Placodermi (from the Greek πλάξ = plate and δέρμα = skin, literally "plate-skinned") is a class of armoured prehistoric fish, known from fossils, which lived from the 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. Placoderms are paraphyletic, and consist of several distinct outgroups or sister taxa to all living jawed vertebrates, which originated among their ranks. This is illustrated by a 419-million-year-old fossil, Entelognathus, from China, which is the only known placoderm with a type of bony jaw like that found in modern bony fishes. This includes a dentary bone, which is found in humans and other tetrapods.[1][2] The jaws in other placoderms were simplified and consisted of a single bone.[3] Placoderms were also the first fish to develop pelvic fins, the precursor to hindlimbs in tetrapods, as well as true teeth.[4] Paraphyletic groupings are problematic, as one can not talk precisely about their phylogenic relationships, their characteristic traits and literal extinction.[5][6] 380-million-year-old fossils of three other genera, Incisoscutum, Materpiscis and Austroptyctodus, represent the oldest known examples of live birth.[7]

Temporal range: 430–358.9 Ma
Bothriolepis panderi.jpg
Fossil of Bothriolepis panderi showing its caliper-like pectoral fins
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Infraphylum: Gnathostomata
Class: Placodermi
McCoy, 1848


In contrast, one 2016 analysis concluded that placodermi are likely monophyletic.[8]

The first identifiable placoderms appear in the fossil record during the late Llandovery epoch of the early Silurian.[9] The various groups of placoderms were diverse and abundant during the Devonian, but became extinct at the end-Devonian Hangenberg event 358.9 million years ago [10]


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 Middle to Late Devonian genus, Bothriolepis, known from over 100 valid species.[11] The vast majority of placoderms were predators, many of which lived at or near the substrate. Many, primarily the Arthrodira, were active, nektonic 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 cavities and were made of both bone and dentine. However, the tooth and jaw development were not as closely integrated as in modern gnathostomes. These teeth were likely homologous to the teeth of other gnathostomes.[4]

External anatomy of the placoderm Coccosteus decipiens

One of the largest known arthrodires, Dunkleosteus terrelli , was 6 m (20 ft) long,[12][13] and is presumed to have had a large distribution, as its remains have been found in Europe, North America and possibly Morocco. Some paleontologists regard it as the world's first vertebrate "superpredator", preying upon other predators. Other, smaller arthrodires, such as Fallacosteus and Rolfosteus, both of the Gogo Formation of Western Australia, 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, 8 m (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,[14] 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 12 in) offspring and was fossilized with the umbilical cord intact.[15] The fossil, named Materpiscis attenboroughi (after scientist David Attenborough), had eggs which were fertilized 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"),[7] pushing the date of first viviparity back some 200 million years earlier than had been previously known. Specimens of the arthrodire Incisoscutum ritchei, also from the Gogo Formation, have been found with embryos inside them indicating this group also had live bearing ability.[16] The males reproduced by inserting a long clasper into the female. Elongated basipterygia are also found on the phyllolepid placoderms, such as Austrophyllolepis[17] and Cowralepis, both from the Middle Devonian of Australia, suggesting that the basiptergia were used in copulation.

The placoderm claspers are not homologous with the claspers in cartilaginous fishes. The similarities between the structures has been revealed to be an example of convergent evolution. While the claspers in cartilaginous fishes are specialized parts of their paired pelvic fins that have been modified for copulation due to changes in the hox genes hoxd13, the origin of the mating organs in placoderms most likely relied on different sets of hox genes and were structures that developed further down the body as an extra and independent pair of appendages, but which during development turned into body parts used for reproduction only. Because they were not attached to the pelvic fins, as are the claspers in fish like sharks, they were much more flexible and could probably be rotated forward.[18]

Evolution and extinctionEdit

Evolution and extinction of placoderms. The diagram is based on Michael Benton, 2005.[19]
Fin spine of Eczematolepis, from the Middle Devonian of Wisconsin.

It was thought for a time that placoderms became extinct due to competition from the first bony fish and early sharks, given a combination of the supposed inherent superiority of bony fish and the presumed sluggishness of placoderms. With more accurate summaries of prehistoric organisms, it is now thought that they systematically died out as marine and freshwater ecologies suffered from the environmental catastrophes of the Devonian/Carboniferous extinction events.

Fossil recordEdit

The earliest identifiable placoderm fossils are from China and date to the early Silurian. At that time, they were already differentiated into antiarchs and arthrodires, as well as other, more primitive, groups. Earlier fossils of basal Placodermi have not yet been discovered.

The Silurian fossil record of the placoderms is both literally and figuratively fragmented. Until the discovery of Silurolepis (and then, the discoveries of Entelognathus and Qilinyu), Silurian-aged placoderm specimens consisted of fragments. Some of them have been tentatively identified as antiarch or arthrodire due to histological similarities; and many of them have not yet been formally described or even named. The most commonly cited example of a Silurian placoderm, Wangolepis of Silurian China and possibly Vietnam, is known only from a few fragments that currently defy attempts to place them in any of the recognized placoderm orders. So far, only three officially described Silurian placoderms are known from more than scraps:

  • the basal antiarch Silurolepis, from the Ludlow epoch of Yunnan, China, known from an almost complete thoracic armor
  • Entelognathus, a placoderm incertae sedis that combines features of primitive arthrodires with jaw anatomy otherwise only seen in bony fish and tetrapods.
  • Qilinyu, a close relative of Entelognathus that further links Entelognathus as a transitional form between placoderms and other stem-gnathostomes and crown-group gnathostomes.



During the Devonian, placoderms went on to inhabit and dominate almost all known aquatic ecosystems, both freshwater and saltwater.[21] But this diversity ultimately suffered many casualties during the extinction event at the FrasnianFamennian boundary, the Late Devonian extinctions. The remaining species then died out during the Devonian/Carboniferous extinction event; not a single placoderm species has been confirmed to have survived into the Carboniferous.

History of studyEdit

The earliest studies of placoderms were published by Louis Agassiz, in his five volumes on fossil fishes, 1833–1843. In those days, 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.



Placoderms also share certain anatomical features only with the jawless osteostracans; because of this, the theory that placoderms are the sister group of chondrichthyians has been replaced by the theory that placoderms are a group of stem gnathostomes.

Taxonomy and phylogenyEdit

Currently, Placodermi are divided into eight recognized orders. There are two further controversial orders: One is the monotypic Stensioellida, containing the enigmatic Stensioella; the other 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.

Placoderm orders


Bothriolepis canadensis
Brindabellaspis stensioi
Phyllolepis orvini
Rhamphodopsis threiplandi
Ptyctodontida ("folded teeth") were lightly armoured placoderms with big heads, big eyes and long bodies. They have 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 claspers on the head as well.

Pseudopetalicthys problematica
Stensioella heintzi


The following cladogram shows the interrelationships of placoderms according to Carr et al. (2009):[26]































See alsoEdit


  1. ^ Barford, Eliot (September 25, 2013). "Ancient fish face shows roots of modern jaw". Nature. doi:10.1038/nature.2013.13823. Retrieved September 26, 2013.
  2. ^ Meredith Smith, Moya; Clark, Brett; Goujet, Daniel; Johanson, Zerina (2017-08-17). "Evolutionary origins of teeth in jawed vertebrates: conflicting data from acanthothoracid dental plates ('Placodermi')" (PDF). Palaeontology. 60 (6): 829–836. doi:10.1111/pala.12318. ISSN 0031-0239.
  3. ^ Entelognathus: The Fish with the First Modern Face
  4. ^ a b 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. 491 (7426): 748–751. Bibcode:2012Natur.491..748R. doi:10.1038/nature11555. PMID 23075852.
  5. ^ Schilhab, Theresa; Stjernfelt, Frederik; Deacon, Terrence (2012-03-26). The Symbolic Species Evolved. Springer Science & Business Media. ISBN 9789400723351.
  8. ^ King, Benedict; Qiao, Tuo; Lee, Michael S. Y.; Zhu, Min; Long, John A. (2016-12-05). "Bayesian Morphological Clock Methods Resurrect Placoderm Monophyly and Reveal Rapid Early Evolution in Jawed Vertebrates". Systematic Biology. 66 (4): 499–516. doi:10.1093/sysbio/syw107. ISSN 1063-5157. PMID 27920231.
  9. ^ a b Burrow, Carol & Turner, Susan (September 1998). "A review of placoderm scales, and their significance in placoderm phylogeny". Journal of Vertebrate Paleontology. 19 (2): 204–219. doi:10.1080/02724634.1999.10011135.
  11. ^ Long 1983
  13. ^ Carr, Robert K. (2010). "Paleoecology of Dunkleosteus terrelli (Placodermi: Arthrodira)". Kirtlandia. 57.
  14. ^ Long & Trinajstic 2010
  15. ^ Long et al. 2008
  16. ^ Long et al. 2009
  17. ^ Long 1984
  18. ^ The first vertebrate sexual organs evolved as an extra pair of legs
  20. ^ Wang Junqing (1991). "The Antiarchi from Early Silurian Hunan" (PDF). Vertebrata PalAsiatica. 21 (3): 240–244. Archived from the original (PDF) on 2013-12-12.
  22. ^ Young, G.C. & Goujet, D. & Lelievre, H. (2001) Extraocular muscles and cranial segmentation in primitive gnathostomes – fossil evidence. Journal of Morphology. 248:304.
  25. ^ "Philippe Janvier Tree of Life Contributor Profile".
  26. ^ Carr, RK; Johanson, Z; Ritchie, A (2009). "The phyllolepid placoderm Cowralepis mclachlani: Insights into the evolution of feeding mechanisms in jawed vertebrates" (PDF). Journal of Morphology. 270 (7): 775–804. doi:10.1002/jmor.10719. PMID 19215000. Archived from the original (PDF) on 2013-06-04.


External linksEdit