JAWS, Then and Now

Even in their earliest days, sharks showed some remarkable variations on their basic structural theme. Ancient sharks differed from their modern descendants in several important respects:

The snout of a Devonian shark was typically short and rounded, and the jaws were longish and located at the front of the head. In modern sharks, the snout is typically longish and pointed, the jaws shorter and located underneath the head. Long jaws are structurally weaker than short ones and less able to produce a powerful bite, so early sharks may have plucked prey from the bottom or 'on the fin' with forceps-like delicacy.

Early sharks' upper jaws were fixed to the braincase at both the front and the back (the so-called 'amphistylic' form of jaw suspension), unlike most modern sharks in which the upper jaw is fixed to the braincase at the back only ('hyostylic' jaw suspension). As a result, ancient sharks may have been less able to protrude their jaws than modern sharks, reducing their ability to suck prey into their mouths and restricting the size of their food.

The braincase and olfactory capsules (which house the scent organs) of ancient sharks were relatively small, suggesting that they had a lesser brain and less well-developed sense of smell than their modern descendants. Smaller brain size may also indicate that their other senses were less acute, predatory behavior less flexible, and social dynamics lgess sophisticated than in most modern sharks (especially the whalers and hammerheads).

The teeth of the earliest sharks were smooth-edged and multi-cusped, with a large central blade flanked by two or more smaller cusplets on either side (a tooth type termed cladodont, meaning 'branch-toothed'). Although some of the more conservative modern sharks (such as the six- and seven-gills, nurse sharks and smoothhounds) have multi-cusped teeth, the most recent forms (such as whalers, hammerheads, and the white shark) typically have single-cusped teeth often with serrations. Cladodont teeth are best suited to grasping prey that can be swallowed whole; whereas the sharp-edged or serrated single-cusped teeth of modern sharks opens new dietary options, enabling them to gouge pieces from food items too large to be swallowed whole.

The pectoral fins of ancient sharks were triangular and rigid with broad bases. In contrast, most modern sharks have falcate, highly flexible pectoral fins with narrow bases. Therefore, the fins of ancient sharks were probably somewhat less maneuverable than those of modern sharks, making them less agile.

The backbone of ancient sharks was composed of many, relatively simple vertebrae which were uncalcified and did not constrict the spinal column. The backbone of most modern sharks contains fewer, complexly sculpted vertebrae which have calcified bands and constrict the spinal column at regular intervals. (Exceptions include the squaloid dogfishes and the six- and seven-gilled sharks, most of which inhabit very deep waters. It is not clear whether this is due to retention of primitive characteristics or a secondary adaptation to their nutrient-poor deep-sea environment.) The poorly calcified backbone of ancient sharks may have been less able to withstand the forces generated by the flank muscles, making them less powerful swimmers than most of their modern descendants.

Yet in many respects, ancient sharks were very similar to modern sharks. Like the sharks of today, ancient sharks had a cartilaginous skeleton, replaceable teeth, tooth-like scales called 'dermal denticles', multiple gill slits, two sets of paired fins (pectoral and pelvic), claspers (the paired, cartilage-supported copulatory organs of male sharks, developed along the inner margin of the pelvic fins), a backbone that extended into the upper lobe of the tail, and a strongly heterocercal tail fin (more properly called a caudal fin), in which the upper lobe is considerably longer than the lower.

 

Thus, sharks are like automobiles: new models come and go, but certain basic features remain much like those in the prototype. No point in re-inventing the wheel.

 
 

ReefQuest Centre for Shark Research
Text and illustrations R. Aidan Martin
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