Phacopina


The Phacopina comprise a suborder of the trilobite order Phacopida. Species belonging to the Phacopina lived from the Lower Ordovician through the end of the Upper Devonian. The one unique feature that distinguishes Phacopina from all other trilobites are the very large, separately set lenses without a common cornea of the compound eye.

Habitat

As far as known, all Phacopina species were marine bottom-dwellers.

Origin

The Early Ordovician genus Gyrometopus is probably close to the common ancestor of the Phacopina. Gyrometopus is phacopid in appearance, but a rostral plate is present, unlike in other Phacopina. However, the rostral plate does not divide the cephalic doublure into a left and right section, but instead the rostral suture defines a semicircle in the frontal ¾ of the doublure.

Description

The eyes consist of very large, separately set lenses without a common cornea. However, some Phacopina species lack eyes, such as the species of the genus Ductina. The natural fracture lines of the head run along the top edges of the compound eye. From the back of the eye these cut to the side of the head and not to the back. In front of the eye, the right and left facial sutures connect in front of the inflated glabella and consequently the free cheeks are yoked as a single piece. The part of the skeleton that is ‘tucked under’ has no sutures crossing it to form a rostral plate.
The thorax has 11 segments, the side lobes are furrowed, and the articulating facets distinct.

Systematics

The Phacopina contain 3 superfamilies and 7 families:
Superfamily Phacopoidea
Superfamily Dalmanitoidea
Superfamily Acastoidea