina, it had a complex armoured mosaic of tubercles. The shoulder joints of its armour are similar to other placoderms, and there are superficial similarities in skull plates, and even more superficial similarities between its tubercles and the tubercles of the rhenanids. It is tentatively placed within Placodermi as a primitive placoderms, though some paleontologists believe the rationale for the placement is inadequate. The paleontologist Philippe Janvier has suggested that Stensioellida is not a placoderm but is a holocephalid. If this is true then the holocephalids diverged from sharks before the Chondrichthyan Devonian radiation. Critics of Janvier's position say that aside from a bodyplan similar to primitive holocephalidst here is little else in common.|
See also 
- "Fossil reveals oldest live birth". BBC. May 28, 2008. Retrieved May 30, 2008.
- Long 1983
- Rücklin, M.; Donoghue, P. C. J.; Johanson, Z.; Trinajstic, K.; Marone, F.; Stampanoni, M. (2012). "Development of teeth and jaws in the earliest jawed vertebrates". Nature. doi:10.1038/nature11555.
- Long & Trinajstic 2010
- Long et al. 2008
- Long et al. 2009
- Ahlberg et al. 2009
- Long 1984
- Benton, M. J. (2005) Vertebrate Palaeontology, Blackwell, 3rd edition, Fig 3.25 on page 73.
- Waggoner, Ben. "Introduction to the Placodermi". UCMP. Retrieved 18 July 2011.
- Young, G.C. & Goujet, D. & Lelievre, H. (2001) Extraocular muscles and cranial segmentation in primitive gnathostomes - fossil evidence. J. Morphology. 248:304.
- Goujet, Daniel & Young, Gavin (2004). Placoderm anatomy and phylogeny: new insights. (in) Arratia, Wilson and Cloutier (eds) Recent Advances in the Origin and Early Radiation of Vertebrates. Verlag Dr. Friedrich Pfeil, Munchen, Germany.
- "Fish 380 Million Years Old Found With Unborn Embryo". Science Daily. June 6, 2008.
- Carr RK, Johanson Z and Ritchie A (2009) "The phyllolepid placoderm Cowralepis mclachlani: Insights into the evolution of feeding mechanisms in jawed vertebrates" Journal of morphology, 270 (7): 775–804. doi:10.1002/jmor.10719
- Ahlberg ,P.E., Trinajstic, K., Johanson, Z. & Long, J.A. 2009. Pelvic claspers confirm chondrichthyan-like internal fertilization in arthrodires. Nature 460: 888-889.
- Janvier, P. Early Vertebrates Oxford, New York: Oxford University Press, 1998. ISBN 0-19-854047-7
- LONG, J.A. 1983. New bothriolepid fishes from the Late Devonian of Victoria, Australia. Palaeontology 26: 295-320
- Long, J.A. 1984. New phyllolepids from Victoria and the relationships of the group. Proceedings of the Linnean Society of New South Wales 107: 263-308.
- Long, J.A. The Rise of Fishes: 500 Million Years of Evolution Baltimore: The Johns Hopkins University Press, 1996. ISBN 0-8018-5438-5
- Long, J.A. & Trinajstic, K. 2010. The Late Devonian Gogo Formation Lagerstatte –Exceptional preservation and Diversity in early Vertrebrates. Annual Reviews of Earth and Planetary Sciences 38: 665-680
- Long, J.A., Trinajstic, K., Young, G.C. & Senden, T. 2008. Live birth in the Devonian. Nature 453: 650-652.
- Long, J.A., Trinajstic, K. & Johanson, Z. 2009. Devonian arthrodire embryos and the origin of internal fertilization in vertebrates. Nature 457: 1124-27.
- Zhu M, X Yu, B Choo, J Wang and L Jia (2012) "An antiarch placoderm shows that pelvic girdles arose at the root of jawed vertebrates" Journal of the Royal Society, Biology Letters, 8 (3): 453–456.
|Wikimedia Commons has media related to: Placodermi|
- Annetta Markussen-Brown, "Devonian Armoured Fish" 2000
- Introduction to the Placodermi Extinct armored fishes with jaws
- PALAEOZOIC FOSSILS UK
- Placodermi: Overview
- BBC—report on Dunkleosteus terrelli