Fenestra

Within the Amniota can be found in terms of the morphology of the skull three basic types: anapsider, synapsider and diapsider skull, which differ by the number and position of certain skull window in the top bone of the skull roof. These three types had in the past a relatively high importance for the internal classification of amniotes and are eponymous for the large groups Anapsida, Synapsida and Diapsida. Today, the importance of the skull openings is lower for the classification system, since recent studies have shown that the same or similar skull configurations occur at different Amniotengroßgruppen.

Anapsider skull

The anapside skull is characterized by the absence of cranial windows in the skull roof. This type probably represents the original state of the skull of amniotes and was inherited by their non- amniotic ancestors.

Taxa with anapsidem skulls are represented in two main lines of Reptiles, Reptiles, and Para Eureptilien. With the exception of turtles, these are exclusively very early, long -extinct forms, such as the Captorhiniden ( Eureptilia ) or the Pareiasaurier ( Parareptilia ).

The turtles are the only extant Amniotengruppe with anapsidem skull structure. However, their anapsider skull is probably not the original state is, but has come through secondary closure formerly existing cranial window materialize.

Since the anapside skull is an original feature, occurs in two main lines of reptiles and was apparently even secondarily acquired new to the turtles, the absence of temporal windows no longer possesses systematic relevance in amniotes today. Hence the term " Anapsida " increasingly disappearing from the zoological and paleontological literature.

Synapsider skull

The synapside skull has a single skull window in the temporal region ( temporal or temporal window ) on each side of the skull. It is framed by the skull bone Jugale, postorbital and squamosal and in some cases also from Quadratojugale. The formed of Jugale and Quadratojugale or squamosal in shape, usually a relatively narrow bone ridge lower boundary of the temporal window is also called lower temporal or temporal arch.

This configuration can be found in the basic blueprint of the skull of Synapsida, the group that includes mammals. In mammals, the skull window is closed, however, secondary, and refers to his former presence only by the ongoing presence of inferior temporal arch, now as zygoma ( zygomatic arch ), recognizable.

Even with some representatives of the Parareptilia ( Bolosaurier, Acleistorhinus and possibly Mesosaurus ) occur such temporal window. Therefore, these alone are not a reliable criterion for the assignment of amniotes to the synapsids.

Diapsider skull

The diapside skull is characterized by two cranial windows in the temporal region on each side of the skull: An upper temporal window ( fenestra supratemporalis ), which usually sits on top of the skull and is framed by the skull bone parietal, postorbital and squamosal and originally also postfrontal, and a lower temporal window ( fenestra infratemporal ) in the cranial side wall, framed by Jugale, postorbital, squamosal and Quadratojugale. The lower temporal arch is formed as in the synapsids from Jugale and Quadratojugale. The often also relatively narrow design bone ridge that separates lower and upper temporal window of each other and formed by the postorbital and squamosal, ie upper temporal arch.

This configuration probably represents the cranial structure of the common ancestor of all modern reptiles, including birds.

In the tuatara, the Quadratojugale is excluded in favor of an involvement of the Squamosums of the formation of the lower temporal arch. Therefore, it is assumed that tuatara of ancestors descended with reduced lower Temporalbogen and the lower temporal arch of the tuatara is newly created. Even the lower Temporalbogen of archosaurs (birds, crocodiles and their fossil relatives ) is probably a neoplasm (see katapsider skull).

" Triapsider " Skull

The " triapside " skull is a sub-form of diapsiden. It is characterized by another skull window in front of the eye socket, the so-called pre-or Antorbitalfenster. This is partly framed as a rule of lacrimal and maxilla also from the front portion of the Jugale. In addition, this skull type often also has an enlarged nostril. This type is the type of skull ursprungliche Archosauria, which extant major groups are the crocodiles and birds. Both crocodiles and birds of triapside skull is modified secondary: Crocodiles have no Präorbitalfenster more and the bird skull is so strong reshuffled that none of the cranial window is still clearly visible. Clearly recognizable as such triapside skull are therefore found only in fossil archosaurs, such as in early closer relatives of crocodiles (among the Rauisuchiden ) or when the dinosaurs.

Euryapsider skull

The euryapside skull is a diapsider skull, in which the lower temporal window is secondary closed. The " Euryapsiden ", whose name is derived from this Fensterungstyp, are a group of large, extinct marine reptiles of the Mesozoic era, which includes the ichthyosaurs and the Sauropterygier ( nothosaurs, plesiosaurs and Placodontier ). Some recent studies of the skull anatomy, however, have furnished that ichthyosaurs and Sauropterygier may not go back to a common ancestor, the euryapside skull could be so incurred from each other in both groups independently. The relationship of " Euryapsida " even seems to be so extensive that the group would have to be regarded as polyphyletic. But the land-dwelling reptile Araeoscelis from the Permian of Texas, which is neither more closely related ichthyosaurs with Sauropterygiern, has a euryapsiden skull.

Parapsider or metapsider skull

When parapsiden or metapsiden skull is also a modified Diapsidenschädel with exclusively upper temporal window. The lower or rather outer ( lateral) boundary of the superior temporal window is formed here not of postorbital and squamosal but by postfrontal and supratemporal so it is expected that the " real" euryapside and parapside skulls are developed independently from each. The parapside configuration is typical of ichthyosaurs.

In the literature of the early 20th century, the term " parapsid " is synonymous with " euryapsid " and " katapsid " (see below) is used to refer all reptiles skull without lower Temporalbogen, regardless of whether this condition the result of a reduction of Temporalbogens or complete obstruction of the inferior temporal window is. The Taxonname " Parapsida " derives from this meaning of the term and refers to a group that includes both ichthyosaurs and Araeoscelis and the Squamata ( Squamata ). Such a grouping has long been considered but not as natural and the term " Parapsida " is outdated accordingly.

" Katapsider " Skull

Also the katapside skull is a derived Diapsidenschädel. With him the lower temporal arch is reduced. He comes before tart exclusively at Schuppenkriechtieren ( Squamata ), but is believed to have originated relatively early in the evolution of Diapsiden and then in various lines of development ( Archosauria, tuatara ) been reshuffled convergent back to a "real " diapsiden skull. The reduction of the lower temporal arch was associated with a reduction in size of Quadratojugale. A special feature of the skull katapsiden squamates is the articulation between the quadrate and squamosal and a complete reduction of the Quadratojugale.