saurischian

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Citipati osmolskae

Mongolia, Late Cretaceous Period (Djadochta Formation)

I made this illustration for Earth Archives - you can read more about the strange and wonderful dinosaurs of the Gobi Desert there.

The inflatable display structure on the male Citipati is highly speculative, but not outside the realm of possibility. Modern birds like frigatebirds and prairie chickens have similar structures, but evidence for them would be difficult to recover from the fossil record.

[Please don’t use or reproduce without permission, and thanks for viewing!]

EORAPTOR
“Dawn thief”
Early Triassic, 231.4 million years ago

Eoraptor was only about 3 feet long, but this little guy may have been the ancestor of half the dinosaurs! Living in the late Triassic period, this small, bipedal omnivore (that’s right – omnivore!) has been classified by some as a basal saurischian – the taxonomical order that includes theropods and sauropods. Quite the legacy, indeed! It was given the specific name Eoraptor lunensis (meaning “dawn thief from the Valley of the Moon”), presumably because it was discovered by LARPers.

plosone.org
PLOS ONE: What Lies Beneath: Sub-Articular Long Bone Shape Scaling in Eutherian Mammals and Saurischian Dinosaurs Suggests Different Locomotor Adaptations for Gigantism

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Abstract

Eutherian mammals and saurischian dinosaurs both evolved lineages of huge terrestrial herbivores. Although significantly more saurischian dinosaurs were giants than eutherians, the long bones of both taxa scale similarly and suggest that locomotion was dynamically similar. However, articular cartilage is thin in eutherian mammals but thick in saurischian dinosaurs, differences that could have contributed to, or limited, how frequently gigantism evolved. Therefore, we tested the hypothesis that sub-articular bone, which supports the articular cartilage, changes shape in different ways between terrestrial mammals and dinosaurs with increasing size. Our sample consisted of giant mammal and reptile taxa (i.e., elephants, rhinos, sauropods) plus erect and non-erect outgroups with thin and thick articular cartilage. Our results show that eutherian mammal sub-articular shape becomes narrow with well-defined surface features as size increases. In contrast, this region in saurischian dinosaurs expands and remains gently convex with increasing size. Similar trends were observed in non-erect outgroup taxa (monotremes, alligators), showing that the trends we report are posture-independent. These differences support our hypothesis that sub-articular shape scales differently between eutherian mammals and saurischian dinosaurs. Our results show that articular cartilage thickness and sub-articular shape are correlated. In mammals, joints become ever more congruent and thinner with increasing size, whereas archosaur joints remained both congruent and thick, especially in sauropods. We suggest that gigantism occurs less frequently in mammals, in part, because joints composed of thin articular cartilage can only become so congruent before stress cannot be effectively alleviated. In contrast, frequent gigantism in saurischian dinosaurs may be explained, in part, by joints with thick articular cartilage that can deform across large areas with increasing load.

Excavating the River of Giants
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In the American Museum of Natural History’s Hall of Saurischian Dinosaurs, there is a great fossil mismatch. You can find the deceptive pairing in the Apatosaurus exhibit. Set in the floor behind the enormous dinosaur is a set of trackways—the Apatosaurus is posed as if the skeletal sauropod has just left the tracks behind. But there is no way that Apatosaurus left those tracks. The footprints and the long-necked dinosaur on display were separated by tens of millions of years.

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