Tuesday, September 13, 2011

Oldest Shark Nursery Found—Predators Lived in Lakes?

Even baby sharks need a safe haven—and now scientists have found the oldest known nursery for the predatory fish.

Several 230-million-year-old teeth and egg capsules uncovered at a fossil site in southwestern Kyrgyzstan suggest hundreds of young sharks once congregated in a shallow lake, a new study says.

Called hybodontids, the animals were likely bottom feeders, like modern-day nurse sharks.

Mothers would've attached their eggs to horsetails and other marshy plants along the lakeshore. Once born, the Triassic-era babies would've had their pick from a rich food supply of tiny invertebrates, while dense vegetation offered protection from predators.

Yet there's no evidence of any fin that rocked the cradle, so to speak—the babies were likely on their own, said study leader Jan Fischer, a paleontologist at the Geologisches Institut at TU Bergakademie Freiberg in Germany.

(See "Shark Nursery Yields Secrets of Breeding.")

In general, "shark-nursery areas are very important, because they are essential habitats for sharks' survival," Catalina Pimiento, a biologist at the Florida Museum of Natural History, said in an email.

"This study expands the time range in which sharks [are known to] have used nursery areas in order to protect their young," noted Pimiento, who wasn't involved in the study. "This expansion of the range of time reinforces the importance of such zones."

Ancient Sharks Lived in Fresh Water?

When Fischer and colleagues first found shark egg capsules at the field site, they knew teeth shed by the newborns must be also embedded in the earth.

So the team collected several sediment samples, took them to Germany, and dissolved the sediments in the lab. The work yielded about 60 teeth, all of which belonged to babies, except for a single adult tooth.

The high number of baby teeth plus freshwater chemical signatures in those teeth suggest the ancient sharks spawned in fresh water, far from the ocean, according to the study, published in the September issue of the Journal of Vertebrate Paleontology.

Fischer also suspects that the ancient sharks spent their whole lives in lakes and rivers, in contrast with modern egg-laying sharks, whose life cycles are exclusively marine, he noted.

It's possible that, like modern-day salmon, shark adults could have migrated hundreds of kilometers upstream between the ocean and the nursery to spawn. But Fischer finds this scenario "improbable," mainly because of the sheer distance that the fish would have to cover.

(Related: "Sharks Travel 'Superhighways,' Visit 'Cafes.'")

Shark Fossils a Rare Find

Whatever the answer, learning more about ancient sharks via fossils is rare, the study authors noted. Sharks' cartilaginous skeletons decay quickly, leaving just tiny clues as to their lifestyles.

(Also see "Oldest Shark Braincase Shakes Up Vertebrate Evolution.")

The "fact they got these shark teeth fossils with the egg capsules is what makes it really neat," noted Andrew Heckert, a vertebrate paleontologist at Appalachian State University in North Carolina.

"Usually you find either a trace fossil [such as a skin impression] or a body fossil, and you're always trying to make the argument that these represent one or the other" theory, said Heckert, who was not involved in the study.

In other words, having only one type of fossil is often not enough to draw definitive conclusions about an ancient species' behavior.

"Those egg capsules," he added, "are spectacular."

For more interesting topics related to archaeology, visit archaeology excavations.

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