Placodermi

Placodermi Fossil range: Early/Mid Silurian - Late Devonian
Conservation status
Extinct (fossil)
Scientific classification
Kingdom: Animalia Phylum: Chordata Subphylum: Vertebrata Infraphylum: Gnathostomata Class: Placodermi McCoy, 1848
Orders
Antiarchi † Arthrodira † Brindabellaspida † Petalichthyida † Phyllolepida † Ptyctodontida † Rhenanida † Acanthothoraci † ?Pseudopetalichthyida † ?Stensioellida †

The Placodermi are armoured prehistoric fishes known from fossils dating 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 of the jawed fish, their jaws likely evolving from the first of their gill arches. The first identifiable Placoderms evolved in the late Silurian; they then began a dramatic decline during the Late Devonian extinctions, with most species disappearing, until the very end of the Devonian, when the class had gone extinct. The first appearance of late Silurian placoderm fossils, in China, show the fishes already differentiated into Antiarchs and Arthrodires; apparently Placoderm diversity originated long before the Devonian, somewhere in the early to middle Silurian, possibly in China (where the aforementioned Late Silurian fragments have been found), though earlier fossils of basal Placodermi have yet to be discovered in these particular strata.

Fossil record

The first identifiable placoderms left remains that date back to the mid to late Silurian; the placoderms ultimately disappeared in the Late Devonian extinctions. The first appearance of late Silurian placoderm fossils, in China, show the fishes already differentiated into Antiarchs and Arthrodires, along with the other, more primitive groups; apparently Placoderm diversity originated long before the Devonian, somewhere in the middle or early Silurian, though earlier fossils of basal Placodermi, have yet to be discovered in these particular strata. A caveat to the reader: the Silurian fossil record of the placoderms is literally fragmented. All known Silurian placoderms are known only from fragments, either as 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, all of the Silurian arthrodire and antiarch species have yet to be formally described or even named. Paradoxically, the best known, or rather, most commonly cited example of an 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 specialist suspect that the scarcity of the Silurian fossil record of placoderms is so due to placoderms living in environments unconductive to their preservation, rather than a genuine scarcity. This hypothesis helps to explain the placoderms' seemingly miraculous appearance and diversity at the very beginning of the Devonian.

In stark contrast to the Silurian, during the Devonian, the placoderms went on to inhabit and dominate almost all known aquatic ecosystems, both freshwater and saltwater. Despite their dominance, the diversity of the placoderms ultimately suffered many casualties during the extinction event at the Frasnian-Famennian boundary, during the Late Devonian. The rest of the surviving species then died out entirely during the Devonian/Carboniferous extinction event, with not a single species surviving into the Carboniferous.

Ecology and lifestyles

Many placoderms, particularly the Rhenanida, Petalichthyida, Phyllolepida, and Antiarchi, were bottom-dwellers. As such, to paraphrase from what was said in Palaeos, Placodermi has been popularly misinterpreted as being a tribe of bottom-feeding snails and garbage trucks, nevermind that the placoderms were the dominant vertebrate group during the Devonian. One must remember that the vast majority of placoderms were predators, many of which lived at or near the bottom. Many, primarily the Arthrodira were mid- to upperwater dwellers, and were active predators. The largest known arthrodire, Dunkleosteus telleri, was an 8 to 11 meter long predator and was presumed to have a nearly worldwide distribution, as its remains have been found in Europe, North America and Morocco. In fact, it is regarded 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 crediting the idea that many, if not most, arthrodires were active swimmers, rather than passive ambush-hunters whose armor practically anchored them to the seafloor.

It was originally thought that the placoderms went extinct due to competition from the first bony fish, as well as the early sharks, due to a combination of the supposed inherent superiority of the bony fish and sharks, as well as the presumed sluggishness of the placoderms themselves. Since then, though, as more accurate summaries of prehistoric organisms have been developed, it is now presumed that the last placoderms died out one by one as each of their ecological communities suffered due to the environmental catastrophes during the Devonian/Carboniferous extinction event.

Placoderms and specialists

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 ironically, completely destroyed as a result), 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, especially the exceptionally well-preserved fossils from the Gogo Reef formation in Australia, Stensiö's theory of sharks and placoderms as sister groups is accepted as fact.

See also

• Ostracoderm
• Pterichthyodes - a genus of antiarch placoderm.
• Gemuendina stuertzi - a species of flattened placoderm
• Titanichthys agassizi - a species of planktivorous placoderm
• Dunkleosteus terrelli - a species of giant, carnivorous placoderm
• Stensioella heintzi - possible placoderm

External links

• Annetta Markussen-Brown, "Devonian Armoured Fish" 2000
• Introduction to the Placodermi Extinct armored fishes with jaws
• PALAEOZOIC FOSSILS UK
• Placoderms
• Placodermi: Overview
• BBC - report on Dunkleosteus terrelli

References

• Janvier, Philippe. Early Vertebrates Oxford, New York: Oxford University Press, 1998. ISBN 0-19-854047-7
• Long, John A. The Rise of Fishes: 500 Million Years of Evolution Baltimore: The Johns Hopkins University Press, 1996. ISBN 0-8018-5438-5

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Placodermi

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