Evolution Education Articles
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Oldest Live-Birth Fossil Found; Fish Had Umbilical Cord
A remarkably well-preserved fossil dating from between 375 to 380 million years ago shows an embryo connected to a mother by an umbilical cord, as seen in an artist's rendering (above). The fossil (bottom), was found in the Australian outback and is the earliest evidence of a vertebrate mother giving birth to live young—shifting back the date some 200 million years, a new study found. Images courtesy Museum Victoria
Carolyn Barry in Sydney, Australia
for National Geographic News
May 28, 2008
Remains of the world's oldest known mother have been unearthed in the Australian outback, scientists say.
The remarkably well-preserved fossil—about 375 to 380 million years old—shows an embryo connected to its mother fish by an umbilical cord.
It is the earliest evidence of a vertebrate giving birth to live young, shifting back the date some 200 million years, said John Long, head of sciences at Museum Victoria in Melbourne, Australia, and lead author of a new study describing the find.
(See a prehistoric time line.)
The fossil is also the earliest record of vertebrate sex, since live birth occurs when an ovum, or egg, has been fertilized internally by male sex cells.
"Having such advanced reproduction for a fish that primitive is amazing," Long said.
Evidence of live birth—as opposed to egg laying—is extremely rare and has only been found in a few fossils of dolphin-like reptiles called ichthyosaurs and marine lizards known as mosasaurs, Long said.
The new fossil captures a long-extinct placoderm, a primitive, shark-like armored fish.
(Related: "Shark Ate Amphibian Ate Fish: First 'Food-Chain Fossil'" [November 8, 2007].)
Dinosaurs of the Sea
Often called the "dinosaurs of the sea," placoderms were the ruling class of marine creatures for 70 million years—in the middle of the Paleozoic period—until their extinction about 360 million years ago.
Paleontologists believe they are the most primitive jawed vertebrates, even predating sharks. (Related: "Fossil Meat Found in 380-Million-Year-Old Fish" [February 12, 2007].)
The newfound mother fish measures 10 inches (25 centimeters) long, but other placoderms can grow to 20 feet (6 meters)—"some gargantuan in size," Long said.
Much of the fish's soft tissue has been preserved in a three-dimensional state, making the fossil "basically an exact replica of the living animal," said study co-author Kate Trinajstic, a paleontologist at the University of Western Australia.
(Read about a dinosaur fossil found with intact skin in China.)
"The material was so well preserved that we were able to pick up subtle details," Trinajstic said.
Such details helped the scientists determine that the prehistoric mother and baby are a new species of ptyctodont, a type of placoderm that has plates around the head and neck rather than the extensive body armor of its relatives.
They named the species Materpiscis attenboroughi—a combination of "mother fish" and a nod to world-renowned naturalist Sir David Attenborough.
Attenborough's 1979 TV series Life on Earth first brought to light the scientific value of the Gogo area in the Kimberley region of Western Australia.
The area is the site of an ancient barrier reef that once teemed with marine life.
Fossils in the Gogo are so immaculately preserved because the reef became devoid of oxygen, which quickly killed the fish and the scavengers that would otherwise devour them, Trinajstic said.
Rapid burial and a stable tectonic continent made for near-perfect fossil preservation conditions.
A description of the fossil is published in this week's issue of the journal Nature.
Michael Lee, an evolutionary biologist at the South Australian Museum, was not involved in the new research.
"Live-bearing and maternal nourishment of embryos is a very important evolutionary innovation, which we ourselves exhibit," Lee said.
"The evidence that the included individual is an embryo [rather than ingested prey] is very strong—it's the same species, the right size to be an embryo, in the correct location within the body, and has what appear to be umbilical structures."
Live birth "might be preserved more commonly than we thought. Now that we know what to look for, it might be noticed more often," he added.
In fact, a reevaluation of a fossil found in 1986 reveals that it is a second placoderm fossil with three embryos nestled inside the mother. Study author Long had found the second specimen, a Gogonasus fossil, on an expedition to Gogo funded by a National Geographic Society grant. (National Geographic News is part of the National Geographic Society).
At the time, Long thought the embyros were scales.
(Related: "Ancient Fish Fossil May Rewrite Story of Animal Evolution" [October 18, 2006].)
"There are still lots of things to discover," Long said. "Gogo is giving us a picture not just of reproduction, but of the whole lifestyle of these creatures."
Evolution in Action: Lizard Moving from Eggs to Live Birth
A yellow-bellied three-toed skink carrying embryos, visible as light orbs inside its body. Photograph courtesy Rebecca A. Pyles
Brian Handwerk for National Geographic News
Published September 1, 2010
Evolution has been caught in the act, according to scientists who are decoding how a species of Australian lizard is abandoning egg-laying in favor of live birth.
Along the warm coastal lowlands of New South Wales (map), the yellow-bellied three-toed skink lays eggs to reproduce. But individuals of the same species living in the state's higher, colder mountains are almost all giving birth to live young.
Only two other modern reptiles—another skink species and a European lizard—use both types of reproduction. (Related: "Virgin Birth Expected at Christmas—By Komodo Dragon.")
Evolutionary records shows that nearly a hundred reptile lineages have independently made the transition from egg-laying to live birth in the past, and today about 20 percent of all living snakes and lizards give birth to live young only.
(See "Oldest Live-Birth Fossil Found; Fish Had Umbilical Cord.")
But modern reptiles that have live young provide only a single snapshot on a long evolutionary time line, said study co-author James Stewart, a biologist at East Tennessee State University. The dual behavior of the yellow-bellied three-toed skink therefore offers scientists a rare opportunity.
"By studying differences among populations that are in different stages of this process, you can begin to put together what looks like the transition from one [birth style] to the other."
Eggs-to-Baby Switch Creates Nutrient Problem
One of the mysteries of how reptiles switch from eggs to live babies is how the young get their nourishment before birth.
In mammals a highly specialized placenta connects the fetus to the uterus wall, allowing the baby to take up oxygen and nutrients from the mother's blood and pass back waste. (See related pictures of "extreme" animals in the womb.)
In egg-laying species, the embryo gets nourishment from the yolk, but calcium absorbed from the porous shell is also an important nutrient source.
Some fish and reptiles, meanwhile, use a mix of both birthing styles. The mother forms eggs, but then retains them inside her body until the very last stages of embryonic development. (Related: "Dinosaur Eggs Discovered Inside Mother—A First.")
The shells of these eggs thin dramatically so that the embryos can breathe, until live babies are born covered with only thin membranes—all that remains of the shells.
This adaptation presents a potential nourishment problem: A thinner shell has less calcium, which could cause deficiencies for the young reptiles.
Stewart and colleagues, who have studied skinks for years, decided to look for clues to the nutrient problem in the structure and chemistry of the yellow-bellied three-toed skink's uterus.
"Now we can see that the uterus secretes calcium that becomes incorporated into the embryo—it's basically the early stages of the evolution of a placenta in reptiles," Stewart explained.
Evolutionary Transition Surprisingly Simple
Both birthing styles come with evolutionary tradeoffs: Eggs are more vulnerable to external threats, such as extreme weather and predators, but internal fetuses can be more taxing for the mother.
(Related: "Human Sperm Gene Traced to Dawn of Animal Evolution.")
For the skinks, moms in balmier climates may opt to conserve their own bodies' resources by depositing eggs on the ground for the final week or so of development. Moms in harsh mountain climates, by contrast, might find that it's more efficient to protect their young by keeping them longer inside their bodies.
In general, the results suggest the move from egg-laying to live birth in reptiles is fairly common—at least in historic terms—because it's relatively easy to make the switch, Stewart said.
"We tend to think of this as a very complex transition," he said, "but it's looking like it might be much simpler in some cases than we thought."
The skink-evolution research was published online August 16 by the Journal of Morphology.
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