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.
Bothriolepis. Late Devonian - worldwide. Length about 20 to 30 cm
With their armored head shield and trunk shield composed of overlapping bony plates, the placoderms appear at first glance extremely similar to the ostracoderms (armoured jawless
fish). They are easily distinguished however by their paired fins and presence of jaws, an adaptation that gave them a tremendous advantage over their earlier jawless cousins, for it enabled them to bite solid food rather than simply suck up organic particles from the mud. Yet still these creatures were very primitive compared to other
fish. The fourth pair of gill arches had not yet become supports for the jaws, and the backbone consisted of a notochord (an embryonic feature in higher vertebrates) that persisted throughout life and vertebra consisting only of Y-shaped spines above and below it. These were often cartilaginous, like the skeleton of sharks (Chondrichthyes), a group with which the Placoderms are considered to share a common ancestry. Placoderms also lacked teeth, but biting or grinding structures are often be found in the dermal bones lining their mouths. The head and trunk shields of most placoderms were articulated by bony joints, allowing the forward part of the skull to tilt up, increasing the gape and allowing them to take in larger food
Although the head and thorax were heavily armored with dermal (skin) bone, the rest of their bodies were quite vulnerable, covered with small bony scales or lacking even that. It is not clear what purpose this armour
may have served. It has been suggested that it was a defense against the giant sea-scorpions that inhabited brackish water environments around this
time. Another explanation is that it
may have served as an exoskeleton and (as with arthropods and mollusks) a support for the external organs (in view of the very weak cartilaginous skeleton this
may not be unlikely)
Placoderms evolved into a variety of ecomorphs (body types), including torpedo-shaped swimmers, flattened
bottom-dwellers, and armoured box-like types. They included both carnivorous and detritivorous types. The pared fins and a heterocercal tail indicates they could swim quite efficiently when they wanted to, but the heavy armour would've weighed them down, and it is likely they spent much of their
time sitting on the
bottom..
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.
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. 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.
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
The ambitious Animal Crossing allows players to interact with a virtual village of animals who are doing something different every minute of every
day. With Animal Crossing's 24-hour clock, the game's unique events can be in synch with real
time -- as day turns to night in the real world, the sun
will also begin to set in the game. Seasons
will change and special events
will occur in real
time, too. The dynamic setup
will have players coming back every day to carry out their daily routines, build relationships with villagers, celebrate special days, collect furnishings for their homes and just live life in an innovative experience that's a world of its own.
When you're playing Animal Crossing and link your GCN to the GBA, you'll be able to sail to a special island that appears only when you connect the two systems. Once you've finished scouring the island, you can sail back to the GCN game with all the unique goodies you've found on your voyage. Gamers
will also be able to load their personalized character from the Memory Card onto a friend's Animal Crossing game and visit their pal's unique village.The ambitious Animal Crossing allows players to interact with a virtual village of animals who are doing something different every minute of every day. With Animal Crossing's 24-hour clock, the game's unique events can be in synch with real
time -- as day turns to night in the real world, the sun
will also begin to set in the game. Seasons
will change and special events
will occur in real
time, too. The dynamic setup
will have players coming back every day to carry out their daily routines, build relationships with villagers, celebrate special days, collect furnishings for their homes and just live life in an innovative experience that's a world of its own.
When you're playing Animal Crossing and link your GCN to the GBA, you'll be able to sail to a special island that appears only when you connect the two systems. Once you've finished scouring the island, you can sail back to the GCN game with all the unique goodies you've found on your voyage. Gamers
will also be able to load their personalized character from the Memory Card onto a friend's Animal Crossing game and visit their pal's unique village.
