The ptyctodontids ("folded-teeth") are

placoderm Placoderms (from Ancient Greek πλάξ 'plax'', ''plakos'''Plate (animal anatomy), plate' and δέρμα 'derma'''skin') are vertebrate animals of the class (biology), class Placodermi, an extinct group of prehistoric fish known from Pal ...

s of the

order Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood ...

Ptyctodontida, containing the

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

Ptyctodontidae. With their big heads, big eyes, reduced armor and long bodies, the ptyctodontids bore a superficial resemblance to modern day chimaeras (

Holocephali Holocephali (Sometimes spelled Holocephala; Romanization of Greek, Greek for "complete head" in reference to the fusion of Palatoquadrate, upper jaw with the rest of the skull) is a Subclass (biology), subclass of Chondrichthyes, cartilaginous fi ...

). Their armor was reduced to a pattern of small plates around the head and neck. Like the extinct and related

acanthothoracids Acanthothoraci (''spine chests'') is an extinct group of chimaera-like placoderms closely related to the rhenanid placoderms. Superficially, the acanthoracids resembled scaly chimaeras and (relatively) heavily armored ptyctodonts. They were ...

, and the living and unrelated holocephalians, most of the ptyctodontids are thought to have lived near the sea bottom and preyed on

shellfish Shellfish, in colloquial and fisheries usage, are exoskeleton-bearing Aquatic animal, aquatic invertebrates used as Human food, food, including various species of Mollusca, molluscs, crustaceans, and echinoderms. Although most kinds of shellfish ...

. On account of their radically reduced armor, some paleontologists have suggested that the Ptyctodontida were not actually placoderms, but actual holocephalians, some primitive group of

elasmobranch Elasmobranchii () is a subclass of Chondrichthyes or cartilaginous fish, including modern sharks ( division Selachii), and batomorphs (division Batomorphi, including rays, skates, and sawfish). Members of this subclass are characterised by h ...

fish, or even were the ancestors of the holocephalians, including the chimaeras. Thorough anatomical examinations of whole fossil specimens reveal that the profound similarities between these two groups are actually very superficial. The major differences between them were that holocephalians have

shagreen Shagreen is a type of rawhide consisting of rough untanned skin, historically from a horse's or onager's back, or from shark or ray. Etymology The word derives from the French ' and is related to Italian and Venetian ''sagrin'', deriv ...

on their skin and ptyctodontids did not, that the armored plates and scales of holocephalians are made of

dentine Dentin ( ) (American English) or dentine ( or ) (British English) () is a calcified tissue of the body and, along with enamel, cementum, and pulp, is one of the four major components of teeth. It is usually covered by enamel on the crown ...

, and the armored plates and scales of ptyctodontids were made of bone, the anatomy of the craniums of holocephalians is more similar to sharks, and that of ptyctodontids were more similar to those of other placoderms, and, most importantly, the holocephalians have true teeth, while the ptyctodonts had beak-like tooth-plates. The Ptyctodontida were the only known group of placoderms that were recognizably

sexually dimorphic Sexual dimorphism is the condition where sexes of the same species exhibit different Morphology (biology), morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecy, di ...

, in that the males had hook-like growths on their pelvic fins that were analogous to the clasping organs found in male sharks, and chimaeras. Paleontologists believe that the males of the ancestral placoderm had pelvic claspers, but the claspers were lost in the evolutionary development of each of the placoderm orders, save for the ptyctodontids (there are too few whole specimens of the primitive '' Stensioella heintzi'' to tell if the males of that species had claspers or not). Because they had reduced armor, the ptyctodontids were once thought to be the most primitive of the placoderms. Indeed, there has been the idea that the placoderms had a gradient, of sorts, from the least armored, and most primitive forms, to the heavily armored, most advanced forms. During the 1980s and '90's, ptyctodont skulls were compared with skulls from other orders. From these analyses, this idea of a gradient from least armored to most armored in placoderms was discarded. Now, the ptyctodonts are regarded as the sister group of the

Arthrodira Arthrodira (Greek for "jointed neck") is an order of extinct armored, jawed fishes of the class Placodermi that flourished in the Devonian period before their sudden extinction, surviving for about 50 million years and penetrating most marine eco ...

and

Phyllolepida Phyllolepida ("leaf scales") is an extinct taxon of flattened placoderms found throughout the world, with fossils being found in Devonian strata. Like other flattened placoderms, the phyllolepids were bottom-dwelling predators that ambushed prey ...

Timeline of genera

ImageSize = width:1000px height:auto barincrement:15px PlotArea = left:10px bottom:50px top:10px right:10px Period = from:-416 till:-299 TimeAxis = orientation:horizontal ScaleMajor = unit:year increment:25 start:-416 ScaleMinor = unit:year increment:5 start:-416 TimeAxis = orientation:hor AlignBars = justify Colors = #legends id:CAR value:claret id:ANK value:rgb(0.4,0.3,0.196) id:HER value:teal id:HAD value:green id:OMN value:blue id:black value:black id:white value:white id:paleozoic value:rgb(0.6,0.75,0.55) id:cambrian value:rgb(0.49,0.63,0.33) id:ordovician value:rgb(0,0.57,0.44) id:silurian value:rgb(0.70,0.88,0.71) id:devonian value:rgb(0.8,0.55,0.22) id:earlydevonian value:rgb(0.90,0.71,0.43) id:middledevonian value:rgb(0.96,0.81,0.51) id:latedevonian value:rgb(0.96,0.89,0.71) id:carboniferous value:rgb(0.4,0.65,0.6) id:mississippian value:rgb(0.4,0.56,0.4) id:pennsylvanian value:rgb(0.8,0.77,0.53) id:permian value:rgb(0.94,0.25,0.24) id:mesozoic value:rgb(0.38,0.77,0.79) id:triassic value:rgb(0.51,0.17,0.57) id:jurassic value:rgb(0.2,0.7,0.79) id:cretaceous value:rgb(0.5,0.78,0.31) id:cenozoic value:rgb(0.95,0.98,0.11) id:paleogene value:rgb(0.99,0.6,0.32) id:neogene value:rgb(0.999999,0.9,0.1) id:quaternary value:rgb(0.98,0.98,0.50) BarData= bar:eratop bar:space bar:periodtop bar:space bar:NAM1 bar:NAM2 bar:NAM3 bar:NAM4 bar:NAM5 bar:NAM6 bar:NAM7 bar:NAM8 bar:NAM9 bar:NAM10 bar:NAM11 bar:NAM12 bar:space bar:period bar:space bar:era PlotData= align:center textcolor:black fontsize:M mark:(line,black) width:25 shift:(7,-4) bar:periodtop from: -416 till: -411.2 color:earlydevonian text: Loch. from: -411.2 till: -407 color:earlydevonian text: Prag. from: -407 till: -397.5 color:earlydevonian text: Ems. from: -397.5 till: -391.8 color:middledevonian text: Eif. from: -391.8 till: -385.3 color:middledevonian text:

Givetian The Givetian is one of two faunal stages in the Middle Devonian Period. It lasted from million years ago to million years ago. It was preceded by the Eifelian Stage and followed by the Frasnian Stage. It is named after the town of Givet in ...

from: -385.3 till: -374.5 color:latedevonian text:

Frasnian The Frasnian is one of two faunal stages in the Late Devonian Period. It lasted from million years ago to million years ago. It was preceded by the Givetian Stage and followed by the Famennian Stage. Major reef-building was under way during ...

from: -374.5 till: -359.2 color:latedevonian text:

Famennian The Famennian is the later of two faunal stages in the Late Devonian epoch. The most recent estimate for its duration is that it lasted from around 371.1 to 359.3 million years ago. An earlier 2012 estimate, still used by the International Commis ...

bar:eratop from: -416 till: -359.2 color:devonian text:

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

PlotData= align:left fontsize:M mark:(line,white) width:5 anchor:till align:left color:earlydevonian bar:NAM1 from: -411.2 till: -407 text: Tollodus color:middledevonian bar:NAM2 from: -397.5 till: -391.8 text: Goniosteus color:middledevonian bar:NAM3 from: -397.5 till: -385.3 text: Desmoporella color:middledevonian bar:NAM4 from: -397.5 till: -378.1 text: Campbellodus color:middledevonian bar:NAM5 from: -397.5 till: -374.5 text: Ctenurella color:middledevonian bar:NAM6 from: -397.5 till: -359.2 text: Denisonodus color:middledevonian bar:NAM7 from: -397.5 till: -359.2 text: Ptyctodopsis color:latedevonian bar:NAM8 from: -385.3 till: -381.7 text:

Chelyophorus The ptyctodontids ("folded-teeth") are placoderms of the order Ptyctodontida, containing the family Ptyctodontidae. With their big heads, big eyes, reduced armor and long bodies, the ptyctodontids bore a superficial resemblance to modern day chim ...

color:latedevonian bar:NAM9 from: -385.3 till: -381.7 text: Eczematolepis color:latedevonian bar:NAM10 from: -385.3 till: -381.7 text: Palaeomylus color:latedevonian bar:NAM11 from: -385.3 till: -381.7 text: Ptyctodus color:latedevonian bar:NAM12 from: -385.3 till: -359.2 text:

Rhynchodus The ptyctodontids ("folded-teeth") are placoderms of the order Ptyctodontida, containing the family Ptyctodontidae. With their big heads, big eyes, reduced armor and long bodies, the ptyctodontids bore a superficial resemblance to modern day chim ...

PlotData= align:center textcolor:black fontsize:M mark:(line,black) width:25 bar:period from: -416 till: -411.2 color:earlydevonian text: Loch. from: -411.2 till: -407 color:earlydevonian text: Prag. from: -407 till: -397.5 color:earlydevonian text: Ems. from: -397.5 till: -391.8 color:middledevonian text: Eif. from: -391.8 till: -385.3 color:middledevonian text:

from: -385.3 till: -374.5 color:latedevonian text:

from: -374.5 till: -359.2 color:latedevonian text:

bar:era from: -416 till: -359.2 color:devonian text:

Gallery

Image:Gamphacanthus.jpg, Fin spine of the ptyctodont, ''Gamphacanthus'', showing color patterns; from the Middle Devonian of Wisconsin. Image:Ptyctodus ferox.jpg, Tooth plates from the ptyctodont ''Ptyctodus ferox'', from the Middle Devonian of Wisconsin. Note the grinding surfaces. Image:Ptyctodont tooth plate.jpg, Beak-like tooth plate of a ptyctodont from the Middle Devonian of Wisconsin.

References

* (1996): ''The Rise of Fishes: 500 Million Years of Evolution''. Johns Hopkins University Press, Baltimore. * {{Taxonbar, from1=Q918188, from2=Q18618797 * Early Devonian first appearances Late Devonian animals Late Devonian extinctions Placoderm orders Monotypic vertebrate orders