Available at http://planet.uwc.ac.za/nisl/Eco_people/Presentations/

Inhabitance of Ancient Seas: Inhabitance of Ancient Seas: Cartilaginous fishCartilaginous fish

(Sharks).(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

Introduction 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

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

Evolution of a super predator: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

Prehistoric sharksPrehistoric 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

Xenacanths and Xenacanths and LeonodusLeonodus::

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.

http://www.elasmo-research.org

Rise of the Modern shark.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 Worldhttp://www.elasmo-research.org

Hybodonts 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.

McmurdodusMcmurdodus

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

Fossilized tooth of Mcmurdodus

PaleospinaxPaleospinax

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

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

Neoselachians Neoselachians

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.

http://www.elasmo-research.org

Molecular evidence of cartilaginous Molecular evidence of cartilaginous fish evolution.fish evolution.

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.

http://www.elasmo-research.org

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

Primitive or Not ?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.

Sharks then and now.Sharks then and now.

The modern descendants of ancient sharks differ with respect to several very important features.

http://www.elasmo-research.org

Snout structure and shapeSnout 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

JawsJaws

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

Braincase and Olfactory capsulesBraincase 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

Modern shark braincasehttp://www.elasmo-research.org

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

Pectoral fins: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 finhttp://www.elasmo-research.org

Vertebrae 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 sharkhttp://www.elasmo-research.org

Conclusion Conclusion 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.

References: 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