The Many Faces of Eoraptor, Dinosaur of the Moon

The “terrible reptiles” didn’t drop out of the sky, but one of the first was found in a lunar-like landscape.

Argentina’s Ischigualasto Badlands certainly look like “the Valley of the Moon” today, but they seemed even more alien back then, when Earth was still one giant supercontinent called Pangaea, surrounded by the impossibly vast Tethys Ocean.

It’s here, on this lush volcanic floodplain 228 million years ago, that Eoraptor took its first steps, ancestor to one of the three dinosaur groups – the meat-eating theropods like T-Rex, the long-necked sauropods like Brachiosaurus, and the bird- rather than lizard-hipped plant-eaters, like Triceratops. But which one?

Grandma- or Grandpa-Rex?

At just 0.5 m (1.5 ft) tall at the hip, Eoraptor was dinky and definitely not the top predator. It wasn’t the oldest dinosaur either – that crown belongs to Herrerasaurus – but it walked on two legs, had sharp front teeth and three clawed fingers with two vestigial bumps, because having five fingers would have made it horrifying. So it made sense that it was the origin of the two-legged theropods. But some of its other features nudged palaeontologists in a different direction.

Not a “real” dinosaur….

“Archosaur” is an umbrella term for several lizard-like groups at the time, including dinosaurs, winged pterosaurs, and the other early reptiles trundling about. Since Eoraptor looks so primitive, some argued that it’s not a dinosaur at all and closer to one of the other archosaur groups. This has largely been discounted, but for some, Eoraptor still hasn’t grown into its dinosaur boots.

…in the grownup sense?

This small evolutionary upstart had very large eyes, and some of its skull bones weren’t completely fused. To some palaeontologists this screams teenage awkwardness, so the most complete skeleton we have may be a juvenile. The same bones also set us on the trail of another possible revelation, which would affect more than just the size of Eoraptor itself.

Dawn snatcher rather than hunter?

Paul Sereno and his team discovered Eoraptor in 1991 when its teeth were spotted glinting from a rock. Amazingly the aforementioned skeleton was attached to them, which itself had felt someone else’s teeth before being buried in sediment. Closer examination showed that while the front teeth were sharp, those near the back were more suited for grinding up plant matter. Could it be that the dawn hunter was an opportunistic omnivore, content to grab whatever food was around? Dinosaurs hadn’t yet split into the different groups, so it makes sense that one of the earliest would eat plants, small prey and possibly insects. However, one of its cohorts threw a spanner in the works.

Earth-shaker, not meat-maker

Contemporary dino Eodromaeus, described in 2011, had the sharp serrated teeth and unmistakeable athletic features of a meat-eating theropod, throwing Eoraptor’s ambiguous teeth into sharp relief. When Sereno and his team examined Eoraptor’s skeleton further in 2013, they found evidence of arched nostrils, and an inset lower first tooth – traits common to the long-necked sauropods instead. So Eoraptor may have gone from ancestor of the carnivores to that of the biggest animals that ever walked the Earth.

Both are awesome descendants, so who could complain with either one in their family tree?

 

TLDR

Meaning: “Dawn hunter of the moon” (Eoraptor lunensis)

What? A small two-legged dinosaur, possibly one of the first ever

Where? Argentina, 228 million years ago during the Triassic Period

How big? 0.5 metre / 1.5 feet high (at the hip), 1.4 metre / 4.5 ft long

Probable motto: I raised so many questions, I may as well have come from the Moon.

Just to prove I’m not fibbing:

Eoraptor“. No date. Natural History Museum.

Kaplan, Matt. 2011. “Move over Eoraptor“. Nature.com.

Leary, Warren E. 1993. “Team finds bones of early dinosaur“. The New York Times.

Letters from Gondwana. 2016. “A brief introduction to the early dinosaurs of Argentina“.

Logan, Alan. No date. “Triassic Period“. Britannica.com.

Pellien, Jessica. 2010. “PGS daily dinosaur – Eoraptor lunensis“. Princeton University Press blog.

Sabrina, 2016. “I Know Dino Podcast Show Notes: Eoraptor (Episode 60)“. Iknowdino.com.

Sereno, Paul. No date. “Eoraptor lunensis“. University of Chicago.

Sereno, P.C. et al. 2013. “Osteology of Eoraptor lunensis (Dinosauria, Sauropodomorpha)“. Journal of Vertebrate Paleontology 32: Memoir 12:83-179.

Speer, B.R. 1997. “Introduction to Eoraptor“. University of California, Museum of Paleontology.

Speer, B.R. 1998. “Ischigualasto Formation, Argentina“. University of California, Museum of Paleontology.

Strauss, Bob. 2017. “Facts about Eoraptor, the world’s first dinosaur“. Thoughtco.com.

 

Featured image credit: “Eoraptor”, by Luis V Rey.

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