1. The Evolution Of Jaws: Agnathans Gnathostomes
By: Sherita Granderson
&
Whittney Rhodes

2. History Agnathans Jawless vertebrates A= no + Gnatho= jaws
Examples: lamprey & hagfish
They have sucker disc mouths used to stick to prey to ingest their insides
Gnathostomes
Modern fish; jawed fish
Gnathos= jaws + Stoma= mouth
1st appeared in the late Silurian period of the Paleozoic Era over 400 m.y.a.
Not only acquired jaws, but also developed paired nostrils, reduced number of gill slits, and a series of pharyngeal arches

3. Timeline From Agnathans to Gnathastomes

4. Neural Crest Cells
Cells that emerge from the dorsal neural tube that will form parts of the nervous system, cartilage, and bones
Migrates to 3 areas:
Trigeminal (mandibular)
Hyoid (pre-otic)
Common Branchial (Post-otic)
Trigeminal neural crest is part of the mesenchymal derivatives of the 1st mandibular arch
Neural crest cells condense to form a dorsal primordium (maxillary) below the eye and ventral primordium (mandibular) below the oral cavity

5. Pharyngeal Arches 2nd arch: Hyoid Arch 1st arch: Mandibular arch
Divided into the Palatoquadrate (Maxillary) & Meckel’s Cartilage (Mandibular)
When neural crest cells make it to this arch, a dorsal maxillary & ventral mandibular articulates
2nd arch: Hyoid Arch
Helps to suspend the mandibular arch from the cranium

6. Hox Genes
Plays a role in directing the neural crest cells to their destination of each pharyngeal arch
There is a Hox- free state found in the mandibular arch
-This may have been the key to the evolution of jaws
Question still unanswered is: “…whether the Hox-free state of the mandibular arch was established at the outset of the gnathostomes evolution, or if it was already present in agnathans, or even in cephalorchordates (amphioxus)?”

7. Specialization in Jaws
Agnathans
Primitive Jaws
Didn’t have specailized palatoquadrate and Meckel’s cartilage
There is no articulation between the arches
Only possessed segmentation of pharyngeal arches, premandibular arch, and the spiracle
Combined the palatoquadrate and Meckel’s cartilage, which moved forward pushing the premandibular arch above the mandibular arch and towards the brain case
The hyoid arch also pushes cranially to begin suspending the mandibular arch, which allows the jaws to open and close the mouth
-It also pushes the spiracle from
alignment to slightly above the
hyoid arch

8. Specialization in Jaws Cont…
Advanced Jaws
No Jaws>Primitive Jaws>Advanced Jaws
Mandibular arch and hyoid arch increased in size
Hyoid arch articulates directly with the brain case making the jaws functional for eating and chewing
Teeth developed
Spiracle become enclosed between the mandibular and hyoid arch
The palatoquadrate cartilage is now in contact with the premandibular arch

9. Jaw Suspensions Primitive Autostylic Suspension Amphistylic Suspension
Hyostylic Suspension
Secondary Autostylic Suspension
Found in placoderms
Found in early bony fish & chondricthyans
Found in modern Condricthyans and osteichthyes
Found in mammals, amphibians, and holocephalans
Palatoquadrate firmly attaches to the underside of the chondrodranium
The hyomandibula extends as a prop from the otic capsule to the caudal end of the palatoquadrate
More flexible jaws that allo to project more downward, forward, or both during eating
Palatoquadrate re-fused to the chondrocranium, but without any help from the hyomandibula
Helped to only open & close mouth to trap in smaller fish to eat
Allowed a small amount of chomping and chewing of smaller prey
Palatoquadrate lost its connection to the chondrocranium; it’s now only held together by the hyomandibula
Instead, the hyomandibula was used for other fuctions; such as forming the stapes of middle ear bones
Made Jaws easier to open & close, tug on food, and chew
Allowed prey to be chewed and completely broken into pieces and have strong grasps on food when biting down

10. Jaw Suspensions Cont…

12. Works Cited
Cerny, Robert, Perter Lwigale, Rolf Ericsson, Daniel Meulemans, Hans-Henning Epperlein, and Marianne Bronner-Fraser. Developmental origins and evolutuion of jaws: new interpretation of “maxillary” and “mandibular”. Developmental Biology edition 276 pg Aug
Cohn, M.J. Lamprey Hox genes and the Evolution of Jaws Nature edition 416 pg May 2004.
Kuratani, Shigeru. Evolution of the vertebrate jaw: comparative embryology and molecular developmental biology reveal the factors behind evolutionary novelty. Journal of Anatomy edition 205 pg Sept
Leim, Bemis, Walker, and Grande. Diversity and Phylogenetic History of Craniates chapter 3. Function Anatomy of the Vertebrates and Evolutionary Perspectives edition 3 pg. 58. Brooks/Cole 2001.
Leim, Bemis, Walker, and Grande. The Cranial Skeleton chapter 7. Function Anatomy of the Vertebrates and Evolutionary Perspectives edition 3 pg. 237 & 240. Brooks/Cole 2001.
Schilling, T. Making Jaws. Heredity pg

13. QUESTIONS ANYONE?!?!