Self-study
Dinosaur expert: Some paleontologists have claimed that birds are descendants of a group of dinosaurs called dromeosaurs. ████ ██████ ██ ███ ██████ ███████ █████ █████████ ████ ███████████ ████ ███████████████ ████ ███████ ██ █████ ████ ██ ████ ██████████ ███ █████ ██ █ █████ ████ ██ █████ █████████ ███ ████████ ████ ███████ ████ ████ ████ ██████████ ████ ████ ████ ██ ████████ ██ █████ ███████ ████ ███ ██████ █████ ██████████ ████████ ████ ███ ████████████████ █████ ██ ██████
Summary
The author concludes that birds are not descendants of dinosaurs called dromeosaurs. Why? Because the earliest bird fossils that have been discovered date much further back in history than the oldest known dromeosaur fossils.
Notable Assumptions
The author assumes that the fact the earlier bird fossils are older than the earlier dromeosaur fossils proves that birds originated before dromeosaurs did.
The author assumes that there do not exist undiscovered dromeosaur fossils that are older than the earliest bird fossils that have been discovered.
22.
The expert's argument depends on ████████ █████ ███ ██ ███ ██████████
a
Having similar characteristics ██ ███ █ ████ ████ █████ ██ ███████ ███ ██████████████ ████████
The author doesn’t deny the idea that there’s an evolutionary relationship between birds and dromeosaurs. For example, the author could believe that dromeosaurs evolved from birds, or that they each evolved from a common ancestor.
b
Dromeosaurs and birds █████ ████ ██████ ██████████
Not necessary, because the author might believe there are no common ancestors between birds and dromeosaurs. All that the author is committed to about the relationship between the two is that birds did not evolve from dromeosaurs.
c
Knowledge of dromeosaur ███████ ███ ███ ████████ ████ ███████ ██ █████████
Too extreme to be necessary. The author doesn’t have to assume that our knowledge of the fossils is “complete.” The author just needs to assume that our knowledge of the fossils tells us that birds originated before dromeosaurs. Whether we’re missing a tooth from a skeleton or whether we’re missing some more recent fossils doesn’t affect the reasoning.
d
Known fossils indicate ███ ████████ █████ ██ ██████ ██ █████ ███ ████████████
Necessary, because if known fossils do NOT indicate the relative dates of origin, then what we know about bird and dromeosaur fossils doesn’t support a conclusion that birds couldn’t have evolved from dromeosaurs. After all, dromeosaurs could have originated before birds, so birds still could have evolved from them.
e
Dromeosaurs are dissimilar ██ █████ ██ ████ ███████████ █████
Not necessary, because the author could agree that dromeosaurs are actually very similar to birds. That similarity may be due to dromeosaurs evolving from birds, or they both evolved from a common ancestor, or to pure coincidence.