The largest Shark ever to live
was Carcharocles megalodon, which
reached sizes of about eighteen meters and survived from the Middle Miocene
until the end of the Pliocene. This was formerly thought to be closely related
to the living Great White Shark, Carcharodon
carcharias, and was placed in the same genus (i.e. known as Carcharodon megalodon), but is now
recognized as a member of the extinct family Otodontidae, collectively known as
the Megatooth Sharks. The Megashark was a remarkably long-lived species (or
morphospecies, since it is impossible to tell whether members of an extinct
‘species’ could have reproduced together, but we can tell they were the same
size and shape, and presumably filled the same ecological niche), surviving for
around 13.3 million years, and had a global distribution, which makes it
possible to study these Sharks over both a long period of time and a wide
geographical range, potentially enabling palaeobiologists to understand what,
if any, evolutionary pressure the species was under while it was alive.
In a
paper published in the
journal
Paleobiology on 4 June 2015,
Catalina Pimiento of the
Florida Museum of Natural History and
Department of Biology at the
University of Florida and
Smithsonian Tropical Research Institute, and
Meghan Balk of the
University of New Mexico examine a large selection of Megashark teeth from museum collections
in order to determine whether the species changed in size over its long fossil
record. Lamniforme Sharks (‘Mackerel Sharks’ – the wider group that includes
both the extinct Megasharks and living Great White Sharks) show heterodont
dentition, that is to say their teeth are not all the same, and it is possible
to determine which jaw a tooth came from and where on that jaw it sat through
morphometric analysis (a mathematical analysis which compares the ratios of
different measurements on a bone, tooth or shell). This enables direct
comparison of isolated teeth from different parts of the mouth, and the
calculation of the overall size of the living Shark from such teeth.
Size, as well as being the most
obvious and dramatic characteristic feature of species such as the Megatooth
Sharks, is an important indicator of ecological role, and therefore useful for
comparing specimens assigned to the same species collected from widely
different places and times. All Megatooth Sharks are thought to have been apex
predators (i.e. they were the largest predators in their environments, lacking
anything which might have fed on them), and the comprises a series of
chronospecies (species which replace one-another over time, and which are
thought to have evolved into one-another rather than dying out) which grow
progressively in time, culminating in the Megashark. This is roughly what is
expected from marine apex predators, as larger animals are able to tackle a
wider range of prey without becoming more vulnerable to attack by predators
higher up the food chain. Based upon this observation Pimiento and Balk
predicted that the species Carcharocles
megalodon was likely to have grown over time.
Schematic representation of the changes in tooth morphology within the megatooth
lineage: cusplet loss, broadening of tooth crowns, and size increase. Pimiento
& Balk (2015).
Pimiento and Balk were able to
examine a large number of teeth from collections around the world online (i.e.
without the need to visit widely-distributed museums personally, which could
have taken years). Teeth which were heavily worn, and therefore could have been
reworked (i.e. buried once, then eroded out of sediments, relocated and
reburied) were excluded from the study, as were teeth which could not be dated
with a reasonable level of accuracy. This meant that some areas where the Megashark
was known to be present were excluded from the end results of the study,
notably northern Europe and Africa, while other areas where severely
under-represented, particularly the tropical Atlantic and Caribbean, and Indian
Ocean.
The results were sorted into
three broad time categories of approximate equal length, the Middle Miocene,
Late Miocene and Pliocene, as well as the Northern and Southern Hemispheres and
Atlantic, Pacific and Indian Ocean Basins.
Contrary to the predicted
outcome, the species Carcharocles
megalodon did not appear to grow over time, with the largest specimens from
the Pliocene being approximately the same size as the largest Middle Miocene
specimens. However the distribution of sizes did vary over time, with a wide
distribution of tooth sizes in the Middle Miocene and a heavy skewing towards
larger specimens in the Late Miocene and Pliocene. There was also a slight
difference in the size of Sharks from different locations, with Sharks from the
Southern Hemisphere being slightly larger in the Middle and Late Miocene, but
not the Pliocene.
It is possible that the
distribution of sample sizes has been affected by sampling bias, as larger
Sharks teeth are more attractive to collectors than smaller teeth. In
particular Pimiento and Balk note that one of the largest collections from the
Southern Hemisphere, originating from the Bahia Formation in Chile, is made up
largely of specimens confiscated from an illegal trade in the teeth, and that
this is likely to have pushed the average size of Southern Hemisphere specimens
upwards.
The early attaining of the
maximum size by Carcharocles megalodon
strongly suggests that the species was unable to grow any larger. The genetic
and physiological underpinning of size in Sharks is not well understood, but
clearly there must be a maximum size which can be reached without dramatic
physiological change not possible through gradual genetic drift, and it is
likely that the Megashark reached this size very early in its history.
The preference for larger
specimens later in the species history may be a result of sampling bias, but
may also be a result of evolutionary pressure favouring larger Sharks. This is
not beyond the bounds of possibility, as many Lamniforme Sharks give birth to
live young rather than laying eggs, and larger Sharks are able to give birth to
larger offspring, giving these juveniles a head start in achieving larger sizes
themselves; since large predatory Sharks are typically willing to eat smaller
members of their own species there is a distinct advantage in an apex predator
Shark reaching its maximum size quickly.
Under this scenario the species
reached its maximum possible size early in its 13.3-million-year history, being
simply unable to grow beyond about 18 m due to biological constraints. However
the species was still subject to ecological pressures favouring larger
specimens, and over time a wide size distribution was replaced with a narrower
distribution, with the Megashark population dominated by larger individuals.
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