COMPARATIVE ANATOMY HOME
COMPARATIVE ANATOMY TABLE OF CONTENTS
  OBL HOME OBL REFERENCES
THE SPHENOID AND ETHMOID BONES

     In humans, the sphenoid and ethmoid bones contribute to the floor of the braincase.  The ethmoid is difficult to observe since it attaches the cranium to the face. 

human skull human skull
human skull human skull
human skull human skull
human skull human skull
human skull human skull

     The sphenoid and ethmoid regions of the human skull existed as a multiple cartilaginous structures in more primitive vertebrates which would later fuse and ossify.  Placoderms possess a cartilaginous floor of their braincase which included a parasphenoid component (Carroll, p. 48, 65).  Cartilaginous fish possess also possess a cartilaginous floor of the braincase and a cartilaginous ethmoid region as well (Romer, p. 192).   Acanthodians possessed homologs of the parasphenoid and basisphenoid (Carroll, p. 87).

     In bony fish, the ethmoid region is ossified (Carroll, p. 87) and the parasphenoid forms ventral surface of sphenethmoid (Carroll, p. 142).  In bony fish, the sphenoid region is composed of sphenethmoid, orbitosphenoid, basisphenoid, and pleurosphenoid components (the pleurosphenoid is absent in mammals) (Kardong, 2002, p. 235).

     In sarcopterygian fish the ethmoid is more ossified (Carroll, p. 142).  In amphibians the parasphenoid is larger and the ethmoid part of the braincase is a single unit made from a single ossification center (Carroll).   Anthracosaurs developed a transverse phlange on the pterygoid (a component of the sphenoid) (Carroll, p. 194) and the parashenoid grew in size (Carroll).

     In therapsids, the ectopterygoid and pterygoid attach to basisphenoid of braincase (Romer, p. 251).   Some therapsids still possessed the primitive characteristic of teeth on pterygoid (Carroll, p. 370).  In cynodonts, the epyptergoid became larger, forming the lateral wall of the braincase.  As such, it is referred to as the alisphenoid, which is the only part of the braincase to develop from the splanchnocranium  (Carroll, p. 392).  Much of the lateral wall of braincase was still composed of cartilage in cynodonts (Carroll, p. 393).  Early cynodonts lost the tuber of the basisphenoid  (Kemp, 1982, p. 181) and the cultriform process of the parasphenoid was reduced (Kemp, 1982, p. 191).  In intermediate cynodonts (such as Thrinaxodon), the pterygoids met at midline (Carroll, p. 381).   In advanced cynodonts, the alisphenoid moved closer to the basisphenoid (Kemp, 1982).

   

 

human skull

  In mammals, the pterygoid was reduced, the orbitosphenoid center of ossification filled in area of medial orbit which was previously open, forming the optic foramen (Carroll, p. 403), the basisphenoid fused to the pterygoid and parasphenoid, (Carroll), the ectopterygoid expanded superiorly to make a lateral wall which contacted dermal bones (Romer, p. 251), and the parasphenoid was greatly reduced in size (Kemp, 1982, p. 305).  In therian mammals, the maxillary and mandibular branches of the trigeminal pass through alisphenoid (through the foramen rotundum and foramen ovale) as opposed to between alisphenoid and petrosal (Carroll, p. 403).  The  ectopterygoid was no longer part of the palate, (Kemp, 1982, p. 280) the alisphenoid was expanded (Carroll, p. 421), and, after protoeutherians, a groove existed in the sphenoid for the promontory artery (Carroll, p. 446).  In eutherians, the orbitosphenoid was reduced and the optic foramen enlarged (Carroll). Primitive mammals possessed a cavum epiptericum which surrounded the trigeminal nerve ganglion.  In modern mammals it is still identificable, although it has fused to the surrounding bone (Gauthier, 1988).

     In anthropoid primates, the sphenoid and ethmoid contribute to the enclosing of the orbit with bone (Carroll).  In catarrhine primates, the sphenoidal recess is absent (Loo, 1973) and apes developed a sphenoidal sinus (Loo, 1973).  In great apes, the sphenosquamosal suture may divide the foramen ovale (Braga, 1998).

n

One significant change that occurred between the archaic and modern humans was a shortening of the sphenoid bone in the braincase.  Externally, this change resulted in jaws, which did not protrude to the same degree.  Internally, the change could have had a much more profound effect: it would have affected the dimensions of the oropharynx and the variety of sounds that humans could have made in speech.  (Chimps and gorillas are capable of large vocabularies when using sign language and their use of signs suggests a degree of complex thought which they were previously thought to be incapable of.  Apes do not speak because they do not have the anatomy for it.  Thus changes in the vocal tract could have had a much larger impact on language than increases in brain size.) (Hublin, 1996; Rightmire, 1997; Rightmire, 1976; Holden, 1998).

Salamander

salamandersalamander

salamander

salamander

salamander

frog

frog

turtleturtle

opossum

opossum

cat

cat

cat

rhesus

rhesus monkey


THE ETHMOID BONE

frog

frog

salamander

salamander

cat

cat

 

monkey

monkey