Everything about Phacopida totally explained
Phacopida ("
Lens-face") is an
order of
trilobite that lived from the
Ordovician to the
Devonian. It is made up of a morphologically diverse group of related suborders.
The origin of the Phacopida order is uncertain. It comprises three suborders (
Phacopina,
Calymenina, and
Cheirurina) which share a distinctive
protaspis (developmental stage lacking segments) type. The Cheirurina and Calymenina retain a
rostral plate (an
apomorphy) but in virtually all Phacopina the free cheeks are yoked as a single piece. This sort of similarity in development suggests phylogenetic unity. The suborder Calymenina is the most primitive of the Phacopida order and shares some characteristics with the order
Ptychopariida, though it isn't included in the subclass
Libristoma.
Phacopida had 8 to 19 thoracic segments and are distinguishable by the expanded
glabella, short or absent
preglabellar area, and
schizochroal (Phacopina) or holochroal (Cheirurina and Calymenina) eyes. Schizochroal eyes are compound eyes with up to around 700 separate lenses. Each lens has an individual
cornea which extended into a rather large
sclera.
The development of schizochroal eyes in phacopid trilobites is an example of post-displacement
paedomorphosis. The eyes of immature holochroal
Cambrian trilobites were basically miniature schizochroal eyes. In Phacopida, these were retained, via delayed growth of these immature structures (post-displacement), into the adult form.
Phacops rana and
Dalmanites limulurus are two of the most well-known members of this order. Other known phacopids include
Cheirurus,
Deiphon,
Calymene and
Ceraurinella.
Further Information
Get more info on 'Phacopida'.
|
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://phacopida.totallyexplained.com">Phacopida 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. |
