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December 08, 2015

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Dinosaurs arose quickly from predecessors, study finds

Dec. 8, 2015
Courtesy of PNAS
and World Science staff

Di­no­saurs evolved from their pre­de­ces­sors much faster than was pre­vi­ously thought, ac­cord­ing to a new stu­dy.

The anal­y­sis puts the time gap be­tween the di­no­saurs and these pre­de­ces­sors, called di­no­saur­o­morphs, at an es­ti­mat­ed 5 mil­lion years in­stead of 10 mil­lion.

An artist's re­con­struc­tion of an­i­mals es­cap­ing from an erupt­ing vol­ca­no 235 mil­lion years ago in north­west­ern Ar­gen­ti­na. These spe­cies, found as fos­sils in the Chañares For­ma­tion, in­clude ear­ly mam­mal rel­a­tives (Din­odon­tosaurus in the left back­ground, and Mas­se­tog­nathus in the left fore­ground) and ear­ly di­no­saur pre­cur­sors (Lewisuchus in the right back­ground, and Lager­peton in the right fore­ground). By meas­ur­ing ra­di­o­ac­t­ive iso­topes in zir­cons crys­tals from the vol­can­ic ash, sci­en­tists ob­tained what they said was a more pre­cise age esti­mate of the fos­sils. (Cred­it: Vic­tor Leshyk)


Di­no­saur­o­morphs look rath­er like di­no­saurs but were rel­a­tively small, of­ten no more than a me­ter (yard) in length, and had var­i­ous skele­tal dif­fer­ences in­clud­ing more un­iform back­bones di­vid­ed in­to few­er seg­ments. They mostly walked on two legs.

Un­cer­tain­ty clouds di­no­saurs’ early his­to­ry, in large part be­cause it’s of­ten hard to pre­cisely meas­ure where and when fos­sils come from. 

In the new work, Ran­dall B. Ir­mis of the Nat­u­ral His­to­ry Mu­se­um of Utah and col­leagues fo­cused on a ge­o­log­ic forma­t­ion in Ar­gen­ti­na where fos­sils of early di­no­saur rel­a­tives ap­pear in lay­ers of rock, cor­re­spond­ing to dif­fer­ent ages.

They al­so es­ti­mat­ed those ages us­ing a newly de­vel­oped form of ra­di­o­i­so­top­ic dat­ing, a meth­od that is based on the rate at which con­centra­t­ions of cer­tain ra­di­o­ac­t­ive el­e­ments change over time. It turned out “these early di­no­saur rel­a­tives were ge­o­log­ic­ally much young­er than pre­vi­ously thought,” said Ir­mis, which “was to­tally un­ex­pect­ed.”

These ages, com­bined with oth­er chronolo­gies from the same forma­t­ion, re­veal that the first clear ev­i­dence of di­no­saurs ap­pears just 5 mil­lion years af­ter di­no­sauro­morphs, the re­search­ers added.

De­spite the rap­id emer­gence, the finding sug­gests that di­no­saurs rose to prom­i­nence in a fairly smooth and grad­u­al way, with­out caus­ing a fun­da­men­tal shift in the bal­ance of the ec­o­sys­tem, the in­ves­ti­ga­tors said. The find­ings may chal­lenge sev­er­al the­o­ries about di­no­saur ori­gins based on less rig­or­ously dat­ed fos­sil ev­i­dence, they added.

The find­ings are pub­lished in this week’s is­sue of the re­search jour­nal Pro­ceed­ings of the Na­tio­n­al Aca­de­my of Sci­en­ces.

Ir­mis and col­leagues stud­ied the Cha­ñares Forma­t­ion, a roughly 75-me­ter-thick-(250-foot) forma­t­ion made of sed­i­ments de­posited by riv­ers, streams and lakes dur­ing the Tri­as­sic Pe­ri­od in pre­s­ent-day La Ri­oja Prov­ince, north­west­ern Ar­gen­ti­na.

The team found that the forma­t­ion, and there­fore the fos­sils in it, is 234 to 236 mil­lion years old, or 5-10 mil­lion years young­er than pre­vi­ous es­ti­mate of a Mid­dle Tri­as­sic age.


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Dinosaurs evolved from their evolutionary predecessors much faster than was previously thought, according to a new study. The analysis puts the time gap between the dinosaurs and these predecessors, called dinosauromorphs, at an estimated 5 million years instead of 10 million. Dinosauromorphs look rather like dinosaurs but were relatively small, often no more than a meter (yard) in height and had various skeletal differences including more uniform backbones divided into fewer segments. They mostly walked on two legs. Uncertainty clouds dinosaurs’ early history, in large part because it’s often hard to precisely measure where and when fossils come from. In the new research, Randall B. Irmis of the Natural History Museum of Utah focused on a geologic formation in Argentina where fossils of early dinosaur relatives appear in layers of rock, corresponding to different ages. They also estimated those ages using a newly developed form of radioisotopic dating, a method that is based on the rate at which concentrations of certain radioactive elements change over time. It turned out “these early dinosaur relatives were geologically much younger than previously thought,” said Irmis, which “was totally unexpected.” These ages, combined with other chronologies from the same formation, reveal that the first clear evidence of dinosaurs appears just 5 million years after dinosauromorphs, the researchers added. Despite the rapid emergence, this latter result suggests that dinosaurs rose to prominence in a relatively smooth and gradual way, without causing a fundamental shift in the balance of the ecosystem, the investigators said. The findings may challenge several theories about dinosaur origins based on less rigorously dated fossil evidence, they added. The findings are published in this week’s issue of the research journal pnas. Irmis and colleagues studied the Chañares Formation, a roughly 75-meter-thick-(250-foot) formation made of sediments deposited by rivers, streams and lakes during the Triassic Period in present-day La Rioja Province, northwestern Argentina. The team found that the formation, and therefore the fossils found in it, is 234 to 236 million years old, from the Late Triassic Period; this is 5-10 million years younger than previous estimate of a Middle Triassic age.