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? pharyngeal slit
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 The cilia have been replaced by longer, thinner rays that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”): st
the first two visceral arches begin to move forward: the 1 visceral arch (now called the
mandibular arch) and the 2 nd 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 branchial arches
Note: the term “visceral arch” refers to a pharyngeal slit plus its pair of cartilaginous rods
2. reduction in number of visceral arches. The arches are larger and further apart. The
rods are jointed and made of cellular cartilage. Cilia are replaced by long filaments.
Function: respiration (true gills)
4. mandibular arch forms upper and
lower jaw (function: feeding), hyoid arch supports gills (function:
branchial arches remain as gills (function: respiration)
1. many closely placed pharyngeal slits each supported by a pair of acellular cartilaginous
rods covered in dense cilia Function: filter feeding
you do not need to know the tree, just be aware that the oldest fossil sharks are
represented by scales and teeth (about 455 my). Since that time, Chondrichthyes has
undergone several radiations, followed by extinctions.
How do you know it is Chondrichthyes???
1st dorsal fin
2nd dorsal fin
anal fin clasper caudal fin
*Claspers: male intromittent (copulatory) organ formed by a modification of some
cartilaginous rays in the pelvic fin. The clasper is only unique to Chondrichthyes among
living species. Some extinct jawed fishes called placoderms also had a clasper, so this
structure is not an autapomorphy for Chondrichthyes.
Spermatozoa travel from the testis, through a duct to the sperm sac where they are stored.
During copulation the spermatozoa are ejected from the sac through the urogenital papilla
into the clasper and from there into the female:
all Chondrichthyes have internal fertilization via the clasper
male (with claspers) on left, femal