Skull roof

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The full complement of bones of the tetrapod skull roof, as seen in the temnospondyl Xenotosuchus

The skull roof or the roofing bones of the

land-living vertebrates. The bones are derived from dermal bone and are part of the dermatocranium
.

In

human anatomy, the skull roof often refers specifically to the skullcap
.

Origin

Dermal armour in Dunkleosteus, a placoderm.

Early armoured fish (such as jawless

placoderms) did not have a skull in the common understanding of the word, but instead had a cartilaginous endocranium that was partially open from above. The loose cartilage was topped by dermal bones forming armour. The dermal bones gradually evolved into a fixed unit overlaying the endocranium like a heavy "lid", protecting the animal's head and brain from above. A more or less full shield of fused dermal bones was common in early bony fishes of the Devonian, and particularly well developed in shallow water species.[3]

Cartilaginous fish, such as sharks, have a skeleton which is entirely formed from cartilage. They lack a continuous dermal armour and thus have no proper skull roof.

Bony fishes

Skull of Platycephalichthys, a sarcopterygian. Most of the roofing over the cheek region is formed by the operculum.

In early sarcopterygians ("lobe-finned fish"), the skull roof was composed of numerous bony plates, particularly around the nostrils and behind each eye. The skull proper was joined by the bones of the operculum. The skull itself was composed rather loosely, with a joint between the bones covering the brain and the snout.

The skull roof in lungfish is composed of a number of bony plates that are not readily compared to those found in early amphibians.[4] In most ray-finned fishes, the skull is often reduced to a series of loose elements, and a skull roof as such is not found.[3]

Early tetrapods

The skull roof in Cheliderpeton, a temnospondyl amphibian

As tetrapodomorph fish trended towards greater adaptations for life on land, the skull became more tightly integrated. At the same time, the number of bones were reduced, the skull bones separated from the shoulder girdle, and the operculum disappeared.[1] The earliest limbed tetrapods ("amphibians" in the broad sense) solidified a pattern of plates which formed the basis for that seen in all land-living vertebrates. These early tetrapods (including temnospondyls, embolomeres, and various minor groups) have historically been termed "labyrinthodonts" ("maze teeth") or "stegocephalians" ("roof heads"). Not including marginal or cheek bones (such as the premaxillae, maxillae, jugals, quadratojugals, and squamosals), the skull roof bones established by these early tetrapods include the following:

The skull roof itself formed a continuous cover over the whole of the head, leaving only openings for nostrils (nares), eyes (orbits), and a small parietal eye (also known as a pineal foramen) between the parietal bones. This type of skull was inherited by the first amniotes (fully terrestrial tetrapods), which evolved in the Carboniferous. This type of skull roof without any above openings behind the eyes is called anapsid. Today, the only reptiles with anapsid skulls are turtles, though this is likely a case of secondary loss of the temporal fenestrae.[5]

In modern amphibians such as

frogs and salamanders, the skull roof is further reduced and has large openings. Only in caecilians can a full covering skull roof be found, an adaption for burrowing.[6]

The debate over skull roof homology

One of the most persistent debates in 20th-century paleontology was how to homologize the skull roof of fish with that of tetrapods.[7] In practically all tetrapods, the midline of the skull roof comprises at least three pairs of plate-like bones. From front-to-back, these bones are the nasals (top of the snout), frontals (between the eyes), and parietals (behind the frontals). Early tetrapods also possessed postparietals, additional paired or singular bones at the rim of the skull, behind the parietals.[8] Lobe-finned fish and early ray-finned fish usually lack paired bones at the top of the snout, instead presenting a mosaic of smaller plates. Nevertheless, two pairs of bones do consistently occur further back.[9] The front pair are positioned between the eyes and surround a pineal foramen, when it is present. The latter pair are elongated and abut the extrascapular bones, which lie behind the skull. The "traditional" or "orthodox" hypothesis considers these two pairs to be equivalent to the frontal and parietal bones, respectively. This was mainly justified by their position in regards to the eyes and brain, in accordance with mammal anatomy.[10][11]

An alternative interpretation was proposed by T.S. Westoll (1938, 1943)[12][13] and A.S. Romer (1941).[14] Their interpretation noted that the tetrapods with a pineal foramen almost always have the hole surrounded by the parietal bones. This would indicate that the "frontal" bones of fish are actually parietal bones. By extension, the "parietals" of fish are actually postparietals, while the tetrapod nasal and frontal bones develop from fused snout ossicles. According to this hypothesis, the eyes shift forwards, the snout expands, and the postorbital region (behind the eyes) contracts at the origin of tetrapods. Newly discovered "transitional" fossils such as Ichthyostega, Elpistostege, and Panderichthys were used as further evidence in support of their interpretation.[12][13]

The Westoll/Romer hypothesis was initially controversial, with the strongest critiques coming from

paleoichthyologists.[15][16][10][11] Proponents of the orthodox hypothesis argued that the "parietals" of fish are too strongly connected to the underlying brain anatomy to justify the W/R scenario, which posits that the "parietals" (= postparietals) diminish into oblivion over the course of evolution. Another concern was that the W/R hypothesis necessitates the complete loss of the extrascapulars in tetrapods. According to the orthodox hypothesis, the tetrapod postparietals develop from extrascapulars which have acquired a stronger connection to the skull.[15][16][10][11] Despite the criticism, the W/R scenario is now accepted by most paleontologists studying the fish-tetrapod transition.[17][18][8][9][7] Developmental studies show support for the novel evolution of the frontal bones in tetrapods and their lack of homology with the "frontals" of fish.[19] Further tetrapodomorph discoveries, such as Acanthostega and Tiktaalik,[20]
provide more concrete evidence for the anatomical shifts hypothesized by Westoll and Romer.

Diapsids and synapsids

Tuatara skull, showing the double openings behind the eye.

In two groups of early amniotes, the skull roof evolved temporal fenestrae to allow for greater movement of the jaw muscles. The two groups evolved the openings independently:

References

  1. ^ a b Romer, A.S. & T.S. Parsons. 1977. The Vertebrate Body. 5th ed. Saunders, Philadelphia. (6th ed. 1985)
  2. ISBN 0-07-303869-5
    .
  3. ^
    ISBN 0697243788.{{cite book}}: CS1 maint: multiple names: authors list (link
    )
  4. ^ Carroll, R.L. (1988): Vertebrate Paleontology and Evolution, W.H. Freeman & Co, New York NY, p.148
  5. ^ deBraga, M. and Rieppel, O. (1997). "Reptile phylogeny and the interrelationships of turtles." Zoological Journal of the Linnean Society, 120: 281-354.
  6. ISBN 0-8018-4780-X.{{cite book}}: CS1 maint: multiple names: authors list (link
    )
  7. ^
    ISBN 978-3-89937-080-5
    .
  8. ^
    S2CID 83672017
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  9. ^
    ISSN 0024-4082
    .
  10. ^
    ISBN 8200376605
    .
  11. ^
    ISBN 978-1-119-28643-1
    , retrieved 2023-09-24
  12. ^
    S2CID 4086668
    .
  13. ^
    S2CID 85648376
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  14. S2CID 85150870
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  15. ^ a b Jarvik, E. (1967). "The homology of frontal and parietal bones in fishes and tetrapods". Coll. Internatn. C.N.R.S. 163: 181–213.
  16. ^ a b Borgen, Ulf J. (1983). "Homologizations of skull roofing bones between tetrapods and osteolepiform fishes" (PDF). Paleontology. 26 (4): 735–753.
  17. ISSN 0024-4082
    .
  18. ^ Schultze, H.P.; Arsenault, M. (1985). "The Panderichthyid fish Elpistostege: a close relative of tetrapods?" (PDF). Paleontology. 28 (2): 293–309.
  19. PMID 31869306
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  20. PMID 16598249
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