Sharks are among the most popular animals featured in television and cinema. And today among sharks, the undisputed king is the great white, a giant predator that can exceed 20 feet in length. Despite the popularity of great whites, relatively little is known about their biology, and even less is known about their evolutionary origins. A new 4-million-year-old fossil from Peru described in the Journal of Vertebrate Paleontology provides important evidence suggesting the shark’s origins may be more humble than previously believed.
Fossil shark skeletons are extremely rare because sharks do not have bony skeletons like most fishes – instead their skeleton is made of cartilage. The new specimen is the most complete fossil known from a white shark. It includes parts of the spinal column, the head skeleton, and a mouthful of 222 teeth.
“It is very unusual for a shark, which has a cartilaginous skeleton, to preserve these details in the fossil record,” said Mr. Dana Ehret, a graduate student at the Florida Museum of Natural History and lead author of the article.
Photograph of the specimen as it appears in the Journal of Vertebrate Paleontology, with nose toward the top of the photo and spinal column toward the bottom. Photo by Dana J. Ehret
In sharp contrast to the wide expanses of open water that it used to dominate, the new fossil was discovered in the dry desert region of coastal Peru. In 1988, Dr. Gordon Hubbell, a co-author of the paper, and local collectors were examining the 4-million-year-old sediments of the Pisco Formation when they came across the shark remains. These large areas of sediments in southwestern Peru are becoming increasingly well known for a variety of fossils of whales, birds, and even an aquatic sloth, Thalassocnus.
Frequently only isolated teeth of shark are present in these ancient sediments. To get a complete picture of a shark’s dentition, scientists have to put these isolated teeth together using information from modern sharks. Having a fossil such as this one with its teeth in their natural positions is important because the shapes of particular teeth and their orientations in the jaw help determine how shark species are related to one another.
“The completeness of this specimen allows us to take a closer look at the interrelationships between white and mako sharks,” said Ehret.
The undoubted all-time kings of the shark world were the so-called “megatooth” sharks. The largest of these, such as Carcharocles (“Carcharodon”) megalodon were contemporaries of the Peruvian shark and may have reached lengths of 60 feet. With a jaw gape of more than nine feet, they would have put great whites in the shade.
Nevertheless, many scientists have argued that the megatooths were close cousins of the great whites. However, the new specimen suggests that the modern great white shark is more closely related to the modern mako shark – a smaller shark that feeds mostly on fishes – than to the prehistoric giant ‘megatooth’ sharks. If true, then the modern great white and the megatooth sharks might have evolved to large size independently. The new fossil specimen was probably 17 feet long in life, similar in size to a large modern great white.
Because this specimen also preserved part of the spinal column, scientists were able to determine that the individual was at least 20 years old when it died. The age determination was based on counting the alternating light and dark bands present in the vertebrae, which calcify with age. Such bands have been shown to represent seasonal changes in modern sharks; this was tested in the fossil by examining difference in the isotopic composition of the dark and light bands, which reflects seasonal temperature changes. A modern great white shark of similar age likely would have been larger, suggesting that this fossil species grew at a slower rate.
“With the exceptional preservation of this Pisco Formation specimen we have a unique opportunity to advance knowledge about the ancient paleobiology of white sharks and their extinct relatives,” said Dr. Bruce MacFadden of the Florida Museum of Natural History, a co-author of the study.
Citation: Ehret, D. J., G. Hubbell, and B. J. MacFadden. 2009. Exceptional preservation of the white shark Carcharodon (Lamniformes, Lamnidae) from the early Pliocene of Peru. Journal of Vertebrate Paleontology, volume 29, No. 1.
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