Gnathostomata: The jawed vertebrates. Quantum leap forward.

• Synapomorphies
  
  • The mandibular arch - i.e., the jaws, consisting of upper elements - the palatoquadrates and lower elements - Meckel's cartilages.
    
  • The hyoid arch, that helps connect the mandibular arch to the braincase.
    
  • Branchial skeleton internal to gills. (In contrast to the external branchial basket of lampreys.)
    
  • Enlarged forebrain that is flexed ventrally with respect to hind brain. (Compare with ammocoetes larva of lamprey.)
    
  • Pharyngeal calcifications involving endodermal interaction with neural crest ectoderm - i.e. phayrngeal teeth.
    
  • Paired pelvic appendages
    
  • Horizontal (third) semicircular canal

• Start with the neurocranium:
  

• Add the branchial arches:
  

• Add the hyoid arch:
  

• Add the mandibular arch - palatoquadrate and Meckel's cartilage:
  

• Finally, add the dermal bone of the skull roof:
  

• Jaws themselves - palatoquadrate and Meckel's cartilage.
  
• Internal branchial skeleton.
  
• Inductive formation of pharyngeal calcifications.

Four general hypotheses for this radical transformation:

• Gegenbaur's Serial homology:
  

  Carl Gegenbaur in 1870 proposed that the jaws and hyoid arch represented modified anterior branchial arches. In many modern gnathostomes, the jaws and hyoid arch enclose an opening called the spiracle (=eustacean tube in land verts) Originally, in Gegenbaur's scenario, they originally would have been separated by a complete gill slit. Gegenbaur predicted that fossil forms would be found with a complete gill slit between the mandibular and hyoid arches, but after a century of careful scrutiny, this has not happened. Ironically, Gegenbaur's hypothesis was so widely accepted that in some textbooks it is still treated as revealed wisdom.

• The New Mouth:
  
  From Jon Mallatt's web site

  Jon Mallatt, in the 1984, noted that the branchial arches of gnathostomes and lampreys can't be homologous, as they are medial to and lateral to the gills, respectively. (indeed, sharks retain extrabranchial cartilages lateral to their gills.) He agreed with Gegenbaur's opinion that the mandibular arch had originated as anterior (front) branchial arches. In 1996, he noted that although lampreys and fossil jawless vertebrates may lack jaws, they have cheeks and lips that enclose a large oral cavity. He proposed that the original mandibular arch had been behind the "old mouth," at the front of the pharynx, and had functioned in ventilation of the gills and sucking in prey. As the jaws became specialized for feeding, they moved forward, crowding the "old mouth" into the small space between the jaws and lips, and creating a "new mouth" - the oral cavity between the jaws and pharynx. The connection between breathing and feeding came from the use of the pharynx in suction feeding. Starting with the arches having had a secondary food manipulation function, they became specialized as primary feeding structures. This tied up one loose end, the presence of labial cartilages in sharks. Still, paleontological evidence of the transition has not been forthcoming.

• The Velar skeleton:
  
  From palaeos

  In 1996, Phillippe Janvier proposed that the skeleton of the velum (as seen in lampreys) was the precursor to the mandibular arch. Like jaws, the cartilagenous velar elements of lampreys have upper and lower segments articulating at a posteriorly facing hinge. Alas, here, too, paleontological evidence currently falls short, as it it not obvious that other jawless vertebrates had a velar skeleton. Of course, such a structure would be hard to preserve.

• Neomorphic jaws: Mid 1970s, Bobb Schaeffer of the American Museum of Natural History proposed that jaws were neomorphs, inductively derived from the interaction of the endoderm of the pharynx and neural crest ectodermal tissue when the downturn and expansion of the forebrain brought the pharyngeal endoderm and neural tube ectoderm into close proximity. This has the advantage of explaining the absence of intermediate forms and the simultaneity of the appearance of jaws and the increase in brain size. Possible cause - yet another hox gene duplication. All gnathostomes have four HOX gene clusters - one more cluster than lampreys or hagfish.

Placodermi: Silurian to Devonian armored gnathostomes experienced a rapid worldwide diversification and sudden decline.

Coccosteus from Wikipedia (sorry)

• Synapomorphies:
  
  1. Distinct cranial and thoracic armor with unique ossification pattern.
     
  2. Jaws lined with self-sharpening occluding bony plates. (Whether or not these represent fused teeth is open to debate. Some placoderms, at least, had proper teeth.)
     
  3. Adductor muscles (that close the jaw) pass medial to (inside) palatoquadrate.

• Shellfish feeding with crushing dental plates.
  
• Bottom feeding.
  
• Predation; Some of the largest predatory fish ever , such as Dunkleosteus, were placoderms, These include some of the largest predatory fish ever. Many seem suited to resting on the bottom and engulfing prey by rapidly raising their head and opening jaws. Some localities, however, preserve streamlined pelagic (i.e. open ocean) placoderms.
  
• Some placoderm detritus feeders had greatly expanded box-shaped thoracic armor and pectoral fins enclosed in arthropod-like armor as well. Serial sectioning of some specimens suggests the possible presence of lungs. Perhaps the weird pectoral fins were used form crawling briefly on land?

What placoderms lacked:

From University of Southern Maine - Biology

The myomeres of living gnathostomes differ from those of lampreys and hagfish in that there myomeres are divided into upper and lower halves by a horizontal septum of connective tissue. Lampreys' are not. Recent work on placoderm fossils with soft tissue preservation show that the placoderms, like lampreys, lacked the horizontal septum. This feature, therefore, is a synapomorphy of the two remaining gnathostome groups: Chondrichthyes and Osteichthyes.

Chondrichthyes - Cartilaginous fish (Silurian - Rec.)

Ctenacanthus from Julius T. Csotonyi web site

Basic definitions:

• For our purposes, Chondrichthyes (cartilaginous fish) is defined as all organisms more closely related to modern sharks and chimaeras than to any other gnathostome.

• Living chondrichthyan diversity include:

  Sampling of diversity:
  

  • Holocephali: Chimaeras and fossil relatives
    
  • Elasmobranchii: Shark-like chondrichthyans.

• Synapomorphy: It is often said that chondrichthyans are characterized by lack of internal bone. While true, it is not quite diagnostic. In fact, the unambiguous synapomorphy of Chondrichthyes is the presence of prismatic calcification of the cartilage. In contrast with bone, prismatic calcification takes the form of chains of tiny apatite crystals covering the surface of cartilage, likend together by collagen.
  Note: Chondrichthyans do not lack other hard tissues. Various groups make teeth, fin spines, and dermal armor out of dentine and enamel. To recap:
  

Fossil Material: Sharks are arguably the oldest known gnathostomes, but their fossil record is poor because their bodies lack preservable hard parts that stay articulated when they die. Two notable exceptions:

• Teeth

• Scales - Skin denticles

  These structures are similar in histology (microstructure) and differ mostly in size. Teeth are generated in a conveyor belt of gum tissue which moves them into place, then causes them to be shed and replaced by the next member of their tooth family in line. Modern sharks do this very quickly, with individual teeth being functional for a few days or weeks. This ability has evolved slowly and earlier sharks grew and shed teeth at a lower rate.

• Bottom feeders:
  

  Creatures live on the bottom, in essence grazing on a food source that does not have to be hunted - Detritus or slow moving organsims. It helps to have a body that stays in place and doesn't get wafted away by the current, hence these creatures are often flat or have broad supports. Being flat also makes one cryptic. We have already seen examples: some Heterostraci (above) and placoderms.

• Ambush predators:
  

  Creatures that stalk prey until they are in close range, then pounce on it, or those that lurk, waiting for prey to blunder into biting range. Examples we have seen include some of the large predatory placoderms. These tend to be critters that can accelerate quickly but have low top speed. Among fish, typically use torso for propulsion. This results from a trade off between drag and acceleration. For maximum acceleration, it is best to push with as much of the body surface as possible. Thus ambush predators tend to have deep bodies and tails that are not terribly distinct.

• Pursuit predators:
  

  Creatures that move faster than their prey, overtake, and overpower it. Possible examples include some of the sleek arthrodirans from Gogo. Needn't accelerate quickly, but have high top speed. To achieve high speeds, drag must be minimized. Thus, among aquatic predators, typically the tail is tall and half-moon shaped, like a propeller blade. Drag from the torso is reduced by making its lateral profile narrow. In fact, some animals have horoizontal finlets that enable the tail to swish back and forth with even less drag.

Phylogeny:

From Discovery Education Ancient Sharks

Basal chondrichthyans: The earliest unambiguous chondrichthyan remains are from the Early Silurian, but consist of isolates scales. Some controversy over where on the tree the basal members of chondrichthyes belong, however we get a good idea of what a primitive chondrichthyan looked like from Cladoselache: (Devonian) Earliest known chondrichthyan from numerous well preserved skeletons. Roughly half meter adult length. Superficially shark like, but hardly a proper modern shark. Anatomically suited as fast pursuit predator with tall tail, narrow trunk, and finlets to reduce drag of tail base. Stomach contents can include fish swallowed tail first, conodont animals, and invertebrates. One odd feature is the reduction of the pelvic fins and absence of anal fins.

Chimaeras and kin:

• Holocephali: (Mississippian - Recent) Chimaeras and their relatives. Synapomorphies:
  
  1. Palatoquadrate fused to braincase.
     
  2. Teeth specialized for crushing - often fusing into broad tooth plates.
     
  3. Branchial arches located beneath braincase and covered by a single gill cover.
     

In addition, many holocephalians develop dermal skull plates made not of bone, but of dentine. Generally speaking, holocephalians are slow moving and specialize on crushing hard-shelled organisms.