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.
See also…
Whale Sharks (Rhincodon typus) are the largest extant Shark
species, and indeed the largest living Fish...
Sawsharks (Pristiophoridae) are highly specialized Sharks related to
Skates and Rays (Batoids). They...
Sharks appear in the fossil record between 450 and 420 million years ago
(all possible specimens older than...
