Proetida
Encyclopedia
Proetida is an order of trilobite
that lived from the Ordovician
to the Permian
. It was the last order of trilobite to go extinct, finally dying out in the Permian extinction.
These typically small trilobites resemble those of the order Ptychopariida
, from which the new order Proetida was only recently separated in 1975 by Fortey
and Owens. Like the order Phacopida
, the proetids have exoskeleton
s that sometime have pits or small tubercles, especially on the glabella
(middle portion of the head). Because of their resemblance to the Ptychopariida
in some features, the proetids are included in the subclass Librostoma.
Unlike some trilobites of the order Phacopida, whose eyes are schizochroal, the proetids have the more common Holochroal eye
s. These eyes are characterized by close packing of biconvex lenses
beneath a single corneal
layer that covers all of the lenses. Each lens is generally hexagonal in outline and in direct contact with the others. They range in number from one to more than 15,000 per eye. Eyes are usually large, and because the individual lenses are hard to make out, they look smooth and sometimes bead-like.
The thorax
of proetids was made up of anywhere between 8-22 segments, but most commonly 10. Many also have genal spines coming off either side of the head, though in some they are blunted. These two features can aid in distinguishing proetids from some trilobites in the order Phacopida
, to which they can be very similar.
The following superfamilies and families are recognised:
Trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...
that lived from the Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
to the Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
. It was the last order of trilobite to go extinct, finally dying out in the Permian extinction.
These typically small trilobites resemble those of the order Ptychopariida
Ptychopariida
Ptychopariida is a large, heterogeneous order of trilobite containing some of the most primitive species known. Many date to the Early Cambrian Period, but the order was extant through the Late Ordovician...
, from which the new order Proetida was only recently separated in 1975 by Fortey
Richard Fortey
Richard A. Fortey FRS is a British palaeontologist and writer.-Career:Richard Fortey studied geology at the University of Cambridge and had a long career as a palaeontologist at the Natural History Museum in London. Prof. Fortey’s research interests include, above all, trilobites...
and Owens. Like the order Phacopida
Phacopida
Phacopida 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 proetids have exoskeleton
Exoskeleton
An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...
s that sometime have pits or small tubercles, especially on the glabella
Glabella
The glabella, in humans, is the space between the eyebrows and above the nose. It is slightly elevated, and joins the two superciliary ridges.-Etymology:The term is derived from the Latin glabellus, meaning smooth, as this area is usually hairless....
(middle portion of the head). Because of their resemblance to the Ptychopariida
Ptychopariida
Ptychopariida is a large, heterogeneous order of trilobite containing some of the most primitive species known. Many date to the Early Cambrian Period, but the order was extant through the Late Ordovician...
in some features, the proetids are included in the subclass Librostoma.
Unlike some trilobites of the order Phacopida, whose eyes are schizochroal, the proetids have the more common Holochroal eye
Holochroal eye
Holochroal eyes are compound eyes with a many tiny lenses . They are the oldest and most widespread type of trilobite eye, and found in all orders of trilobite from the Cambrian to the Permian periods. Lenses show hexagonal close packing, and a single corneal membrane covered all lenses...
s. These eyes are characterized by close packing of biconvex lenses
Lens (anatomy)
The crystalline lens is a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a...
beneath a single corneal
Cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...
layer that covers all of the lenses. Each lens is generally hexagonal in outline and in direct contact with the others. They range in number from one to more than 15,000 per eye. Eyes are usually large, and because the individual lenses are hard to make out, they look smooth and sometimes bead-like.
The thorax
Thorax
The thorax is a division of an animal's body that lies between the head and the abdomen.-In tetrapods:...
of proetids was made up of anywhere between 8-22 segments, but most commonly 10. Many also have genal spines coming off either side of the head, though in some they are blunted. These two features can aid in distinguishing proetids from some trilobites in the order Phacopida
Phacopida
Phacopida 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....
, to which they can be very similar.
The following superfamilies and families are recognised:
- Aulacopleuroidea
- Aulacopleuridae
- Brachymetopidae
- Rorringtoniidae
- Bathyuroidea
- Bathyuridae
- Dimeropygidae
- Holotrachelidae
- Hystricuridae
- Raymondinidae
- Telephinidae
- Toernquistiidae
- Proetoidea
- ProetidaeProetidaeProetidae is a family of trilobites. The first appeared in the Upper Ordovician, and were among the last surviving trilobites, occurring until the Carboniferous. The family Proetidae contains the following...
- PhillipsiidaePhillipsiidaeThe Phillipsiidae comprise the last of the trilobites, with a range that extended from the Lower Mississippian , to the Upper Permian .The Phillipsiidae are sometimes defined as a subfamily, the Phillipsiinae, within the Proetidae...
- Tropidocoryphidae