Bones show ancient marine reptile was a big baby | Science News for Students

Bones show ancient marine reptile was a big baby

Plesiosaurs likely needed this survival advantage in predator-infested seas
Mar 8, 2019 — 6:45 am EST
a photo of a fossil of developing baby plesiosaur skeleton inside the mother's skeleton

The bones of a developing baby (encircled) can be seen inside the skeleton of this plesiosaur.


Sometimes it’s good to be a big baby. That’s what scientists are learning from a skeleton at the Natural History Museum of Los Angeles, in California. It belongs to a marine reptile known as Polycotylus (Pahl-ee-KOAT-eh-luss). Inside the fossil skeleton, scientists found the bones of a developing baby. By comparing that fossil to other young plesiosaurs, researchers are figuring out how these animals grew.

Polycotylus is a type of plesiosaur (PLEES-ee-oh-sawr). These reptiles swam the seas between 203 million and 66 million years ago. They weren’t dinosaurs, although dinos lived at the same time. Plesiosaurs were enormous — growing to 5 meters (16 feet) in length. We’ve known about they since 1821. That’s when English paleontologist William Conybeare first described one. Back then, no one was sure how these enormous swimmers reproduced.

Instead of laying eggs like other reptiles, a mother Polycotylus gave birth to a single, large offspring. Here, an artist imagines what that looked like.
Stephanie Abramowicz/NHMLA

“We really had no idea,” says Robin O’Keefe. He’s a paleontologist at Marshall University in Huntington, W.V. Today’s sea turtles, also ocean reptiles, lay their eggs on land. No clues existed that plesiosaurs might have had live births. But they seemed too big to venture ashore to lay eggs.

The mystery remained unsolved until 2011. That’s when O’Keefe and his colleagues first studied the Polycotylus skeleton in Los Angeles. As luck would have it, this plesiosaur had been pregnant. That showed scientists that Polycotylus gave birth to live young, instead of laying eggs. And it let O’Keefe compare the baby to other fossils for the new study.

“The fetus of Polycotylus is an extraordinary finding,” says Corinna Fleischle. She’s a paleontologist at the University of Bonn in Germany. Although not involved in this study, she does study plesiosaurs. And she says the discovery “for the first time [makes it possible] to investigate the bone microstructure in an unborn plesiosaur.” That’s what O’Keefe’s team did in its new research.

Boning up on its life history

Scientists had found Polycotylus bones before. Those fossils belonged to reptiles that lived about 75 million years ago. They were found in a seaway that split North America in half. O’Keefe and his colleagues compared these bones to those of the pregnant one. They used a branch of science called histology. It’s the study of microscopic details in body tissues. Such details can point to how these animals grew.

O’Keefe’s team saw one Polycotylus fossil that was similar in size to the unborn baby. This one, however, had already been swimming on its own. The researchers knew that from a line in its bone tissue that likely formed soon after the reptile’s birth. The pressures of having to swim on its own had changed how that baby’s bones were growing.

These data now suggest that a newborn Polycotylus would have been four-tenths its mother’s length. So a baby would have been about 2 meters (7 feet) long at birth. O’Keefe and his colleagues shared their new findings January 2, 2019, in Integrated Organismal Biology.

“Zooming in to the actual bone cells, and seeing the direction of the bone fibers” was very important, O’Keefe says. It helped him to understand how the bones grew. And that revealed more about the ancient reptile’s life story.

Plesiosaurs, the new data show, had “an absolutely unique bone histology,” Fleischle says. It “is not comparable to any other species, living or extinct.” That’s why it’s critical to compare fossil bones to others from the same species, she says.

One big question remains: Why were these creatures born so large?

“We don’t have a lot of hard evidence [to go on],” O’Keefe admits. Still, he has some ideas. Large size and fast growth might have been a form of protection. The reptiles lived during the Mesozoic period, when the “sea was a very dangerous place.” Huge sharks, fish, and other hungry marine reptiles were hunting a meal. In these unfriendly waters, being born big might have helped Polycotylus avoid becoming lunch.

Power Words

(more about Power Words)

biology     The study of living things. The scientists who study them are known as biologists.

cell     The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells.

colleague     Someone who works with another; a co-worker or team member.

extinct     An adjective that describes a species for which there are no living members.

fetus     (Adj. fetal ) The term for a mammal or other large animal    during its later-stages of development in the womb. For humans, this term is usually applied after the eighth week of development.

fiber     Something whose shape resembles a thread or filament.

fossil     Any preserved remains or traces of ancient life. There are many different types of fossils: The bones and other body parts of dinosaurs are called “body fossils.” Things like footprints are called “trace fossils.” Even specimens of dinosaur poop are fossils. The process of forming fossils is called fossilization.

histology     The anatomical study of the microscopic structure of animal and plant tissues. The microscopic structure of tissue.

marine     Having to do with the ocean world or environment.

Mesozoic      An era in geologic history that contained three related periods that became renowned for their large reptiles: the Triassic (which spanned from 251 to 199.6 million years ago), the Jurassic (which spanned from 199.6 to 145.5 million years ago), and the Cretaceous (which spanned from 145.5 to 65.5 million years ago).

microscopic     An adjective for things too small to be seen by the unaided eye. It takes a microscope to view objects this small, such as bacteria or other one-celled organisms.

paleontologist     A scientist who specializes in studying fossils, the remains of ancient organisms.

plesiosaur     A type of extinct marine reptile that lived at the same time as dinosaurs and is noted for having a very long neck.

reptile     Cold-blooded vertebrate animals, whose skin is covered with scales or horny plates. Snakes, turtles, lizards and alligators are all reptiles.

sea     An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.

shark     A type of predatory fish that has survived in one form or another for hundreds of millions of years. Cartilage, not bone, gives its body structure.

species     A group of similar organisms capable of producing offspring that can survive and reproduce.

tissue     Made of cells, it is any of the distinct types of materials that make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues.

unique     Something that is unlike anything else; the only one of its kind.


Journal: F.R. O’Keefe et al. Ontogeny of polycotylid long bone microanatomy and histology. Integrative Organismal Biology. Vol. 1, January 2, 2019 (online). doi: 10/1093/iob/oby007.

Journal: F.R. O’Keefe and L.M. Chiappe. Viviparity and K-selected life history in a Mesozoic marine plesiosaur (Reptilia, Sauropterygia). Science.  Vol. 333, August 12, 2011, p. 870. doi: 10.1126/science.1205689.