Placodermi
From Palaeos
| Vertebrata - The Vertebrates | |
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Phylum: Chordata
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Craniata `--o Vertebrata |?- †Myllokunmingia fenjiaoa |?- †Haikouichthys ercaicunensis |--Conodonta `--+--Hyperoartia `--+--Anaspida `--+--Pteraspidomorphi `==Thelodonti |--+--Cephalaspidomorphi | |--Galeaspida | `--Pituriaspida `--o Gnathostomata |--Placodermi |--Chondrichthyes `--o Teleostomi |--Acanthodii `--Osteichthyes `--Tetrapoda `--o Amniota |--Sauropsida `--Synapsida |
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[edit] Introduction
The placoderms are among the most ancient of fish, and, along with the Acanthodii, the only class of (gnathostomes) to become completely extinct. The name "Placoderm" is from the Greek and means "tablet + skin", referring to the heavy armoured bony plates that completely covered the head and thorax of these curious prehistoric fish.
The Class Placodermi is composed of a group of armoured prehistoric fishes known from fossils dating from the Mid or 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. Placoderms were one of the first jawed fish, their jaws likely evolving from the first of their gill arches. Starting with the the studies of Dr Erik Stensio, and supported by uncrushed fossils that preserve their 3-dimensional structures from the Gogo Reef Formation in Australia, it is presumed that sharks share a common ancestry with placoderms.
Although they first appeared during the early Silurian period, the placoderms did not become common or widespread until the early Devonian, when they underwent an extraordinary evolutionary radiation. Or, at the very least, until the begining of the Devonian, they did not live in enough environments conductive to fossilizing their remains. By the start of the Devonian periond, they soon came to dominate most brackish and near-shore ecosystems, and also spread to marine and freshwater environments. More than 250 genera are known in all, making them the most diverse and important of early vertebrates.
Most placoderms were quite small, about 10 or 15 cm in length, but one specialized group of carnivorous types, Dinichthyidae included a few genera reached 4 to 9 meters in length, making them the largest animals of their time, and the most fearsome.
The placoderms were seemingly unaffected by the great Frasnian-Famennian mass extinction that rocked the late Devonian, yet all suddenly died out at the Devonian/Carboniferous boundary, without leaving any descendents.
| PLACODERMI | |
| Taxonomy | Phylogeny |
Subphylum: Vertebrata
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Gnathostomata `--o PLACODERMI | ?--Stensioella | ?--Pseudopetalichthyida `==Acanthothoraci (paraphyletic) |--Rhenanida |--Antiarchi `--+--Ptyctodontida |--Petalichthyida `--Arthrodira `--Phyllolepida |
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Placoderm topics: | Characteristics | Ecology and Lifestyle | References | Links |
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[edit] Discovery and Interpretation
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 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. Using the Sollas method, 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 with the discovery of more fossil specimens, 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 due to a whole host of similarities between the skull anatomies of the two groups.
[edit] Earliest Appearance in the Fossil Record
The first identifiable placoderm fossils are known from the Middle Silurian (Wenlock). The first appearance of 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 Silurian, though earlier fossils of basal Placodermi, have yet to be discovered in these particular strata. At least, that's what scientists assume so far. The Silurian placoderm fossil record is literally fragmented, as they are known only from fragments of plates and armor. Some of these fragments bear a strong enough resemblence (due to ornamental and or histological details) to certain groups (i.e., the Antiarchs and Arthrodires) to be identified as being members of those groups, though none of these fragments have been officially described, or even named. Ironically, the best known, or at least, most commonly cited example of a Silurian placoderm, Wangolepis, is known from fragments that currently defy attempts at placement in any of the known orders of placoderms.
Palaeos com - Placodermi
Credits: Stanton 060924, MAK060926, MAK060930
