Phacopida
| Phacopida | ||||||||||||
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Dalmanites limulurus from the suborder Phacopina |
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| Temporal occurrence | ||||||||||||
| Ordovician to Devon | ||||||||||||
| 488.3 to 359.2 million years | ||||||||||||
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Phacopida | ||||||||||||
| Salter , 1864 |
Phacopida is an order of trilobites that was widespread worldwide and existed from the Ordovician to the Devonian . The Phacopida trilobites have a very variable body shape.
features
The trilobites of this order have 8 to 19 thorax segments. In some species, these segments can have distinct central furrows. The central lobe is very wide in some species. The glabella on the cephalon has up to four pairs of furrows, sometimes fused into one another, but which do not divide the glabella in praise . Compound eyes may be present. Sometimes they contain several thousand individual eyes (so-called holochroal compound eyes, up to 15,000 closely lined up eyes) and those that only consist of a few hundred individual eyes (so-called schizochroal compound eyes, up to 700 larger, individually set eyes). A so-called rostral plate can be found on the underside of the front . The pygidium is very small. But there are also species with larger pygidia. There are sometimes praises on the pygidium, and it can be spiky or pointed.
Submissions
This trilobite order has three suborders: Calymenina , Cheirurina and Phacopina .
Features of calymenina
The cephalon is semicircular to triangular and mostly has small, holochroal compound eyes. There is a rostral plate at the bottom. The glabella is narrowed towards the front. The thorax typically has thirteen segments with rounded pleural ends ; sometimes there are eleven to thirteen segments. The pygidium is semicircular to triangular with no spines on the border.
Please refer:
Features of Cheirurina
The cephalon has a rostral plate on the underside. The cheeks are often pitted with dimples. The glabella of these animals has up to four furrows and usually becomes wider towards the front or it is barrel-shaped. The trilobites have holochroal compound eyes. The thorax consists of 8 to 19 segments that often end in a spike shape. The pygidium consists of two to 16 pairs of ribs and is sometimes pointed. Very often these animals are covered with tubercles .
Features of Phacopina
These animals have schizochroal compound eyes. The glabella is widened towards the front. The free cheeks ( Librigenae ) are fused into one piece like a yoke and sometimes they have thorn-like extensions pointing backwards. Some do not have a rostral plate below. The thorax sometimes has ten but typically eleven furrowed segments. The semicircular, triangular or pointed pygidium is smaller than the cephalon and has either a smooth or a prickly border.
