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Gruesome group death of young dinos analyzed

March 16, 2009
Courtesy University of Chicago
and World Science staff

A mud­dy lake­side some 90 mil­lion years ago drew a herd of young, bird­like di­no­saurs to a ter­ri­fy­ing end, say pa­le­on­tol­ogists who ex­ca­vat­ed the site in Inner Mon­go­li­a’s Go­bi Des­ert.

“These an­i­mals died a slow death in a mud trap, their flail­ing only serv­ing to at­tract a near­by scav­en­ger or preda­tor,” said the Uni­ver­s­ity of Chica­go’s Paul Sereno, one of the lead­ers of the dig. He added that the site pro­vides some of the best ev­i­dence to date for any di­no­saur’s cause of death.

A map of Inner Mon­golia in north­ern China show­ing the site of the dis­cov­ery of a herd of young Si­nor­nith­o­mi­mus di­no­saurs, a place near the out­post Su­hong­tu. (Cour­tesy Pro­ject Ex­plor­a­tion)

Com­posed of ju­ve­niles of the spe­cies Sin­or­nith­o­mi­mus dongi, the find sug­gests young­sters were left to fend for them­selves when adults were busy nest­ing or brood­ing; “there were no adults or hatch­lings,” Sereno re­marked. 

One pair of the skele­tons, pre­pared for dis­play in Sereno’s lab and air­lifted back to Chi­na in late Feb­ru­ary, pre­serve the an­i­mals’ last meals in the stom­achs, sci­en­tists said.

Sereno, along with Tan Lin of the In­ner Mon­go­lia De­part­ment of Land and Re­sources and Zhao Xi­jin of the Chin­ese Acad­e­my of Sci­ences, led the 2001 ex­pe­di­tion that found the fos­sils. The find­ings are pub­lished in the De­cem­ber 2008 is­sue of the re­search jour­nal Ac­ta Pa­lae­on­to­log­ica Po­lon­ica.

“Find­ing a mired herd is ex­ceed­ingly rare among liv­ing an­i­mals,” said team mem­ber Da­vid Var­ric­chio of Mon­tana State Uni­ver­s­ity. “The best ex­am­ples are from hoofed mam­mals.”

The first bones from the herd were spot­ted by a Chin­ese ge­olo­g­ist in 1978 at the base of a small hill in a des­o­late, wind­swept re­gion of the Go­bi Des­ert. Some 20 years lat­er, a Sino-Japanese team ex­ca­vat­ed the first skele­tons, giv­ing the di­no­saur its La­tin sci­en­ti­fic name, which means Chin­ese bird mim­ic.

Sereno and as­so­ci­ates then opened an ex­pan­sive dig. They fol­lowed one ske­l­e­ton af­ter an­oth­er deep in­to the base of the hill to ex­tract more than 25 in­di­vid­u­als, rang­ing from one to sev­en years old as cal­cu­lat­ed by an­nu­al growth rings in the bones.

In this image by artist Todd Mar­shall, a band of young Sin­or­ni­tho­mi­mus di­no­saurs finds itself trapped in mud. (Courtesy Project Exploration)

The team recorded the po­si­tion of all of the bones and the de­tails of the rock lay­ers to try to un­der­stand the long-ago events. The skele­tons showed si­m­i­lar ex­quis­ite pre­serva­t­ion and were mostly fac­ing the same di­rec­tion, the re­search­ers said, sug­gest­ing that they died to­geth­er and rath­er quick­ly.

The de­tails pro­vid­ed key ev­i­dence of an an­cient trag­e­dy, the sci­en­tists ar­gue. Two skele­tons fell one right over the oth­er. Al­though most of their skele­tons lay on a flat hor­i­zon­tal plane, their hind legs were stuck deeply in the mud be­low. Only their hip bones were mis­sing, the likely hand­i­work of a scav­en­ger work­ing over the meat­i­est part of the body bod­ies shortly af­ter the an­i­mals died.

Plung­ing marks in mud sur­round­ing the skele­tons recorded their failed at­tempts to es­cape. Var­ric­chio said he was both ex­cit­ed and down­cast by the grim ev­i­dence. “I was sad­dened be­cause I knew how the an­i­mals had per­ished. It was a strange sensa­t­ion and the only time I had felt that way at a dig,” he said.

In ad­di­tion to herd compo­si­tion and be­hav­ior, the site al­so pro­vides en­cy­clo­pe­dic knowl­edge of even the ti­ni­est bones in the skull and skel­e­ton: “we even know the size of its eye­bal­l,” Sereno said. “Sin­or­nith­o­mi­mus is des­tined to be­come one of the best-un­der­stood di­no­saurs.”

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A muddy lakeside some 90 million years ago drew a herd of young, birdlike dinosaurs to a terrifying end, say paleontologists who excavated the site in the Mongolia’s Gobi Desert. “These animals died a slow death in a mud trap, their flailing only serving to attract a nearby scavenger or predator,” said the University of Chicago’s Paul Sereno, one of the leaders of the dig. He added that the site provides some of the best evidence to date of a dinosaur’s cause of death. Composed of juveniles of the species Sinornithomimus dongi, the find suggests that immature individuals were left to fend for themselves when adults were busy nesting or brooding; “there were no adults or hatchlings,” Sereno remarked. One pair of the skeletons, prepared for display in Sereno’s lab and airlifted back to China in late February, preserve the animal’s’ last meals in the stomachs, scientists said. Sereno, along with Tan Lin of the Inner Mongolia Department of Land and Resources and Zhao Xijin of the Chinese Academy of Sciences, led the 2001 expedition that found the fossils. The findings are published in the December 2008 issue of the research journal Acta Palaeontologica Polonica. “Finding a mired herd is exceedingly rare among living animals,” said team member David Varricchio of Montana State University. “The best examples are from hoofed mammals.” The first bones from the herd were spotted by a Chinese geologist in 1978 at the base of a small hill in a desolate, windswept region of the Gobi Desert. Some 20 years later, a Sino-Japanese team excavated the first skeletons, naming the dinosaur Sinornithomimus (“Chinese bird mimic”). Sereno and associates then opened an expansive dig. They followed one skeleton after another deep into the base of the hill to extract more than 25 individuals, ranging from one to seven years old as calculated by annual growth rings in the bones. The team recorded the position of all of the bones and the details of the rock layers to try to understand what had happened. The skeletons showed similar exquisite preservation and were mostly facing the same direction, the researchers said, suggesting that they died together and rather quickly. The details provided key evidence of an ancient tragedy, the scientists argue. Two skeletons fell one right over the other. Although most of their skeletons lay on a flat horizontal plane, their hind legs were stuck deeply in the mud below. Only their hip bones were missing, the likely handiwork of a scavenger working over the meatiest part of the body bodies shortly after the animals died. Plunging marks in mud surrounding the skeletons recorded their failed attempts to escape. Varricchio said he was both excited and downcast by the grim evidence. “I was saddened because I knew how the animals had perished. It was a strange sensation and the only time I had felt that way at a dig,” he said. In addition to herd composition and behavior, the site also provides encyclopedic knowledge of even the tiniest bones in the skull and skeleton: “we even know the size of its eyeball,” Sereno said. “Sinornithomimus is destined to become one of the best- understood dinosaurs.”