Acid Dissolution of Fossil Dinosaur Eggs (2023)

Acid dissolution is a technique of removing a fossil fromthe surrounding rock matrix in which it is encased by dissolvingthat matrix with acid. Fossilized bone, though strong enough to bepreserved for thousands or millions of years, is often more delicatethan rock. Once a fossil is discovered, scientists must remove thefossil from its surroundings without damaging the fossil itself.Scientists have used chemicals to expose vertebrate fossils sincethe 1930s, and in the late 1990s Terry Manning, an amateur scientistand technician working in England, adapted the technology todinosaur eggs. Manning used acid dissolution on dinosaur eggs toexpose the embryos beneath the rock and fossil shell. Manning's aciddissolution enabled scientists to better study the remains ofdinosaur embryos otherwise hidden beneath layers of eggshell androck, revealing previously unrecorded aspects of dinosaur growth anddevelopment.

Acid dissolution is a technique that scientists have used since themid-nineteenth century, first used to expose invertebrate fossils.In the 1930s, scientists Harry Ashley Toombs and Errol Ivor White ofthe British Museum of Natural History in London, England, began toexperiment with acid dissolution to prepare the remains ofvertebrate fossils for viewing. White and Toombs experimented withdifferent levels of acid to remove the red sandstone matrix fromaround the ancient armored jawless fish (ostracoderms)that they were studying. They discovered that acetic acid, when diluted inwater, was the most successful in dissolving the matrix of calciumcarbonate without compromising the fossilized bone, which wascomposed of calcium phosphate. Toombs published the methods ofacid dissolution for vertebrate fossils in 1948, and afterwards manyscientists used similar methods to prepare fossils.

Since the 1930s, paleontologists experimented with a variety ofacid dissolution techniques to remove fossils from various kinds of stonematrices, including fossilized eggs containing potential embryos.Scientists estimated that fewer than one in 500 eggs contain theremains of an embryo. Bones of dinosaur embryos are not welldeveloped (ossified), and therefore are more fragile and more likelyto decay over millions of years than a fossil of an adult dinosaurwith a fully developed skeleton. Most dinosaur eggs are crushed anderoded due to scavenging by predators or as a result of havinghatched.

Prior to the use of acid dissolution, scientists attempted a numberof unsuccessful methods to view embryos within eggs, including the use ofx-ray and CT scan technology to attempt to peer through the bonematrix and eggshell. However, those technologies could detect onlydifferences in density within the egg, but not the structures withinthe egg. Scientists hypothesized that those differences indicatedembryonic bones contrasted against other parts of the fossilizedegg. Even when later methods revealed the presence ofembryonic remains, the density differences picked up by CT scans andx-rays were often either pieces of eggshell inside the egg ordifferential mineralization of the matrix within the egg.

In the 1990s, Manning turned to acid dissolution as an alternative methodto reveal fossilized dinosaur embryos. During that decade, thousandsof dinosaur eggs were exported and sold from China, and Manningpurchased eggs that were the least crushed and most likely tocontain embryonic remains. Manning aimed to dissolve both theeggshell and the stone matrix without damaging the delicateembryonic bones within the egg. In his 1995 co-authored publication TheDinosaur Egg and Embryo Project: Exhibition Guide, Manningdescribed his technique for acid dissolution using dilute aceticacid to dissolve the matrix composed of calcite and silt withoutdissolving the fossilized bones.

To conduct acid dissolution, Manning first partially submerged awhole dinosaur egg in a solution of 5 percent acetic acid forseveral weeks. During that time, he removed the egg every few daysand soaked it in distilled water to remove any acid that seeped deepinto the egg. The egg was then air dried for several days andexamined under a microscope. If any bone was present, Manningcleaned it using a small paint brush or a needle. He then coated thebone with a clear preservative, such as acrylloid b67, to ensurethat the bone didn't decay when he returned the eggs to the acidbath and the process was repeated. Those steps continued for severalmonths, until the entire embryo was visible. Fewer than twentypercent of the eggs that went through Manning's acid dissolutionprocess revealed an embryo.

In 1997 Manning described his acid dissolution technique in"Observations of Microstructures within Dinosaur Eggs from HenanProvince, Peoples' Republic of China," which examined an eggpreviously assigned to the family Dendroolithidae. Through acid dissolution, Manning discovered that the egg was fromLate Cretaceous period, 94 to 66 million years ago. It belonged to along-necked Therizinosaurid dinosaur. The egg had been excavatedfrom the Nanyang Valley of the Henan Province, China, along withthousands of other dinosaur eggs during the late twentieth century.Upon receiving the egg, Manning first attempted to x-ray andCAT-scan the egg, but after the images proved inconclusive, heturned to acid dissolution to reveal the embryo within.

In the paper Manning described how he began with the standardtechnique of acid dissolution described in a widely used manual forpreparing fossils, British Museum paleontologist Arthur E. Rixon's1976 Fossil Animal Remains. Manning discovered that thestandard concentration of acid, five percent, was too strong anderoded away some of the fragile embryonic bones. He then tried morediluted solutions ranging from less than 1 percent to 2 percent acid,which scientists initially thought were too weak, until theydiscovered that the process was inhibited because the solution waslocally stratified. Manning adjusted for that error by shaking thesolution to redistribute and keep it in direct contact with theexposed bone.

Once bone was exposed, Manning and his team covered it with a dilutesolution of less than 1 percent of Paraloid B72 in acetone. Thatprocedure sealed part of the embryo so that when the egg wasre-immersed in the acid, the fossilized bone did not disintegrate.Additionally, the team photographed the embryos during each stage ofthe acid dissolution process. The 1997 paper notes that in additionto uncovering embryonic bones, that process can uncover fossilizedsoft tissue like muscle and cartilage, as well as parts of the egglike the surrounding yolk sac.

The embryos Manning uncovered using acid dissolution enabledscientists to explain much about dinosaur evolution and development.In 2008, a team of scientists led by biologist Martin Kundrát atMcGill University in Montreal, Canada, published a paper analyzingthe embryos he had processed. The team describes the features of thetwo Theropod embryos, including the toothless beak and massive face,larger than in any other described dinosaur egg, indicating that embryos of that speciesspent a longer time incubating than other kinds of dinosaurs. Kundrát also argued thatthe Theropod embryos appeared to have reached a more advanced stageof ossification than other embryos previously found, with thedinosaurs born relatively mature and able to move around on theirown immediately upon hatching.

In the early decades of the twentieth century, Manning was theprimary technician using acid dissolution on dinosaur eggs to revealpotential embryos, with the majority of other technicians preparingembryos by manually removing stone matrices, under microscopes, onegrain at a time.