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FRONTAL AND PARIETAL BONES

Cartilaginous fish possess a cartilaginous roof of the braincase. In many fossil jawless fish and in placoderms the skull roof was composed of large bony plates. The braincase of acanthodians was composed of a series of smaller dermal bones. Primitive actinopterygian and sarcopterygian fish are the first vertebrates in which frontal and parietal bones can be identified. Although these primitive fish possess frontal and parietal bones and there is even a pineal foramen between the parietal bones (as in primitive tetrapods and amniotes), these bones lack many of the features seen in their homologs in higher vertebrates (Carroll, p.95). The frontal and parietal bones originated small flat plates. The frontal bones do not comprise the orbit nor form a postorbital bar as in humans. In fact, in primitive vertebrates these regions can be filled by several small bones (the prefrontal, postfrontal, and postorbital) which have been lost in the lineage which led to humans. The first frontal bones lacked sinuses and were not completely fused (and some did not even contact each other at the midline of the cranium). The skull roof is flat in pirimitive vertebrates, unlike the domed crania of higher vertebrates. Frontal bones (blue) and parietal bones (green) in the fossil bony fish, the actinopterygian Cheirolepis and sarcopterygian Eusthenopteron

Cheirolepis

Eusthenopteron

bowfin

The frontal bones of the sarcopterygian Eusthenopteron and basal amphibian Ichthyostega are evident in the following photo.

The frontal bones were larger and fused in the early amphibians.

In amphibians, the braincase became more solid and the ancestral joint between the parietals and postparietals was lost (Carroll).

In frogs, the frontal and parietal bones are fused.

frog

salamander

In the first reptiles (Paleothyris is depicted in the images below), the frontal bones had become larger and composed part of the orbit. The parietal also became larger (Carroll, p. 194).

Anapsid reptiles (such as Paleothyris in back and center) gave rise to diapsid (left) and synapsid (right) reptiles. During this early evolution of reptilian lineages, there was little modification of the frontal and parietal bones.

In a turtle skull, some of the bones lining the orbit have been lost in mammals.

There was little modification in the frontal bone as primitive pelycosaurs (Archeothyris), early therapsids (Biarmosuchus), later therapsids (Rubidgina), and cynodonts (Thrinaxodon) evolved. In ophiacodont pelycosaurs, the frontal bone developed a slope unlike the flat ancestral bone. (Kemp, 1982, p. 18)

In intermediate cynodonts (such as Thrinaxodon) the parietal bone extended into an area of the lateral wall of the braincase which was previously cartilaginous (Carroll, p. 392). The pineal foramen between the two parietal bones was lost (Carroll, p. 389). Although the frontal bone changed little from the cynodonts to the first mammals, three ancestral bones around the orbit were--the prefrontal, postfrontal, and postorbital. In later mammals, the frontal bone was be modified to form part of the orbit, and, in primates, a postorbital bar.

Plesiadapiformes, which are either primitive primates or a sister group to primates, possessed a postorbital ligament which protected the eye laterally. Among other things, such support may ensure better vision while chewing. (Rayosa, 2000). The frontal bones of the primitive plesiadapiform Palaecthon formed a process over the lateral portion of the orbit. This process was not present in basal mammals such as Morganucodon. In more advanced primates, this forms postorbital process from the frontal bone fused with the jugal (zygomatic) bone to form a complete postorbital bar (Carroll, p. 468).

opossum

armadillo

cat--note the postorbital bar is not complete

Sinuses evolved in eutherian mammals. (Weichert, 1970, p.712)

mink

In the adapid Smilodectes and omomyid Teutonius, the frontal formed the superior and lateral borders of the orbit.

In anthropoid primates, the orbits are more forward-facing, the braincase is larger, and the two frontal bones fuse later in life (Carroll).

In Aegyptopithecus (ancestor of Old World monkeys and Apes) and Proconsul (the first ape), the two frontal bones fused to form one frontal bone in adults

lemur

tarsier

New World monkey

marmoset

New World monkey Cebus

howler monkey

rhesus monkey

gibbon

female gorilla

female bonobo

human fetus