Earth Sciences 2266F/G Lecture Notes - Lecture 19: Metatarsal Bones, Tetanurae, Compsognathus

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Lec 19
After the previous group, tetanurans were developed and they were huge; their skulls were
deeper and thus bigger prey-eating
Limb bone and back bone got very big as well
In the height of tetanuran evolution, it includes the Allosaurus; during the Jurassic, this evolved
into the largest carnivore (Cretaceous was T. Rex)
In all evolution of the theropods and to some degree the plant-eating ornithipods, their hind legs
get very big, but front limbs get very small
Their front legs aren’t “shrinking,” it’s the same as the previous evolutionary group, just the hind
legs got bigger
A tiny piece of bone starts to evolve at this stage of evolution, called the furcula (fused
clavicles), which is a bird feature (in us, we have the shoulder bones)
In Coelophysis, clavicles were separate, but in the tetanurans it has fused
Later on in velociraptor stage, the furcula becomes smaller
And then in birds, the furcula becomes more V-shaped to control flying
At the tetanuran evolutionary stage, that is the first appearance of the true furcula which is a bird
feature
This is at the same time that archaeopteryx was also flying around at the same time, which had a
much better developed furcula
That means furcula evolved much earlier, and the Allosaurus is a crown group since Allosaurus’
furcula is still very primitive
Next group that evolved was the coelurosaurs; coelo implies hollow, so their bones start to
become hollow, which is a bird feature (e.g. in chickens, their bones are quite hollow)
Many more bird features will add on as evolution continues, incl. development of feathers and
wings; these may develop within the coelurosaurs
Basal coelurosaurs have feathers developed on the tail and front body
Sinosauropteryx: Sino = Chinese, sauro = reptile, teryx = wings
Despite its primitive look, it’s already a crown group
First bird, Archaeopteryx, is already in Jurassic, and this is Cretaceous so its ancestors already
developed in the Jurassic
Also legs lengthen, which happens through the elongation of metatarsal bones
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2 groups: Sinosauropteryx and Compsognathus
Compsognathus was fairly small, and before, it was constructed with skin but now
reconstructions have feathers since they have the same skeleton as Sinosauropteryx
So this is early stage of coelosaurus evolution
No contoured feathers, just down feathers throughout the body; so feathers not for gliding, just
for keeping themselves warm
Recent studies: most feathers were black and not colourful
Black colour absorbs heat very well and most dinosaurs were quite small, not running with the
Allosaurus, so in marginalized areas, they evolved to those marginalized environments which
may be quite colder
How does T. Rex fit in the sequence of evolution of birds?
Its legs: metatarsal portion stood up, and the 3 metatarsal bones start to fuse together
This is a bird feature (e.g. chicken leg bones are fused together)
Fused metatarsals is called the arctometatarsus state, which is seen in the T. Rex
Not all members in T. Rex group got big, just T. Rex did
Also got a long ilium (in pelvic girdle), which fused with vert column (like birds); ilium no
longer separate from sacrum
These 2 features in T. Rex: prominent bird-like features
- No feathers ever found on T. Rex bones, but if this reconstruction is right, they inherited
feathers
Things started to look more like ostriches than dinosaurs
Limb bones start to fuse together; ulna/radius/hands start to fuse
Elongation of legs, so good runner
Caudipteryx: bundles of feathers preserved and elongation of legs
This is called the Raptor group, beginning to see feathers very well
Down feathers, but now there’s starting to have contoured feathers (symmetrical)
A feather that is asymmetrical (one side bigger) is good for flying
But here, it’s still symmetrical
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