Placodermi
Encyclopedia
Placodermi is a 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 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.
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.
└─Infraphylum Gnathostomata
├─Class Placodermi
— extinct (armored gnathostomes)
└Microphylum Eugnathostomata
(true jawed vertebrates)
├─Class Chondrichthyes
(cartilaginous fish)
└─(unranked) Teleostomi
(Acanthodii & Osteichthyes)
├─Class Acanthodii
— extinct ("spiny sharks")
└Superclass Osteichthyes
(bony fish)
├─Class Actinopterygii
(ray-finned fish)
└─Class Sarcopterygii
(lobe-finned fish)
└Superclass Tetrapoda
├─Class Amphibia (amphibians)
└(unranked) Amniota (amniotic egg)
├─Class Sauropsida
(reptiles or sauropsids)
│ └─Class Aves
(birds)
└─Class Synapsida
└─Class Mammal
ia (mammals)
Note: lines show evolutionary relationships.
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. 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.
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 extinction
s. The remaining species then died out during the Devonian/Carboniferous
extinction event; not a single species survived into the Carboniferous.
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 (10 in) in length, which died in the process of giving birth to a 6 cm ( 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 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.
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 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. 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.
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
of armoured prehistoric fish
Prehistoric fish
Prehistoric fish refers to early fish that are known only from fossil records. They are the earliest known vertebrates, and include the first and extinct fish that lived through the Cambrian to the Tertiary. The study of prehistoric fish is called paleoichthyology...
, known from fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s, which lived from the late Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
to the end of the Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
Period. Their head
Head
In anatomy, the head of an animal is the rostral part that usually comprises the brain, eyes, ears, nose and mouth . Some very simple animals may not have a head, but many bilaterally symmetric forms do....
and thorax
Thorax
The thorax is a division of an animal's body that lies between the head and the abdomen.-In tetrapods:...
were covered by articulated armoured plates and the rest of the body was scaled
Scale (zoology)
In most biological nomenclature, a scale is a small rigid plate that grows out of an animal's skin to provide protection. In lepidopteran species, scales are plates on the surface of the insect wing, and provide coloration...
or naked, depending on the species. Placoderms were among the first jaw
Jaw
The jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. The term jaws is also broadly applied to the whole of the structures constituting the vault of the mouth and serving to open and close it and is part of the body plan of...
ed fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
; their jaws likely evolved from the first of their gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...
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 extinction
Late Devonian extinction
The Late Devonian extinction was one of five major extinction events in the history of the Earth's biota. A major extinction, the Kellwasser Event, occurred at the boundary that marks the beginning of the last phase of the Devonian period, the Famennian faunal stage, , about 374 million years ago...
s, and the class was entirely extinct by the end of the Devonian.
Taxonomy and phylogeny
Subphylum Vertebrata└─Infraphylum Gnathostomata
├─Class Placodermi
Placodermi
Placodermi is a 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...
— extinct (armored gnathostomes)
└Microphylum Eugnathostomata
Eugnathostomata
-Taxonomy and phylogeny: Subphylum Vertebrata └─Infraphylum Gnathostomata ├─Class Placodermi — extinct └Microphylum Eugnathostomata...
(true jawed vertebrates)
├─Class Chondrichthyes
Chondrichthyes
Chondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone...
(cartilaginous fish)
└─(unranked) Teleostomi
Teleostomi
Teleostomi is a clade of jawed vertebrates that includes the tetrapods, bony fish, and the wholly extinct acanthodian fish. Key characters of this group include an operculum and a single pair of respiratory openings, features which were lost or modified in some later representatives...
(Acanthodii & Osteichthyes)
├─Class Acanthodii
Acanthodii
Acanthodii is a class of extinct fishes, sharing features with both bony fish and cartilaginous fish. In form they resembled sharks, but their epidermis was covered with tiny rhomboid platelets like the scales of holosteans...
— extinct ("spiny sharks")
└Superclass Osteichthyes
Osteichthyes
Osteichthyes , also called bony fish, are a taxonomic group of fish that have bony, as opposed to cartilaginous, skeletons. The vast majority of fish are osteichthyes, which is an extremely diverse and abundant group consisting of over 29,000 species...
(bony fish)
├─Class Actinopterygii
Actinopterygii
The Actinopterygii or ray-finned fishes constitute a class or sub-class of the bony fishes.The ray-finned fishes are so called because they possess lepidotrichia or "fin rays", their fins being webs of skin supported by bony or horny spines , as opposed to the fleshy, lobed fins that characterize...
(ray-finned fish)
└─Class Sarcopterygii
Sarcopterygii
The Sarcopterygii or lobe-finned fishes – sometimes considered synonymous with Crossopterygii constitute a clade of the bony fishes, though a strict classification would include the terrestrial vertebrates...
(lobe-finned fish)
└Superclass Tetrapoda
├─Class Amphibia (amphibians)
└(unranked) Amniota (amniotic egg)
├─Class Sauropsida
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
(reptiles or sauropsids)
│ └─Class Aves
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
(birds)
└─Class Synapsida
└─Class Mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
ia (mammals)
Note: lines show evolutionary relationships.
Fossil record
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
. They are already differentiated into antiarchs
Antiarchi
The Antiarchi were the 2nd most successful order of placoderms known, after the Arthrodira. The order's name was coined by Edward Drinker Cope, who, when examining some fossils that he thought were armored tunicates related to Chelysoma, mistakenly thought that the eye-hole was the mouth, and...
and arthrodires
Arthrodira
Arthrodira is an order of extinct armoured jawed fishes of the Placodermi class that flourished in the Devonian period before their sudden extinction, surviving for about 50 million years and penetrating most marine ecological niches....
, 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
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...
Placodermi have not been discovered in these particular strata.
The Silurian fossil record of the placoderms is both literally and figuratively fragmented. 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.
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
Freshwater
Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...
and saltwater
Seawater
Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% . This means that every kilogram of seawater has approximately of dissolved salts . The average density of seawater at the ocean surface is 1.025 g/ml...
. But this diversity ultimately suffered many casualties during the extinction event at the Frasnian
Frasnian
The Frasnian is one of two faunal stages in the Late Devonian epoch. It lasted from 385.3 ± 2.6 million years ago to 374.5 ± 2.6 million years ago. It was preceded by the Givetian stage and followed by the Famennian stage...
–Famennian
Famennian
The Famennian is one of two faunal stages in the Late Devonian epoch. It lasted from 374.5 ± 2.6 million years ago to 359.2 ± 2.5 million years ago. It was preceded by the Frasnian stage and followed by the Tournaisian stage and is named after Famenne, a natural region in southern Belgium.It was...
boundary, the Late Devonian extinction
Late Devonian extinction
The Late Devonian extinction was one of five major extinction events in the history of the Earth's biota. A major extinction, the Kellwasser Event, occurred at the boundary that marks the beginning of the last phase of the Devonian period, the Famennian faunal stage, , about 374 million years ago...
s. The remaining species then died out during the Devonian/Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
extinction event; not a single species survived into the Carboniferous.
Ecology and lifestyles
Rhenanida
Rhenanida was an order of primitive, lightly armored placoderms. Unlike most other placoderms, the rhenanids' armor was made up of a mosaic of unfused scales and tubercles...
, Petalichthyida
Petalichthyida
The Petalichthyida was an order of small, flattened placoderm fish. They were typified by their splayed fins, and numerous tubercles that decorated all of the plates and scales of their armor. They reached a peak in diversity during the Early Devonian and were found throughout the world,...
, Phyllolepida
Phyllolepida
The order Phyllolepida was an order 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...
, and Antiarchi
Antiarchi
The Antiarchi were the 2nd most successful order of placoderms known, after the Arthrodira. The order's name was coined by Edward Drinker Cope, who, when examining some fossils that he thought were armored tunicates related to Chelysoma, mistakenly thought that the eye-hole was the mouth, and...
, 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
Bothriolepis
Bothriolepis was the most successful genus of antiarch placoderms, if not the most successful genus of any placoderm, with over 100 species found on every continent.-Description and palaeobiology:...
, 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
Arthrodira
Arthrodira is an order of extinct armoured jawed fishes of the Placodermi class that flourished in the Devonian period before their sudden extinction, surviving for about 50 million years and penetrating most marine ecological niches....
, were active, pelagic predators that dwelled in the middle to upper portions of the water column. The largest known arthrodire, Dunkleosteus
Dunkleosteus
Dunkleosteus is a genus of prehistoric fish, one of the largest arthrodire placoderms ever to have lived, existing during the Late Devonian period, about 380-360 million years ago.This hunter, measuring up to and weighing , was a hypercarnivorous apex predator...
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
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
"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
Holonema
Holonema is an extinct genus of relatively large, barrel-shaped placoderms that were found in oceans throughout the world from the Mid to Late Devonian, when the last species perished in the Frasnian-Fammian extinction event. Most species of the genus are known from fragments of the armor, but the...
, and some were planktivores, such as the gigantic, 7 to- long arthrodire, Titanichthys
Titanichthys
Titanichthys agassizi was a giant, aberrant marine placoderm from the Late Devonian. It approached Dunkleosteus in size and build. Unlike its relative, however, T. agassizi had small, ineffective-looking mouth-plates that lacked a sharp cutting edge...
.
Extraordinary evidence of internal fertilization in a placoderm was afforded by the discovery in the Gogo Formation
Gogo Formation
The Gogo Formation in the Kimberley region of Western Australia is a Lagerstätte that exhibits exceptional preservation of a Devonian reef community.- Sedimentology :...
, 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 ( in) live young one and was fossilized with the umbilical cord intact. The fossil, named Materpiscis attenboroughi (after scientist David Attenborough
David Attenborough
Sir David Frederick Attenborough OM, CH, CVO, CBE, FRS, FZS, FSA is a British broadcaster and naturalist. His career as the face and voice of natural history programmes has endured for more than 50 years...
), 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
Vivipary
Vivipary has two different meanings. In animals, it means development of the embryo inside the body of the mother, eventually leading to live birth, as opposed to laying eggs...
"), pushing the date of first viviparity back some 200 million years earlier than had been previously known. The arthrodire Incisoscutum
Incisoscutum
Incisoscutum is an extinct genus of placoderm. Well preserved fossil embryos in the body cavity of Incisoscutum showed that these fishes, close to the common origin of all jawed vertebrates, gave birth to live young in a manner similar to modern sharks...
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
Austrophyllolepis
Austrophyllolepis is an extinct genus of placoderm.-References:*...
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
Osteichthyes
Osteichthyes , also called bony fish, are a taxonomic group of fish that have bony, as opposed to cartilaginous, skeletons. The vast majority of fish are osteichthyes, which is an extremely diverse and abundant group consisting of over 29,000 species...
, and the early shark
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....
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
Louis Agassiz
Jean Louis Rodolphe Agassiz was a Swiss paleontologist, glaciologist, geologist and a prominent innovator in the study of the Earth's natural history. He grew up in Switzerland and became a professor of natural history at University of Neuchâtel...
, 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
Turtle
Turtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield...
-like vertebrates. The work of Dr. Erik Stensiö
Erik Stensiö
Erik Helge Osvald Stensiö was a Swedish paleozoologist.Erik Andersson, as his original name was, was born in the village of Stensjö in Döderhult parish in Kalmar County; he later took his new surname from his place of origin and is occasionally referred to with both names...
, at the Swedish Museum of Natural History
Swedish Museum of Natural History
The Swedish Museum of Natural History , in Stockholm, is one of two major museums of natural history in Sweden, the other one being located in Gothenburg....
, Stockholm, from the late 1920s established the details of placoderm anatomy, and identified them as true jawed fishes related to shark
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....
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
Wax
thumb|right|[[Cetyl palmitate]], a typical wax ester.Wax refers to a class of chemical compounds that are plastic near ambient temperatures. Characteristically, they melt above 45 °C to give a low viscosity liquid. Waxes are insoluble in water but soluble in organic, nonpolar solvents...
. 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
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....
s, but with more fossil specimens found, the theory of placoderms being the sister-group of chondrichthyian
Chondrichthyes
Chondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone...
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. 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.