Types of skulls of the amniotes

from Wikipedia, the free encyclopedia

With the skull of the Amniota , three basic types can be distinguished based on the number and location of certain skull windows in the cover bone of the skull roof: anapsid, synapsid and diapsid skulls. In the past, these three types were of relatively high importance for the internal systematics of the amniotes (or reptiles in the traditional sense) and are eponymous for the large groups Anapsida , Synapsida and Diapsida . Today, the importance of the skull openings for the system is somewhat less, as recent studies have shown that the same or similar skull configurations occur in different large groups of amniotes.

Anapsid skull

Anapsid skull. Names of the bones: Jugale (j), Postorbital (po), Parietale (p), Squamosum (sq), Quadratojugale (qj), Quadratum (q). It should also be noted that this skull is missing some bones in the posterior area of ​​the cranial roof, which are present separately in the basic plan of the amniotic skull: postparietals, supratemporals and tabulars.
Anapsid skull of Labidosaurus , a captorhinid.

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

Taxa with anapsid skull are represented in both main lines of reptiles, parareptiles and eureptiles . With the exception of the turtles, these are exclusively very early, long-extinct forms such as the captorhinids (Eureptilia) or the pareiasaurs (Parareptilia).

The turtles are the only recent reptile group with anapsid skull structure. However, their anapsid skull probably does not represent the original state, but was created by secondary closure of previously existing skull windows.

Since the anapsic skull is an original feature, occurs in both main lines of the reptiles and was apparently acquired secondarily in the turtles, the absence of temporal windows in amniotes is no longer systematic today. The term “Anapsida” is therefore increasingly disappearing from the zoological and paleontological literature.

Synapsid skull

Synapsid skull. Names of the bones: Jugale (j), Postorbital (po), Parietale (p), Squamosum (sq), Quadratojugale (qj), Quadratum (q).
Synapsid skull of Dimetrodon .

The synapside skull has a single skull window in the temple area (temporal or temple window) on each side of the skull. It is framed by the skull bones Jugale, Postorbitale and Squamosum and in some cases also by the Quadratojugale. The lower boundary of the temporal window, formed by Jugale and Quadratojugale or Squamosum in the form of a mostly relatively narrow bone bridge, is also called the lower temporal arch.

This configuration is found in the basic plan of the skull of the Synapsida , the group that also includes the mammals . In mammals, however, the cranial window is secondarily closed and its previous presence can only be recognized from the lower temporal arch that is still present, now called the zygomatic arch (zygomatic arc).

Such temporal windows also occur in some representatives of the Parareptilia ( Bolosaurs , Acleistorhinus and possibly Mesosaurus ). Therefore, these alone are not a reliable feature for assigning an amniote to the synapsids.

Diapsid skull

"Basic form"

Diapsid skull. Names of the bones: Jugale (j), Postorbital (po), Parietale (p), Squamosum (sq), Quadratojugale (qj), Quadratum (q).
Youngina from the Upper Permian of South Africa has an original diapsid skull (reconstruction).

The diapside skull is characterized by two skull windows in the temporal region on each side of the skull: An upper temple window ( Fenestra supratemporalis , supratemporal window), which usually sits on top of the skull roof and is framed by the cranial bones parietals, postorbital and squamosum and originally also postfrontals, and a lower temporal window ( Fenestra infratemporalis , infratemporal window) in the side wall of the skull, framed by the jugale, postorbital, squamosum and quadratojugale. As with the synapsids, the lower temporal arch is formed by the Jugale and Quadratojugale. The bone bridge, which is also often relatively narrow and separates the lower and upper temporal windows and is formed by the postorbital and squamosum, is called the upper temporal arch.

This configuration represents the skull structure of the common ancestor probably of all reptiles today , including birds ( diapsids - crown group ).

In the bridge lizards , the quadratojugale is excluded from the formation of the lower temporal arch in favor of involvement of the squamosum. It is therefore assumed that bridge lizards descend from ancestors with a reduced lower temporal arch and that the lower temporal arch of the bridge lizards emerged from scratch. The lower temporal arch of the archosaurs (birds, crocodiles and their fossil relatives) is probably a new formation (see catapsid skull ).

"Triapsider" skull

Diapsid skull with additional pre-orbital window: "Triapsid" skull. Names of the bones: Maxillare (m), Lacrimale (l), Jugale (j), Postorbital (po), Parietale (p), Squamosum (sq), Quadratojugale (qj), Quadratum (q).
Triapsid skull of Plateosaurus , a member of the sauropod line of dinosaurs. Skull openings from left to right (back to front): lower temporal window, eye socket, pre-orbital window, outer nostril.

The "triapside" skull is a sub-form of the diapsid. It is characterized by another skull window in front of the eye socket, the so-called pre- or antorbital window. This is usually framed by the lacrimale and maxillary , and sometimes also by the front part of the jugale. This type of skull also often has an enlarged nostril. This type is the original type of skull of the Archosauria , whose recent large groups are crocodiles and birds . In both crocodiles and birds, the triapside skull is secondarily modified: crocodiles no longer have a pre-orbital window and the bird's skull is so strongly remodeled that none of the skull windows are clearly recognizable. Clearly recognizable as such triapsid skulls can therefore only be found in fossil archosaurs, e.g. B. in early close relatives of the crocodiles (including the Rauisuchids ) and in many dinosaurs .

Euryapsid skull

Diapsid skull with closed lower temporal window: Euryapsid skull. Names of the bones: Jugale (j), Postorbital (po), Parietale (p), Squamosum (sq), Quadratojugale (qj), Quadratum (q).
Euryapsider skull of Zarafasaura , a representative of the plesiosaurs.

The euryapsid skull is a diapsid skull in which the lower temporal window is secondarily closed. The " Euryapsids ", whose name is derived from this type of fenestration, are a group of large, extinct marine reptiles from the Mesozoic era , which include the ichthyosaurs and the sauropterygians ( nothosaurs , plesiosaurs and placodon animals ). However, some recent studies of the anatomy of the skull have shown that ichthyosaurs and sauropterygians may not go back to a common ancestor, i.e. the euryapsid skull could have arisen in both groups independently of one another. The relationship of the "Euryapsida" even seems to be so extensive that the group should be regarded as polyphyletic . But the land-living reptile Araeoscelis from the Permian of Texas, which is not closely related to either ichthyosaurs or sauropterygians, has a euryapsid skull.

Parapsid or metapsid skull

Diapsid skull with closed lower temporal window: parapsid or metapsid skull. Designations of the bones: Jugale (j), Postfrontale (pof), Postorbital (po), Parietale (p), Squamosum (sq), Supratemporale (st), Quadratojugale (qj), Quadratum (q).
Relatively well recognizable pair of upper temporal windows in the dorsal cranial roof of a flattened and otherwise incompletely preserved skull (on the right in the picture) of Athabascasaurus .

The parapsid or metapsid skull is also a modified diapsid skull with only the upper temporal window. The lower or rather outer (lateral) boundary of the upper temporal window is not formed by postorbital and squamosum, but by postfrontal and supratemporal, which is why it is assumed that the "real" euryapsid and parapsid skulls developed independently of each other. The parapside configuration is typical of ichthyosaurs.

In the literature of the early 20th century, the term “parapsid” is used synonymously with “euryapsid” and “katapsid” (see below) and denotes all reptile skulls without a lower temporal arch, regardless of whether this condition is the result of a reduction in the temporal arch or one complete closure of the lower temporal window. The Taxonname "Parapsida" is derived from the meaning of the term and refers to a group containing both Ichthyosaurs and araeoscelis and Squamata comprises (squamates). Such a grouping has long been regarded as not natural and the term “Parapsida” is accordingly out of date.

"Katapsid" skull

Diapsid skull with reduced lower temporal arch: "Katapsid" skull. Designations of the bones: Jugale (j), Postorbital (po), Parietale (p), Squamosum (sq), Quadratum (q).
Catapsid skull of Estesia , a fossil relative of the crustacean lizards . The horizontal bone element that can be seen in the lower part of the lower cranial window is not the lower temporal arch, but the quadratus process of the pterygoid, a part of the roof of the mouth that extends far back.

The catapside skull is also a derived diapsid skull. The lower temporal arch is reduced in him. It occurs recently only in the squamata, but it is believed to have originated relatively early in the evolution of the diapsids and then converged in various lines of development (archosauria, bridge lizards) to form a "real" diapsid skull. The reduction of the lower temporal arch was accompanied by a reduction in the size of the quadratojugale. A special feature of the catapsid skull of the Squamats is the articulated connection between the quadratum and the squamosum as well as a complete reduction of the quadratojugale.

See also

literature

  • Michael J. Benton: Paleontology of the vertebrates. Translation of the 3rd English edition (Translator: Hans-Ulrich Pfretzschner). Publishing house Dr. Friedrich Pfeil, Munich 2007, ISBN 978-3-89937-072-0

Individual evidence

  1. ↑ Rafael Zardoya, Axel Meyer: Complete mitochondrial genome Suggests diapsid affinity of turtles. Proceedings of the National Academy of Sciences of the United States of America. Vol. 25, No. 19, 1998, pp. 14226-14231, doi: 10.1073 / pnas.95.24.14226
  2. ↑ Ylenia Chiari, Vincent Cahais, Nicolas Galtier, Frédéric Delsuc: Phylogenomic analyzes support the position of turtles as the sister group of birds and crocodiles (Archosauria). BMC Biology 2012, 10:65, doi: 10.1186 / 1741-7007-10-65 ( Open Access )
  3. ↑ Graciela Piñeiro, Jorge Ferigolo, Alejandro Ramos, Michel Laurin: Cranial morphology of the Early Permian mesosaurid Mesosaurus tenuidens and the evolution of the lower temporal fenestration reassessed. Comptes Rendus Palevol. Vol. 11, No. 5, 2012, pp. 379-391, doi: 10.1016 / j.crpv.2012.02.001
  4. ↑ a b Johannes Müller: Early loss and multiple return of the lower temporal arcade in diapsid reptiles. Natural sciences. Vol. 90, No. 10, 2003, pp. 473-476, doi: 10.1007 / s00114-003-0461-0
  5. ↑ a b Michel Laurin, Jacques A. Gauthier: Diapsid Phylogeny. Tree Of Life web project, 2000
  6. ↑ Michael W. Maisch: Phylogeny, systematics, and origin of the Ichthyosauria - the state of the art. Paleodiversity. Vol. 3, 2010, pp. 151–214, online (PDF; 1.3 MB)
  7. ^ Samuel W. Williston: The Phylogeny and Classification of Reptiles. The Journal of Geology. Vol. 25, No. 5, 1917, pp. 411-421