SCIENCE – A Shark Joins The Great Beings That Lived With Dinosaurs

A 91 million-year-old fossil shark, recently called Cretodus houghtonorum and discovered in Kansas, joins a list of large animals from the dinosaur age.

Preserved in sediments deposited in an ancient ocean called Western Interior Seaway that covered central North America during the Late Cretaceous Period (144 million to 66 million years ago), Cretodus houghtonorum was an impressive shark approximately five meters long, according to a new study that appears in the Journal of Vertebrate Paleontology.

The fossil shark was discovered and excavated in 2010 on a ranch by researchers Kenshu Shimada, a professor of paleobiology at DePaul University in Chicago, and Michael Everhart, both research associates attached to the Sternberg Museum of Natural History, Fort Hays State University . The name of the Houghtonorum species is in honor of Keith and Deborah Houghton, the landowners who donated the specimen to the science museum.

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Although it is a largely disarticulated and incomplete skeleton, it represents the best Cretodus specimen discovered in North America, according to Shimada. The discovery consists of 134 teeth, 61 vertebrae, 23 placoid scales and fragments of calcified cartilage, which when analyzed by scientists provided a great deal of biological information about the extinct shark. In addition to his estimated large body size, anatomical data suggested that he was a fairly slow shark, belonged to a group of sharks called Lamniformes that includes the great modern white and sand tiger sharks as distant cousins, and had a fairly tooth pattern different for a lamniform shark.

"Much of what we know about extinct sharks is based on isolated teeth, but an associated specimen that represents a single shark individual like the one we describe provides a great deal of anatomical information that in turn offers a better understanding of their ecology, "said Shimada, the lead author of the study.

"As important ecological components in marine ecosystems, the understanding of sharks in the past and present is essential to assess the roles they have played in their environments and biodiversity over time, and more importantly, how they can affect future marine ecosystem if they become extinct, "he said.

During the excavation, Shimada and Everhart believed that they had a specimen of Cretodus crassidens, a species originally described in England and subsequently commonly reported from North America. However, not a single tooth matched the tooth shape of the original Cretodus crassidens specimen or any other known Cretodus species, Shimada said.

"It was then that we realized that almost all North American teeth that were previously reported as Cretodus crassidens belong to a different species new to science," he said.

The shark growth model calibrated from the observed vertebral growth rings indicates that the shark could have theoretically reached up to about 6.8 meters.

"What is more exciting is its large size inferred at birth, almost 1.2 meters in length, which suggests that the cannibal behavior for the breeding of embryos commonly observed inside the uterus of modern female lamiforms should already have evolved at the end of Late Cretaceous, "Shimada added.

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