A sobering view of a Two-toed Sloth as it makes its way along utility lines on our way to Monteverde Preserve. This is what can happen to animals faced with disappearing habitat.
The Evolution of Sharks
Over 400 million years ago, innumerable species of bony fish and marine animals inhabited the Earth’s seas. This time period, known in history as the Devonian Era, also gave birth to a family of fish unlike any other of the time, known as Chondrichthyes. These primitive fish evolved a truly unique skeleton composed almost completely of cartilage. Divergent evolution began to further separate the Chondrichthyans from the primitive fish, creating an array of beings that would come to be known as the possible progenitors of all shark species. These animals are considered to have been highly advanced for their time, possessing a great deal of the same characteristics seen in modern shark species. Now nearly 400,000 millennia of evolution have generated over 1,100 highly advance species of sharks and rays that swim the oceans in place of their ancient ancestors
Chondrichthyes or cartilaginous fish are so named because their internal framework is composed of almost entirely of cartilage rather than bone. Cartilage, unlike bone, does not fossilize well when an animal dies. The exposed collagen fibers in the cartilage degrade and break apart, fragmenting the skeleton into thousands of pieces, and leaving only teeth, scales and spines behind. This explains why scientists have had such difficulty pinpointing the approximate time that these creatures began their life on Earth. The oldest shark-like scale found in Siberia date back to the Silurian era around 420 million years ago. The scales belonged to a fish of the genus Elegestolepis and very little is known of its appearance. In Mongolia more shark –like scales from the similar age were found and given the genera Mongolepis and Polymerolepis. Again there are only names to go by; there is no knowledge of their physical structure. The oldest shark teeth discovered date back to the early Devonian period, about 400 million years old, found in Europe. They are small (3-4mm) two pronged teeth and may belong to a shark known as Leonodus. The two shape have led many to believe that Leonodus was an early the Xenocanth, or freshwater dwelling shark. Again very little is known of this early shark.
The many of oldest articulated fossilized shark remains have been found in Antarctica and Australia leading some paleontologists to project that shark originated in the southern hemisphere. Geologist Gavin Young discovered some of these ancient fossils in deposits in the Lashley Range of Antarctica. The structure and features of these remains have led to the theory that they were also from an early xenocanth. The shark had a spine in front of an elongated dorsal fin and two-pronged teeth and was later called Antarctilamna. The shark is believed to have been 16 inches in length with a long eel-like body. This shark is known to be over 400 million years old, making one of the oldest verified ancestors to modern sharks.
In a historic scientific find last century, scientists discovered several entire shark skeleton imprints dating back to the mid-Devonian Era, persevered in the Cleveland shales in the USA. Among these fossils were the remnants of the one of the oldest known shark genus, the Cladoselache. Scientists found that Cladoselache exhibited several anatomical features that are exclusive to modern sharks. This fish had a very streamlined body structure and grew up to 2 meters (6.5 ft) in length. It had five gill slits and was equipped with all of the same fins, with the exception of an anal fin, as modern sharks. It had an elongated snout and a “terminal” mouth at the front of the head. It also had strong spines composed of dentine and enamel that were positioned in front of the two dorsal fins. The positioning of these spines indicates that they functioned to cut water in front of the dorsal fin, making swimming easier and faster. These spine structures became more common in later sharks and are still found in some species today. Cladoselache had a large pectoral fins and a heterocercal or asymmetrical tail structure, with the top was larger than the bottom, this suggests that it was an excellent swimmer and highly maneuverable. This resembles the modern sharks of the family Lamnidae, a group which includes the white shark and makos which are two of the most efficient swimmers of all pelagic shark species. Remnants of fish fossilized swallowed hole, tail first are indicative of this early shark’s speed and agility. The teeth of Cladoselache also displayed similarities in structure to modern sharks. They had a shallow root and a crown with a central cusp flanked by two smaller cusps, designed to prey on small fish. Unlike most modern and ancient sharks, Cladoselache was almost completely devoid of scales, with exception of a few cusped scales on the edges of the fins, mouth and eyes. One particular characteristic of Cladoselache that was unusual for a shark of that time was that it lacks claspers. The claspers are organ that transfer sperm during copulation. These structures were present on most early sharks, including xenocanths, and evident on all modern shark species. Also Cladoselache had a jaw that was fused with the cranium under the snout and the eye. Modern sharks have a hinged jaw that is connected the skull through several series of ligaments. The latest fossil of Cladoselache have led scientists to believe that it went extinct about 250 million years ago.
Ctenacanths were another important group of that emerged around the same time as Cladoselache. Ctenacanths had many of the same anatomical characteristics as Cladoselache. Among these were the same structure teeth, a jaw fused with the skull at front and back, spines in front of the dorsal fins and broad pectoral fins. They also had developed more advanced features not found on Cladoselache, but present on many modern sharks. These adaptations included three blocks of cartilage that supported the pectoral fins, allowing for greater flexibility. They also had deeply imbedded, combed dorsal fin spines characteristic of modern bullhead sharks and spiny dogfish.
During Carboniferous era, 320 million years ago, most of present day Europe and North America was covered by shallow, warm seas. This is widely considered as the prime of the cartilaginous fish existence. At that point in time sharks are considered to have been as numerous and diverse as present-day reef-dwelling fish (Maisey, 2002). Sharks were once able to divide up the highly productive, warm-water reef environment into many distinct niches in ways their present-day descendants do not (Maisey, 2002). Some of the notable evolutionary adaptations present in sharks at this time were specialized secondary sexual features, including spines and barbs on fins and tails. Others developed complex mechanisms for rapidly replacing teeth after just weeks or days of use. The trait has been maintained throughout the age of dinosaurs to the present.
The Hybodontidae came into existence in the Carboniferous era and may be considered a more immediate ancestor to modern sharks. They possessed many key similarities, including copulatory claspers, which the more primitive Cladoselache did not. The hybodonts became the dominant group during the Mesozoic Era spanning the late Permian to the Jurassic period (220 to 65 million years ago). The Hybodus was a common genus of the hybodont group and is the oldest known representative of the order Selachii. It’s fin orientation and body structure may show links between the more primitive cladoselachii order and neoselachii or modern sharks such as the bullhead. The Hybodontidae are believed to have become extinct about the same as the dinosaurs, around 65 million years ago.
Scientists have also found fossils of mako and mackerel sharks dating back to this era putting the arrival of most modern sharks somewhere in the lower Cretaceous period (150- 100 million years ago). Two paleontologist Detlev Thies and Wolf-Ernst Reif paleontologist proposed a theory called neoselachian radiation which stated that the development of the modern shark or neoselachian was the result of an opportunistic response to an abundant new food source. Populations of these new bony fish exploded in the seas providing plenty of food to predators that possessed the speed, flexible jaws and maneuverability necessary to catch them. Thies and Reif went on to suggest that these sharks began to branch off from near-shore hunting and began competing with the large marine reptiles of the time in pelagic predation.
The “Jurassic Explosion”, beginning approximately 100 million years ago, marker the emergence of thousands of shark that slowly evolved into the species that we see today including the great white. Around 65 to 60 million years ago, the white shark, Carcharodon Carcharias arrived. These sharks were born into existence so highly evolved for their niche on the Earth that they have hardly been altered by nature. The great whites seen today are virtually a mirror image of the white shark of the late-Jurassic/ early Tertiary period.
Also around this time (60- 55 million years ago) the greatest of all sharks inhabited the Earth, Carcharodon megalodon. Megalodon was the biggest predator known to have existed in the oceans of the planet. This immense beast could reach 65 feet in length and could weigh 30 tons, comparable to an average-sized sperm whale. The teeth of megalodon closely resembled that of its smaller cousin the white shark, but three times the size, over 6 inches long. Due to similarities in serrated edges and size of the teeth, a popular theory emerged that the white shark and megalodon were very closely related, if not actually representative of the same species (Ferrari, 1999). This theory suggested that due to a reduction in prey size, the megalodon became smaller eventually evolving into the great white shark. Currently a new popular theory maintains that the two enormous predators diverged from a common line and one containing C. megalodon became extinct and the other has survived to this day. Others still think that it is possible that C. megalodon may still inhabit the most remote parts of the oceans. Scientists have also found partially fossilized giant teeth of C. megalodon a great depths of the ocean causing some to believe that megalodon was still alive 2 million years ago.
Not long ago it was commonly accepted that sharks are simply primitive, mindless predators. We now know that this couldn’t be further from the truth. Humans have recently come to appreciate these creatures for their astonishing history and deep ancestry. Sharks have evolved from modest means hundreds of millions of years ago, to the present day apex predators of the sea. The fact that many modern shark species evolved several million years ago and have remained unchanged in all that time is simply remarkable. Research and compassion for these magnificent animals has helped develop our understanding of their history and place on the Earth.
1. Ferrari, Andrea & Ferrari, Antonella. Sharks. 2002. Firefly Books, Buffalo,
2. Readers Digest- various authors. Sharks: Silent Hunters of the Deep.1986,
Readers Digest Publishing. Surry Hills, NSW.
4. Maisey, John. G. Natural History. Voracious Evolution.v 107 no.5. June
1998. pg. 38-41.
5. Monastersky, Richard. Science News. The first shark: to bite or not to bite.
v.149. Feb 1996. pg 101.
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