The Placodermi were 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 jawed fishes; their jaws likely evolved from the first of their gill arches. The first identifiable Placoderms evolved in the late Silurian; they began a dramatic decline during the Late Devonian extinctions, and most species disappeared; the class was extinct by the end of the Devonian. The earliest known fossils were found in China, and are already differentiated into Antiarchs and Arthrodires. Apparently they diversified long before the Devonian, somewhere in early to middle Silurian, possibly in the same area in China, though earlier fossils of basal Placodermi have not been discovered in these strata.
Fossil record
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 diversified long before the Devonian, somewhere in 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 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 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 the Silurian fossil record of placoderms is due to placoderms living in environments unconductive to fossil 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. But the diversity ultimately suffered many casualties during the extinction event at the Frasnian-Famennian boundary, the Late Devonian extinctions. The surviving species died out during the Devonian/Carboniferous extinction event; not a single species survived 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, even though they were actually the dominant vertebrate group during the Devonian. 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 sea floor.
It was thought that placoderms went extinct due to competition from the first bony fish, and early sharks, due to a combination of the supposed inherent superiority of bony fish and sharks, and the presumed sluggishness of placoderms. But after making 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 during the Devonian/Carboniferous extinction event.
History of study
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 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 sharks, but when more fossil specimens were 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
External links
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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