Placodermi

{{Automatic taxobox
| fossil_range = {{fossil range|430|360}}Early/Mid [[Silurian]] – Late [[Devonian]] 
| image = DunkleosteusSannoble.JPG
| image_width = 250px
| image_caption = ''Dunkleosteus'' skull at [[Sam Noble Oklahoma Museum of Natural History|Sam Noble Museum]]  
| authority = [[Frederick McCoy|McCoy]], 1848
| subdivision_ranks = [[Order (biology)|Order]]s
| subdivision = 
[[Antiarchi]] [[extinction|†]]<br />
[[Arthrodira]] †<br />
[[Brindabellaspida]] †<br />
[[Petalichthyida]] †<br />
[[Phyllolepida]] †<br />
[[Ptyctodontida]] †<br />
[[Rhenanida]] †<br />
[[Acanthothoraci]] †<br />
?[[Pseudopetalichthyida]] †<br />
?[[Stensioellida]] †
}}

'''Placodermi''' (from the Greek πλάξ = plate and δέρμα = skin, literally "plate-skinned") is an [[Extinction (biology)|extinct]] [[Class (biology)|class]] of armoured [[prehistoric fish]], known from [[fossil]]s, 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 [[scale (zoology)|scaled]] or naked, depending on the species. Placoderms were among the first [[jaw]]ed [[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.<ref name="BBC">{{cite web | publisher = BBC | url = http://news.bbc.co.uk/2/hi/science/nature/7424281.stm | title =  Fossil reveals oldest live birth | date = May 28, 2008 |accessdate = May 30, 2008 }}</ref> 

The first identifiable placoderms evolved in the late Silurian; they began a dramatic decline during the [[Late Devonian extinction]]s, and the class was entirely extinct by the end of the Devonian.

==Taxonomy and phylogeny==
Currently, Placodermi is divided into 9 or 10 recognized [[order (biology)|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 [[Acanthothoraci]]ds.

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 (biology)|family]], [[Weejasperaspidae]], is considered [[Monophyly|monophyletic]], being the sister-group of Brindabellaspida.  Another family, Palaeacanthaspidae, however, is considered [[paraphyly|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 [[Batoidea|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 [[superpredator]]s, including ''[[Dunkleosteus]]''.  According to fossils of ''[[Incisoscutum]]'' containing unborn fetuses, it is strongly implied that arthrodires gave birth to live young<ref>{{cite news |url=http://www.sciencedaily.com/releases/2008/06/080606104814.htm |publisher=[[Science Daily]] |title=Fish 380 Million Years Old Found With Unborn Embryo |date=June 6, 2008}}</ref>.

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 [[Wuttagoonaspidae|Wuttagoonaspids]].

The '''[[Ptyctodontida]]''' are a group of lightly armored placoderms that bear a strong, albeit superficial likeness to [[chimaera]]s.  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):<ref name=CJR09>{{cite doi|10.1002/jmor.10719}}</ref>

{{clade| style=font-size:100%;line-height:85%
|1={{clade
   |1=''[[Stensioella]]''
   |label2='''Placodermi'''
   |2={{clade
      |1=''[[Pseudopetalichthys]]''
      |2={{clade
         |1=''[[Brindabellaspis]]''
         |2=[[Acanthothoraci]]}}
      |3={{clade
         |1=[[Rhenanida]]
         |2={{clade
            |1={{clade
               |1=''[[Yunnanolepis]]''
               |2=[[Euantiarcha]]}}
            |2={{clade
               |1={{clade
                  |1=[[Petalichthyida]]
                  |2=[[Ptyctodontida]]}}
               |label2=[[Arthrodira]]
               |2={{clade
                  |1=''[[Wuttagoonaspis]]''
                  |2=[[Actinolepidae]]
                  |3={{clade
                     |1=[[Phyllolepida]]
                     |2={{clade
                        |1=[[Phlyctaeniida]]
                        |label2=[[Brachythoraci]]
                        |2={{clade
                           |1={{clade
                              |1=''[[Holonema]]''
                              |2=''[[Antineosteus]]''
                              |3=[[Buchanosteidae]]}}
                           |label2=[[Eubrachythoraci]]
                           |2={{clade
                              |1={{clade
                                 |1={{clade
                                    |1=''[[Pholidosteus]]''
                                    |2=''[[Tapinosteus]]''}}
                                 |2={{clade
                                    |1=''[[Coccosteus]]''
                                    |2=''[[Torosteus]]''}} }}
                              |2=''[[Plourdosteus]]''
                              |3={{clade
                                 |1=''[[Dunkleosteus]]''
                                 |2={{clade
                                    |1=''[[Brachyosteus]]''
                                    |2=''[[Erromenosteus]]''
                                    |3=''[[Gorgonichthys]]''
                                    |4=''[[Titanichtys]]''
}} }} }} }} }} }} }} }} }} }} }} }} }}

==Fossil record==
[[File:Dunkleosteus BW.jpg|thumb|left|220px|''[[Dunkleosteus]]'']]
The earliest identifiable placoderm fossils are from China and date to the mid to late [[Silurian]].  They are already differentiated into [[Antiarchi|antiarchs]] and [[Arthrodira|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 (phylogenetics)|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 [[seawater|saltwater]].<ref name=PlacodermUCMP>{{cite web|last=Waggoner|first=Ben|title=Introduction to the Placodermi|url=http://www.ucmp.berkeley.edu/vertebrates/basalfish/placodermi.html|publisher=UCMP|accessdate=18 July 2011}}</ref>  But this diversity ultimately suffered many casualties during the extinction event at the [[Frasnian]]–[[Famennian]] boundary, the [[Late Devonian extinction]]s.  The remaining species then died out during the Devonian/[[Carboniferous]] extinction event; not a single species survived into the Carboniferous.

==Ecology and lifestyles==
[[File:Phyllolepis12DB.jpg|thumb|left|150px|''[[Phyllolepis]]'']]
[[File:Coccosteus BW.jpg|thumb|right|250px|''[[Coccosteus]]'']] 
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.<ref>Long 1983</ref>   The vast majority of placoderms were [[predator]]s, 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 {{convert|8|to-|11|m|abbr=on}} 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, {{convert|7|to-|8|m|abbr=on}} 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)|Kimberley]], Western Australia,<ref>Long & Trinajstic 2010</ref> of a small female placoderm, about {{convert|25|cm|0|abbr=on}} in length, which died in the process of giving birth to a 6&nbsp;cm ({{frac|2|1|2}}&nbsp;in) live young one and was fossilized with the umbilical cord intact.<ref>Long et al. 2008</ref> 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 ("[[Vivipary|viviparous]]"),<ref name="BBC"/> 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.<ref>Long et al. 2009</ref> The males reproduced by inserting a long clasper that was fused to part of the pelvic girdle, the basipterygium.<ref>Ahlberg et al. 2009</ref> Long basipterygia are also found on the phyllolepid placoderms, such as ''[[Austrophyllolepis]]''<ref>Long 1984</ref> 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 [[Osteichthyes|bony fish]], and the early [[shark]]s, 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==

[[File:Homostius2DB.jpg|thumb|''[[Homostius]]'' and [[Pterichthys]]'']]
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 [[shark]]s.  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 [[shark]]s, but with more fossil specimens found, the theory of placoderms being the sister-group of [[Chondrichthyes|chondrichthyian]]s 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.<ref>Young, G.C. & Goujet, D. & Lelievre, H. (2001) Extraocular muscles and cranial segmentation in primitive gnathostomes - fossil evidence. J. Morphology. 248:304.</ref><ref>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.</ref>  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 [[osteostracan]]s, 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.

==See also==
{{Portal|Paleontology}}
* [[List of placoderms]]
* [[Ostracoderm]]
* [[Acanthodii]]

==Notes==
{{Reflist}}

==External links==
{{Commons category|Placodermi}}
* [http://hoopermuseum.earthsci.carleton.ca/placoderms/first.html Annetta Markussen-Brown, "Devonian Armoured Fish" 2000]
* [http://www.ucmp.berkeley.edu/vertebrates/basalfish/placodermi.html Introduction to the Placodermi Extinct armored fishes with jaws]
* [http://www.btinternet.com/~vendian/FOSSILWEB/paleozoic_fish.htm PALAEOZOIC FOSSILS UK]
* [http://www.toyen.uio.no/palmus/galleri/montre/english/m_panserhai_e.htm Placoderms]
* [http://www.palaeos.com/Vertebrates/Units/Unit060/060.000.html Placodermi: Overview]
* [http://news.bbc.co.uk/1/hi/sci/tech/6195188.stm BBC—report on ''Dunkleosteus terrelli'']

==References==
* 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.

{{Placodermi}}