Jurassic-dinosaur med fuglehøfte var en af ​​de første, der levede i flokke

Fossiler afsløret i Sydafrika kan afsløre nogle af de tidligste beviser på social adfærd blandt ornitiske dinosaurer.

Da verden kom sig efter en vulkansk udryddelse, kan Lesothosaurus have slået sig sammen for at hjælpe med at afværge rovdyr.

Et af de tidligst kendte medlemmer af en større dinosaurgruppe har vist, at krybdyrene kan have været sociale i det meste af deres evolutionære historie.

Lesothosaurus diagnosticus var en af ​​de tidligste ornithische dinosaurer, den gruppe, der senere skulle omfatte arter som Triceratops horridus og Iguanodon bernissartensis.

Udgravninger i Sydafrika, ledet af palæontologer fra Evolutionary Studies Institute, Johannesburg og Natural History Museum, afslørede et "benbed" af Lesothosaurus-individer, med dele af mange skeletter rodet og pakket tæt sammen.

En ny undersøgelse af disse fossiler tyder på, at Lesothosaurus levede i flokke, som strejfede rundt i det antikke superkontinent Pangaea, hvilket giver nogle af de tidligste beviser for socialt liv i disse dinosaurer.

Professor Paul Barrett, en dinosaurekspert ved museet, der var medforfatter til undersøgelsen, siger:"Indtil nu havde vi ikke direkte beviser for, at Lesothosaurus levede sammen i grupper."

"Vi ved, at senere ornithischians havde iøjnefaldende træk, såsom horn og dikkedarer fra Triceratops, som blev brugt til kommunikation og anden social adfærd, men Lesothosaurus mangler disse åbenlyse knoglekarakteristika. Dette viser, at Lesothosaurus allerede udviklede aspekter af sin sociale biologi før disse strukturer udviklet sig."

"At leve i en flok giver disse dinosaurer de samme fordele, som vi ville forvente af flokliv i dag. Besætninger giver beskyttelse mod prædation ved at reducere chancen for, at et individ bliver angrebet, samt ved at virke mere intimiderende for et rovdyr."

Resultaterne af undersøgelsen blev offentliggjort i tidsskriftet Palaeontology .

Hvad er Lesothosaurus?

Lesothosaurus levede for mere end 190 millioner år siden i den tidlige jura og levede i det nuværende Sydafrika og Lesotho. Det var en relativt lille tobenet dinosaur, sammenlignet med nogle af dens efterkommere, og var omkring to meter lang.

Det dukkede først op kort efter begyndelsen af ​​Jurassic Perioden, da verden var ved at komme sig efter en tid med intens vulkansk aktivitet, som menes at have været ansvarlig for slutningen af ​​trias-masseudryddelsen. Denne begivenhed banede vejen for, at dinosaurerne kunne blive dominerende i de næste 135 millioner år.

I den tidlige jura havde mange dinosaurgrupper dog endnu ikke nået enorme størrelser eller udviklet en række kropsformer og strukturer. Lesothosaurus betragtes ofte som en referenceart af videnskabsmænd for at udlede, hvordan de første ornithische dinosaurer ville have set ud.

"I øjeblikket har vi ikke beviser for nogen trias ornithischians," forklarer Paul. "Kommer fra den tidlige jura, gør dette Lesothosaurus til et af de tidligste medlemmer af gruppen, som vi kender til."

"Andre tidlige ornithischere er ret specialiserede eller kendt fra begrænsede mængder fossilt materiale, så Lesothosaurus er i øjeblikket den bedste model til at forstå, hvordan de tidligste medlemmer af denne gruppe ville have set ud."

Paul has previously published a paper, based on the remains of three Lesothosaurus specimens, which suggested that the dinosaur was a social animal. However, it could not be ruled out if the individuals were all buried at the same time, or if the remains had accumulated in the same place separately over time.

Excavations carried out by Paul and other scientists between 2009 and 2015 at Bramleys Hoek Farm in South Africa revealed a bed of at least 17 Lesothosaurus specimens clustered together, which all appear to have been buried at the same time.

"Until fairly recently, these dinosaurs were regarded as quite rare animals in these ecosystems, and the idea that they lived in groups, although it wasn't impossible, hadn't really been shown," Paul says. "We're gradually building a picture that these animals were actually abundant and quite sociable."

How did Lesothosaurus grow?

These fossils are also helping to reveal other information about how Lesothosaurus lived. The researchers analyzed the Bramleys Hoek Farm remains alongside other specimens to try and assess how these dinosaurs grew up.

Lead author Dr. Jennifer Botha, from South Africa's National Museum, Bloemfontein, investigated the bone microstructure preserved in the fossils.

"When an animal is fully grown, their bone microstructure changes as growth slows down," Jennifer explains. "There are certain features that characterize this, such as an absence of, or very few, blood vessels, as well as closely spaced lines of arrested growth similar to the annual rings found in a tree."

"While it may continue to grow slightly every year, by a millimeter or so, it will never grow significantly larger."

Collectively, this feature is known as an external fundamental system (EFS). A 2010 study suggested that such a structure was found in Lesothosaurus, indicating its maximum size. However, there was some evidence of growth after the formation of this structure, which may have meant it wasn't an EFS after all.

By looking at a group of 27 Lesothosaurus specimens, the researchers searched for further signs of an EFS to infer how large the dinosaur could grow.

"We didn't find evidence of an EFS in any of the fossils," Jennifer says. "The previous researchers may have found the beginnings of an EFS, but it could have represented a temporary, rather than permanent, halt in the animal's growth."

"Instead, we found that Lesothosaurus grew rapidly until it reached reproductive maturity, where it then invested more energy in reproduction over growth, between the ages of two and four. It is likely it reached its full size at around six years old, but as we didn't find an EFS it may have grown slightly larger."

The researchers are continuing fieldwork in South Africa in an attempt to uncover further Lesothosaurus specimens to try and address some of the ongoing puzzles over its biology. + Udforsk yderligere

A busy year for a little dinosaur

Denne historie er genudgivet med tilladelse fra Natural History Museum. Læs den originale historie her.