EEB267H1 Lecture Notes - Pharyngeal Slit, Pharyngeal Arch, Fish Fin
DepartmentEcology & Evolutionary Biology
ProfessorDeborah Mc Lennan
Behold the Gnathostomata: gnathos (jaw) + stoma (mouth)
Myxiniformes Petromyzontiformes Chondrichthyes Osteichthyes
proto-vertebrae, true gills, two eyes, olfactory lobes
many molecular characters
dorsal hollow nerve cord, notochord, post-anal tail
Before we get to jaws, we need to know something about hard tissues.
The first type of hard tissue to evolve was cartilage. It is composed of:
a matrix of collagen (a type of protein) fibers+proteoglycans (proteins coupled with
long chains of sugars [e.g., chondroitin]) +
~ 75% water (bound to the sugars)
Specialized cells (called chondrocytes) produce the matrix and lay it down in thin layers.
lymphocytes have unique antigen receptors
In the basal Chordata, the chondrocytes are not part of the matrix so this type of cartilage
is “acellular” and is therefore “nonliving”.
In the ancestor of the Cristozoa, the chondrocytes deposit the matrix around themselves.
Because these cells are now part of the matrix, cartilage is living tissue in these animals.
Cartilage is flexible, but strong, and light. It has no blood vessels or nerve fibers running
What do you think this means in terms of cartilage’s ability to repair itself?
Evolution of jaws: STEP 1 one visceral arch:
tentacles around mouth
acellular cartilage rod
In the ancestor of the Deuterostomia (remember, pharyngeal slits are older than the
•origin of pharyngeal slits with reinforcing acellular cartilage deposited as a pair of rods,
one on either side of each slit, forming an arch, holding the slit open
•a pharyngeal slit + its pair of solid, cartilaginous rods is called a visceral arch
•these animals have numerous visceral arches (sometimes up to 200). The slits are very
close together, the cartilaginous rods are slender and the openings are
covered in thick layers of tiny cilia •function: filter feeding (remember the
Cephalochordata and the Urochordata)
fewer, larger tentacles
•rods are now made of cellular cartilage
•there was a dramatic reduction in the number of visceral arches
•the visceral arches are larger and further apart, each rod in the arch is jointed, (not
solid) and larger. The cilia have been replaced by longer, thinner rays that
are not packed so closely together •function: respiration (true gills)
In the ancestor of the Cristozoa:
In many extinct vertebrates without jaws (called “jawless fishes”):
the first two visceral arches begin to move forward: the 1st visceral arch (now called the
mandibular arch) and the 2nd visceral arch (the hyoid arch) separate from the
remaining visceral arches (branchial arches)
In the ancestor of the Gnathostomata (many extinct jawed fishes):parts of the
mandibular arch are modified to form the main components of
the upper and lower jaw (feeding) •the upper jaw is attached to the skull by ligaments
•the rays on the hyoid arch provides support for the gills (respiration) •the branchial
arches still function as gills (respiration)
STEP 4: JAWS !!!!!
So let’s place the preceding information on an expanded tree for the Chordata
that includes many extinct lineages of jawless and jawed fishes:
†Cephalochordata Urochordata Haikouella Myxiniformes Petromyzontiformes
[ extinct jawless fishes ] † † †
[ ------- Gnathostomata -------- ] † †
[ extinct jawed fishes ]
3. somewhere during this time the mandibular and hyoid arches separate from the