Sciency Thoughts: The demise of the Megashark.

The Giant Shark, Carcharocles megalodon, is one of the more charismatic creatures of the recent fossil record, a relative of modern Mackerel Sharks that is thought to have been able to reach about 18 m in length, known from the Middle Miocene to the end of the Pliocene, with some claims of the species persisting into the Pleistocene. It is interpreted to have had a life-style similar to the modern Great White Shark, which preys primarily on Marine Mammals, and teeth of Carcharocles megalodon are frequently found with fossil Whales from the Miocene and Pliocene.

A set of jaws of the Giant Shark, Carcharocles megalodon, from the National Aquarium of New Zealand. Wikipedia.

Large predators are thought to have a profound effect on marine ecosystems, and it has been shown in modern habitats that the removal of such top predators often leads to a chain reaction which can completely reshape the local biological community. Given the size of Carcharocles megalodon, its removal from marine ecosystems can be expected to have had a major impact, however it is difficult to determine when fossil species actually went extinct, making it hard to detect changes in marine ecosystems associated with the loss of particular species.

In a paper published in the journal PLoS One on 22 October 2014, Catalina Pimiento of the Florida Museum of Natural History and Department of Biology at the University of Florida and Christopher Clements of the Institute ofEvolutionary Biology and Environmental Studies at The University of Zurich, use a statistical method called the Optimal Linear Estimation model to attempt to determine exactly when Carcharocles megalodonwent extinct.

The Optimal Linear Estimation model was developed by conservation biologists studying extinction in modern species. It uses a statistical method based around recorded sightings of a species to determine the probability of an endangered species still being alive, or when it is most likely to be extinct. Since Carcharocles megalodon is known only from fossil specimens and has never been seen by human eyes fossil records were used as a proxy for sightings. Like all Sharks Carcharocles megalodon lacked a bony skeleton (Shark skeletons are formed entirely of cartilage) but produce new teeth throughout it life, losing some when attacking prey and shedding others as they become worn. This makes for a very good fossil record (tooth is denser than bone, and preserves more easily as a fossil) for all Sharks, particularly large and distinctive species that have lived in the not-to-distant past.

Using the Optimal Linear Estimation model Pimento and Clements determined that the extinction of Carcharocles megalodon is most likely to have occurred 2.6 million years ago exactly at the end of the Pliocene. This would coincide with a dramatic expansion of Baleen Whales (filter-feeding Whales) at the beginning of the Pleistocene; up until this point these Whales were a minor part of marine ecosystems and were quite small compared to modern species, but in the early Pleistocene they began to diversify rapidly and grow dramatically in size, and today include the largest species of animal ever known to have lived (the Blue Whale, Balaenoptera musculus).

The association between the demise of Carcharocles megalodon and the rise of the great Baleen Whales has been made in the past, but has been hard to prove or to compare to the alternative theory that the expansion of the Baleen Whales was driven by a re-organization of plankton diversity at the end of the Pliocene driven by the closure of the Panama Seaway, which connected North and South America, and which in turn led to a major re-organization of the Earth’s ocean currents.

Temporal distribution of the inferred dates of extinction of Carcharocles megalodon using the Optimal Linear Estimation model bootstrapped 10,000 times. The orange area shows the distribution of inferred dates of extinction through time, whereas the green line shows the cumulative frequency of inferred dates of extinction. The modal peak represents the point in time by which the species was most likely to have gone extinct (2.6 million years ago). Approximately 50% of simulations fell before the modal peak of inferred dates of extinction (2.6 million years ago), whereas the remaining 50% are roughly evenly distributed between the mode and the present day. The two vertical dashed lines indicate the most recent and oldest inferred dates of extinction (160,000 years in the future and 3.5 million years ago respectively). The horizontal bars represent the time range of each fossil occurrence. The blue bars are the occurrences used in the Optimal Linear Estimation model analysis. The grey bar represents the occurrences that failed the age evaluation process and were not used in the analysis. Pimento & Clements (2014).

Pimento and Clements also estimated that there is a 0.06% chance that Carcharocles megalodon has not gone extinct at all, and is still found in today’s oceans; however they strongly emphasize that this is an artefact of the statistical methodology used, and that they do not believe that there is any chance of the species still being in existence. While surviving Megasharks makes for a good movie plot, it is highly unlikely that an 18 m Shark could have escaped detection into the twenty-first century, no matter how cryptic its behaviour. In order for this to be the case the Shark would not just have to avoid detection by all observers (including military sonar systems developed during the Cold War which looked for things far smaller than 18 m), it would have to avoid leaving any distinctive marks on prey species from attacks (bite marks several meters across would be likely to be recorded), and would need to avoid losing any teeth in places where humans could find them; as noted before Sharks lose teeth throughout their lives, and Sharks’ teeth are widely collected by humans for ornamental purposes, making it highly unlikely that recent teeth of Carcharocles megalodon would escape detection.

(Left) A tooth from Carcharocles megalodon, and (right) pendants made from modern Sharks teeth. Wikipedia/ebay.

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Tuesday, 4 November 2014

The demise of the Megashark.