Sunday 19 July 2015

Carcharocles megalodon: Did the Megashark get bigger over time?

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.

See also…

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