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January 24, 2012

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“Oldest known” dinosaur nesting site found

Jan. 24, 2012
Courtesy of the University of Toronto
and World Science staff

Ex­cava­t­ions in South Af­ri­ca have un­earthed a 190-mil­lion-year-old di­no­saur nest­ing site that sheds light on the ev­o­lu­tion of com­plex re­pro­duc­tive be­hav­ior in early di­no­saurs, re­search­ers say.

Made by di­no­saurs of a spe­cies called Mas­sos­pon­dy­lus, the nests pre­date pre­vi­ously known nest­ing sites by an es­ti­mat­ed 100 mil­lion years.

This paint­ing by art­ist Jul­ius Csotonyi il­lus­trates how the nests, eggs, hatch­lings and adults of the prosauro­pod di­no­saur Mas­sospondy­lus might have looked.

A study led by pa­le­on­tol­ogist Rob­ert Reisz of the Un­ivers­ity of To­ron­to Mis­sis­sau­ga de­scribes clutches of eggs—many with em­bryos—as well as ti­ny di­no­saur foot­prints. These are said to pro­vide the old­est known ev­i­dence that hatch­lings stayed at the nest­ing site long enough to at least dou­ble in size.

At least ten nests turned up at sev­er­al lev­els of rock, each with up to 34 round eggs in tightly clus­tered clutches, sci­en­tists said. The lay­out in­di­cates these di­no­saurs re­turned re­peat­edly to this site, a be­hav­ior known as “nest­ing fi­del­ity,” and probably as­sem­bled in groups to lay their eggs, a hab­it called “colo­nial nest­ing,” the re­search­ers added—the old­est known fos­sil ev­i­dence of such be­hav­iors.

An em­bry­on­ic ske­l­e­ton of Mas­so­spon­dy­lus from clutch of eggs at the nest­ing site. (Pho­to by D. Scott )

The sci­en­tists con­tend that the moth­ers’ large size, at six me­ters or yards in length; the small, ten­nis ball-like size of the eggs; and the nests’ highly or­gan­ized na­ture sug­gest moth­ers may have ar­ranged the eggs care­ful­ly. 

“The eggs, em­bryos, and nests come from the rocks of a nearly ver­ti­cal road cut only 25 me­ters long,” said Reisz. “Even so, we found ten nests, sug­gesting that there are a lot more in the cliff, still cov­ered by tons of rock. We pre­dict that many more nests will be erod­ed out in time as nat­u­ral weath­er­ing pro­cesses con­tin­ue.”

A handprint of a baby Mas­so­spon­dy­lus from the nesting site in South Africa. The print shows  the hatch­lings walked on all fours, whereas adults walked on two legs. (Pho­to by D. Scott)

The fos­sils were found in sed­i­men­ta­ry rocks from the Early Ju­ras­sic Pe­ri­od in the Gold­en Gate High­lands Na­t­ional Park in South Af­ri­ca. This site has pre­vi­ously yielded the old­est known em­bryos of Mas­sos­pon­dy­lus. The di­no­saur is part of a line­age known as “pro­sau­ro­pods” be­cause of a kin­ship with gi­ant, long-necked di­no­saurs from the Ju­ras­sic and Cre­ta­ceous pe­ri­ods, known as sau­ro­pods.

“Even though the fos­sil rec­ord of di­no­saurs is ex­ten­sive, we ac­tu­ally have very lit­tle fos­sil in­forma­t­ion about their re­pro­duc­tive bi­ol­o­gy, par­tic­u­larly for early di­no­saurs,” said Da­vid Ev­ans, as­so­ci­ate cu­ra­tor of Ver­te­brate Pal­ae­on­tol­ogy at the Roy­al On­tar­i­o Mu­se­um, where some of the eggs and em­bryos will be on dis­play through May. “This amaz­ing se­ries of... nests gives us the first de­tailed look at di­no­saur re­pro­duc­tion early in their ev­o­lu­tion­ary his­to­ry, and doc­u­ments the an­ti­qu­ity of nest­ing strate­gies that are only known much lat­er in the di­no­saur rec­ord.”

The study is pub­lished in the jour­nal Pro­ceed­ings of the Na­t­ional Acad­e­my of Sci­ences.

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Excavations in South Africa have unearthed a 190-million-year-old dinosaur nesting site that sheds light on the evolution of complex reproductive behavior in early dinosaurs, researchers say. Made by dinosaurs of a species called Massospondylus, the nests predate previously known nesting sites by an estimated 100 million years. A study led by paleontologist Robert Reisz of the University of Toronto Mississauga describes clutches of eggs—many with embryos—as well as tiny dinosaur footprints, said to provide the oldest known evidence that the hatchlings remained at the nesting site long enough to at least double in size. At least ten nests turned up at several levels of rock, each with up to 34 round eggs in tightly clustered clutches, scientists said. The layout indicates these dinosaurs returned repeatedly to this site, a behaviour known as “nesting fidelity,” and probably assembled in groups to lay their eggs, a habit called “colonial nesting,” the researchers added—the oldest known fossil evidence of such behaviours. The scientists contend that the mothers’ large size, at six metres or yards in length; the small, tennis ball-like size of the eggs; and the highly organized nature of the nest suggest the mother may have arranged the eggs carefully. “The eggs, embryos, and nests come from the rocks of a nearly vertical road cut only 25 metres long,” said Reisz. “Even so, we found ten nests, suggesting that there are a lot more in the cliff, still covered by tons of rock. We predict that many more nests will be eroded out in time as natural weathering processes continue.” The fossils were found in sedimentary rocks from the Early Jurassic Period in the Golden Gate Highlands National Park in South Africa. This site has previously yielded the oldest known embryos of Massospondylus. The dinosaur is part of a group known as “prosauropods” because of a kinship with giant, long-necked dinosaurs from the Jurassic and Cretaceous periods, known as sauropods. “Even though the fossil record of dinosaurs is extensive, we actually have very little fossil information about their reproductive biology, particularly for early dinosaurs,” said David Evans, associate curator of Vertebrate Palaeontology at the Royal Ontario Museum, where some of the eggs and embryos will be on display through May. “This amazing series of 190 million year old nests gives us the first detailed look at dinosaur reproduction early in their evolutionary history, and documents the antiquity of nesting strategies that are only known much later in the dinosaur record.” The study is published in the journal Proceedings of the National Academy of Sciences.