Paleontologists from the University of Florida have identified an ancient intermediate form of the white shark, which shows present-day white shark likely descended from smaller mako sharks and not from megatooth sharks as previously thought.
A new study published in the journal Palaeontology deviates from the white shark’s original classification as a relative of megatooth sharks such as the extinct Carcharocles megalodon, the largest carnivorous shark that ever lived. Using zircon U-Pb dating and strontium-ratio isotopic analysis, the scientists have concluded that the new shark species, named Carcharodon hubbelli, lived about 6-8 million years ago during the late Miocene – at least 2 million years earlier than previously believed.
“We can look at white sharks today a little bit differently ecologically if we know that they come from a mako shark ancestor,” said lead author Dr Dana Ehret of Monmouth University in New Jersey. “That 2-million-year pushback is pretty significant because in the evolutionary history of white sharks, that puts this species in a more appropriate time category to be ancestral or kind of an intermediate form of white shark.”
Most ancient shark species are named using isolated teeth, but analysis of C. hubbelli, also known as Hubbell’s white shark, was based on a complete set of jaws with 222 teeth intact and 45 vertebrae. The species was named for Gainesville resident Gordon Hubbell, a collector who recovered the fossils from a farmer who discovered them in the Pisco Formation of southern Peru in 1988. Hubbell donated the specimens to the Florida Museum of Natural History in December 2009.
“The impetus of this project was really the fact that Gordon Hubbell donated a majority of his fossil shark collection to the Florida Museum,” Dr Ehret said. “Naming the shark in his honor is a small tip of the hat to all the great things he has done to advance paleontology.”
The same team published an initial study describing the shark specimens in the Journal of Vertebrate Paleontology in 2009 (full paper in pdf), but dates for the site reflected information from a 1985 study about the Pisco Formation. With Hubbell’s hand-drawn maps and descriptions of the landscape, researchers returned to the site and found the exact spot the fossils were discovered.
The team extracted more accurate age estimates from mollusk shells in the fossil horizon to determine the shark species was from the late Miocene, about 6.5 million years ago, rather than the early Pliocene, about 4.5 million years ago. The new dates will also be useful for better understanding other fossils found in the rich Pisco Formation, which include new whale, marine sloth and terrestrial vertebrate species.
The paleontologists determined Hubbell’s white shark was related to ancient broad-toothed mako sharks by comparing the physical shapes of shark teeth to one another. While modern white sharks have serrations on their teeth for consuming marine mammals, mako sharks do not have serrations because they primarily feed on fish. Hubbell’s white shark has coarse serrations indicative of a transition from broad-toothed mako sharks to modern white sharks.
Bibliographic information: Ehret D.J. et al. 2012. Origin of the white shark Carcharodon (Lamniformes: Lamnidae) based on recalibration of the Upper Neogene Pisco Formation of Peru. Palaeontology, vol. 55, no. 6, 1139–1153; doi: 10.1111/j.1475-4983.2012.01201.x