Forskere opdager uddøde forhistoriske krybdyr, der levede blandt dinosaurer

Smithsonian-forskere har opdaget en ny uddød art af firbenlignende krybdyr, der tilhører den samme gamle slægt som New Zealands levende tuatara. Et team af videnskabsmænd, herunder National Museum of Natural Historys kurator for Dinosauria Matthew Carrano og forskningsassistent David DeMar Jr. samt University College London og Natural History Museum, Londons videnskabelige associerede Marc Jones, beskriver den nye art Opisthiamimus gregori, som engang beboede Jurassic North America for omkring 150 millioner år siden sammen med dinosaurer som Stegosaurus og Allosaurus, i et papir offentliggjort i dag i Journal of Systematic Palaeontology . I livet ville dette forhistoriske krybdyr have været omkring 16 centimeter (ca. 6 tommer) fra næse til hale - og ville passe krøllet sammen i håndfladen på en voksen menneskehånd - og sandsynligvis overlevet på en diæt af insekter og andre hvirvelløse dyr.

"Det, der er vigtigt ved tuataraen, er, at den repræsenterer denne enorme evolutionære historie, som vi er så heldige at fange i, hvad der sandsynligvis er dens afsluttende handling," sagde Carrano. "Selvom det ligner et relativt simpelt firben, legemliggør det et helt evolutionært epos, der går mere end 200 millioner år tilbage."

Opdagelsen kommer fra en håndfuld eksemplarer, herunder et ekstraordinært komplet og velbevaret fossilt skelet udgravet fra et sted centreret omkring en Allosaurus-rede i det nordlige Wyomings Morrison Formation. Yderligere undersøgelser af fundet kunne hjælpe med at afsløre, hvorfor dette dyrs gamle orden af ​​krybdyr blev væltet ned fra at være mangfoldig og talrig i jura til kun New Zealands tuatara, der overlevede i dag.

Tuataraen ligner lidt en særlig kraftig leguan, men tuataraen og dens nyopdagede slægtning er faktisk slet ikke firben. De er faktisk rhynchocephalians, en orden, der afveg fra firben for mindst 230 millioner år siden, sagde Carrano.

I deres Jurassic storhedstid blev rhynchocephalians fundet næsten over hele verden, kom i størrelser store og små og udfyldte økologiske roller lige fra vandfiskejægere til voluminøse plantegængere. Men af ​​årsager, der stadig ikke er fuldt ud forstået, forsvandt rhynchocephalians næsten, da firben og slanger voksede til at være de mere almindelige og mere forskelligartede krybdyr over hele kloden.

Denne evolutionære kløft mellem firben og rhynchocephalians hjælper med at forklare tuataraens mærkelige træk, såsom tænder smeltet sammen med kæbeknoglen, en unik tyggebevægelse, der glider underkæben frem og tilbage som en savklinge, en 100 års plus levetid og en tolerance for koldere klimaer.

Efter O. gregoris formelle beskrivelse sagde Carrano, at fossilet er blevet føjet til museets samlinger, hvor det vil forblive tilgængeligt til fremtidig undersøgelse, måske en dag hjælpe forskere med at finde ud af, hvorfor tuataraen er alt, der er tilbage af rhynchocephalians, mens firben nu er fundet over hele kloden.

"These animals may have disappeared partly because of competition from lizards but perhaps also due to global shifts in climate and changing habitats," Carrano said. "It's fascinating when you have the dominance of one group giving way to another group over evolutionary time, and we still need more evidence to explain exactly what happened, but fossils like this one are how we will put it together."

The researchers named the new species after museum volunteer Joseph Gregor who spent hundreds of hours meticulously scraping and chiseling the bones from a block of stone that first caught museum fossil preparator Pete Kroehler's eye back in 2010.

"Pete is one of those people who has a kind of X-ray vision for this sort of thing," Carrano said. "He noticed two tiny specks of bone on the side of this block and marked it to be brought back with no real idea what was in it. As it turns out, he hit the jackpot."

The fossil is almost entirely complete, with the exception of the tail and parts of the hind legs. Carrano said that such a complete skeleton is rare for small prehistoric creatures like this because their frail bones were often destroyed either before they fossilized or as they emerge from an eroding rock formation in the present day. As a result, rhynchocephalians are mostly known to paleontologists from small fragments of their jaws and teeth.

After Kroehler, Gregor and others had freed as much of the tiny fossil from the rock as was practical given its fragility, the team, led by DeMar, set about scanning the fossil with high-resolution computerized tomography (CT), a method that uses multiple X-ray images from different angles to create a 3D representation of the specimen. The team used three separate CT scanning facilities, including one housed at the National Museum of Natural History, to capture everything they possibly could about the fossil.

Once the fossil's bones had been digitally rendered with accuracy smaller than a millimeter, DeMar set about reassembling the digitized bones of the skull, some of which were crushed, out of place or missing on one side, using software to eventually create a nearly complete 3D reconstruction. The reconstructed 3D skull now provides researchers an unprecedented look at this Jurassic-age reptile's head.

Given Opisthiamimus's diminutive size, tooth shape and rigid skull, it likely ate insects, said DeMar, adding that prey with harder shells such as beetles or water bugs might have also been on the menu. Broadly speaking, the new species looks quite a bit like a miniaturized version of its only surviving relative (tuataras are about five times longer).

"Such a complete specimen has huge potential for making comparisons with fossils collected in the future and for identifying or reclassifying specimens already sitting in a museum drawer somewhere," DeMar said. "With the 3D models we have, at some point we could also do studies that use software to look at this critter's jaw mechanics." + Udforsk yderligere

Ancient salamander was hidden inside mystery rock for 50 years—new research