Placoderm

? Placodermi Conservation status: Fossil
Scientific classification
Kingdom: Animalia Phylum: Chordata Subphylum: Vertebrata Class: Placodermi McCoy, 1848
Orders
Antiarchi † Arthrodira † Petalichthyda † Phyllolepida † Ptyctodontida † Rhenanida † Brindabellaspida † Acanthothoraci † ?Pseudopetalichthyida † ?Stensioellida †

The placoderms (class Placodermi) are armored prehistoric fishes known from fossils dating from the late Silurian to the end of the Devonian period. Placoderms were among the first of the jawed fish and were dominant vertebrates in the Devonian. Their head and thorax were covered by articulated armored plates and the rest of the body was scaled or naked. They lacked teeth.

According to the fossil record, placoderms were rare in the Silurian (444-416 million years ago), but common in the Devonian (416-360 million years ago). Placoderms largely disappeared in the Late Devonian extinctions about 364 million years ago, a mass extinction event in which an estmated 22% of all families of marine animals disappeared and some 57% of genera (McGhee 1996).

While the placoderms were highly successful in the Devonian, diversifying into many forms and dominating brackish, saltwater and freshwater aquatic ecosystems, they had a relatively short history of only about 50 million years. During their time on Earth, this earliest branch of jawed fishes were considered to have been the most diverse and important Devonian vertebrates, with over 250 genera identified and almost 100 species of one genus, Bothriolepis (Kazlev 2005). They may have been ancestral to modern Chondrichthyes (cartilaginous fish) and Osteichthyes (bony fish).

"Placoderm" comes from the Greek, meaning "tablet" and "skin," referencing the armored plates covering the body.

Fossil record

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.

The Silurian fossil record of the placoderms is 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 specialists suspect that the Silurian fossil record of placoderms is 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 "Age of Fish," the placoderms went on to inhabit and dominate almost all known aquatic ecosystems, both freshwater and saltwater. Despite their dominance, the placoderms died out during the Devonian/Carboniferous extinction event, with species, or at least almost all species, not surviving into the Carboniferous.

It has been suggested that there was no single cause to the placoderms' downfall and disappearance. 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 Late Devonian extinction events.

Ecology and lifestyles

Many placoderms, particularly the orders Rhenanida, Petalichthyida, Phyllolepida, and Antiarchi, were bottom-dwellers. 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. Other, smaller arthrodires, such as Fallacosteus and Rolfosteus (from Australia's Gogo Reef formation), 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.

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 (armored, jawless fish). Some naturalists even suggested that they were shelled invertebrates, or even turtle-like vertebrates.

The work of Dr. Erik Stensio, 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 Stensio 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, Stensio's theory of sharks and placoderms as sister groups is accepted as fact.

External links

• Introduction to the Placodermi Extinct armored fishes with jaws

References

ISBN links support NWE through referral fees

• Janvier, P. 1998. Early Vertebrates. Oxford: Oxford University Press. ISBN 0-19-854047-7
• Long, J. A. 1996. The Rise of Fishes: 500 Million Years of Evolution Baltimore: The John Hopkins University Press. ISBN 0-8018-5438-5
• Kazlev, M. A. 2005. Placodermi: Overview. PALAEOS: The Trace of Life on Earth website (www.palaeos.com). Retrieved January 16, 2007.
• McGhee, G. R. 1996. The Late Devonian Mass Extinction: The Frasnian/Famennian Crisis. New York: Columbia University Press. ISBN 0231075057

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