ulna/radius tibia/fibula

Q: I was wondering how to distinguish the difference between ulna and radius and between tibia and fibula?

A: For our purposes, lets just say the ulna is closer to the higher digits (4 and 5) and it projects to form the elbow protrusion. It is also broader proximally and narrower distally. The radius tends to be smaller and closer to the lower digits (1 and 2). For the hindlimb, just go with the tibia is bigger than the fibula.

determing absolute age by half lives

Q: I was wondering how to determine the absolute age by using half lives. I looked at the
final practice questions and was unable to answer the first five or so questions. The
powerpoint on stratigraphy doesn't really explain how to solve these types of problems
very well. I looked over the slides a few times and am still confused. Thanks.

A: Radioactive decay occurs at a constant rate. As a result, you can use it like a clock. The rate of decay is measured in half lives. The half life of carbon is roughly 5000 years. Carbon-14 decays into Nitrogen-14. Carbon-14 is called the parent isotope, Nitrogen-14, the daughter isotope.

Say you have 16 atoms of carbon 14. After ~5000 years, half of the carbon has decayed away, so there are 8 atoms of carbon 14, 8 atoms of nitrogen. What about after 10,000 years? How many atoms of Carbon 14? (Answer: 4). How many total atoms of N-14? (Answer: 12 = 8 oldones, 4 new ones). After 15,000 years?

A Problem: You have find a fossil snail shell. It has carbon in it, so you can date it using Carbon-14 (C-14) methods. Let’s say you find that you have 15 grams of nitrogen-14 (N-14), and 1 gram of carbon 14. Assume all N-14 that’s there is the result of carbon 14 decay. So, when the snail was alive, it didn’t have any N-14 in it. How old is the snail?

Answer:

OK: how much C 14 was there to start?

15 + 1 grams = 16 grams.

How much carbon is left?

1 out of 16 original grams.

How many half lives is that?

Half-life #1: Carbon goes from 16 -> 8

Half-life #2: 8 -> 4

Half-life #3: 4 -> 2

Half-life #4: 2 -> 1

Four half-lives, each 5000 years: 4x5000 = 20,000 years.

Final Review Question 21

Q: Does a mesotarsal ankle provide us with enough evidence to determine
whether an archosaur with a mesotarsal ankle could be quadrapedal or
bipedal?  Since it's an evolutionary trait for dinosaurs, and some
dinosaurs were quadrapedal and others were bipedal, does that mean a
Marasuchus, who had a mesotarsal ankle, could have been either?

A:  Good Point!  The question relating to this is misleading.  The question is supposed to be about the ankle joint telling us that marasuchus-like archosaurs are very closely related to dinosaurs, and therefore, since marasuchus-like dinos are all bipedal, the first dinosaurs are also thought to be bipedal.   This is also supported by the earliest known saurischians and ornithischians all being bipedal.

UPDATED: 1-2-3 hand and 2-3-4 foot - whose novelties?!

Q:  On the cladograms, a "1-2-3" hand and "2-3-4" foot are evolutionary
traits for dinosaurs but for theropods on the saurischian chart, these
evolutionary traits are also listed for theropods.  Since theropods
are dinosaurs, is it correct to name these traits again?  Is there a
reason they are named again?

A:  There is no good reason!  And it is incorrect since they are supposed to be evolutionary novelties labeled on the cladograms.  They are novelties of dinosaurs, not theropods.  I did talk in lecture about how many of the features of theropods being discussed were not novelties, but they should not have made it onto the cladogram.  Sorry about that!

Q: In regards to the "1-2-3 hand" and "2-3-4 foot" novelty, does that mean there's no real answer to question 41 on the final review?  I am confused however because theropods are known for the loss of the 4-5 finger and 1-5 toe, but since this is a novelty for all dinosaurs, that means it is no longer exclusive to theropods right?

A: OK you caught me again, I jumped the gun saying that the novelty was entirely with the dinosauria rather than the theropoda.  What I should have said was the dinosaur novelties were more asymmetrical hands and feet: reduction of digits 4 and 5 of the hand and digits 1 & 5 of the foot.  Then theropods lose hand digit number 5 entirely, and later tetanurans lose hand digit number 4 entirely.  Foot digit number 1 is severely reduced in theropods and moved to a separate position (and later becomes bigger again in the avian lineages).  Foot digit number 5 is severely reduced and then I believe completely lost in some lineages, but I'm not sure which. 

Thanks for pushing me to get it right! 

Final Review Question 42

Q: I was able to label digit 1 as the one in the back but was unable to figure out what group of theropoda it belonged to.

A: This is a bird since it has a reversed hallux.
See also the post above this about digit numbers.

Final Review Question 26

Q: When you ask coexist, do you mean same area or same time? Because according to your slides, they were alive at the same times but not in the same areas. Wouldn't both of them have met a Tyrannosaurus because both were alive in the Cretaceous?

A: I didn't specifically say this in lecture, but you can be pretty sure that the ankylosaurs would have seen a Tyrannosaur as they were so widespread in the cretaceous. So it must be the stegosaurs and it must be because Tyrannsaurs have not been found in India.

Final Review Question 16

Q: I understand that the pubis is more anterior than the ischium in the pictures. For the second part of the question it asks whether this is always the case and according to the pictures in the textbook it is but I wanted to make sure I wasn't missing anything.

A: Yes, the pubis is always more anterior than the ischium