♪♪
This is the enormous skull of Tyrannosaurus rex.
Many, many times bigger than mine.
And these are the bones of its forelimbs...
...relatively tiny.
And there are other dinosaurs in which the difference in size is even more marked.
Why should we think that these tiny limbs got anything to do with display?
(Roaring)
Dr. Darren Haish: Everyone knows that T-rex had small arms.
Dr. Darren Haish: But dozens of other predatory dinosaurs
from all around the world had small arms too.
♪♪
Dr. Darren Haish: They all evolved from earlier dinosaurs that had much longer arms.
Dr. Darren Haish: But over millions of years these dinosaurs
Dr. Darren Haish: came to rely more on their mouths for holding and killing prey.
Dr. Darren Haish: So their heads became more powerful and their arms became smaller.
♪♪
David Attenborough: Some of the smallest arms relative to body size
David Attenborough: belong to the South American dinosaur Carnotaurus.
David Attenborough: When the bones of these arms were first discovered
David Attenborough: it was assumed that they would be of little use
David Attenborough: and were evolutionarily leftovers,
David Attenborough: much like a human's tiny tail bone.
♪♪
David Attenborough: The closer look at the bones revealed
David Attenborough: that these tiny arms also probably lacked any claws.
♪♪
Carnotaurus's upper arm bone, the humerus,
up here, has this weird ball and socket joint.
Prof. John Hutchinson: It's very round at the upper end.
And that suggests that its shoulder joint had quite a bit of mobility.
David Attenborough: And they were attached to the shoulder with lots of powerful muscles.
♪♪
David Attenborough: Evidence that they were used for something.
♪♪
David Attenborough: But could they have been used during courtship?
♪♪
Carnotaurus's tiny arms were too small for anything like digging or fighting opponents.
Dr. Darren Haish: The most likely explanation for such small yet mobile arms
Dr. Darren Haish: is that they were used in display.
In the natural world today,
highly specialized body parts that appear to have no other function
often turn out to be used for display and to attract mates.
♪♪
David Attenborough: These range from colorful feather plumes...
♪♪
David Attenborough: to long cumbersome horns and antlers.
♪♪
David Attenborough: In many cases, the owner of such appendages puts up with the cost of having them
David Attenborough: because they're so attractive to potential mates.
♪♪
David Attenborough: In modern day birds and reptiles
David Attenborough: iridescent colors also help physical displays to stand out.
David Attenborough: And the same could be true for Carnotaurus's arms.
♪♪
David Attenborough: A wide range of arm movement and a show of bright colors
David Attenborough: is certainly eye catching.
♪♪
David Attenborough: But in the end...
♪♪
David Attenborough: ...potential mates can still be very choosy.
♪♪