Inhabitance of Ancient Seas: Cartilaginous fish (Sharks).

1. Inhabitance of Ancient Seas: Cartilaginous fish(Sharks). Marlee J. De Vries, 2234552 Department Biodiversity and Conservation Biology , University of the Western Cape, Private Bag X17, Bellville, 7535, South Africa. 2234552@uwc.ac.za

2. Introduction • 12 000 years ago any human venturing into the sea, faced an encounter with the largest fish-eating shark ever to inhabit the oceans, a 15 m monster known as Carcharodon megalodon. • This is not the first cartilaginous fish on earth however, their lineages can be traced back to the Silurian era, 440 million years ago. http:// www.sharkattacks.com

3. Cartilaginous fish are among the oldest surviving vertebrate groups. • The popular belief that sharks have not evolved and have remained virtually unchanged over the centuries, is incorrect. • Sharks have in fact evolved slowly relative to mammals for example, throughout the centuries. • Are they thus primitive animals? http://www.elasmo-research.org

4. Evolution of a super predator: • Sharks have undergone two major adaptive radiations and at least several smaller ones and have survived some five mass extinctions. • Modern day sharks are the products of hundreds of millions of years of evolutionary tinkering. http://www.elasmo-research.org

5. Prehistoric sharks. • The first major shark radiation occurred during the Carboniferous Period, 360 to 286 million years ago. • This era was dominated by lungfishes and xenacanth sharks. Reconstruction of Xenacanthus. A freshwater shark from the early Permian era (about 280 million years ago). http:// www.sharkattacks.com

6. Xenacanthus and Leonodus are the most well document prehistoric shark species. First discovered in the early Devonian deposits, about 400 million years ago. Of the two species Leonodus is the most mysterious and scientists believe that Leonodus may have been a sub-species of the freshwater species Xenacanthus, and have arisen some 3-4 million years ago. Xenacanths and Leonodus: http://www.elasmo-research.org

7. Rise of the Modern shark. • The second occurred during the Jurassic Period some 208-144 million years ago. • It was in this world were modern sharks first originated with species such as Hybodus sp., Mcmurdodus sp., and Paleospinax sp. The Jurassic World http://www.elasmo-research.org

8. Hybodonts • Hybodus sp. had a length of 2.5 m with • Lived in shallow seas 180 million years ago. • Representatives of the species were also found in freshwater and brackish water . • According to J G. Maisey the Hybodonts was most likely a side-branch of the group that gave rise to the “Modern Shark”. Hybodus sp.

9. Mcmurdodus • The earliest known modern shark, originating from the early-Devonian period. • Its status is based on its multi-layered tooth structure. • This type of teeth can be found in all living sharks, but are commonly lacking in the ancient types. The Devonian World http://www.elasmo-research.org

10. Fossilized tooth of Mcmurdodus

11. Paleospinax • This species arose about 140 million years after Mcmurdodus. • Fossilized teeth have been found to range from 250-60 million years of age. http://www.elasmo-research.org

12. Paleospinax shared a number of morphological features with modern, living sharks, including: • Longish snout • Sub-terminal mouth • Short jaws, loosely attached to the cranium • Teeth with dense enameloid • Well developed vertebrae. http://www.elasmo-research.org

13. These animals first radiated in the mid-Cretaceous Period, about 100 million years ago. The earliest members of this group were near-shore predators. In the mid-Cretaceous Period they evolved and became fast offshore hunters. They were among the few to survive the global catastrophe that wiped out many other species at the end of the Cretaceous period, including the hybodonts and he dinosaurs. Neoselachians http://www.elasmo-research.org

14. Anthropologist Rebecca Cann first used mitochondrial DNA (mtDNA) as a kind of molecular clock to estimate the age at which the ancestors of modern humans first appeared. The molecular clock of sharks was determined by relating the genetic changes of two populations of small hammerhead species which diverged by the rise of the Isthmus 7-3 million years ago. This research showed that the rate of change in sharks are some 7-8 times slower than mammals. Molecular evidence of cartilaginous fish evolution. http://www.elasmo-research.org

15. To date, research performed by Andrew P. Martin shows that in a period of 6 million years sharks have only undergone a 1% change in genetic sequence. A comparison of all three genera of Lamnidae, suggests the following: Lamna is the most divergent species with 7.6% difference from Carcharodon. Separation of the two species would then have occurred 35 million years ago Isurus differs from Carcharodon by 7.1% thus divergence would have occurred about 60-35 million years ago. The white shark for example can thus be trace back to 60 million years and would thus constitute as a relatively old species. http://www.elasmo-research.org

16. Primitive or Not ? • Based on the evidence give can cartilaginous fish be considered as primitive animals and if so how primitive are they? • The answer is no, they are not primitive at all, they are however, ancient.

17. Sharks then and now. • The modern descendants of ancient sharks differ with respect to several very important features. http://www.elasmo-research.org

18. Snout structure and shape • Ancient sharks typical of the Devonian Period possessed a short, rounded snout, with an elongated jaw. • Their modern counter parts have a elongated, pointed snout, with shorter jaws located sub-terminally. • In terms of structure ancient sharks had much weaker jaws and would have had a typically bottom feeding existence. http://www.elasmo-research.org

19. Jaws • Early sharks had their jaws fixed to their braincase anteriorly as well as posteriorly. • Present day sharks have their upper jaw fixed to their braincase posteriorly. • As a result early sharks were not able to protrude their upper jaws as commonly seen with the modern sharks (Great White sharks) also they would have been restricted in the size of pray. http://www.elasmo-research.org

20. Braincase and Olfactory capsules • These features were relatively small in ancient sharks, which would suggest that they had a smaller brain and a less well developed sense of smell than their modern counterparts. • This could also have had implications on predatory behaviour as well as social dynamics. Ancient shark braincase http://www.elasmo-research.org Modern shark braincase

21. Teeth • Ancient sharks had smooth-edged, multi-cusped teeth with a large central blade, flanked by two or more cusplets on either side. • Multi-cusped teeth is still evident today in some species, most modern day sharks have single-cusped teeth. Multi-cusped tooth of early sharks Single-cusped tooth of modern sharks. http://www.elasmo-research.org

22. Pectoral fins: • Ancient sharks typically possessed triangular fins with broad based ridges. • In contrast modern sharks have highly flexible falcate pectoral fins. • As a result of this difference ancient sharks were most likely less agile in their environment than modern sharks. Ancient shark fin Modern shark fin http://www.elasmo-research.org

23. Vertebrae • Ancient sharks had a relatively simple backbone which was uncalcified and thus had no constriction on the spinal column. • Most modern sharks posses less and more complexly sculpted vertebrae, with calcified bands at regular intervals. Spinal column of ancient sharks Spinal column of modern shark http://www.elasmo-research.org

24. In conclusion although ancient species share many common features with their modern day counterpart such as a cartilaginous skeleton, replicable teeth, dermal denticles etc., the divergence in this order is evident, showing clear evolutionary traits. Thus although they are an ancient species the sharks observed today are clearly a modern day version and thus not primitive. Conclusion

25. http://www.elaso-research.org/ biosketch.htm http://www.sharkattacks.com/ chat.htm Anderson M K, Sun X, Miracle A L, and Rothenberg G W and Rothenberg E V (2001 ) Evolution of hematopoiesis: Three members of the PU.1 transcription factor family in a cartilaginous fish, Raja eglanteria. PNAS,98: 553-558 References: