Everything about Acanthodii totally explained
Acanthodii (sometimes called
spiny sharks) is a class of extinct
fishes, having features of both bony fish (
Osteichthyes) and cartilaginous fish (
Chondrichthyes). In form they resembled
sharks, but their
epidermis was covered with tiny rhomboid platelets like the scales of holosteans (
gars,
bowfins). They may have been an independent phylogenetic branch of fishes, which had evolved from little-specialized forms close to Recent
Chondrichthyes.
Acanthodians did, in fact, have a cartilaginous
skeleton, but their fins had a wide, bony base and were reinforced on their anterior margin with a dentine spine.
The earliest acanthodians were marine, but during the
Devonian, freshwater species became predominant. They are distinguished in two respects: they were the earliest known jawed
vertebrates, and they'd stout
spines supporting their
fins, fixed in place and non-movable (like a
shark's
dorsal fin).
There were three orders: Climatiiformes, Ischnacanthiformes and Acanthodiformes. Climatiiforma had shoulder armor and many small sharp spines, Ischnacanthiforma with
teeth fused to the jaw, and the Acanthodiforma were
filter feeders, with no teeth in the jaw, but long
gill rakers. Overall, the acanthodians' jaws are presumed to have evolved from the first
gill arch of some ancestral jawless fishes that had a gill skeleton made of pieces of jointed cartilage.
The popular name "spiny sharks" is really a misnomer for these early jawed fishes. The name was coined because they were superficially shark-shaped, with a streamlined body, paired fins, and a strongly upturned tail; stout bony spines supported all the fins except the tail - hence, "spiny sharks". Fossilized
spines and scales are often all that remains of these fishes in ancient
sedimentary rocks.
The
scales of Acanthodii have distinctive ornamentation peculiar to each order. Because of this, the scales are often used in determining relative age of sedimentary rock. The scales are tiny, with a bulbous base, a neck, and a flat or slightly curved diamond-shaped crown.
Despite being called "spiny sharks," acanthodians predate sharks. They evolved in the sea at the beginning of the
Silurian Period, some 50 MYA before the first sharks appeared. Later the acanthodians colonized fresh waters, and thrived in the rivers and lakes during the
Devonian and in the
coal swamps of
Carboniferous. But the first
bony fishes were already showing their potential to dominate the waters of the world, and their competition proved too much for the spiny sharks, which died out in
Permian times (approximately 250 MYA).
Many paleonthologists consider that the acanthodians were close to the ancestors of the bony fishes. Although their interior
skeletons were made of
cartilage, a bonelike material had developed in the skins of these fishes, in the form of closely fitting scales (see above). Some scales were greatly enlarged and formed a bony covering on top of the head and over the lower
shoulder girdle. Others developed a bony flap over the gill openings analogous to the
operculum in later bony fishes.
Further Information
Get more info on 'Acanthodii'.
|
External Link Exchanges
Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:
<a href="http://acanthodii.totallyexplained.com">Acanthodii Totally Explained</a>
Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned. |
